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• Peggy Jan Qorban

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Catalog No. L51990

Extended Model for Pipe Soil Interaction Contract PR- 271-0184

Prepared for the Design, Construction & Operations Technical Committee of Pipeline Research Council International, Inc.

Prepared by the following Research Agencies: C-CORE & Doug Honegger, D.G. Honegger Consulting Publication Date: August 2003

Extended Model for Pipe Soil Interaction - Final Report

TABLE OF CONTENTS EXECUTIVE SUMMARY ................................................................................................II 1 REVIEW OF STUDIES RELEVANT TO PIPE/SOIL INTERACTION .................. 1 1.1 Representation of Soil-Pipeline Interaction Forces: Theoretical Analysis and Laboratory Testing .......................................................................................... 1 1.1.1 Longitudinal Interaction.................................................................... 2 1.1.2 Transverse Horizontal Interaction..................................................... 7 1.1.3 Transverse Vertical Interaction ....................................................... 16 1.2 Validation ...................................................................................................... 19 1.2.1 Field Tests ....................................................................................... 19 1.2.2 Laboratory Tests ............................................................................. 20 1.2.3 Centrifuge Modelling ...................................................................... 20 1.3 Numerical Modelling of Pipe/Soil Interaction .............................................. 22 1.3.1 Finite Element Analysis – Structural Models ................................. 22 1.3.2 Finite Element Analysis – Continuum Models ............................... 23 1.4 Summary of Past Efforts to Characterize Pipe-soil Interaction..................... 23 1.4.1 Current State of Practice ................................................................. 23 1.4.2 State-of-the-Art ............................................................................... 24 1.4.3 Open Issues/Challenges .................................................................. 26 1.5 Issues Addressed by This Research Project .................................................. 28 1.5.1 Introduction ..................................................................................... 28 1.5.2 Non-linear Soil-Spring Characteristics ........................................... 29 1.5.3 Axial Pipe-Soil Interaction.............................................................. 29 1.5.4 Lateral Pipe-Soil Interaction ........................................................... 29 1.5.5 Complex Loading............................................................................ 30 1.5.6 Quantifying Mitigative Measures ................................................... 30 2 SUMMARY OF METHODS TO ESTIMATE AXIAL FRICTION ....................... 41 2.1 Comparisons of Proposed Adhesion Factor Relationships with Test Data... 42 2.2 Variation of Axial Load with Axial Displacement........................................ 44 3 CONSTITUTIVE MODELS .................................................................................... 49 3.1 Constitutive Models....................................................................................... 49 3.1.1 Constitutive Model for Sand ........................................................... 49 3.1.2 Constitutive Models for Clay.......................................................... 49 3.2 Analysis Procedure........................................................................................ 50 3.3 Treatment of Pipe-Soil Interface ................................................................... 50 3.4 Numerical Model Verification ...................................................................... 51 3.4.1 Pipe in Sand: Lateral Loading......................................................... 51

Extended Model for Pipe Soil Interaction - Final Report

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3.4.2 Pipe in Clay: Lateral Loading ......................................................... 52 3.5 Comments...................................................................................................... 52 EFFECT OF COVER DEPTH AND SOIL STRENGTH: HORIZONTAL LOADING ................................................................................................................ 61 4.1 Introduction ................................................................................................... 61 4.2 Finite Element Analysis Set-up ..................................................................... 61 4.2.1 Finite Element Set-up...................................................................... 61 4.2.2 Material Properties .......................................................................... 62 4.2.3 Analysis Procedure and Cases Analyzed ........................................ 62 4.3 Pipes in Uniform Clay ................................................................................... 63 4.3.1 Ultimate Soil Resistance: Bearing Capacity Factor........................ 63 4.3.2 Failure Mechanisms ........................................................................ 63 4.3.3 Discussion ....................................................................................... 64 4.3.4 Force-Displacement Curves and Pipe Displacement at ultimate state ......................................................................................................... 65 RATE EFFECT ON SOIL-PIPELINE INTERACTION ......................................... 91 5.1 Effective Stress Analysis............................................................................... 91 5.2 Representative Soil Parameters ..................................................................... 91 5.2.1 Soil consolidation during loading ................................................... 92 5.2.2 Effect of loading rate on pipe responses ......................................... 92 TRENCH EFFECT ON SOIL-PIPELINE INTERACTION .................................. 103 6.1 Studied Cases............................................................................................... 103 6.2 Effect of Trench Width................................................................................ 103 6.3 Effect of Burial Depth at a Given Trench Width ........................................ 104 6.4 Effect of Backfill Strength Relative to Native Soil Strength ...................... 105 6.5 Trench with Inclined Walls ......................................................................... 105 PIPE/SOIL INTERACTION UNDER COMPLEX LOADING.............................. 126 7.1 Studied Cases............................................................................................... 126 7.2 Effect of the Angle of Pipe Translation on Pipe-Soil Interaction ................. 127 7.3 Effect of Soil Strength on Pipe-Soil Interaction with Combined Pipe Movement .................................................................................................... 127 7.4 Effect of Pipe-Soil Frictional Angle δ and Burial Depth Ratio................... 128 PHYSICAL MODELLING .................................................................................... 142 8.1 Introduction ................................................................................................. 142 8.2 Summary Program....................................................................................... 142 8.3 Experimental Procedure and Testing........................................................... 143 8.3.1 Pipelines ........................................................................................ 143 8.3.2 Soil Preparation and Testing ......................................................... 144 8.3.3 Instrumentation and Measurement................................................ 144

Extended Model for Pipe Soil Interaction - Final Report

8.3.4 Test Procedure............................................................................... 145 8.4 Shear Strength and Water Content of Native Soil....................................... 145 9 QUANTIFICATION OF MITIGATIVE METHODS............................................ 155 9.1 Summary of Experimental Results.............................................................. 155 9.1.1 Force-Displacement Curves .......................................................... 155 9.1.2 Comparison of Test Results with Existing Methods..................... 156 9.1.3 Mitigative Effects.......................................................................... 158 9.2 Comparison of Experimental and Numerical Results ................................. 160 9.3 Numerical Back-Analysis............................................................................ 162 10 DISCUSSION AND RECOMMENDATIONS...................................................... 187 10.1 Complex Loading ........................................................................................ 187 10.2 Axial Pipe-Soil Interaction .......................................................................... 188 10.3 Lateral Pipe-Soil Interaction........................................................................ 189 10.4 Quantifying Mitigative Measures................................................................ 190 10.5 Future Work................................................................................................. 191 11 REFERENCES ....................................................................................................... 202 APPENDIX A EXAMPLE FOR PIPELINE RESPONSE TO P-Y CURVE INITIAL STIFFNESS............................................................................................. 213 APPENDIX B RECOMMENDED CHANGES TO STRUCTURAL ANALYSIS DESIGN GUIDELINES ......................................................................................... 216

Extended Model for Pipe Soil Interaction - Final Report

LIST OF FIGURES

Figure 1-1

(a) Schematic illustration of continuum pipe/soil interaction, (b) Idealization of pipe/soil interaction based on structural model. .................. 31 Figure 1-2 Generalized load−displacement relationships for modelling soil behaviour ..................................................................................................... 32 Figure 1-3 Plotted values for the adhesion factor, α ..................................................... 33 Figure 1-4 ASCE horizontal bearing capacity factor: adapted from Hansen (1961) .... 34 Figure 1-5 ASCE horizontal bearing capacity factor: after Trautmann and O'Rourke (1983a) ......................................................................................................... 35 Figure 1-6 Lateral bearing coefficients recommended in PRCI guidelines .................. 36 Figure 1-7 Definition of ε50 ........................................................................................... 36 Figure 1-8 Comparison of p-y curves for clays: ASCE (1984) for pipes and lateral loaded pile in soft clay ................................................................................. 37 Figure 1-9 Differences between ASCE (1984) for pipes and the practice of pile engineering................................................................................................... 38 Figure 1-10 Dependency of soil force on loading rate: pipelines buried in saturated sand ......................................................................................................... 39 Figure 1-11 Dependency of soil force on loading rate: pipelines buried in saturated clay ......................................................................................................... 40 Figure 2-1 Adhesion Factors Recommended in API RP2A and ALA Guidelines........ 47 Figure 2-2 Comparison of Adhesion Factor Relationships with Test Data................... 48 Figure 3-1 Finite element mesh for verification analyses ............................................. 53 Figure 3-2 Model parameters: (a) dependency of soil elastic modulus on effective mean stress used in finite element analysis, and (b) mobilization of soil strength parameters during deformation (adapted from Nobahar et al., 2000 and Popescu et al., 2001) .................................................................... 54 Figure 3-3 Experimental and calculated force-displacement curves for lateral loading of a rigid pipe in dense sand: using a perfectly elasto-plastic model with constant dilation angle and E = Eave ......................................... 55 Figure 3-4 Experimental and calculated force-displacement curves for lateral loading of a rigid pipe in dense sand: elasto-plastic hardening model with variable dilation angle: (a) constant Young’s modulus; and (b) variable Young’s modulus ........................................................................... 56 Figure 3-5 Experimental and calculated force-displacement curves of rigid pipe in loose sand..................................................................................................... 57 Figure 3-6 Recorded and predicted force-displacement relations for large-scale tests in clay, using the Von-Mises soil model: (a) soft clay; and (b) stiff clay.... 58 Figure 3-7 Comparison between predicted (a) and observed (b) failure in stiff clay.... 59 Figure 3-8 Recorded and predicted force-displacement relations for large-scale tests in clay, using the Cam-Clay model: (a) soft clay; (b) stiff clay................... 60 Figure 4-1 Typical finite element mesh and boundary conditions ................................ 67 Figure 4-2 Predicted force-displace curves for pipelines in uniform clay at different burial depth: (a) cu = 10 kPa, (b) cu = 20 kPa, and (c) cu = 45 kPa.............. 68

Extended Model for Pipe Soil Interaction - Final Report

Figure 4-3 Figure 4-4 Figure 4-5 Figure 4-6 Figure 4-7 Figure 4-8 Figure 4-9 Figure 4-10 Figure 4-11 Figure 4-12 Figure 4-13 Figure 4-14 Figure 4-15 Figure 4-16 Figure 4-17 Figure 4-18 Figure 4-19 Figure 4-20 Figure 4-21 Figure 4-22 Figure 4-23 Figure 5-1 Figure 5-2 Figure 5-3 Figure 5-4

Effect of burial depth on the bearing capacity factor of pipe in uniform clays 69 Effect of burial depth ratio on soil displacement distribution around a pipeline: cu = 20 kPa, δ/D = 0.35................................................................. 70 Effect of burial depth ratio on plastic strains (PEMAG) in soil around a pipeline: cu = 20 kPa, δ/D = 0.35................................................................. 71 Effect of burial depth ratio on soil displacement distribution around a pipeline: cu = 45 kPa, δ/D = 0.35................................................................. 72 Variation of plastic zone with burial depth: cu = 45 kPa, δ/D = 0.35 .......... 73 Effect of burial depth ratio on soil displacement distribution around a pipeline: cu = 10 kPa, δ/D = 0.35................................................................. 74 Effect of burial depth ratio on plastic strains in soil around a pipeline: cu = 10 kPa, δ/D = 0.35 .................................................................................... 75 Effects of soil strength on soil displacement distribution around a pipeline: H/D = 1.97, δ/D = 0.35 ................................................................. 76 Effect of burial depth ratio on plastic strains, PEMAG in soil around a pipeline: H/D = 1.97, δ/D = 0.35 ................................................................. 77 Onset of ultimate deformation: cu = 45kPa.................................................. 78 Contours of plastic strains, PEMAG: Influence of burial depth ratio on plastic deformation zone at the onset of ultimate soil deformation using cu = 45 kPa ................................................................................................... 79 The relationship between (H/D)cr and Nch ................................................... 81 Hansen’s failure mechanism for an anchor slab: the basic case .................. 82 Effects of soil unit weight of bearing capacity factor.................................. 83 Lateral interaction factor for weightless cohesive soil ................................ 84 Values of β, increase in the lateral interaction factor vs. γh/cu .................... 85 Comparison of calculated force-displacement curves and recommendations of ASCE (1984) for cu = 20kPa: (a) H/D =1.03; (b) H/D = 4.08 ................................................................................................... 86 Comparison of calculated force-displacement curves and recommendations of ASCE (1984) for cu = 45kPa: (a) H/D =1.03; (b) H/D = 4.08 ................................................................................................... 87 Variation of A '' yu and B” with burial depth ratio H/D at cu = 10 and 45 kPa 88 Calculated force-displacement curves and the curve fitting with A = 0.075 and B = 0.925: cu = 10kPa ................................................................. 89 Pipe displacement yu: predicted and recommended by ASCE (1984)......... 90 Effective and undrained strength comparison of some clays ...................... 95 Mean effective stress distribution in soil mass under drained conditions (relative pipe displacement δ/D = 0.45)....................................................... 96 Strengths of soil around the pipeline in total and effective stress analyses . 97 Distributions of mean effective stresses in soil under (a) drained, (b) undrained and (c) partially drained conditions: H/D = 1.3, c′ = 35kPa, φ′ = 30°, relative pipe displacement δ/D = 0.15 .......................................... 98

Extended Model for Pipe Soil Interaction - Final Report

Figure 5-5 Figure 5-6 Figure 5-7 Figure 5-8 Figure 6-1 Figure 6-2 Figure 6-3 Figure 6-4 Figure 6-5 Figure 6-6 Figure 6-7 Figure 6-8 Figure 6-9 Figure 6-10 Figure 6-11 Figure 6-12 Figure 6-13 Figure 6-14 Figure 6-15 Figure 6-16 Figure 6-17 Figure 6-18 Figure 6-19 Figure 6-20 Figure 6-21 Figure 7-1

Effect of loading rate on predicted force-displacement curves with different clays, burial depth ratio H/D = 1.3................................................ 99 Force displace responses at small displacement at different loading rates 100 Dependency of ultimate pipe force on loading rate................................... 101 Mobilization of cohesion and friction angle with strain ............................ 102 Definition of trench configuration ............................................................. 107 Effect of trench width: cun/cub=45kPa/20kPa, H/D = 1.03......................... 108 Effect of trench width on soil deformation: displacement distribution (relative pipe displacement δ/D = 0.08)..................................................... 109 Effect of trench width on soil deformation: plastic strain distribution, PEMAG (relative pipe displacement, δ/D = 0.08)..................................... 110 Soil deformations and contours of shear strain (PEMAG) in uniform soft clay (cu = 20 kPa) at relative pipe displacement δ/D = 0.08; B/D = 3.16.. 111 Effect of trench width at cun/cub =45kPa/20kPa: (a) H/D = 1.30 and (b) H/D = 2.0 ................................................................................................... 112 Differences of force-displacement curves of trenched pipes and pipes in uniform native soil ..................................................................................... 113 Plastic strain distribution, PEMAG and deformations of native soil: B/D = 2.1, H/D = 1.03, at pipe relative displacement, δ/D = 0.79 ................... 114 Plastic strain distribution, PEMAG and deformations of native soil: B/D = 1.6, H/D = 1.03, at pipe relative displacement, δ/D = 0.38 .................... 115 Plastic strain distribution, PEMAG and deformations of soil for a pipe buried in uniform native clay of cu = 45kPa: at pipe relative displacement, δ/D = 0.21 ........................................................................... 116 Effect of burial depth at different trench widths: (a) B/D = 1.6; (b) B/D = 2.1 117 Influence of burial depth ratio on bearing capacity factor for trenched pipelines: cun/cub = 45kPa/20kPa ............................................................... 118 Effects of burial depth on the distribution of plastic strain developed in soil at δ/D = 0.08: trenched pipelines, B/D = 2.16, cub/cun = 20kPa/45kPa119 Effects of burial depth on soil displacements at δ/D = 0.08: trenched pipelines, B/D = 2.16, cub/cun = 20kPa/45kPa............................................ 120 Calculated force-displacement curves for backfill strength of cun/cub = 45kPa/10kPa: H/D =1.3 ............................................................................. 121 Effects of backfill strength on force-displacement curves: (a) H/D = 1.3, B/D = 1.58; and (b) H/D = 2.0, B/D = 2.11 ............................................... 122 Effects of backfill strength on bearing capacity of trenched pipelines...... 123 Influence of burial depth ratio on bearing capacity factor for trenched pipelines: cun/cub = 45kPa/10kPa................................................................ 123 Effects of backfill strength on the bearing capacity factor of trenched pipelines ..................................................................................................... 124 The configuration of trench with inclined walls........................................ 124 Influence of trench wall inclination on calculated force-displacement curves ....................................................................................................... 125 Buried pipe subjected to combined axial and lateral translation ............... 130

Figure 7-2

Simulation of a buried rigid pipe subjected to complex loading: a & b. finite element meshes; c. positioning of the pipe and result output section ....................................................................................................... 131 Figure 7-3 The maximum normalized axial interaction forces (a) and lateral forces (b) predicted for the central zone of the pipe: H/D = 1.8, δ = 20° and cu = 45kPa ...................................................................................................... 132 Figure 7-4 Contact between pipe and soil at relative axial displacement z/D = 0.35: cu = 45kPa, H/D = 1.8 ............................................................................... 133 Figure 7-5 Relationship between calculated axial interaction factor Nz and lateral interaction factor Nx: H/D = 1.8, δ = 20° and cu = 45kPa.......................... 134 Figure 7-6 The maximum normalized axial interaction forces predicted for the central zone of the pipe: H/D =1.8, δ = 20°, cu = 20, 45, 100 and 200kPa 135 Figure 7-7 The maximum normalized lateral forces predicted for the central zone of the pipe: H/D = 1.8, δ = 20°, cu = 20, 45, 100 and 200kPa....................... 136 Figure 7-8 Variation of axial forces with soil strength in axial loading: (a) normalized force and (b) axial force on 1m long pipe segment ................ 137 Figure 7-9 Effect of pipe-soil frictional angle δ on pipe-soil interactions with combined axial and lateral loading: H/D = 1.8, cu = 45kPa ...................... 138 Figure 7-10 Effect of pipe-soil frictional angle on the interaction diagrams: H/D = 1.8, cu = 45kPa........................................................................................... 139 Figure 7-11 Effect of burial depth ratio on pipe-soil interactions with combined axial and lateral loading: cu = 45kPa, δ = 20° .................................................... 140 Figure 7-12 Effect of burial depth ratio on the interaction diagrams: cu = 45kPa, δ = 20° ....................................................................................................... 141 Figure 8-1 Typical layout of centrifuge model test after Paulin (1998)...................... 147 Figure 8-2 (a) Layout of Pipes in Test bed-2; and (b) location of pipe in the trench. . 148 Figure 8-3 Consolidated soil testbed before placing pipelines.................................... 149 Figure 8-4 Testbed surface is covered by Vaseline..................................................... 150 Figure 8-5 Test 5: undrained shear strength using Equation 9-2 (left) and Equation 9-3 (right)................................................................................................... 151 Figure 8-6 Laterally loaded pipeline and surrounding soil after test........................... 152 Figure 8-7 Soil deformations in front of pipeline captured by spaghetti mesh ........... 153 Figure 8-8 Sketch of pipe location after test for T2P3 ................................................ 154 Figure 9-1 Force-displacement curves for test 1, pipes 1-4 ........................................ 164 Figure 9-2 Force-displacement curves for test 2, pipes 1-4 ........................................ 165 Figure 9-3 Force-displacement curves for test 3, pipes 1-4 ........................................ 166 Figure 9-4 Force-displacement curves for test 4, pipes 1-4 ........................................ 167 Figure 9-5 Force-displacement curves for test 5, pipes 1-4 ........................................ 168 Figure 9-6 Force-displacement curves for all centrifuge tests .................................... 169 Figure 9-7 Horizontal Interaction Factor Comparison ................................................ 170 Figure 9-8 Comparison of test T2P1 (uniform soil) in terms of normalized force – normalized displacement with ASCE and PRCI guidelines...................... 171 Figure 9-9 Comparison of test T2P4 (uniform soil) in terms of normalized force – normalized displacement with ASCE and PRCI guidelines...................... 172

Figure 9-10 Comparison of pipe response in terms of normalized force vs. normalized displacement for pipe buried in uniform soil (T2P1) and pipe with trench (T2P3) ..................................................................................... 173 Figure 9-11 Comparison of pipe response in terms of normalized force vs. normalized displacement for pipes with vertical trench wall .................... 174 Figure 9-12 Comparison of pipe response in terms of normalized force vs. normalized displacement for pipes T1P1 to 4 ........................................... 175 Figure 9-13 Normalized force-displacement curves for pipelines buried in inclined trench ....................................................................................................... 176 Figure 9-14 Comparison of soil reactions in terms of normalized force vs. normalized displacement for T2P2 buried in inclined trench (45 degrees) and T2P3 buried in trench with vertical wall............................................. 177 Figure 9-15 Comparison of soil reactions in terms of normalized force vs. normalized displacement for T3P1 buried in inclined trench (60 degrees) and T3P4 buried in trench with vertical wall............................................. 178 Figure 9-16 Normalized force-displacement for pipelines buried in trenches with sand backfill............................................................................................... 179 Figure 9-17 Comparison of soil reactions for test T2P3 (cub = 20 kPa) and T4P3 (sand backfill) ............................................................................................ 180 Figure 9-18 Comparison of soil reactions for test T5P3 (vertical trench –sand backfill) and T5P2 (inclined trench – sand backfill) ................................. 181 Figure 9-19 Force-displacement curve comparisons, Tests T1P3 and T1P4 ................ 182 Figure 9-20 Force-displacement curve comparisons, Test series T2 ............................ 183 Figure 9-21 Force-displacement curve comparisons, Test series T3 ............................ 184 Figure 9-22 Force-displacement curve comparisons, Tests T2P1 and T4P1 ................ 185 Figure 9-23 Back-analysis of T2P3 using finite element method ................................. 186 Figure 10-1 Typical axial - lateral resistance interaction diagram ................................ 193 Figure 10-2 Proposed maximum normalized axial force .............................................. 194 Figure 10-3 Proposed maximum normalized lateral force ............................................ 195 Figure 10-4 Comparison of lateral interaction factor accounting for soil weight effects 196 Figure 10-5 Comparison of lateral interaction factor for weightless soil...................... 197 Figure 10-6 Normalized force-displacement relationship: (a) experimentally recorded shown by continuous lines; and (b) proposed approximation shown by dashed lines ............................................................................... 198 Figure 10-7 Normalized force-displacement relationship: (a) experimentally recorded shown by continuous lines; and (b) proposed approximation shown by dashed lines ............................................................................... 199 Figure 10-8 Normalized force-displacement relationship: (a) experimentally recorded shown by continuous lines; and (b) proposed approximation shown by dashed blue line ......................................................................... 200 Figure 10-9 Normalized force-displacement relationship: (a) experimentally recorded shown by continuous lines; and (b) proposed approximation shown by dashed blue line ......................................................................... 201 Figure B - 1 Plotted values for the adhesion factor, α ................................................... 221

Extended Model for Pipe Soil Interaction - Final Report

Figure B - 2 Lateral interaction factor for weightless cohesive soil .............................. 222 Figure B - 3 Values of β, increase in the lateral interaction factor vs. γh/cu .................. 223 Figure B - 4 ASCE horizontal bearing capacity factor: after Trautmann and O'Rourke (1983a) 224 Figure B - 5 Effects of lateral spring load on axial spring capacity based on angle of loading 225 Figure B - 6 Comparison of lateral interaction factors estimated from replacement model (dashed-line) and 3D finite element analysis.................................. 226 Figure B - 7 Comparison of axial interaction factors estimated from replacement model (dashed-line) and 3D finite element analysis.................................. 227 Figure B - 8 Proposed p-y curve for laterally loaded trenched pipeline ........................ 228

Extended Model for Pipe Soil Interaction - Final Report

LIST OF TABLES Table 2-1 Table 2-2 Table 4-1 Table 5-1 Table 6-1 Table 7-1 Table 8-1 Table 8-2 Table 9-1 Table 9-2 Table 9-3

Key Parameters for Tests Discussed in Honegger (1999) ........................... 44 Key Information from Tests Performed by Honegger in Sandy Soil .......... 45 Summary of cases studied............................................................................ 61 Studied case: rate effects.............................................................................. 94 Studied cases: trench effects ...................................................................... 107 Studied cases: combined axial and lateral loading .................................... 129 Summary of experimental tests.................................................................. 143 Shear strength and water content of native soil before and after acceleration ................................................................................................ 146 Experimental results and theoretical predictions ....................................... 155 Initial stiffness: experimental and theoretical values................................. 158 Estimated responses for T2P3 and T4P3 ................................................... 159

Table A - 1 Parameters used for the analysis................................................................ 214 Table A - 2 Finite element results in terms of peak values........................................... 215