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Raft Foundations Design and Analysis with a Practical Approach

SHARAT CHANDRA CUPTA Advisor, Indian Buildings Congress, Former Chief Engineer Central Public Works Department

PUBLISHING FOR ONE WORLD

NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS

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New Delhi Bangalore Calcutta Chennai Guwahati Hyderabad Lukhnow Mumbai Pune

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Copyright O 1997 New Age International (P) Limited, Publishers NEW A G E INTERNATIONAL (P) LIMITED, PUBLISHERS NEW DELHI BANGALORE

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CALCUTTA CHENNAI

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GUWAHATI HYDERABAD

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LUCKNOW MUMBAI

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PUNE

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4835124, Ansari Road, Daryaganj, New Delhi-110 002 35, Annapoorna Complex, South End Road, Basavangudy, Bangalore-560 004 4018, Ballygunge Circular Road, Calcutta-700 019 20, IInd Main Road Kasthuribai Nagar, Adyar, Chennai-600 020 Pan Bazar, Rani Bari, Guwahati-781 001 1-2-41219, Gaganmahal, Near A.V. College, Domalguda, Hyderabad-500 029 18, Madan Mohan Malviya Marg, Lucknow-226 001 1281A. Noorani Building, Block No. 3, First Floor. L.J. Road, Mahim, Mumbai-400 016. 44, Prashant Housing Society, Lane No. 6, Paud Road, Kothrud, Pune-4 1 1029.

This book cr any-part there of may not be reproduced in any form without the written permission of the publisher

This book is not to be sold outside the country to which it is consigned by New Age International (P) Limited

ISBN :81-224-1078-2

Published by H.S. Poplai for New Age International (P) Limited, 4835124, Ansari Road, Daryaganj, New Delhi- 110002. Typeset by EPTECH, and printed ai Ram Printograph, C-114, Okhla Industrial Area, Phase I, New Delhi-110020. Printed in India Production : M.I. Thomas

CONTENTS Preface i

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1.

INTRODUCTION

2.

NEED OF RAFT FOUNDATION

3.

TYPES OF RAFT FOUNDATION

4.

SURVEY OF AVAILABLE LITERATURE Foundation Engineering by Peck, Hansen and Thornburn 1 Foundation Design and Practice by Elwyn. E.S. Seelye 4.2 4.3 Foundation Design by Teng Foundation of Structures by Dunham 4.4 4.5 Indian Standard Code of Practice for Design and Construction of Raft Foundation - IS 2950-1965 Raft Foundation - The Soil Line Method of Design by A.L.L. Baker Indian Standard Code of Practice for Design and Construction of Raft Foundation 1.S : 2950 (Part-I) 1973 Foundation Engineering Handbook Edited by' Hans F. Winterkorn & Hsaiyang Fang Foundation Analysis and Design by Joseph. E. Bowels Building Code Requirements for Reinforced Concrete (ACI 318 - 77) Foundation Design and Construction by M.J. Tomlinson Design of Combined Footings and Mats ACI Committee 336 Pile Foundation Analysis and Design by H.G.Poulos and E.H. Davis 1980 Reinforced Concrete Designers Handbook by Charles E. Reynolds and James C. Steedman - 9th Edition 1981 IS 2950 (Part I) 1981 -Code for Design and Construction of Raft Foundation Part I ~esi~n Eleventh International Conference of Soil Mechanics and Foundation Engineering San Francisco, August 12 - 16,1985 Foundation Design and Construction by M.J. Tomlinson, 5th Edition, 1986

CONTENTS

Handbook of Concrete Engineering -Mark Fintel - 2nd Edition, 1986 Reinforced Concrete Designer Handbook by Charles E. Reynolds and James Steedman, 10th Edition, 1988 Building Code Requirements in Reinforced Concrete - ACI - 3 18 - 1989 Foundation Engineering Hand book by Hsai-Yang-Fang 2nd Edition, 1991 Design of Combined Footings and Mats - ACI committee 336 2R - 88 Published in ACI Manual 1993 Foundation Analysis and Design by Bowles, 4th Edition, 1988 Proceedings of Indian Geo-Technical Conference 1992, Calcutta, December, 1992 Designs of Foundation Systems - Principles and Practices by Nainan P. Kwian, 1992 13th International Conference on Soil Mechanics and Foundation Engineering, New Delhi, January, 1994 Soil Structure Inter-action -The Real Behaviour of Structures, published by the Institution of Structural Engineers, U.K. The Institution of Civil Engineers, U.K. International Association for Bridge and Structural Engineering in March, 1989

5.

DESIGN APPROACH AND CONSIDERATIONS 5.1 Rigid Approach 5.2 Flexible Approach 5.3 Parameters for Raft Design Pressure Distribution Under the Raft 5.4 5.5 Rigidity Criteria 5.5.1 Proposed by IS : 2950 (Part I) 1981 5.5.2 ACI Committee, 336 5.5.3 Hetenyi's Criteria 5.6 Modulus of Sub-Grade Reaction 5.6.1 Recommended by Bowles 5.6.2 IS : 2950 Part I Indian Standard Code of Practice for Design and Construction of Raft Foundation 2950 - 1981 5.6.3 I.S. 9214-1979 - Method of Determination of Modulus of Subgrade Reaction (k value) of Soils in Field 5.6.4. IS 8009 - Part I - 1978. Code of Practice for Calculation of Settlements of Foundations - Part I - Shallow Foundations. Subjected to Sy_mmetrical Static Vertical Load. 5.6.5 Recommendation by Alpan and Prof. Alarn Singh 5.6.6 Summary

6.

STRUCTURALDESIGNERS DILEMMA

7.

STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERSON DESIGN OF RAFT 38 7.1

Study 1 7.1.1 Examples Selected 7.1.2 Raft Size

40 41 41

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CONTENTS

7.1.3

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Soil Investigation

7.1.4 Load Considered in Study 7.1.5 Analysis 7.1.6 Discussions of Results 7.1.7 Conclusions Study 2 -Effect of Horizontal Loads 7.2.1 Example Selected 7.2.2 Discussion of Results 7.2.3 Conclusion Study 3: Comparison with Conventional Rigid Methods Details of Conventional Method: Combined Footing Approach 7.3.1 7.3.2 Examples Selected 7.3.3 Discussion of Results 7.3.4 Inverted Floor Method 7.3.5 Conclusions Study 4. Another Office Building 7.4.1 Example Details 7.4.2 Comparison of Results 7.4.3 Discussions of Results 7.4.4 Conclusions '

7.2

7.3

7.4

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STUDIES CARRIED OUT ON ANALYSIS AND DESIGN OF PILED RAFTS 8.1 8.2

Design Procedures being Used Example Selected

8.3 8.4

Soil Data Methods of Analysis Studied 8.4.1 Conventional Rigid Method with Simplified Models 8.4.1.1 Combined footing approach 8.4.1.2 Continuous beam analogy :inverted floor 8.4.1.3 Comparison of results 8.4.2 Piled RafPAnalysis Based on Finite Element Approach Study of Parameters Influencing the Raft Behaviour 8.5.1 Effect of Raft Stiffness on the Pile Loads and Raft Moments 8.5.2 Effect of Superstructure and Retaining Walls on Foundation Stiffness 8.5.3 Effect of Earthquake Loads and Moments 8.5.4 Effect of End Bearing and Friction Piles 8.5.5 Summary of Results

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8.6 8.7

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Discussions Conclusions

JOINTS IN RAFl'S SUMMARY OF STUDIES

CONTENTS

11.

FACTORS AFFECTING CHOICE OF MET,HODOF ANALYSIS

12. GUIDELINES APENDM - ILLUSTRATIVE EXAMPLES A.l Conventional Rigid Method - Combined footing approach A.2 Flexible Raft - Beam on elastic foundation A.3 Piled Raft-Plate on elastic foundation

INTRODUCTION I i t

In 1957, when the author was a student of Civil Engineering at the Indian Institute of Technology, Kharagpur, the first institute of national importance, one of his professors of Civil Engineering at his first lecture in the class said: "Civil Engineering is 50% common sense but common sense is that sense which is quite uncommon. " After 34 years of experience in Civil Engineering construction and design, the author only wonders how true the statement of his Professor was and how much more it is true in case of foundation engineering.

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1.1 Foundation engineering has been practised as an art, without help of science, since time immemorial upto 1920 when it had achieved a considerable amount of refinement. It was in the earlier 1920s that a concerted effort was made to study and undentand the physical laks governing the behaviour of sub surface materials, i.e.. soil from which foundations derived their support and on whose behaviour its own behaviour depends. This is the time when study of soil mechanics was started and it was in 1919 when Karl Terzaghi, popularly known as 'father of soil mechanics', made successful attempt to explain the phenomenon of settlement oti a scientific basis. Though study of soil mechanics has provided us with new techniques for selecting appropriate type of foundation and predicting the behaviour of completed structures, it has not been able to decrease the importance of the accumulated experience of the ages. Amount of uncertainty and degree of variation in the properties of soil and number of parameters on which performance of a foundation depends, make exact solution impractical, if not impossible. With so much of advancement in science and computer application, structural design is still defined as:I5 a creation of a structuralfonn to satisfy a number of requirements. It is a combination of art and science. As a rule, there is no direct procedure leading to the solution of a specific problem. An engineer uses all his resources of knowledge experience and imagination to produce a trial scheme. He then constructs a mathematical model of such a solution to assess its adequacy and ifnecessary, modifies the original concept in the light of analytical results. The process is repeated until the designer is satisfied with thefinalproduct, taking into account not only structural adequacy but also such non-quantifiablefactors as aesthetics, ease of construction and performance. The design process is characterised by a complex interaction of parameters and the need to arrive at decisions based on incomplete data Intuitive decisions which have to be taken, appear to be diametrically opposite to the logical nature of ... '

2

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

Foundation design and analysis is, at a stage behind structural analysis and design for superstructure, and even now continues to be practised more as an art and will probably continue to be done so, for many years to come.

1.2 Available textbooks, handbooks, various publications and papers give widely different approaches to design of raft foundations. A designer, when faced with a task of designing a raft foundation, finds himself in a precarious position where he has to balance the time available for design, the cost of design, the need of adequate safety and, above all, acceptance of the design by the client and the professional community in general and decide the method of design to be followed by him. Generally, it is not practical for any designer to go through the various approaches as available in engineering literature at a particular time, compare their merits and demerits and select the most suitable for his purpose. He, therefore, perforce selects a particular textbook and applies the same to his problem, quite often little realising that the theoretical problem dealt with in the textbook is widely different from his practical problem relating to an actual building. Resulting solution may not be as satisfactory as he feels. An effort has been made in the following chapters to explain the various approaches suggested in literature, give their comparative limitations, examine the implications of the so-called more sophisticated approaches and finally make recommendation for the method which can be followed by a designer till he accumulates enough experience so as to select his own method particularly applicable to his problem. Intention of this publication is not to hinder initiative of an individual in going deeper in any problem, but to give him a comparative idea of available approaches with sufficient number of references which he can study during the beginning of his profession and formulate his own opinion in due course but still continuing to design satisfactory raft foundations. This publication should, therefore, be studied in this background.

NEED OF RAFT FOUNDATION. Raft or Mat foundation is a combined footing that covers the entire area beneath a structure and supports all walls and columns. This raft or mat normally rests directly on soil or rock, but can also be supported on piles as well. Raft foundation is generally suggested in the following situations: (a) Whenever building loads are so heavy or the allowable pressure on soil so small that individual footings would cover more than floor area. (b) Whenever soil contains compressible lenses or the soil is sufficiently erratic and it is difficult to define and assess the extent of each of the weak pockets or cavities and, thus, estimate the overall and differential settlement. (c) When structures and equipment to be supported are very sensitive to differential settlement. (d) Where structures naturally lend themselves for the use of raft foundation such as silos, chimneys, water towers, elc. (e) Floating foundation cases wherein soil is having very poor bearing capacity and the weight of the super-structure is proposed to be balanced by the weight of the soil removed. (f) Buildings where basements are to be provided or pits located below ground water table. (g) Buildings where individual foundation, if provided, will be subjected to large widely varying bending moments which may result in differential rotation and differential settlement of individual footings causing distress in the building. Let us now examine each of the above situations in greater detail.

2.1 In case of soil having low bearing pressure, use of raft foundation gives three-fold advantage: (a) Ultimate bearing capacity increases with increasing width of the foundation bringing deeper soil layers in the effective zone. (b) Settlement decreases with increased depth. (c) Raft foundation equalises the differential settlement and bridges over the cavities. Every structure has a limiting differential settlement which it can undergo without damage. The amount of differential settlement between various parts of a structure supported on a mat foundation is much lower than that if the sarne.structure was supported on individual footings and had undergone the same amount of maximum settlement. With these considerations, maximum total settlement which

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

can be allowed for a particular structure on mat foundation is more than what is permitted when the structure is resting on individual footings. This, therefore, allows a higher bearing capacity for such situations. It may, however, be noted that if in a case deeper layers of soil are of very poor quality, increase in width of the foundation may not always lead to higher bearing capacity. In situation where comparatively s h a l l y top layers of soil are underlain with deeper layers of much poorer soils, it may be advantageous to provide individual footings so that the zone of influence of the footings remains within the top stronger layer. In such a situation, provision of a mat foundation may be disadvantageous.

2.2 Some designers work on the rule that if more than 50%of the area of the structure is occupied by individual footings, it is necessary to provide an overall raft. This is not true and quite often, the quantity of reinforcing steel and concrete required to avoid excessive deflection and cracking of a raft carrying unequal column loads, necessitating carry-over of stresses from one part of the raft to the other part, may be large and may make raft foundation uneconomical. In such situations, it may be more economical to excavate the entire site to a level formation, construct individual closed space footings (sometimes touching each other) and then backfill around them. In these cases, however, one must weigh form work costs against the extra footing material required by using mat foundation. It should be considered that it is possible to construct alternate footings by using spacer pads against already laid footings and thus save form work cost. Quite often, doubt exists about the structural behaviour of individual footings touching each other. This problem of interaction of footings has been studied by many researchers. It has been reported that the effect of adjacent footings may vary considerably with the angle of shearing resistance. For low values, they are negligible though for high values they appear to be significant, particularly if a footing is surrounded by other footings on both sides. It is also stated that these effects are considerably reduced as length over breadth ratio of the footings approaches unity. There are practically no such effects in the case of punching shear failure. For these and other reasons, it has been recommended that interference effects need not be considered in designs. Adesigner should, however, be aware of the possibility of their existence in some special circumstances 11. 2.3 Situations exist in practice w h p a soil stratum contains compressible lenses or the soils have a formation where individual layers of soil are neither parallel nor can be reasonably stratified into different layers of known properties to enable calculations of settlement to a reasonable accuracy. In such situations, individual footings, if provided, would undergo widely varying settlements resulting in large differential settlement which cannot be tolerated by the structure. 2.4 Situations, as mentioned in (c) and (d) above, are explicit and do not require further explanation. These are special cases, and adoption of raft foundation is more or less necessary by the particular nature of the problem involved. 2.5 In cases where soil is very soft and highly compressible and the buildings cannot be founded on such soils in normal circumstances, it may be possible to provide the building with a basement in such a manner that weight of the structure is equal to the weight of the soil removed and, thus, there being no change in the stresses in the soil beneath the basement and, therefore, little settlement. However, in practice it is rarely possible to balance the loading so that no additional pressure comes on the soil. However, in such cases still, it is only a part of the total load which comes on the bottom soil and, thus, it is possible to construct a building inducing a much larger load than the soil would have otherwise supported. The basement provided, gives additional space in the building for the owner and can be made use of. However while constructing such foundations,

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NEED OF RAFT FOUNDATION

5

reconsolidation of the soil, which has swelled as a result of removal of over burden pressure in excavating for the sub-structure, should always be considered and necessary steps be taken to prevent detrimental effects.

2.6 Basements located below ground water table should use a mat as their base to provide water tight

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c.onstruction.The alternative of having individual columns footings connected by thin slabs has not proved to be successful in most of the cases; presents difficulties in water proofing; causes concentration of stresses at the junction of the thin slabs and footings and also at the junction of basement walls and raft causing cracks to develop. This arrangement, therefore, should not be resorted to unless the economy is of such a magnitude as to outweigh all other considerations. Even in cases where sub-soil water level is low and basement does not extend below ground water table, long-term built up of surface water accumulating against basement walls and bottom should be allowed for. This is particularly so in case of impermeable soils (permeability co-efficient below 0.1 mm per second) or of large surface areas draining towards the building. i.e., areas on sloping ground near hillocks. The basement walls should also normally be designed as self-supporting cantilever retaining walls even though they may eventually be strutted by floor construction. It is inconvenient and often impossible to provide temporary raking struts to support a basement retaining wall until such time as strutting given by ground floor or intermediate basement floor is completed.

2.7 Situations also arise when isolated footings are subjected to very large eccentric loadings, and one is faced with the possibility of excessive footing rotation, excessive differential settlement or possibility of exceeding the allowable bearing capacity of the soil at some location. This can happen when the building consists of shear walls and columns, shear walls sharing most of the horizontal load subjecting its footings to larger settlements and rotation, decreasing the effectiveness of the shear walls and also creating difficulties by way of large differential settlements. Raft, if provided, will even out these deformations. Mats or rafts are supported on piles'in cases where sub-soil conditions warrant provision of piles, but one has to have the basement. In such situations, raft also helps in making the basement water tight. It would, therefore, be seen that it is not possible to lay down hard and fast rules defining situations wherein a raft foundation is required. The author, therefore, opines that every designer should learn all that he can within reason about the conditions at site, determine the types of foundations that are practical, compare their cost, suitability, ease of construction, safety and select a type which in his judgement would serve the purpose well. There can always be differences of opinion about the solution decided by him, but as already mentioned in chapter I , it cannot be helped because foundation design still continues to be practised more as an art than an exact science. Two artists seldom agree.

TYPES OF RAFT FOUNDATION Raft can be classified into various types on the basis of criteria used for classification.

3.1 Based on the method of their support, raft can be: (a) Raft supported on soil, (b) Raft supported on piles, and (c) Buoyancy raft. 3.2 On the basis of structural system adopted for the structure of the raft, these can be classified as: (a) Plain slab rafts which are flat concrete slabs having a uniform thickness throughout. This can be with pedestals or without pedestals. (b) Beam and slab raft which can be designed with down stand beam or upstand beam systems. (c) Cellular raft or framed raft with foundation slab, walls, columns and one of the floor slabs acting together to give a very rigid structure. Raft of uniform depth is most popular due to its simplicity of design and construction. This type is most suitable where the column loads are moderate and the column spacing fairly small and uniform. Pedestals are utilised to distribute the load on a bigger area in case of heavy column loads. 3.3 Slab and beam raft is used as a foundation for heavy buildings where stiffness is the principal requirement to avoid excessive distortion of the super structure as a-resultof variation in the load distribution over the raft or the compressibility of the supporting soil. These rafts, however, have many obvious difficulties. If the beams are deep, ribs placed below the basement floor or raft, the bottom of the excavation becomes badly cut up with trenches, impairing the bearing value of the soil because of its disturbance.Water proofing in case of basements becomes more complicated arid involved. If the beams are projecting up, usefulness of the basement is destroyed unless the entire foundation is lowered and the gap filled up or an upper slab is provided supported on these inverted beams to form the ground floor of the structure.

3.4 Buoyancy raft are necessarily to be provided with a basement so that the weight of the soil removed balances to a large extent, the imposed load. Cellular raft consisting of foundation slabs, walls, columns and ground floor slab can be designed, but it creates considerable amount of uncertainties, difficulty of construction and quite often even in such cases, raft is designed as a slab of uniform rhickncss.

7

TYPES OF RAFT FOUNDATION

Raft, as a slab of uniform thickness, has an additional advantage of providing better water-proofing treatment ease of reinforcement fabrication and laying of concrete. This type of raft is most commonly used. Various types of rafts are shown in Fig. 3.1

F T S U P P O R T E D ON SOIL --------------------- R--A-------------------

RAFT SUPPORTED ON PILE

BUOYANCY RAFT -------------

FLAT PLATE RAFT

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FLAT PLATE WITH PEDESTALS

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BEAM AND SLAB RAFT ------------------

Fig. 3.1 Various types of rafts

FRAMED RAFT

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SURVEY OF AVAILABLE LITERATURE Testbooks and design manuals by various authors suggest varying approaches to analysis and design of raft foundation. Differences of opinion exist in the method of analysis proposed to be adopted while determining moments, shear forces for the design of raft. Once the bending moments and shear forces are known, structural design does not present any difficulty and there exists no difference of opinion in this respect except very minor difference relating to desired thickness of slab and the effectiveness of the shear reinforcement Methods suggested by different authors are summarised below. These have been arranged chronologically with reference to date of publication of the testbooktdesign handbook. 4.1

Foundation Engineering b y Peck, Hansen and

hornb burn^

Raft is usually regarded and designed as an inverted continuqus flat slab floor supported without any upward deflections at the columns and walls. The soil pressure acting against the slab is commonly assumed to be uniformly distributed and equal to the total of all column loads multiplied by appropriate load factors and divided by the area of the raft. The moment and shears in the slabs are determined by the use of appropriate coefficient listed in the specifications for the design of flat slab floors. On account of erratic variation in compressibility in almost every soil deposit, there are likely to be correspondingly erratic deviations of the soil pressure from the average value. Since the moment and the shears are determined on the basis of the average pressure, it is considered good practice to provide this slab with more than theoretical amount of reinforcement and to use the same percentage of steel at top and bottom. This method has been widely used, often with complete success. On the other hand, it has also sometimes led to structural failure not only of the slab but also of the super structure. Therefore, its limitations must be clearly understood. The analogy follows only if the differential settlement between columns will be small and if the pattern of the differential settlement will be erratic rather systematic. The method is valid when the columns are more or less equally loaded and equally spaced. If the downward loads on some areas are on the average much heavier than on others, differential settlementsmay lead to substantial re-distribution of moments in the slabs resulting in unconservative design. Rafts are sometimes designed as if they rested on a bed of closely and equally spaced elastic springs of equal stiffness. The contact pressure beneath any small area is then proportional to the deflection of the spring

SURVEY OF AVAILABLE LITERATURE

9

in that area and thus to the settlement. The constant of proportionality 'K' is called the modulus of sub-grade reaction. Although, the theory has been well developed but the value of 'K' for real soils is not constant and depends not only on the stress deformation characteristics of the soil but also in a complex manner on the shape and size of the loaded area and the magnitude and position of nearby loaded areas. Evaluation of 'K'for design is difficult and fraught with uncertainty. Whatever method may be adopted for design, there is no guarantee that the deflections of the raft will actually be unimportant. In case, the structure covers a fairly large area with possibilities of differential settlements, it is not enough to provide great strength in the slab. It is also necessary to provide sufficient stiffness. However, a stiff foundation is likely to be subjected to bending moments far in excess of those corresponding to the flat slabsubgrade modulus analysis. There appears to.be no further edition of this book after 1954. 4.2

Foundation Design and Practice b y Elwyn. E.S. seelye9

According to Seelye after determining the soil pressures at various points of raft, shear and moment diagrams can be constructed for bands assumed from centre of bay to centre of bay. However, 65% of the moment is assumed to be resjsted by half the width of the band. There has not been any further edition of this book after 1956. 4.3

Foundation Design b y

en^'

In the conventional method, it is assumed that the mat is infinitely rigid and that the bearing pressure against bottom of the mat follows the planner distribution. The mat is analysed as a whole in each of two perpendicular directions. Thus the total shear forces acting on any section cutting across the entire mat is equal to the arithmetic sum of all forces and reactions (bearing pressure) to the left orright of the section. The total bending moments acting on such section is equal to the sum of all moments to the left or right of this section. Although the total shear and moments can be determined by the principles of simple statics, the distribution along this section is a problem of highly indeterminate nature, the average moment not being indicative of the sign and the magnitude of the bending moments in the individual strip in either direction. In order to obtain some idea as to the upper limit of these values, each strip bounded on central line of the column bays, may be analysed as independent continuous or combined footings. If the column loads are used, the soil reaction under each strip is determined without reference to the planner distribution determination for the mat as a whole. This method, undoubtedly, gives very high stress because it ignores the two way action of the mat. Therefore, certain arbitrary reduction in values (15% to 33113%) is made. The author gives other method like Finite Difference Method also for the design of the raft. There has not been any further edition of this book after 1962. The book, however, has been reprinted in 1992.

The recommendation in this book can be summarised in the following words: A great refinement of calculations is not always justified or practicable in case of raft.foundations because of the uncertainties of the action of soil and of short thick members that are arranged in complicated and multiple systems. It is reasonable to assume that the mat is so stiff and the load so constant that plastic soil will compress and adjust itself so that each column load will spread almost uniformly under the mat in the general vicinity of that particular column. For example, the total unit pressure under the rectangular area D, E, F, G shown in Fig. 4.1 may be assumed equal to 114th of the total loads on the columns at D, E, F and G divided by the area of D, E, F, G plus the weight of the mat per sq m. For the purpose of computing average pressure

10

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

under the slabs, near the walls, the outer column loads are treated as though they were concentrated at the columns. For this method, however, the load on adjacent columns should not differ very much and the bays in either direction should be reasonably, equal in length, the larger spacing not exceeding 1.2 time, the smaller one and the columns should be arringed in reasonably straight rows.

Fig. 4.1 Plan of assumed columns strips and distribution of loads One method of making a preliminary analysis of such a mat is on the basis of an assumed supporting system of columns strips that constitute a grid of beam along the column rows in each direction. The portion of the slabs in the central areas is taken up to be supported by this grid. The effective width of these strips or shallow beams has to be assumed and it is normal to take it slightly more than, what is determined by 45 degrees fiom the pedestal or column, to the lower reinforcement in themat. Technically the top reinforcement of a central panel may be less than of the bottom. However, it may be advisable to reinforce both sides equally because any yielding of end restraint will increase the, tension in the top of the mat above the computed value. Each column strip may be analysed by moment dishbution if the variation of loading or spans make this desirable, the entire thing being designed as an inverted floor. The effect of hydrostatic pressure has to be considered wherever it is present. There has been no further edition of this book after 1962. 4.5

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Indian Standard Code of Practicefor Design and Construction of Raft Foundation IS 2950-1965'

There are two approaches for design-conventional method and the elastic method. In the conventional method, the foundation is considered infinitely rigid and pressure distribution independent of the deflection of the raft. Soil pressures are also assumed to be planner so that the centroid of the soil pressure coincides with the line of action of the resulting forces of all the loads acting on the foundation. The method is normally used in design because of its simplicity . A generous amount of reinforcement is provided to safeguard uncertainties caused

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SURVEY OF AVAILABLE LITERATURE

by differential settlement. The raft is anabjsed as a whole in each of the two perpendicular directions. Thus, total shear forces and total bending moments acting on any section cutting across the entire raft is equal to the arithmetic sum of all forces and reactions/moments to the left or right of the section. The actual reinforcement provided shall be twice that worked out theoretically. Elastic method has two approaches. In one, the soil is replaced by an infinite number of isolated springs. In the other, the soil is assumed as a continuous elastic medium obeying Hook's Law. These methods are applicable in case the foundation is comparatively flexible and the loads tend to concentrate over small areas. The actual reinforcement can be one-and-a-half times that required theoretically.The famous soil line method falls in this category. As limitations to applicability of the methods, code mentions that the coda1 provisions: (1) do not apply to large and heavy industrial construction where special considerations of the base pressure distribution will be required. (2) apply only to fairly uniform soil conditions and for fairly horizontal planes of separation of layer below. (3) foundations in seismic area and/or to vibrating load shall be given special considerations. This code has been revised in 1973. Kindly see para 4.7. 4.6

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RafL Foundation The Soil Line Method of Design by A.L.L. ~ a k e q

According to Mr. Baker, the design of raft as a reversed floor is dangerous. Engineers being aware of this, who. therefore, normally adopt the second method in which earth pressure is assumed to be uniform throughout and moments are obtained at any section by statics. He, however, feels that in the second method also high values of moments are obtained, which may or may not be present, and it is irrational or wasteful to provide for such moments without investigating the deflections and variation in soil pressure. Mr. Baker has, therefore, suggested the soil line method which takes into account the variations in soil pressure and its relation to deflection but in order to simplify the calculations, it is assumed that the earth pressure varies throughout a beam according to straight line law. There is no further edition of this book after 1969.

4.7

Indian Standard Code of Practice for Design and Construction of Raft Foundation 1.S :2950 (Part-I) 1973~

In the revised version of the code, following methods of analysis have been proposed: (a) Assumption of linearly varying contact pressure (b) Perfectly rigid structures (c) Perfectly flexible structures (d) Structures stiffened along one axis (e) Structures stiffened along both the axis (f) General methods: (i) Based on modulus of subgrade reaction, and (ii) Based on modulus of compressibility (half space theory). Method (a) corresponds to the conventional method in the earlier version of the code and has similar limitations. In method (b), contact pressure distribution is to be calculated based on Boussineq's Equation for Elastic Isotropic half space and is applicable when deformations of raft under loads are small as compared to the mean settlement of the structure.

12

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

Method (c) is applicable for structures which have relatively less stiffening members specially resting on very stiff foundation soil. In this case, the deflections of the raft are same as the settlements of the foundation soil under external load. Method (d) is something in between methods (b) and (c) . Here in the direction of the stiffened axis the contact pressure distribution is determined by Boussineq's Equation as in method (b). In perpendicular direction distribution is determined as given in (f). Method (e) is same as method (b). The two methods under (f) are elastic methods and are used when simplified methods from (a) to (e) are not applicable. Details given in the codedo not provide enough guidance to enable the analysis and design 10 be completed by the designer. Apart from the limitations applicable in earlier version of the code it is stated that: (i) Allowable settlement both total and differential shall satisfy the requirement of the super-structure (ii) The approximate values of permissible settlements as given in earlier code have been deleted. This code has further been revised. Please see para 4.15. 4.8

Foundation Engineering Handbook Edited by Hans F. Witerkorn & Hsaiyang

an^''

Dr. Joseph E. Bowles and Wayne C. Teng are authors of chapters on spread footings, combined and special footings and mat foundation respectively. Chapter on floating foundation has been written by Dr. H.Q. Golder. This book classifies the method of design of mat foundation according to assumptions used. The rigid method which is the conventional method assumes that: (a) Mat is extremely rigid as compared to the sub-soil and, therefore, the flexural deflection of the mat, does not alter the contact pressure. (b) The contact pressure or the pile reaction are distributed in a straight line or a plain surface such that the centroid of the contact pressure coincides with the line of action of the resultant force of all the loads acting on the mat. When mat foundation is supported on piles, piles are assumed to be perfectly elastic. Raft is considered to be rigid when the column spacing is less than 1.751h or when the mat is supporting a rigid super-structure. his same as defined by Heteny. The mat is analysed as a whole in each of the two perpehdicular directions. The mat is divided into perpendicular bands of width between centre lines of adjacent column rows. Each band is assumed to act as an independent beam subjected to common contact pressure and known column loads. The simplified elastic method assumes that the soil behaves like an infinite number of individual elastic springs each of which is not affected by others. This foundation model is also referred to as Winkler foundation. Analysis procedures have also been developed for the beams on the simplified elastic foundation concept. The mat is considered as a plate and the effect of each column load is considered in area surrounding the load. Using the method of super-imposition,effect of all the column loads within the zone of influence is calculated. Among computer-oriented methods suggested is finite difference method, based on the assumption that the sub-grade can be substituted by a bed of uniformly distributed elastic springs with a spring constant equal to coefficient of sub-grade reaction. For this purpose, the mat is divided into square areas. The deflection at the nodal points of these areas is expressed by a differential equation in terms of deflection at the adjacent points to the right, left, top and bottom. These simultaneous equations are solved with an electronic computer and deflection at all the points are determined. Once deflections are known, the bending moment at any point in each direction is determined from theory of elasticity. The finite element method transforms the problem of plates on elastic foundation into a computer-oriented procedure of matrix structural analysis. The mat is idealised as a mesh of finite elements inter-connected only

13

SURVEY OF AVAILABLE LITERATURE

at the comers and the soil may be modeled as a set of isolated springs or as an elastic isotropic half space. The matrix structural analysis can be extended to include the influence of the super-structure as well, thus the interaction between the super-structure, the foundation and the soil is accounted for. It is further suggested that in a mat supported on hard rock, the column loads are transmitted to the rock on relatively small areas directly under the column. A greater economy may be achieved by designing the mat by elastic methods. On very soft soils, the contact pressure against the mat foundation approaches planer distribution and, therefore, it is commonly justified to design a mat on mud, soft clay, peat, organic soils or even medium clays by the conventional rigid method. A generous amount of reinforcement running in both directions at top and bottom is suggested regardless of method of design used in view of the likelihood that the stresses actually introduced would bedifferent from those calculated irrespective of the method used foi analysis. Second edition of this book is published in 1991. Please see para 4.21.

4.9

"

Foundation Analysis and Design by Joseph. E. ~ o w e l s '

The mat may be designed as rigid structures thereby soil pressure are computed as Q = V/A in the case where the resultant of the forces coincide with the centre of the mat area. If resultant has eccentricity with respect to geometric centre, soil pressure is calculated by the relation

In case, however, if the eccentricity is very large, the resulting internal stresses may be seriously in error. Once the dimensions of the mat are established, soil pressures at various locations beneath the base may be computed. With the pressure distribution known, the mat is sub-divided into a series of continuous beams (strips) centred on the appropriate column lines as shown in Fig. 4.2. For the series of beams, shear and moment diagram may be established using either combined footinglanalysis or beam moment coefficient. The depth is selected to satisfy shear stresses and is usually constant but the steel reinforcement vary from strip to strip. The perpendicular direction is analysed similarly, to complete the design.

Fig. 4.2

14

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

When the soil bearing pressure is low say 0.5 ~ i ~ s l f(25 t 2 K N I ~or ~ less ) and if the deformation of the mat surface can be tolerated, the mat may be designed as an inverted flat slab, using heavy beams from column to column. The portion between beams is designed as a conventional one or two way slabs. When footings are designed as flexible members, the computation takes some form of the solution of a beam on an elastic foundation. The experience has indicated that the solution obtained are generally reliable when the data are satisfactory. Possibly the reasons, as to why the methods have not been widely used in the past, are ease of making conventional solution, which have been generally satisfactory and usually not much different from elastic solution. Second reason is that the soil data are generally obtained using the standard penetration test for which no straight forward conversion to a value of modulus of sub-grade reaction exists. Various methods for elastic analysis like finite element and finite differences have also been explained in this book. New edition of this book is publisheg in 1988. Kindly see para 4.23 4~10 Building Code Requirements for Reinforced Concrete (ACI 318 - 77)18

Matters relating to design of footings are included in this code in Chapter 15. paragraph 15.10 relates to combined footings and mats. This paragraph reads as under: 15.10.1 Footings supporting more than one column, pedestal, or wall (combinedfootings or mats) shall be proportioned to resist the factored loads and induced reactions, iir accordance with appropriated design requirements of this code. 15.10.2 The Direct Design Method of Chapter 13 shall not be usedfor design of combinedfootings and mats. 15.10.3 Distribution of soil pressure under combined footings and mats shall be consistent with propemees of the soil and the structure and with establishedprinciples ofsoil mechanics. It would be seen that this code does not provide for much guidance in design of raft foundation. This code has been revised several times. Final being in 1989. Please see Para 4.20. 4.11

Foundation Design and Construction by M .J . ~ o m l i n s o n ' ~

Mr. Tomlinson states that it is wrong in principal to assume that araft acts as an inverted floor slab on unyielding supports and to design the slab on the assumption that its whole area is loaded to the maximum safe bearing pressure on the soil as this canlead to wasteful and sometimes dangsrous designs. Allowance must be made for deflection under the most favourable combination of dead and live load and variation in soil compressibility. Guidance is required from the soil mechanics engineer on the estimated total and differential settlement for dead and live load considered separately. Some flexibility is desirable to keep bending moments and shear stresses to a minimum, but the degree of flexibility must be related to the allowable distortion of the super-structure.Basement rafts carrying heavy building on weak soils are often founded on piles. The normal function of the piles is to transfer the loading to stronger and less compressible soil at greater depth or if economically possible, to transfer the load to bed rock or other relatively incompressible strata. The piles also have the effect of stiffening the raft and reducing or eliminating re-consolidation of ground heave, thereby reducing differential settlement or tilting. In such cases, considerable heave takes place with further upward movement caused by displacement due to pile driving. After completion of piling, the swelled soil should be trimmed off to the finished level. The basement walls should generally be designed as self-supportingcantilever retaining walls even though they may eventually be supported by the floor construction and additional stability against overturning given by super-structure loading on top of the wall. The basement floor slabs must be able

SURVEY OF AVAILABLE LITERATURE

15

to withstand pressure on the underside of the slab together with stresses caused by differential settlement, non-uniform column loads, reaction from the retaining walls. If the columns are provided with independent bases with only a light slab between them, there would be likelihood of failure of the slab from the pressure of the underlying soil. Fifth edition of this book has been out in 1986. Please see para 4.17.

t g

4.12

i

a

1

The committee observes that no authentic method has been devised that can evaluate all the factors involved in the problem and allow carrying out determination of contact pressures under combined footings and mats. Simplifying assumption must, therefore, be made based on the knowledge of the interaction of the various elements of the system. The following factors should be considered while examining any problem: (1) Soil type immediately below the footing (2) Soil type at the greater depth (3) Size of footing (4) Shape of footing (5) Eccentricity of loading (6) Rigidity of footing (7) Rigidity of the super-structure (8) Modulus of sub-grade reaction The committee suggests procedure to be followed for design of footings under two columns: grid foundations and smp footings supporting more than two columns and mat foundation. Linear soil pressure distribution is suggested for footings which can be considered rigid to the extent that only very small relative deformations result from the loading. The rigidity may result from the spacing of the columns on the footing from the rigidity of the footing itself or the rigidity of the super-structure.Limitations which must be fulfilled to make this assumption valid have been discussed in the report. Distribution of soil pressure by means of sub-grade reaction has been suggested where sub-soils are of such character that the deformations are localised in the general vicinity of the loads and when the maximum contact pressure is smaller than about one and a half times the ultimate bearing capacity. In case of rigid footings, it is suggested that uniform or linear distribution of soil pressure can be assumed and the design based on statics. Flexible footing procedure is divided into 2 parts i.e. uniform condition and general condition. Uniform conditions are considered to be those where the variation in adjacent column loads and spans is not greater than 20%. For cases where supporting columns are at random location with varying intensities of loads a detailed design procedure based on plate theories has been recommended. 4.13

:

Design of Combined Footings and Mats ACI Committee 33614

Pile Foundation Analysis and Design by H.G.Poulos and E.H.Davis 1 9 8 0 ~ ~

In this book, Chapter 10 deals with piled raft systems. The author says that, "in design of foundation for a large building on a deep deposit of clay it may be found that a raft foundation would have an adequate factor of safety against ultimate bearing capacity failure but the settlement would be excessive; traditional practice would then be, to pile the foundation and to choose the number of piles to give an adequate factor of safety assuming the piles take all the load; however it is clearly illogical to design the piles on an ultimate load basis when they have only been introduced in order to reduce the settlement on other-wise satisfactory raft." According to the author, once the have been introduced solely for the purpose of reducing the settlement

16

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

design question becomes not "how many piles are required to carry the weight of the structure" but "how many piles are required to reduce the settlement to an acceptance level". However, in Chapter 5, the settlement behaviour of a free standing pile is obtained from the elastic-based analysis. The pile is divided into number of elements and the expressions for vertical settlement of the pile and the soil at each element in terms of unknown stresses on the piles are obtained and solved, imposing the vertical displacement compatibility condition, to arrive at the settlement behaviour of the pile. As a further extension, the unit consisting of a single pile with an attached cap resting on the soil surface is considered. It is assumed that purely elastic condition prevails upto the load at which the pile would fail if no cap were present and thereafter any additional load is taken entirely by the cap. The book gives charts indicating interaction factor between the raft and the pile for various values of length of the piles, diameter of the pile, poisson ratio of soil, height of soil layer over the rigid stratum and the cap diameter.The method is further extended to group of piles upto about 40 numbers. Curves are drawn which are applicable only for rigid rafts or perfectly flexible rafts. The entire emphasis is to work out the ratio of the load carried out by the piles and the raft soil system. No details are given on &e method to determine the bending moment and shear forces in the raft. It is only mentioned that none of the simple methods are satisfactory and a proper analysis of plate on piles and continuum is desirable.

4.14

Reinforced Concrete Designers Handbook by Charles E. Reynolds and JamesC. Steedman 9th Edition 1981"

-

This book suggests the analysis of a raft foundation supporting a series of symmetrically arranged equal loads on the assumption of uniformly distributed pressure on the ground considering the structure as an inverted reinforced concrete floor acted upon by the load of earth pressure from bottom. It is further suggested that when the columns on the raft are not equally loaded or are not symmetrically arranged, the raft should be so designed that the centroid coincides with the centre of gravity of the loads. If this coincidence of centre of gravity is impracticable owing to the extent of the raft being limited on one or more sides, the plan of the raft should be made so that the eccentricity of the total loading is a minimum, though this may produce a raft which is not rectangular in plan.

4.15

-

IS 2950 (Part I) 1981 Code for Design and Construction of Raft Foundation Part I ~ e s i ~ n ~

In the second revision of the code, two methods of analysis have been suggested depending upon the assumption involved. Conventional method assuming planner distribution of contact pressure is applicable to foundations which are rigid relative to supporting soil and the compressible soil layer is relatively shallow. The rigidity of the foundation is determined with a relative stiffness factor K > 0.5 or columns spacing less than 1.75A. Methods of determining value of K and hare given in the code. Conventionalmethod is applicable when either of the two conditions are satisfied. The value of K depends upon the flexural rigidity of the super-structure, modulus of the compressibilityof the foundationsoil, thickness of the raft, length of the section in the bending axis and length perpendicular to the section under investigation. Value of h depends upon modulus of sub-grade reaction for the footing of the width of the raft, modulus of elasticity of concrete and moment of inertia of the raft. In this method, the raft is analysed as a whole in each of the two perpendicular directions on the basis of statics. In case of flexible footings, simplified methods are applicable when variation in adjacent column load is not more than 20% of the higher value and the structure (combined action of the super-structure and raft) may be considered as flexible, ie., relative stiffness factor K is greater than 0.5. In this method, it is assumed that

17

SURVEY OF AVAILABLE I-ITERATURE

&' i :

the sub-grade conslsts of an infinite array of individual elastic springs each of which is not affected by others. This method is more or less same as the famous soil line method. When conditions, as mentioned above, for flexible foundations are not satisfied ,a method based on closed form of solution of elastic plate theory has been suggested. The distribution of deflection and contact pressure on the raft due to a column load is determined by the plate theory. Since the effect of a column load on the elastic foundation is damped out rapidly. It is possible to determine the total effect at a point of all column loads with~nthe zone of influence by the method of super-imposition. The computation of the effect at any point is restricted to columns of two adjoining bays in all directions. The code also lays down that: (a) Size and shape of the foundation adopted affects the magnitude of subgrade modulus which should be taken into consideration. (b) Consideration must be given to the increased contact pressure developed along the edges of the raft on cohesive soils and the opposite effect on granular soils. (c) Expansion joint should be provided when the structure supported by the raft consists of several parts with varying heights and loads or there is a change in the direction of the raft. (d) This code does not explicitly provide any guidance as to how factors emphasised in (a) and (b) above should be allowed for. The second part of the code relating to construction aspect is still not printed. There has not been any further revision and this code was reaffirmed in 1987.

4.16

Eleventh Intenationul Conference of Soil Mechanics a d Foundation Engineering San Francisco, August 12 1 6 , 1 9 8 5 ~ ~

-

In the conference while two papers were presented on instrumentation of pile raft foundation and cap pile soil interaction, there was no recommendation or paper on design of raft foundation.

4.17

Foundation Design and Construction by M.J. Tomiinson, 5th Edition, 1986"

There is no significant change in this edition from what was recommended in 4th edition

4.18

-

-

Handbook of Concrete Engineering Mark Fintei 2nd Edition, 1986%

This book makes no recommendation about raft foundation.

4.19

Reinforced Concrete Designer Handbook by Charles E. Reynolds and James Steedman, 10th Edition, 1988~'

There is no change in recommendations from what was done in the earlier edition published in 1981

4.20

-

- -

Building Code Requirements in Reinforced Concrete ACI 318 1989~'

Building code requirements since their second edition in 1977 have gone in for further revision 1983, 1989 and 1992. In the latest revision there is no change in the code requirements for design of combir.ed footings and mats, but in commentary a reference has been made to 'design procedure for combined footings and mat i sper report prepared by ACI committee 336'and also to a paper 'simplified design of footings by' Kramrisch, Fritz and Rpgers Paul published in American Society of Civil Engineers Proceeding, V. 87, NOSM 5, October 1961, p. 19.

18

4.21

RAFT FOUNDKTIONS-DESIGN AND ANALYSIS

Foundation Engineering Handbook by Hsai-Yang-Fang2nd Edition, 1 9 9 1 ~ ~

This edition has omitted the chapter on mat foundation which was originally'included in first edition.

4.22

-

-

Design of Combined Footings and Mats ACI committee 336 2R 88 Published in ACI Manual 1 ~ 3 ~ ~

1966 report mentioned in para 4.12 above was reaffirmed in 1980 but has been completely revised and elaborated in 1988. This report suggests that: (a) Maximum unfactored design contact pressure should not exceed the available soil pressure determined by geotechnical engineer. Where wind or earthquake forces form a part of the load combination, the allowable soil pressure may be increased as allowed by the local code and in consultation with geo-technical engineer. (b) Combined footings and mats are sensitive to time dependent sub surface response. Many structural engineers analyse and design mat foundations by computer using the finite element method. Soil response can be estimated by modelling with coupled or uncoupled "Soil springs". The spring properties are usually calculated using a modulus of subgrade reaction, adjusted for footing size, tributary area to the node, effective depth, and change of modulus with depth. The use of uncoupled springs in the model is a simplified approximation. The time dependent characteristics of the soil response, consolidation settlement or partial consolidation settlement, often can significantly influence the subgrade reaction values. Thus, the use of a single constant modulus of subgrade reaction can lead to misleading results. (c) Caution should be exercised when using finite element analysis for soils. Without good empirical results, soil springs derived form values of subgrade reaction may only be a rough approximation of the actual response of soils. Some designers perform several finite element analyses with soil springs calculated from a range of subgrade moduli to obtain an adequate design. (d) The response of a footing is a complex interaction of the footing itself, the superstructure above, and the soil. That interaction may continue for a long time until final equilibrium is established between the superimpos&lloads and the supporting soil reactions. Moments, shears, and deflections can only be computed if these soil reactions can be determined. (e) No analytical method has been devised that can evaluate all of the various factors involved in the problem of soil-structure interaction and allow the accurate determination of the contact pressures and associated subgrade response. (f) For mat foundationsmodulus of subgrade reaction cannot be reliably estimated on the basis of field plate load tests because the scale effects are too severe. (g) Mats may be designed and analysed as either rigid bodies or as flexible plates supported by elastic foundation. A combination analysis is common in current practice. An exact theoretical design of mat as plate on an elastic foundation can be made. However a number of factors like, difficulty in projecting subgrade responses, variation in soil properties both horizontal and vertical, mat shape, variety of superstructure loads and assumption in their development and effect of superstructure stiffness on mat rapidly reduce exactness to a combination of approximations.The design is further affected by excavation heave. (h) After propottioning the mat size, compute the minimum mat thickness based on punching shear at critical columns based on column load and shear perimeter. It is common practice not to use shear reinforcement so that mat depth is maximum.

I

I\

1 I

*

;

i I

19

SURVEY OF AVAILABLE LITERATURE

(i) In case column spacing is less than 1.75 divided by h or the mat is very thick and variation of column loads and spacing is not over 2096, mat may be designed by treating it as a rigid body and considering strips both ways. These strips are analysed as combined footings with multiple column loads and loaded with the soil pressure on the strip and column reactions equal to loads obtained from the superstructure analysis. Since a mat transfers load honzontally, any given strip may not satisfy vertical load summation. Q) In case the criteria is not met with an approximate analysis can be made using the method suggested by ACI Committee 336 in 1966. (k) Computer aided finite differences,finite grid or finite element methods can be used where computers are available. The report gives details of these 3 methods. In any of these 3 methods node pressure should not exceed the safe bearing pressure value recommended by the geotechnical engineer. (1) A mat analysis is only as good as the soil parameters. Since it is very difficult for the geotechnlcal engineer to provide accurate vdues of moGulus of subgrade reaction, the structural designer may do the parametric study, varying the value of K over range of one half the furnished value to 5 or 10 times the furnished value. (m) The analysis and design of combined footings and mats is a soil-structure interaction effort in which there is no unique method to determine mat deflection. The determination of mat deflection extends far beyond the analysis of a beam or finite element model to the prediction of subgrade response. The prediction of subgrade response, though part of the structural analysis of the mat, is more elusive than designers wish to admit. Experience with extensive measurements of both foundation loadings and subgrade response are needed to develop a high degree of confidence in the method selected. A very close working relationship must exist between the geotechnical and structural engineers to properly analyse comb~nedfootings and mats.

4.23

Foundation Analysis and Design by Bowles, 4th Edition, 1 9 8 8 ~ ~

In this edition analysis of mat foundation has further been elaborated considerably. Among the design methods included are conventional or rigid methods as explained in earlier edition stating that this method is not recommended at present because of substantial amount of approximations and the wide availability of computer programmes which are relatively easy to use and mat being generally too expensive and important not to use most refined analytical method available. The approximate flexible procedure suggested by ACI Committee 436 (1966) has been retained and elaborated. Further details have been given for finite difference method, finite element method and finite grid method applicable with computer. 4.24

Proceedings of Indian Geo-Technical Conference 1992, Calcutta, December, 1 9 9 2 ~ ~

This conference does not have papers relating to design and analysis of raft foundation.

4.25

-

Designs of Foundation Systems Principles and Prrictices by Nainan P. Kurian, 1 9 9 2 ~ ~

The book details conventional approach to raft design as a flat slab and beam and slab raft, following the Indian Standard Code of Practice, more on the inverted floor approach. The book only mentions that an integrated analysis of the beam and slab on the computer by the finite element method using package programmes such as SAP IV which will give exact results based on the actual behaviour of the system can be carried out. This book also mentions about the design of raft foundation by the Soil line method stating that this method has

20

RAW FOilNDATlONSDESlGNAND ANALYSIS

1I

rather become obsolete in the wake of possibility of using more refined flexible methods with the aid of computer. 4.26

13th International Conference on Soil Mechanics and Foundation Engineering, New Delhi January, 1 9 9 4 ~ ~

A paper by M.F. Randolph was presented as a special lecture on design methods for Pile Groups and Piled

Rafts. The paper recalls that in majority of the cases where piles form part of the foundation for a building or other structures, the primary reason for inclusion of the piles is to reduce settlements. However, once the decision has been made that piles are required the traditional design approach has been to ensure that the total structural load can be carried out by the piles, with adequate factor of safety against bearing failure. However, there is elastic interaction'between the raft and soil below, between piles and piles as the performance of a pile within a group is affected by the presence of other piles. The key question that arises in the design of pile rafts concerns the relative proportion of load carried out by raft and the piles and the effect of additional pile support on absolute and differentialsettlements.,Thepaper suggests that this distribution of load between the raft and piles be taken into account. The paper also gives methods by which this proportion of load between the two components are carried out. 4.27

-

Soil Structure Inter-action The Real Behaviour of Structures, published by the Institution of Structure Engineers, U.K. The Institution of Civil Engineers, U.K. International Association for Bridge and Structural Engineering in March, 1 9 ~ 9 ~ ~

The above institutions constituted a joint committee under Dr. Sam Thornborn which prepared this report. Pointing out that, (i) Red behaviour of structures in contact with ground involves an inter-active process beginning with the construction phase and ending with a state of balance after a period of adjustment of stresses and strains within the structure and within the ground influenced by the structure. (ii) Actual behaviour of the structure relates to the inherent spatial variations in the ground and it should be appreciated that these variations are not always readily identifiable by occasional and local boring, sampling and testing. The report deals with the question of soil structure interaction in 2 parts. Pari I relates to structures supported by ground and Part I1 for ground supported by structures. (a) Under structures supported by ground, the report points out that engineers could estimate the settlements for a perfectly flexible load or they could estimate the avenge settlement of a rigid load but in between these limits, the engineers could say nothing. (b) Analytical methods have been developing so rapidly over the last few years that it is now possible to obtain solution to many complex problems which a few years ago would have been quite out of reach. If used sensibly and with discernment, these powerful analytical methods can be of considerable assistance enabling a designer to gain a feel for the behaviour of soil structure system. However, if used blindly, such methods cause menace and can be extremely misleading. The key to successful use is to gain a clear understanding of the idealisations that are being made and to be aware of, how far they may be, from reality. (c) For a framed building founded on a raft, during excavation some heave of the soil will occur. The raft will then be constructed and will be influenced by the differential settlement there after. As the

II i

i

,

I

1

I

SURVEY OF AVAILABLE LITERATURE i

t

(d)

(e)

(f)

(g)

structural load is applied short term settlements take place, the part of the structure in existence distorts and the overall stiffness gradually increases. The cladding is then added and may substantially increase the stiffness of the building. Finally, the imposed load is applied. Not all the components of the buildings are subject to the same relative deflection. The relative deflections experienced by the raft will be the largest. Those experienced by the structural members will vary with location and elevation in the building. The likelihood of damage will diminish, the larger the proportion of medium and long-term settlements,the smaller the ratio of imposedldead loads and later the stage at which the finishes are applied. The report has an appendix which has reviewed currently available techniques for the analysis of the total soil structure system. More readily available computer packages that utilise these techniques, have been listed in the appendix. The manner in which and the limitations with which super-structure can be modelled have been singled out. For soil model, it is pointed out that commonly known approach of treating the soil as a set of liner unconnected springs cannot be recommended for the analysis of rafts and continuous footings although this model has the advantage of being easily included in standard computer programmes for structural analysis. It is a poor physical model. The results of analysis based on use of this model may be excessively sensitive to the pattern of applied load. The half space continuum using elastic theory for both stresses and strains has severe limitations because it does not take into account, the soil layering or the variation of soil modulus with depth within a given layer. In an extension of this method where elastic theory is used for strains only and then stresses are calculated using the various deformation moduli of the soil is better approximation. In a further improvement of a layered coniinuum the exact stresses and strains in a layered soil mass are calculated. Super structure stiffness has a marked influence on the behaviour of the raft and should not be ignored although the quantitative assessment of all but the simplest of the wall system connected to the raft may prove difficult. However, often the raft is itself a major contributor to the overall stiffness of the building. Since the raft is in intimate contact with the supporting soil, the inter-active effects are perhaps most marked in consideration of its own behaviour. In the design of raft foundation, it is totally unrealistic to ignore deformation and rely on moment and shears obtained from the analysis of the conventional flat slab method. It is equally unrealistic to compute deformation without consideration of the structural stiffness and then to design on the basis of the corresponding stress resultants. Rational design approach must be based on the results of an interactive analysis.

DESIGN APPROACH AND CONSIDERATIONS Summary of methods suggested by various authors discussed in Chapter 4 would indicate that basically two approaches have been suggested for analysing the behaviour of raft foundation: A. Rigid foundation approach B. Flexible foundation approach 5.1

Rigid Approach

In rigid foundation approach, it is presumed that raft is rigid enough to bridge over non-uniformities of soil structure. Pressure distribution is considered to be either uniform or varying linearly. Design of rigid raft follows convkntional methods where again following two approaches have been suggested: (a) Inverted floor system (b) Combined footing approach In rigid rafts, differential settlements are comparatively low but bending moment and shear forces to which raft is subjected are considerably high.

5.2

Flexible Approach

In flexible foundation approach, raft is considered to distribute load in the area immediately surrounding the column depending upon the soil characteristics. In this approach differential settlements are comparatively larger but bending moments and shear forces to which the raft is subjected are comparatively low. Analysis is suggested basically on two theories (a) Flexible plate supported on elastic foundation, i.e., Hetenyi's Theory (b) Foundation supported on bed of uniformly distributed elastic springs with a spring constant determined using coefficient of sub-grade reaction. Each spring is presumed to behave independently, i.e., Winklers's foundation Based on these two basic approaches, methods suggested include simplified methods subject to certain limitations which can be carried out by manual computation. Also now available are computer based methods

23

DESIGN APPROACH AND CONSIDERATIONS

like finite element and finite differences methods. Finite differences method is based on the second approach uf uniformly distributed elastic springs and can consider one value of sub-grade modulus for the entire area. Finite element method transforms the problem of plates on elastic foundation into a computer oriented method of matrix structural analysis. In this method, plate is idealised as a mesh of finite elements inter-connected only at the nodes (corners), and the soil may be modelled as a set of isolated springs or as an elastic isotropic half space. The matrix structural analysis can be extended to include the influence of the super-structure as well. Thus, the interaction between the super-structure, the foundation and the soil can be accounted for. It is possible to consider different values of sub-grade modulus in different areas of the raft foundation. In case of piled rafts against the usual assumption of entire load being carried by piles alone, emphasis is now being laid on sharing of load between raft supported on soil, i.e., raft soil system and raft pile system. Sufficiently accurate methods for practical distribution of these loads are not yet available. As a simplification of treating the entire raft as a plate, concept of beam on elastic foundation is also being used. For this purpose raft is considered to consist of beams in both the directions. Each of these beams is treated as supported on springs having spring constant calculated using modulus of subgrade reaction and carrying column loads. The beam is then analysed as a bean1 on elastic foundation.

5.3

Parameters for Raft Design

In all these methods, however, three basic parameters, i.e., rigidity of the raft, pressure distribution under the raft and value of sub-grade modulus become important in addition to whatever other info&ation'is received from soil investigation report. These three parameters and method of their determination are discussed in subsequent paragraphs.

A problem which has to be solved while designing a raft foundation is to evaluate the actual contact pressure of the soil against the raft. This problem has occupied many researchers theoretically and a lesser number experimentally with no exact values being known. Contact pressure, settlement of foundation, soil characteristics and its behaviour are so much inter-related and their relationship so complex, that soil foundation structure interaction is not clear even now. Considering all these aspects it can be said that the contact pressure distribution under the raft depends upon: (1) The nature of the soil below the raft, i.e., a single homogenous mass or a layered formation, thicknesses of various layers and their relative locations (2) Properties of the soil (3) The nature of the foundation, i.e., whether rigid, flexible or soft (4) Rigidity of the super-structure (5) The quantum of loads and their relative magnitude (6) Presence of adjoining foundation (7) Size of raft (8) Time at which pressure measurements are taken The total settlement under the raft foundation can be considered to be made up of three components, i.e.,

S = Sd+Sc+Ss where Sd is the immediate or distortion settlement, Sc the consolidation settlement and Ss is the secondary compression settlement. The immediate component is that portion of the settlement which occurs simul-

-,

24

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

taneously with the load application,primarily as aresultof distortion within the foundation soils. Thesettlement is generally not elastic although it is calculated using elastic theory. The remaining components result from the gradual expulsion of water from the void and corresponding compression of the soil skeleton. The distinction between the consolidation and secondary compression settlement is made on the basis of physical process which control the time rate of settlement. Consolidation settlements are largely due to primary consolidation in which the time rate of settlement is controlled by the rate at which water can be expelled horn the void spaces in the soil. The secondary compression settlement,the speed of settlement is controlled largely by the rate at which the soil skeleton itself yields and compresses. The time rate and the relative magnitude of the 3 components differ for different soil types. Water flows so readily through most clean granular soil that the expulsion of water from the pores for all practical purposes is instantaneous and thus foundation settles almost simultaneously with the application of load. In cohesive soil, it takes considerable time for water to escape and thus settlement in cohesive soils continue much longer. In fact, it has been reported that the pressure under a mat foundation on clay may vary from time to time. It is usual to assume that the soil below the foundation is an isotropic homogeneous material for its entire depth. But normally this is not the situation and we get different layers in varying thickness, having different properties below foundation. If the thickness of the upper most layer is large relative to the dimension of the loaded area, it would probably be sufficient if the soils were considered as a homogenous layer of indefinite depth. However, if the upper stratum is relatively thin ignoring theeffect of layering, it may have an appreciable influence on the contact pressure distribution and consequently settlements. This is likely to be of special importance when a compressive stratum is underlain by rock or a very hard or dense soil. Such presence decreases the settlement considerably. It is very significant when this occurs within a depth equal to width of the footings. Incase, there is a stiff stratum underlain by a soft stratum like layer of sand over soft clay layer, effect is negligible if depth is greater or equal to 3.5 b2.1n case of raft, dimensions of raft are generally such that the possibilities of encountering a different soil layer within the significant depth are quite large and as such it would be necessary to account for the different soil layers within the significant depth. Moreover it is to be remembered that properties of soil constituting each layer which determine the shear strength characteristics and settlement characteristicsof the soil become more important as rafts are generally adopted in areas where soils of poorer types are'~ncounteredand which some years ago might have not been taken up for construction at all. Effect of groundwater table is appreciable on the load carrying capacity of the soil and consequently settlements. It is, therefore, necessary to consider the expected ground water table in life time of the structure including the temporary rises as during floods. Even in areas where sub-soil water table is not present, it is necessary to consider long term built up water for design of basement and raft foundation. If permeability coefficient of the soil is below 0.1 mm per second, soil is cohesive and probability of surface water accumulated against basement walls exist'. In such situations, it may be necessary to design raft foundations of basement for water uplift also. The conventional analysis of footings, in general, uses the concept of a rigid fcotings and with rigid footing are associated the concept of uniform soil pressure. Actually to have a uniform soil pressure distribution, we require a very flexible footing. If simultaneously we accept the concept of soil being elastic (modulus of elasticity or coefficient of sub-grade modulus), settlementof rigid footing will be uniform and that for a flexible footing the settlement would be non-uniform and but if this is the case then how can the contact pressure be uniform (under a rigid footing). In reality we have a soil snucture interaction problem and there is a non-uniform soil pressure and differential settlements within the footings. It has been suggested that in case of square footing resting on clay on average contact pressure of 0.6 PIA with additional 0.1 PIAalong edges would be reasonable

25

. DESIGN APPROACH AND CONSIDERATIONS

1 I1 :

i-

i

:

1

:

pressure distribution. For a rectangular footing of large length it is suggested that it would be reasonable to have an average pressure equal to 0.8 P average + 0.1 PIB for the edges. Here P is total load, A, area and B, length of the footing. For footings on sands a pressure distribution of uniform soil pressure is reasonable. Rigidity of foundation gets modified by the rigidity of super-structure. Arigid super-structurewill not allow differential settlement to take place in foundation. Situation can arise when a particular column of the building may be hanging from the super-structure and even transmitting the weight of attached soil mass to the super structure rather than transmitting any load from the super-structure to the foundation soil. In fact, a rigid foundation with a rigid super structure means less differential settlement, large variation of contact pressure and high bending and shear stress in foundation members. A flexible foundation with flexible super structure means large differential settlements, uniform contact pressure and lower values of bending and shear stresses in foundation members. Quantum of loads and their relative magnitude affect the contact pressure. When the loads are so high that bearing pressures are increased to the point of shear failure in the soil, the contact pressure is changed leading to an increase in pressure over the centre of the loaded area in all cases. The consolidation pressure involves expulsion of water from the soil being compressed. This takes time and at any time between the application of the load producing consolidation and the time at which essentially ultimate or 100 per cent consolidation has occurred, the measured settlements and consequently contact pressure distribution would be different. Many times it may take several years to achieve final settlement. There are situations in engineering practice where footings are placed so close to each other that their zones of influence overlap. Studies have shown that effect of adjacent footings may vary considerably with angle of shearing resistance. For low values they are negligible. For higher values they appear to be significant particularly if footing is surrounded by others on all sides. There are practically no effects in case of punching shear failure. It is generally recommended that interference effect may be neglected. In view of various factors affecting the pressure distribution under a raft foundation and difficulties in determining affect of each, it is generally believed that contact pressure distribution under a raft could be of the following type as shown in Fig. 5.1. ,

( c ) SOFT SOIL

-------

Fig. 5.1 Contact pressure distribution under a raft

26

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

Fig. 5.1 (a) is applicable when the mat is supported on hard rock and column loads are transmitted to the rock on areas of relatively small size directly under the columns. If the raft rests on a stiff dense soil, then loads are distributed to the sub-soil in relatively large areas, as shown in Fig. 5.1 (b). It is only on very soft soils that the contact pressure against the mat foundation approaches linear distribution as shown in Fig. 5.1 (c). Therefore, it is commonly justified to design a mat on mud, soft clay, peet or organic soil by the conventional rigid method using uniform pressure. In fact assumption of rigid footings with uniform soil pressure results in designing the raft for assumed bending moments which are larger than the actual bending moments. The resulting design is conservative generally but may not be economical. A greater economy can, perhaps, be achieved by designing the mat with elastic methods, but at what risk and is it really so ? Actual pressure distribution under the raft, therefore, remains unanswered.

5.5

Rigidity Criteria

Whether a structure behaves as rigid or flexible, it depends on the relative stiffness of the structure and the foundation soil.The behaviour of the foundation as rigid or flexible will also depend upon the rigidity of the super-structure above and properties of soil below. In physical terms, a rigid foundation would mean a foundation which is capable of bridging over pockets of soil with different properties and thus try to even out the settlements at various points. A rigid foundation would, therefore, have comparatively lower values of differential settlement but higher values of stresses. A rigid foundation with a rigid super-structure on a comparatively compressible soil will result in uniform settlements of structure. A flexible foundation with a flexible super-structures and a comparatively rigid soil below will behave as a flexible foundation and would result in large differential settlements and low stresses. Thus: (i) Arigid member is characterisedby high bending moments and relatively small, uniform deflections. Over all differential settlements are small. (ii) An intermediate member, as the term implies, has intermediate bending and deflection values. (iii) The flexible member has comparatively smaller bending moments and deflection is maximum in vicinity of the loads and small values else where. Overall differential settlement would be of higher orders.

-

Rigidity criteria proposed by various authorities are discussed below:

5.5.1 Proposed by IS :2950 (Part I) 19813 Appendix C of this standard gives the method of deciding rigidity of super-structure and foundation. This is reproduced below: Rigidity of Superstructure and Foundation

C-1

Determination of the Rigidity of the Structure

C-1.1 Theflexural rigidity El of the structure of any section may be estimatcdaccotding to the relation given below (see also Fig. 5.2):

DESIGN APPROACH AND CONSIDERATIONS

Fig. 5.2 Determination of rigidity of a structure where

El = modulus of elasticity of the infilling material (wall material) in kg/crn2, I, = Moment of,inertia of the infilling in cm4, b = length or breadth of the s ructure in the direction of bending. H = total~heightof the infill In cm, E, =modulus of elasticity of frame material in kg/cm2 Ib = moment of inertia of the beam in cm4

J

/.

where 1 = Spacing of columns in cm, h, = Length of upper column in cm, hl = Length of lower column in cm,

I,, = Moment of inertia of upper column in cm4, Il = Moment of inertia of lower column in cm4 If = hioment of inertia of foundation beam or raft in cm4,

28

RAFT FOUNDATIONS-DESIGNAND ANALYSIS

Note :The summation is to be done over all the storeys, including the foundation beam of raft. In the case of the' foundation I;replaces Pb and 1, becomes zero, whereas for the topmost beam 1'" become zero Relative Sti#hess Factor K:

C-2

C-2.1 Whether a structure behave as rigid orflexible depends on the relative s t i m s s ofthe structure and thefoundation soil. This relation is expressed by the relative stimess factor K given below: (a) For the whole structure

(b) For rectangular rafts or beams

(c) For circular rafts

where El = Flexible rigidity of the structure over the length (a) in kg/cm2 E, = Modulus of compressibility of the foundation soil in kg/cm2 b = Length of the section in the bending axis in cm, a = Length perpendicular to the section under investigation in cm, d = Thickness of the raft or beam in cm, R = Radius of the raft in cm C-2.1.1 For K > 0.5, the foundation may be considered as rigid C-3

Determination of Critical Column Spacing

C-3.1 Evaluation of the characteristics h is made as follows:

where

k = Modulus of sub-grade reaction in kg/cm3 for footing of width B in cm

B = Width of raft in cm, E, = Modulus of elasticity of concrete in kgf/cm2 1 =Moment of inertia of the raft in cm4 Modulus of compressibility of the soil is the additional property required in this particular case. 5.5.2 ACI Committee, 436

Suggested design procedure for combined footings and mats - American Concrete Institute Journal, October, 196614 Relevant extracts from this paper are given below:

I1 a

Ig

DESIGN APPROACH AND CONSIDERATIONS

29

Footings supportingjield structures

Continuous strip footings supporting structures which because of their rigidity will not allow the individual columns to settle differentially should be designed as rigid footings with a linear distribution of soil pressure. This distribution can be determined on the basis of simple statics. To determine the approximate rigidity of the structure, an analysis must be made comparing the combined stiffness of the footings, super-structure framing members, and shear walls with the stiffness of the soil. The relative stiffness will determine whether the footing should be considered rigid or flexible. The following formulas may be used in this analysis :

where

E = Modulus of elasticity of the materials used in the structure, kips per sq.ft (metric tons per sq.m) I, =Moment of inertia of the structure per unit length, ft3 (m3)

IF = Moment of inertia of the footing per unit length, ft3(m3)

Es=Modulus of elasticity of the soil, kips per sq.ft (metric tons per sq.m) b =Width of footings, ft (m)

per unit length of building can bedetermined by summing the flexural rigidity An approximate value of ElIC of the footing (E'L,) the flexural rigidity of the each framed member (FIB)and the flexural rigidity of any shear walls (F3112) where a and h are the thickness and height of the wall, respectively. Computations indicates that as the relative stiffness K, increases, the differential settlement decreases rapidly. For K , = 0 ,the ratio of differential to total settlement is 0.5 for long footing and 0.35 for a square one. For K , = 0.5 , the ratio of differential to total settlement is about 0.1. If the analysis of the relative stiffness of the footing yields a value above 0.5, the footing can be considered rigid and the variation of soil pressure determined on the basis of simple statistics. If the relative stiffness factor is found to be less than 0.5, the footing shall be designed as a flexible member using the foundation modulus approach as described under section 6.4 of the report. Columns Spacing

The column spacing on continuous footings is important in determining the variation in soil pressure distribution. If the average of two adjacent spans in a continuous strip having adjacent loads and column spacings that vary by not more than 20 per cent of the greater value or is less than 1.75/h, the footing can be considered rigid and the variation of soil pressure determined on the basis of simple statics. - If the average of two adjacent span, as limited above, is greater than 1.75/h, the design of the footing shall be governed by subgrade modulus theories. For general cases falling outside the limitation stated above, the critical spacing i t which the subgrade modulus theory becomes effective has to be determined individually. Evaluation of the factor can be made on the basis of the following formulae: '

30

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

K, = SR,r

Where

K, = Coefficient of vertical subgrade reaction, Kips per cu ft (metric tons per cu m)

K',= basic value of coefficient of vertical subgrade reaction for a square area with width b = 1 ft (0.3 m). Kips per cu ft (metric tons per cu m)

b = Width of footings, ft (m) S = Size or shape factor for a footing on a particular type of soil E, = Modulus of elasticity of concrete, Kips per sq ft (metric tons per sq m)

I = Moment of inertia of footings ft4 (m4 For sandy soils the size factor S can be determined from the following formula:

with a limiting value of 0.25 for large footings. As for clay soils, the shape factor S can be determined from the following formula:

When n is the ratio of the longer side to the shorter side of the footing. As for extremely long footings, where n approaches infinity, S can be assumed as 0.67. can be determined from the results of field tests performed on the subgrade of the proposed Values for Kt,, structure or can be estimated on the basis of empirical values in "Evaluation of coefficients of Subgrade Reaction" by Terzaghi.

5.5.3 Hetenyi's Criteria From theory of beams on elastic foundation, Hetenyi proposed rigidity criteria on the basis of hLterm which considers width, length and elastic properties of the media. This term is

hL =

(K,.L ~ ) " ~ 4 El

where

K, = KB = Modulus of sub grade reaction X Width of footing - units of psf. L = Total length of foundation member

E = Modulus of elasticity of footing material I = Moment of inertia of footing If 1 , c W4 footing can be considered as rigid. For value between W4 and l3 semi rigid, and elastic, if > I7 ~ o w l e sfound ' ~ this criteria of very limited application. 5.6

Modulus of Sub-Grade Reaction

One of the important terms required in analysing foundation on the basis of flexible footings is value of modulus of sub-grade reaction also called coefficient of sub-grade reaction for the particular soil in the foundation of the buildings. Mathematically, this can be axpressed as intensity of soil pressure required to create a unit

31

DESIGN APPROACH AND CONSIDERATIONS

3 I

deflection. Theoretically, it can be determined by performing a plate load test and plotting a curve of soil pressure versus deflection. In actual practice, however, many other factors enter and actual value in field is different from what can be determined by a simple plate load'test. Major problems associated are: (a) Soil is not perfectly elastic and results are effected by the magnitudes of soil pressure and deflection (b) Footing size affects the value (c) Footing shape also affects (d) Depth at which footing is located also affects (e) Soil stratificationand other changes with depth which may not show when testing with a small plate (f) In methods where soil modulus is determined in laboratory, site condition can not be exactly duplicated in field laboratory (g) Various authors have suggested different factors to take these problems into account On the other hand, certain authors have suggested very simple values for modulus of sub-grade reaction which can be determined from bearing capacity factors used in Terzaghi bearing capacity equation. 5.6.1 Recommended by ~ o w l e s ' ~

Has related value of modulus of sub-grade reaction with safe bearing capacity by the relation Ks = 36 qa where qa is the allowable bearing capacity in Kips per sq ft. A slightly improved values are also suggested by the equation.

where c is cohesion, Nc and N q are bearing capacity factors, Sc and Sq are shape factors for particular soil in foot units . Moreover:

Sc = I + -

NcxB NcxL

General values suggested by Bowles are given below: Soil

Range of Ks. Kef

~ o o s sand e

30 - 100

Medium sand

60 - 500

~ e n s sand e

100-800

clayey sand (Medium)

200 - 500

Silty sand (Medium)

150 - 300

Clayey soil :

qu 5 4 Ksf 4 50, curve 1 for N < 10 and Terzaghi, Peck curve for N values between 30 and 50. As is apparent there are number of limitations to this method. All suggestions are for cohesionless soil. Original test reported by Terzaghi, Peck and Thornburn cannot be considered to have universal application. 5.6.6 Summary

It would thus be seen that even for the same soil data values of modulus of sub grade reaction determined by methods suggested in the different text books and codes will be different. Which value to be adopted for the correct design, is a million dollar question.

STRUCTURAL

DESIGNER^ DILEMMA

Structural design of raft foundation is being carried out by structural engineers individually, or while working in any consultancy organisation which could be in public or private sector. Competence of these structural designers varies widely. On one extreme are those who have some knowledge of structural design, but do not have much of guidance from their seniors. Such engineers when faced with problem of undertaking design of raft foundation will pick up a text book or manual on reinforced concrete design and follow the procedure laid down which in most cases would be conventional combined footing and would normally be safe ut expensive. On the other extreme are top class engineers who have wide experience and knowledge of structural designs with ability to analyse the problem, carry out alternative analysis and design a raft foundation which would not only be safe, but would be economical. Such designers are, however, very few and seldom undertake design for normal buildings. Majority of the designers have knowledge in between those two extremes. They have read moderately, have some experience of design and would normally try to make a design which is not only safe but should also be economical. These designers will study more than one book or manual on concrete and structural design and will normally find that the opinions and methods of design recommended in various text books and manuals differ widely. They will also find that the examples considered in the text books and manuals mostly are very simple and regular-shaped uniformly loaded rafts which satisfy number of assumptions made in the text book, whereas their problem is much different. They may also have a feeling that conventional methods of raft design are old fashion and may lead to high thickness and high quantity of reinforcement; Flexible methods give low thickness and low values of bending moments and accordingly low cost of reinforcements. They f&e the dilemma as to which of the methods they should adopt. Quite often since no straightaway guidance is available in the books or manuals for practical design of raft, they also finally take up one or two text books and work out a design. Quite often analysis is carried out on computer using flexible approach utilising the value of modulus of subgrade reaction suggested by the soil consultant or worked out by them as per method given in any book and simultaneously design the raft for vertical loads neglecting various other factors which affect the design of raft. Such designs are seldom satisfactory, though structures designed by them, do not show any distress to start with. Structures seldom get loaded to the design loads. How will these structures behave when subjected to full designed loads including seismic effects can be judged only by number of otherwise standing structures failings in such circumstances.

e

STRUCTURAL DESIGNERS'S DILEMMA

$

E

::

Keeping such designers in view, it was felt that enough material should be made available to them to allow them to appreciate the effect on raft foundation of variation in the values of the various parameters adopted by them in design and also the effect of neglecting base moments of the column and horizontal loads. Studies have, therefore, been carried out on the effect of various parameters on the values of bending moments, shear force, contact pressure and deflection in the rafts already constructed for actual buildings. These studies are presented in Chapters 7 and 8. Finally, keeping in view the various constraints under which a designer has to cany out the design, suggestions have been made for methods to be adopted in Chapter 12 for various situations.

STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERS ON DESIGN OF RAFT The usual practice of design being followed is to work out preliminary sizes of the raft, i.e., thickness of the slabs, if it is uniformly thick raft or beam size and slab thickness in case it is beam and slab system on the basis of shear and analyse the raft for vertical loads alone. As an improvement where computer facilities and greater expertise are available, raft is analysed as flexible raft selecting one particular value of modulus of subgrade reaction, one assumed size of the raft and vertical loads alone. Values of bending moments thus obtained are used. In both these designs unless the preliminary sizes selected are found to be structurally unsafe in resisting moments and shears, even after addition of permissible reinforcement, the design is completed and finalised. As already pointed out in previous chapters the real position is not so simple. Different designers may select different preliminary sizes, different values of modulus of subgrade reaction even for the same soil, and pattern of pressure distribution under the raft. In actual buildings, columns have base moments which are resisted by the junction of the raft and the columns. Buildings subjected to earthquake forces have not only increased column base moments but also undergo cyclic effect in which vertical loads in different groups of columns decrease and increase. Studies have, therefore, been carried out to consider on the design of raft foundation the effect of neglecting some of these aspects and making assumptions which in fact are not true. These studies have been carried out in four parts. 7.1

Study 1

In sophisticated flexible analysis, utilising computer, it is soil properties which matter to a large extent. In exact analysis all soil properties matter, but in commonly adopted analysis where soil-raft interaction is idealised as a.spring of known rigidity most important soil property is modulus of sub-grade reaction. The rigidity of raft which is determined by the size of the raft and effect of super-structure on the same, is another vital parameter which comes into play in any analysis. The effect of variation in values of both these parameters on the value of bending moments and shear forces, one gets on an analysis, has been studied in this study. Efforts have been

STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERS

39

made to present results, in numerical values and show the large variation which, one can get .for the same structure, having a particular loading pattern founded on the same soil when different sizes of raft or values of modulus of sub grade reacrion determined by various methods available in literature are adopted. While carrying out this study, only vertical loads have been considered. Contribution made by super structure in the rigidity of raft has been neglected. 7.1.I Examples Selected Most of the text-books on structural engineering and reinforced concrete design,while dealing with examples on raft analysis, generally consider a simple symmetrical shape with more or less symmetricaVunifom loading. But in practice this never happens. Even when the shape may be symmetrical, the loading is not. TOmake the study realistic, raft foundations for actual buildings have been considered in this study. One eight-storeyed block of residential flats consisting of four flats on each floor with a central core having staircase, lift and other service areas, which has already been constructed few years ago, has been considered in this study. The central core goes beyond eight storeys to provide staircase mumty, machineroom and water tank. Ground floor has got part parking. The central core is separate from other blocks. The raft foundation consists of a slab having uniform thickness, one for each side block and other for central core. The central core is symmetrical in shape about one axis. The side block is not symmetrical about any axis. Loading on these blocks are as per actual loads obtained during design process. Example 1 relates to the side block, and Example 2 to central core. The third example considered is the front block of another six-storeyed institutional building, which consists of a front block and rear block separated by expansion joints. Its front block isrectangular in shape but has unsymmetrical loading. The raft consists of beams in both direction and a slab monolithic with the b%ams. In two out of these three examples, raft dimensions have been so adjusted that the centre of gravity of vertical loads and centre of gravity of raft area coincide. The loading can, therefore, be considered to be symmetrical and it is this aspect which is very important. In practical examples, it may generally be possible to coincide CG of raft and load. It is, however, not possible to have simultaneously a symmetrical shape in plan also. 7.1.2 Rafr Size The raft thckness actually provided for Example 1 (Fig. 7.1) is one metre. In this study, it was considered that this thickness could vary from 80 cm to 1.2 m. For Example 2 (Fig. 7.2) actual thickness provided is 1.2 m. This was considered to vary from 1 M to 1.4 M . In Example 3 (Fig. 7.3) the slab thickness is 50 cm and the transverse beams are 80 cm x 150 cm (including slab). Longitudinal beams are 85 cm x 110 cm. While the beam sizes are considered to remain the same, the slab thickness is taken to vary from 50 cm to.90 cm . All these variations are considered in steps of 10 cm each. The effect of rigidity of the super structure has not been taken into account. 7.1.3 Soil Investigation . Soil investigations to determine the safe bearing capacity of soil for purposes of design were done through specialised consultants and their reports obtained. Since in conventional design, properties like modulus of sub-grade reaction are not utilised, these consultants were not requested to intimate value of modulus of sub-grade reactions and they did not do so. Values of modulus of sub grade reaction were, therefore, calculated by variousmethods described in 5.6 above. Details of soil investigationsindicating soil strata at various depths, 'N' values from standard penetration tests, location of water table and values of modulus of sub-grade reaction calculated are indicated in Figs. 7.4 and 7.5 for Examples l , 2 , and 3.

RAFT FOUNDATIONS-DESIGNAND ANALYSIS

t

T16985

RAFT SLAB THICKNESS4OOoMM

Fig. 7.1 -

..

1 0 6 6 0 t 2 2 8 0 t6090

~

+ l 5 8 O b 7500 -tlW+

-10660

~t Fig.7.2

22904

41

STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERS

f5

I

i ~ n JKN n

1N -

I 6213 4m.4

1 SRI ~ K N U N

1767 TKN

~9-

BEAM S I Z E S B1=650X1500mm B2=600X1500mm B3=800X 1 1 00mm

1 g

COLUMN S I Z E =400X800mm

T h i ckness

PLAN OF R A F T AT FRONT BLOCK

Fig. 7.6

STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERS

,-

10720

1 1) :. adopt K, = 1 T, = 0.25 fd = characteristic strength of concrete T, = 0.25 0.968 N / I T ~ ~ Punching stress < Permissible stress

6

m=

which is less than 1.2 m depth of the pedestal already provided. If a punching shear calculation is made to check the pedestal punching through itself it would be found to be safe. As per above calculation raft therefore consists of beam and slab construction with 1000 mm x 1200 mm beam in the transverse direction and 1000 mm x 800 m m beams in the longitudinal direction with a slab thickness of 400 mm. Under each column a pedestal is provided with its top at 1200 mm level and projecting 650 mm on both sides of the beam as shown in Fig.A.2.2 (A). A.2.5. Analysis as a Whole in Long Dirkction The raft is converted into a large beam as shown in Fig. A.2.3. This beam rectangular in cross section has the same width and rigidity as whole of the raft. Calculations for this conversion are as under. Distance of N.A from

A - A = &Ii. Xi &Ii(Refer cross section)

= 0.257 m say 0.26 m Moment of inertia a b u t N.A

APPENDIX-ILLUSTRATIVE EXAMPLES

+9L

W

m

r

3

z IY

w

m r

w

I:

A

I I 1

'rO

'4

@

:4 .

3

11

?

N

% ru ro

'4

N O

'4

N

' . a

m

N a-

a

a?

N -

A

'em

-4 N

+a

!-

2 (4 r

I-

z

a I-

V)

. .!-

+-

z

0 U

u z H

a

v,

120

RAFT FOUNDATIONSDESIGNAND ANALYSIS

Equivalent beam with same width and I, would have depth

Thus equivalent beam is 0.569 m deep and 24.03 m wide. The various point loads on this beam are sum of all column loads in the line perpendicular to the length of the beam at that point. Springs have been provided under each column and also at the mid point of span. More number of springs can be provided but that much more of calculations get involved. Bending moment values under columns and mid point of span are enough. Springs are also located at the two extreme ends. Calculation. of spring constant are indicated below the figure. It would be seen that spring constant at the end of the raft are doubled as recommended by Bowles. This is done to allow for the end effect expected in such rafts. The problem is solved on electronic computer utilising a finite element approach. Portion of beam between the nodes is treated as a finite beam element and the.values of bending moment shear force and pressure at each node are obtained. This computer programme is such that values of SF and BM given are per metre width of beam, i.e., the strip of the raft being analysed or raft as a whole. These are shown in table A.2.1. Bending moment, shear force and pressure diagrams are also drawn in Fig. A.2.4. For Bending moment only values are indicated at each node. Table A.2.1 Table showing values of Shear Force, Bending Moment and Soil Pressure for the raft as

a whole in long direction Node

1

1

Shear Force in KN

Lefi

Right

Moments KNm

Soil Pressure K N / ~ *

I

121

APPENDIX-ILLUSTRATIVE EXAMPLES

Node

Left 20 21 22 23 24 25 26 27 28 29 30 31 32 33

ll6a

- 30.803

- 30.803

2 10.468

- 360.065 - 1 2.364

346.55 1 - 12.364 392.640 25.338 327.233 13.660 265.549 - 5.895 273.322 10.731 160.102 0.001

- 272.932

I -

- 16 6 1 7

-

4 9

I

106.980 116.349 134.040 138.899 145.371 133.947 126.611 11 1.848 106.799 99.622 101.312 98.377 105.381 107.410

- 1 10.908

262.874

25.338 - 317.295 13.660 - 273.860 - 5.895 - 270.123 10.731 - 257.21 0 0.001

1 . 2 1 3 4 1 5 0

Right

- 256.186

- 386.178

lTaP

Soil Pressure K N / ~ '

Moments KNm

Shear Force in KN

174.926 - 337.966 191.919 - 204.69 1 21 8.053 - 132.843 196.190 - 163.853 204.599

'

- 107.410 0.000

SHEAR FORCE DIAGRAM ( I N KN> In75

la4i

lOSB

174.9

I

1 ~ 1 n P W lh

I LtO

m

unz 3lu

till

I

ua9 EOIL m 9 337.9

BENDING MOMENT DIAGRAM ( I N KN-M)

S O I L PRESSURE DIAGRAM ( I N KN/Sq.M)

Fig. A.2.4 Force diagram for raft on a whole (Longer direction)

I

m7.4

l6aa

"

122

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

A.2.6 Analysis as a Whole in the Short Direction

The equivalent beam in the short direction with same rigidity and width with calculation for spring constant and the result obtained are shown in Fig. A.2.5 and Table A.2.2 respectively. Both these values are treated as lower bound. Distance of N.A from

A - A = U,. CX,/ Ui (Refer cross section)

Moment of inertia about N.A

Equivalent beam of same width and I, would have depth

Thus equivalent beam is of cross section 0.905 metre x 80.855 metre.

I 80.855 M CROSS

SPRING CONSTANTS

SECTION

OF

SHORTER

BEAM

N-->

I-

(N)-->

(NOTE)

NODE NO. MEMBER NO.

- THIS

BEAM HAS A CROSS SECTIONAL AREA OF 3.877 M~ ALL DIMENSIONS ARE I N H.

Fig. A.2.5 Idealised beam for the raft as a whole in shorter direction and cross section of the beam

i I

APPENDIX-ILLUSTRATIVE EXAMPLES

408373

4la674

SHEAR FORCE DIAGRAM ( I N KN)

BENDING MUMENT DIAGRAM ( I N KN-M)

SOIL PRESSURE DIAGRAM ( I N KN/Sa.M)

Fig. A.2.6 Force diagram for raft as a whole (Short direction) Equivalent beam of same width and I, would have depth

Thus equivalent beam is of cross section 0.905 metre x 80.855 metre.

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124

RAFF FOUNDATIONS-DESIGN AND ANALYSIS

I

I Table A.2.2 Table showing values of Shear Force, Bending Moment and Soil Pressure for the raft as a whole in shorter direction Moments KNm

Shear Force in KN

Node

Left 0.000 - 256.458 136.840 - 17.253 - 188.110 - 408.373 190.660 0.690 - 187.994 - 4 10.674 184.852 16.539 - I 36.160 -317.372 0.000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Right 0.000 318.806 136.840 - 17.253 - 188.110 416.659 190.660 0.690 - 187.994 401.459 184.852 16.539 - 136.160 256.0 13

0.000 - 254.580

126.530 235.046 66.058 - 434.278 80.579 233.985 82.997 - 424.934 66.848 222.973 124.816 - 254.268

Soil Pressure KE!/~' 113.147 115.651 92.013 83.843 111.143 140.228 117.689 99.110 1 16.220 137.908 109.289 82.794 9 1.470 115.334 113.008

A.2.7 Analysis for a Strip in the Short Direction The equivalent beam for the strip M7 - 57, shown in Fig. A.2.2, position of springs, spring constant and result obtained are shown in Fig. A.2.7 Table A.2.3 and Fig. A.2.8 respectively. It would be noted that while analysing this strip strings have been located at a closer interval as compared to long direction. This is not necessary in each case. These are upper bound value and can be decreased by 30% for design. Calculations for determining depth of equivalent beam, and spring constant, is not shown.

Table A.2.3 Table showing values of Shear Force, Bending Moment and Soil Pressure for the beam M7 - J7 Node

1

Moments KNm

Shear Force in Kn Right 0.000 278.829 124.074 - 25.856 - 185.089 361.155 177.196 0.219 - 177.140 367.425 187.441 24.924 -.130.167 -217.629

' 0.000

Soil Pressure KN/~*

APPENDIX-ILLUSTRATIVE EXAMPLES 2542 KN

3807 KN

SPRING CONSTANTS

/I

3836 KN

2678 KN

N-->

:-

K = 11000 X 5.257 X 1.7525 = 101341.8 KN/M

NODE NO.

(N>-->

MEMBER NO.

SPRING CONSTANTSI-

N

---->

lECleER NwER

ME WER

Fig. A.2.9 Idealised beam on elastic foundation for strip L5-L20 in long direction From these results it is observed that: (a) Values of soil pressure at various nodes in all the four analysis carried out and shown above are generally within safe bearing pressure of 130 KN/m2. Maximum value reached at any point in a strip is 187 KNIsqm. Allowing a reduction of 30% this will be very near to SBC. So O.K. (b) Values of B.M. obtained from analysis of individual strip in long direction are not much different from that obtained from raft as a whole. Either these are low or within 30%. It would thus be O.K. if raft in the long direction is designed for a B.M value of +_ 210 KNmIper metre width throughout and additional reinforcement added in the portion where it goes more.

I

1 ,

I

,

APPENDIX-ILLUSTRATIVE EXAMPLES

SHEAR FORCE DIAGRAM C I N KN

1

>

BENDING MOMENT DIAGRAM C I N KN-M

SOIL PRESSURE DIAGRAM FOR BEAM L5-L20




Fig. A.2.10 Force diagrams for beam LS-L20 (c) B.M. in short direction between strips and raft as a whole is not much different and can be designed for a value of 250 K N d m and additional reinforcement added in central portion to resist a total B.M. value of - 430 K N d m . (d) While designing the raft it is to be seen that the values of B.M. given above are per unit width of the strips, while the raft actually provided consists of beams and slab. Using these values of per unit width total bending moment to be resisted by each strip should first be calculated and then the design carried out for the T beam. Care is to be taken to see that reinforcement detailing in slab is such that it is also capable of functioning as a slab supported on the four edges carrying a load equal to the upward average pressure of soil.

+

128

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

Table A.2.4 Table showing values of Shear Force. Bending Moment and Soil Pressure for the beam L5 - LU) Moments KNm

shear Force in KN

L.ft

Right

Soil Pressure KN/~'

I I 1

'APPENDIX-ILLUSTRATIVE EXAMPLES

A.2.10 !

129

VQliation on Value of K and Seismic Effects

Analysis has been carried out for a single value of K. Effect of earth quake forces has not been considered. It would be therefore necessary to adopt a minimum value of BM's as f w12/10 by treating the raft as inverted floor system with a uniform load of 120 KN/m2 as done in preliminary designs.

1 I

A.3 Example 3: Piled Raft-plate on Elastic Foundation Ideally the raft should be designed as a flexible plate supported on piles and soil taking into account the pile, soil raft and superstructure interaction. The most simple method could be to treat piles being replaced by soil having a bearing capacity higher than the total load canying capacity of piles actually provided, divided by the area of the raft. The raft can then be designed by rigid method as already explained in Example I. As an improvement, it could abo be analysed adopting the approach of beam on elastic foundation as given in Example 11. Further refinement somewhat short of ideal solution could be to treat the raft as a plate supported on springs neglecting the effect of one pile on the other and the effect of soil on the raft. In the present example this method is used. The piled raft has been modelled as a plate consisting of plate bending elements and the piles are modelled as axial elements. The piles being predominantly frictional piles, the axial stiffness of the pile element has been taken as h EAIL where E is modulus of elasticity, A the cross sectional area and L is the length of the pile. h is a factor depending upon the pile characteristics. According to Bowles has a value of I , for end bearing piles and 2 for friction piles. Analysis has been canied out using a general purpose 3 dimensional finite element package (SAP IV). A.3.1 Example Selected

I

:

The building consists of 3 blocks. The two side blocks are two storeyed and the central block is 8 storeyed. The entire building has a basement and has a T shape. Expansion joints have been provided to separate two storeyed blocks from 8 storeyed blocks but the joint is not continued in the raft which is made continuous.A plan of the building at the basement level is given in Fig. A.3.1. Expansion joint as provided are shown in this plan. The building at the groundfloor extend beyond the basement in the portion shown dotted in the above figure. Then again in the 1st floor level building has a set back from the basement line with the result that the columns F13, H13, K13, Ml I, M9, M7, M5, K3, H3, F3, are lightly loaded. 708 bored cast-in-situ 21 metre long piles have been provided. Each pile has a safe load bearing capacity of 46 tonnes. The piles have been spaced as below: Portion PQR W

- 1.54 metres x 1.925 metres.

Portion RRl Wl W - 1.54 x 1.52 metres. Portion WW,UV and RSTR, = 2.40 x 1.52 with some extra piles near column C2 and C14 which are comparatively more loaded in this area. Arrangement of piles in the panel A1, A2, C2, C1 and F10, F12, H12, H10 is shown in Fig. A.3.2 andA.3.3 as a blow up. A.3.2 Type of RaftAdopted The raft adopted in this case is a slab of uniform thickness of 1.2 metres. The depth has been decided on the consideration of punching shear in accordance with the provision of IS: 456-1978 as per calculation given below:

130

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

PANSION JOINT

-------------

+ SHOWS COLUMN LOCATIONS

I

36!l46

I SHOWS TWIN COLUMN LOCATIONS

Fig. A.3.1 Plan of raft showing column locations The permissible shear stress at the critical section, i.e., at dl2 from the column faces is equalto Ks T,, where K , = (0.5 + PC)but not greater than 1,

PCbeing the ratio of short side to longer side of the column.

T, = 0.16%

For concrete of M20 grade

fck

2

= 20 Nlmm

T, = 0.16 &=

0.72 N/mm2

Here the column for which the check is being applied is 750 mm x 750 mm with maximum load being 6095.3 KN Therefore

Thus, Permissible stress

P = 6095.3 KN

APPENDIX-ILLUSTRATIVE EXAMPLES

-

I

7314

I

Fig. A.3.2 Blow up of X showing pile locations and plate elements

-

9-

-

@SHNS

SHOVS PILE LOCATION NODE NUnBER (N)

PLATE ELDENT NUMBER (E)

Fig. A.3.2 Blow up of Y showing pile locations and plate elements

132

RAFT FOUNDATIONSDESIGNAND ANALYSIS

Area of concrete resisting the shear =2{(750+d)+(750+d)}d =4(750+d)d Therefore shear stress

For a clear cover of 60 mm and 20 mm diameter steel; effective cover

Thus overall depth required

= 1127+70= 1197mm

Overall depth provided

= 1200 mm

A.3.3 Division of l a # into Plate Elements

The raft has been divided into plate elements with nodes on each pile. No nodes have been located under the columns. Arrangement of plate element has been shown in the blow up figures A.3.2 and A.3.3. The load on each column has been distributed on the 4 nodes around the columns in proportion to the contributory areas as per calculation shown below: Let the column (here F12) be surrounded by the four nodes (i.e. pile points) N1, N2, N3 and N4 to make a rectanguldplate element of size 1925 mm x 1540 mm. The td;al area of the element gets divided into four parts A,, A2, A3 and A4 as shown in Fig. A.3.4: Now the load on any node Niis given by,

Here

P = Total load of the column A, = Area opposite node Ni

I

APPENDIX-ILLUSTRATIVE EXAMPLES

I

2 9

Fig. A.3.4 Distribution of column load on surrounding nodes Here

Total area of the plate element = 2964500 mm2 Here N1, N2, N3 and N4 are node nos. 590,591,567 and 566 respectively. Here the loads on corresponding nodes are

/

Here the nodes are not getting any load from other columns. In case load from other column(s) is also coming onto a particular node, the sum of all such loads should be taken. One such case is available at nodes between columns H9 and HlO.

134

RAFr FOUNDATIONS-DESIGN AND ANALYSIS

A.3.4 Output Output gives moments about X axis, Y axis and in-plane moment of the element at the centre point of the element. In case, these values are required at any other point other than the centre point, option is available in the SAP programme to define that point as the 5th node. Though the raft is 1 :2 metre thick, but the thickness has not been considered in this model and the modelling is done at the central plane of the thickness. Loads on each pile are also given as an output. The values of the moment in X and Y direction are plotted on the plan and used for design purposes. Care is required to be taken while detailing reinforcement. Care is also to be taken to see that difference between the actual structure and the modelling is taken into account in detailing. However, as already explained in Chapter 8 ,above analysis is carried out considering only vertical loads and neglecting the effect of rigidity of super-structure and retaining walls. Loads on some piles would also 8e found to be exceeding safe bearing capacity by as much as 75%. It would therefore be necessary to select one of the two approaches given in para 12.6. Approach actually to be adopted will depend upon the particular case. In this example there was no possibility of obtaining piles with higher load and proposal was thereyore being processed for additional piles in sections where load was exceeding. Once this is done, question of effect of seismic forces would arise. This is explained in para 8.5.3. In case analysis is not repeated taking into account seismic forces, minimum values of bending moment at any location should be ~ 1 ~ 1 determined 10 by treating the raft as an inverted floor. This would result in a safe design.

+

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j

i

:

; J

1

1