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PROCEEDINGS OF THE 10TH INTERNATIONAL SYMPOSIUM ON TUBULAR STRUCTURES, 18 – 20 SEPTEMBER 2003, MADRID, SPAIN

Tubular Structures X Jaurrieta, M.A., Alonso, A., Chica, J.A. (Eds.)

Welded circular hollow section (CHS) joints in bridges Ulrike Kuhlmann Hans-Peter Günther Reiner Saul Marc-Ulrich Häderle

A.A. BALKEMA PUBLISHERS

LISSE / ABINGDON / EXTON (PA) / TOKYO

Welded circular hollow section (CHS) joints in bridges U. Kuhlmann & H.-P. Günther Institute of Structural Design, University of Stuttgart, Germany

R. Saul & M.-U. Häderle Leonhardt, Andrä und Partner, Consulting Office, Stuttgart, Germany

ABSTRACT: The use of circular hollow section members in bridge design is a relatively new concept. The application of such constructions is strongly influenced by the design and manufacturing of the joints. In general, there are two possibilities: either to use cast steel joints or welded joints. This contribution tries to give an overview about the advantages and disadvantages of both possibilities concerning the aspects of resistance, fatigue, manufacturing and economy in order to help practical engineers in their decisions and to allow for a further application of circular hollow sections in bridge design. As a crucial question, special considerations are given to the fatigue assessment of welded circular hollow section joints. For an example of a recently completed bridge with a typical spatial CHS truss and cast steel joints it has been shown that also welded connections would have been a possible alternative. This conclusion is drawn from numerical studies based on FE calculations applying the hot-spot stress approach for the fatigue assessment of the welded joints.

1 INTRODUCTION

2 CHS IN BRIDGE DESIGN

Spatial truss girders built up with tubular members are more and more used in modern bridge design due to their architectural transparent appearance. Most important for the design and durability of such structures is the design and the construction of the joints. The recently in Germany completed highway bridge “Korntal-Münchingen” design by Leonhardt, Andrä und Partner was constructed using cast steel joints at the intersection of the tubular members. However, during the design process of this bridge, several proposals suggested the use of costeffective welded joints. Due to some lack of information, mainly concerning the durability and fatigue behaviour of this kind of joint and due to some positive experience with cast steel in former CHS bridges, the relevant highway authority finally decided to use cast steel joints. This was the starting point for intensive numerical investigations in order to clarify whether for this specific type of bridge welded joints would have been a possible alternative, see also Kuhlmann et. al. (2002).

2.1 General The use of circular hollow section members as part of the main load-carrying structure of bridge girders is a relatively new constructional concept. During the last couple years several steel-concrete composite bridges had been constructed, see Table 1. The typical cross-section of this type of bridge generally consists of a tubular spatial truss girder carrying the concrete deck slab (Figure 1). The deck slab is connected directly to the steel structure by either shear studs, concrete dowels or in some cases where no top chord exist, as e.g. at the “Nesenbachtal” bridge by saw-tooth connections, see Schlaich et. al. (2000).

Figure 1. Cross-section of the bridge “Korntal-Münchingen”

At the bottom chord of the tubular space truss four brace members have to be connected to the continuous bottom chord. This type of joint is usually named KK-joint (Figure 2). Table 1 summarizes the dimensions of five CHS highway bridges recently built in Switzerland (CH) and Germany (D), as well as their joint characteristics. The table clearly indicates that all five bridges resemble each other concerning their overall global and local dimensions.

3.1.2 Structural Behaviour The casting process allows almost perfectly modeling of the joint according to the flow of internal forces, avoiding large stress concentrations. The cast joints are usually designed in such a way, that the load bearing capacity of the joint is higher than that of the attached tubular sections. This is generally done using three-dimensional FE calculations. During the design of the cast joints one should also consider some special features regarding the casting process. Large differences of the wall thickness for example, significantly influence the solidification and shrinkage process of the melt and may lead to unintentional internal blowholes and micro cracks. In order to avoid this, solidification simulation calculations may help to get the cast free from blowholes and to find the optimal feeder head position for fabrication. 3.1.3 Fatigue

Figure 2. Typical multiplanar KK-joint with notations

3 JOINT CONSTRUCTION 3.1 Cast Steel Joints 3.1.1 Material Properties Through the ongoing development within the manufacturing of cast steel products it is possible nowadays to gain almost the same mechanical and chemical qualities in terms of strength, toughness, weldability and corrosion resistance as for ordinary rolled steel products, see e.g. Schober (2001) or Mang & Herion (2001).

The smooth shape of the cast joint according to the flow of internal forces results in only small stress concentrations, thus making cast steel joints in particular advantageous for structures subjected to repeated loading. The critical part in terms of fatigue are the welds between the cast steel joint and the tubular steel members. Concerning the fatigue behaviour of this type of connection, there are only few investigations documented, some associated with the “Humbolthafen” bridge in Berlin, see Seifried et. al. (1999). Based on experimental investigations on small and large scale test specimens, the fatigue resistance could be classified to detail category 71, according to Eurocode 3 Part 1.9 (2002), similar to a butt welded end-to-end connection of CHS members. 3.1.4 Manufacturing and Quality Assurance The manufacturing of cast joints is quite expensive due to the high costs preparing the cast form-

Table 1. Summary of recently built CHS bridges and joint characteristics

year of completion span length h/L of steel truss type of joint construction joint type brace dimensions chord dimensions joint parameters β = d1/d0 γ = d0/2t0 τ = t1/t0 Θ ; cos(Θ) φ 1)

according to Dauner (1998)

Lully1) (CH) 1997 ≈ 43 m 1 / 14 welded KK 267 / 25 508 / 36

Dättwill2) (CH) 2001 ≈ 38 m 1 / 12 welded KK 267 / 25 508 / 50

0.53 7.06 0.69 60 ; 0.5 69

0.53 5.08 0.50 60 ; 0.5 69

2)

Aarwangen2) (CH) 1997 ≈ 48m 1 / 27 welded K 194 / 28 406 / 36

Nesenbachtal (D) 1999 25 / 50 / 36 m 1 / 11-22 cast steel KK 194 / 10-60 324 / 16-80

Korntal-Münch. (D) 2002 32 / 41 m 1 / 13 cast steel KK 267 / 28-45 457 / 45-67

0.48 5.64 0.78 45 ; 0.71 --

0.60 10.13-2.03 0.63-0.75 46 ; 0.69 102

0.58 5.07-3.51 0.62-0.69 60 ; 0.5 90

according to Schumacher, Nussbaumer & Hirt (2001)

work. Usually, additional machining of the connection device is required in order to fulfill the small tolerances for a precise fitting of the tubular sections. The quality assurance has to be guaranteed for both, the welded connection between the cast joint and the tubular sections and the cast steel material itself. In Germany, non-destructive quality control usually is performed by ultrasonic devices in accordance with DIN 1690 (1991).

peated loading the high stress concentrations may lead to a premature fatigue failure. Thus, the design of welded CHS joints, particular for bridge structures is very much influenced by their fatigue behaviour. CIDECT Serial No. 8 (2000) deals with the fatigue behaviour of welded joints under repeated loading and includes guidelines, charts and provides parametric formulas for the design of welded CHS joints.

3.1.5 Economic Efficiency

3.2.3 Manufacturing and Quality Assurance

The economic efficiency using cast steel joints is very dependent on the manufacturing of the cast formwork. If the design of the bridge allows the use of a large number of equal joints, a high cost effectiveness can be achieved by reducing the cost for the formwork in relation to the single joint. Depending on the number of equal joints and the complexity of the joints the price varies between 3000 € and 6000 € per ton cast steel.

In order to ensure an optimal fitting of the diagonal brace members onto the continuous chord at the very complicated intersection area a precise cutting of the tubular members is required. Nowadays computer-operated profile cutting machines allow for an efficient and very precise cutting process. The welding is mainly manual. For a reliable quality inspection of the welds, especially the critical weld root, the geometrical dimensions of the joint should be designed in a way that enough space for inspection devices is guaranteed.

3.2 Welded Steel Joints 3.2.1 Structural Behaviour

3.2.4 Economic Efficiency

The forces between the members are directly transmitted through the welds, leading to a very complex and multidimensional stress situation with high local stress concentrations. Over the past 30 years a considerable amount of research work on CHS joints has been driven forward by the petroleum industry concerning the design of offshore-structures. Most of this research is published by CIDECT (Comité International pour le Développement et l’Étude de la Construction Tubulaire). CIDECT Serial No. 1 (1991) includes guidelines, charts and provides parametric formulas for the design of welded CHS joints that are subjected to predominately static loading. Thus, allowing a quick and easy design of such joints, without time-consuming FE calculations.

As the chord member normally is continuous at the joint and only the diagonal braces are to be welded to the chord, the number of welded connections is reduced in comparison to cast steel joints leading to decisive savings. However, some of the savings are compensated by the more costly welding procedures. As a conclusion: for simple and standardized joint types such as e.g. T-, X-, K- or KK-joints with easy to reach weld positions, welded joints form the more economic solution. Additionally, as the whole steel construction is manufactured within only one company there are also logistic and organizational advantages. 4 FATIGUE ASSESSMENT OF CHS JOINTS 4.1 General; S-N Concept

3.2.2 Fatigue Welded CHS joints are very sensitive to fatigue because the geometric discontinuities of the welds lead to a high stress concentration. Under static loading, these stress concentrations are less important due to local plastification. However, under re-

Fatigue assessment procedures are usually based on S-N curves which relate a nominal or geometric stress range S to the corresponding number N of load cycles to fatigue failure.

Table 2. Construction detail and detail category of a uniplanar CHS K-joint with gap according to Eurocode 3 Part 1.9 (2002) Detail Category Constructural detail

90 m=5 45 m=5

Description

t0 ≥ 2 .0 t1

- for intermediate values of the ratio t0/t1 interpolate linearly between detail categories - t0 and t1 ≤ 12.5 mm

t0 = 1 .0 t1

- 35°≤ Θ ≤ 50° - d0 /t0 ≤ 25

4.2 Nominal Stress Method In this case fatigue assessment refers to the nominal stress range ∆σnom in a structural member. The fatigue resistance is given according to a classification catalogue in the form of standardized S-N curves. Structural details classified in this catalogue, see e.g. Eurocode 3 Part 1.9 (2002), correspond to a specific situation of stress range, direction, crack position, detail dimension and weld quality which had been characteristic for the tests on which the classification is based. Thus, the application of this method is limited in some extend to the geometrical dimensions or the loading conditions of the classified structural details. Table 2, for example gives the detail category of uniplanar CHS K-joints with gap depending on the relation of the corresponding wall thickness ratio t0/t1. In this special case, the range of application is limited to a wall thickness of t0 and t1 ≤ 12.5 mm and therefore impossible to use for bridge structures. 4.3 Hot-Spot Stress Method The hot-spot stress method also named geometric stress method has been developed for unclassified details and for details where a clear definition of the nominal stress, e.g. due to geometric discontinuities is not possible. The hot-spot stress is defined as the maximum geometric stress occurring at the spots where cracks usually tend to initiate. It characterizes a fictitious measured or calculated stress at the critical section (hot-spot) that is extrapolated to the weld toe, from two or three points at certain distances from the weld toe, see Figure 3. For design, the hot-spot stress is usually calculated by multiplying the nominal stress σnom by the so-called stress concentration factor (SCF) for the appropriate structural discontinuity, see equation (1). σ hs = SCF ⋅ σ nom

In general, the hot-spot stress respectively the stress concentration factor is determined by testing and/or FE analysis. For typical CHS joints CIDECT Serial No. 8 (2000) includes formulae and charts depending on the geometric dimensions, for the evaluation of SCF values. Similar to the nominal stress method the hot-spot method uses Shs-N curves for the fatigue resistance, where Shs is the hot-spot stress range. The advantage of the hot-spot stress method is the possibility of predicting the fatigue lives of many different types of joint configuration using only one single Shs-N curve. 5 STRESS CONCENTRATION FACTORS OF WELDED CHS BRIDGE JOINTS 5.1 Bridge “Korntal-Münchingen” The bridge is located about 30 km north east of Stuttgart nearby the small city of KorntalMünchingen and was opened for traffic in October 2002. The total length of the bridge is about 300 m. The main middle part of the bridge consists of a tubular space truss (Figure 1 and 4) approximately 2.9 m in depth and 4.6 m in width. Piers are typically spaced between 32 m and 41 m. The diameter of the hotrolled CHS members were chosen to be 267 mm for the braces and 457 mm for the chords. The wall thickness of the CHS members varies in order to achieve the necessary strength and stiffness. The maximum wall thickness of the chord is 65 mm and was needed at one of the inner-supports. Although the joints where built in cast steel, these typical dimensions of the bridge structure and the joint geometry were used as an example to clarify whether for this bridge welded joints would have been a possible alternative.

(1)

Figure 3. Definition and extrapolation region of the hot-spot stress σhs

Figure 4. Spatial CHS framework of the bridge KorntalMünchingen

5.2 Finite Element Analysis 5.2.1 General For the given CHS joint geometry it is not possible to verify the fatigue resistance according to the hot-spot approach as recommended in CIDECT Serial No. 8 (2000). The reasons are as follows: - the parameter range of the given SCF formulas and charts is limited to 12 ≤ γ ≤ 24 and 0,3 ≤ β ≤ cos(θ). These conditions are not fulfilled for almost every CHS bridge according to Table 1, - the SCF formulas are restricted to CHS KK-joints with gap but without any eccentricity and - the Shs-N curves are limited to a wall thickness smaller than t = 50 mm. To overcome these restrictions a detailed FE analysis has been performed, determining the critical hot-spot stresses at the joint intersections, Stuba (2002). The hot-spot stresses respectively SCF values were determined in accordance to CIDECT Serial No. 8 (2000) and Niemi (1992) using the FE package ANSYS. Figure 5 shows the three-dimensional FE model of the KK-joint. Geometrical symmetries have not been taken into account. 20-node solid elements with an 3x3x3 integration scheme were used. Various mesh densities were investigated and compared in order to ensure enough convergence of the stresses in the vicinity of the weld toe.

Figure 6. Applied loading and boundary conditions

5.2.3 Stress Concentration Factors (SCF) The stress concentrations factors (SCF) were determined using the quadratic extrapolations method. Studies by Romeijn (1994) have shown that for CHS joints this method gives satisfying results. In circumferential direction the tubular sections have been divided into 36 elements/nodes resulting in altogether 36 x 4 x 2 = 288 SCF values for one joint and one loading condition. Since no experimental results have been available, the numerical FE results of test calculations were compared to the values given in CIDECT Serial No. 8 (2000), leading to an acceptable difference less than 5%. The location of maximum stress concentration (hot-spot) always appeared in the brace near the saddle. Figure 8 shows the variation of the SCF value along the circumferential direction of the brace in case of axial balanced brace loading. Figure 7 reflects the corresponding von Mises stress pattern at the surface. The maximum SCF value appears in the brace at node number 22.

Figure 5. FE model of the multiplanar CHS KK-joint using 20-node elements

5.2.2 Loading and boundary conditions The loads were introduced as unit loads. Two different axial loading conditions have been investigated: - pure axial balanced brace loading and - pure chord loading, see Figure 6. Additional studies showed that in case of in-plane bending the maximum SCF values are somewhat below those of the axial loading condition. Therefore it can be concluded that the axial loading sufficiently covers also in-plane bending.

Figure 7. FE-results, von Mises stress pattern in case of axially loaded brace

4.5

4.5

3.0

3.0

1.5

1.5

0.0

0.0

1

6

11

16

21

26 31 36 node number

1

Figure 8. Stress concentration factor (SCF) in circumferential direction of the brace, brace loading

5.2.4 Influence of Weld Shape For critical connections in terms of fatigue and where the wall thickness is large, full penetration welds should be used. However, a realistic modeling of the weld shape in circumferential direction is a very complex task, because of the ever-changing angle Ψ between the surface of the chord and the diagonal braces that have to be joined, see figure 9. Especially in the “heel” zone of the connection with a small intersection angle full penetration is difficult and leads to a much higher throat thickness compared to the “crown” zone. Two different weld shapes have been considered: - a full penetration weld with a weld foot length tw = 0 (butt weld) and - a full penetration butt weld with a constant weld foot length tw, see figure 9. Figure 10 shows the influence of the these different types of weld shapes on the stress concentration factor. The consideration of an additional foot length tw leads to a reduction of the SCF of about 10 to 15%.

6

11

16

21

foot length tw = 0

26 31 36 node number foot length tw > 0

Figure 10. Influence of the weld shape on the stress concentration factor (SCF)

6 FATIGUE VERIFICATION OF THE KK-JOINT 6.1 General In order to clarify the feasibility of welded joints for the bridge “Korntal-Münchingen”, the fatigue verification was performed according to Eurocode 3 Part 2 –Steel bridges– (1997) and the hot-spot fatigue design method recommended for hollow section joints in CIDECT Serial No. 8 (2000). Three different joints of the aforementioned triangular truss girder were selected. These joints differ regarding their wall thickness t0 and t1 and loading condition because of their different positions in longitudinal direction of the bridge girder. Table 3 summarizes the joint dimensions and parameters. 6.2 Stress Range Fatigue load model 3 (FLM 3) of Eurocode 3 Part 2 (1997) was used to determine the nominal stress ranges in the corresponding tubular members. FLM 3 consists of a single 4-axle vehicle truck with axle loads of 120 kN. According to Eurocode 3 Part 2 (1997) the resulting stress range is transformed into the damage equivalent stress rang ∆σE,2 related to 2·106 cycles in order to make it comparable to the fatigue strength ∆σC. This is realized by the so-called damage equivalent factors λi, see equation (2). These factors depend on fatigue relevant parameters as e.g. the traffic volume, the design life of the bridge or the Table 3. Joint dimensions and parameters

Figure 9. Different weld shapes

do t0 d1 t1 g β = d1/d0 γ = d0/2t0 τ = t1/t0 Θ φ

joint 1

joint 2

joint 3

457 65 267 45 79.0 0.58 3.52 0.69 60 90

457 55 267 36 65.5 0.58 4.15 0.65 60 90

457 45 267 28 52.8 0.58 5.07 0.62 60 90

shape of the influence line. Table 4 summarizes the values applied as they are given in Eurocode 3. ∆σ E , 2 = λ 1 ⋅ λ 2 ⋅ λ 3 ⋅ λ 4 ⋅ ∆σ nom

(2)

6.3 Fatigue Verification Using the hot-spot method, the fatigue limit state can be verified using equation (3): γ Ff ⋅ SCF ⋅ ∆σ E , 2 ≤ ∆σ C.hs γ Mf

(3)

Where γFf and γMf are the partial safety factors for the fatigue limit state, ∆σC.hs is the characteristic value of the fatigue strength against hot-spot stresses for 2·106 number of cycles and SCF is the stress concentration factor. The safety factors were chosen to γFf = 1.0 and γMf = 1.15. Values for the fatigue resistance ∆σC.hs for CHS section joints are given in CIDECT Serial No. 8 (2000) by the following formula: ∆σ C.hs = 1 3 ⋅ (12.476 − log( N f ) ) + 0.06 ⋅ log( N f ) ⋅ log(16 / t )

(4)

Where Nf is the number of cycles to failure and t the wall thickness. Although the given formula is limited to a maximum thickness of t ≤ 50 mm it has been applied in this case. Based on the aforementioned FE calculations, the relevant stress concentration factors (SCF) for the observed three joints are given in Table 5. Herein the first lower index describes the member (ch = chord, b = brace) and the second one the loading condition (ch = chord loading, ax = axial balanced brace loading). In contrast to the given values in Table 5, CIDECT Serial No. 8 (2000) recommends a minimum SCF = 2.0 unless it is negligible. The following reasons are given: - a possible and uncontrollable crack initiation from the weld root, that is not covered within the SCF value determined for the weld toe, - possible deviations of the principle stress direction from the direction perpendicular to the weld toe and - difficulties in FE modeling. Table 4. Assumed damage equivalent factors λi λ description λ1 span length and shape of influence line; mid span, L = 40 m λ2 traffic volume; NObs = 0,5·106 lorries per year λ3 design life of the bridge; N = 100 years λ4 number of lanes with heavy traffic; k = 1 λ = λ1⋅λ2⋅λ3⋅λ4

The recommended minimum SCF value is a reasonable assumption and should especially be applied for thin sections used for buildings or crane structures. However, the authors believe, that for thick sections and high quality full penetration welds with good accessibility SCF values in the range between 1.5 and 2.0 may be acceptable as well. With the above mentioned assumptions and based on the SCF values given in Table 5, the fatigue limit state has been verified for all three joints, thus clearly indicating, that for the specific type of bridge also welded CHS joint would have been possible. 7 CONCLUSIONS This contribution covers the application of circular hollow sections in bridge design and tries to give an overview about the advantages and disadvantages of either cast steel or welded joints in order to help practical engineers in their decisions and to allow for a further application of circular hollow sections in bridge design. Concerning a cost effective and robust design of CHS joints for bridge structures the following conclusions are drawn: - cast steel joints should be used if there are several members to be connected at one joint resulting in a complex joint geometry. For such cases, the casting process allows an optimal design of the joint according to the flow of internal forces, increasing their static and fatigue resistance compared to welded joints. - for standard joint types such as e.g. K- or KKjoints that are typically used for triangular truss girders, welded joints are the more economic solution and, providing a high manufacturing standard especially for the welds, a possible alternative. Through detailed investigations connected to a recently completed CHS truss bridge near Stuttgart, it has been shown, that for this bridge instead of cast steel also welded joints would have been a possible alternative. This conclusion could be drawn from numerical studies based on FE calculations applying the hot-spot stress approach for the fatigue assessment of the welded joints. Table 5. Stress concentrations factors (SCF)

value 2.25

SCF

chord KK ch ,ax

SCF

0.63 1.0 1.0 1.41

brace loading

joint 1 joint 2 joint 3

1.99 2.06 2.08

chord loading

brace

chord

brace

SCFbKK ,ax

SCFchKK,ch

SCFbKK ,ch

1.68 1.78 1.85

1.40 1.39 1.37

0.30 0.36 0.41

Furthermore, these studies clearly indicated that for a wide application of cost effective and robust welded CHS joints, the currently existing design guide CIDECT Serial No. 8 (2000) should be adjusted to the specific situation of bridge structures, e.g. by extending the parameter range of the SCF values or providing values for the fatigue resistance also for a large wall thickness. 8 PREFERENCES, SYMBOLS AND UNITS ANSYS Rev. 5.7. ANSYS Inc., Southpointe, Technnology Drive, Canonsburg, PA 15317. Comité International pour le Développement et l’Étude de la Construction Tubulaire (CIDECT) 1991: Design Guide for Circular Hollow Section (CHS) Joints under Predominantly Static Loading. Construction With Hollow Steel Sections. Serial No. 1. Verlag TÜV Rheinland, Cologne, Germany. Comité International pour le Développement et l’Étude de la Construction Tubulaire (CIDECT) 2000: Design Guide for Circular and Rectangular Hollow Section Welded Joints under Fatigue Loading. Construction With Hollow Steel Sections, Serial No. 8. Verlag TÜV Rheinland, Cologne, Germany. Dauner, H.-G. 1998. Der Viadukt von Lully. Eine Neuheit im Verbundbrückenbau. Stahlbau 67, Vol. 1, pp. 1-14. Eurocode 3 2002: Design of steel structures - Part 1.9: Fatigue. Draft, 7 August 2002. European Committee for Standardisation. Eurocode 3 1997. Design of Steel Structures - Part 2: Steel Bridges. European Committee for Standardisation. German code (DIN 1690) 1991: Technische Lieferbedingungen für Gußstücke aus metallischen Werkstoffen. Ergänzende Festlegungen für Stahlguß für höher beanspruchte Armaturen. DIN. Ausgabe: 1991-01. Kuhlmann, U., Günther, H.-P., Saul, R., Häderle, M.-U. & Stuba, G. 2002. Zur Anwendung geschweißter Hohlprofilverbindungen im Brückenbau. Stahlbau 71, Vol. 7, pp. 507-515. Mang, F. & Herion, S. 2001. Guß im Bauwesen. Kuhlmann, U. (ed.): Stahlbaukalender 2001. Berlin: Ernst & Sohn, pp. 625-667. Niemi, E. 1992. Recommendations concerning stress determination for fatigue analysis of welded components. IIW, Document No. XIII-1458-92/XV-797-92, 1992. Romeijn, A. 1994. Stress and Strain concentration factors of welded multiplanar tubular joints. PhD-Thesis, Delft, The Netherlands. Seifried, G., Angelmaier, V., Wilhelm, G. & Beschorner, K. 1999. Eisenbahnbrücke über den Humboldthafen in Berlin. Stahlbau 68, Vol. 7, pp. 511-519. Schlaich, J., Pötzl, M., Beiche, H., Ehrke, E. & Decker, U. 2000. Die Brücken über das Nesenbachtal im Zuge der Ostumfahrung Stuttgart–Vaihingen. Beton- und Stahlbetonbau 95, Vol. 11, pp. 678-687. Schober, H. 2001. Rohrknoten aus Stahlguß. Der Prüfingenieur, Vol. 17, Bundesvereinigung der Prüfingenieure für Bautechnik e.V. (eds.), pp. 16-36. Schumacher, A., Nussbaumer, A. & Hirt, M.A. 2001. Fatigue behaviour of Welded Circular Hollow Section (CHS) joints in bridges. Puthli, R. & Herion, S. (eds.): Tubular Structures IX, Swets & Zeitlinger, Lisse, pp. 291-297. Stuba, G. 2001. Zur Anwendung geschweißter Hohlprofilknoten im Brückenbau. Universität Stuttgart, Institut für Konstruktion und Entwurf, Diploma-Thesis.

9 ACKNOWLEDGMENT The authors would like to express their gratitude to Mr. G. Stuba who did the FE-Analysis within his diploma-thesis.