Keywords: Composite bridge, circular hollow sections, welded joints, cast steel nodes, fatigue design

Abstract:

Recently, steel-concrete composite bridges have been built using hollow section members for their main load carrying structure. Several examples exist, notably in Germany and Switzerland. The joints in these bridges are complex and require special attention. The connection between hollow section members is characterised by changes of stiffness, non-uniform stress distributions and complex three-dimensional behaviour. This complexity must be considered when performing fatigue checks. In addition, existing fatigue design rules, which have been developed primarily for the offshore industry, do not cover typical member proportions used in bridge design.

Based on recent analytical and experimental studies carried out by the Steel Structures Laboratory (ICOM) at the Swiss Federal Institute of Technology in Lausanne (EPFL), recommendations for the design and fabrication of tubular bridge structures are made. Welded and cast steel solutions for the joints are compared, and key issues for the design and fabrication of both types of nodes are reviewed. Herein, the importance of a balanced design between the various potential crack initiation sites in a joint (weld toes, weld roots, defects in the cast steel) is highlighted. The beneficial influence of post-weld improvement methods on the fatigue strength of tubular joints is also discussed.

Conclusions

The conclusions have been regrouped in two categories, design and fabrication, and are given below.
Design of tubular bridges:

• The hot-spot stress method was found to be the only current method for the fatigue design of tubular joints that is both scientifically acceptable and practical for use by design engineers.
• The engineer should seek a balanced design approach, as little is gained in having sufficient fatigue safety at one location when another is not properly designed or fabricated. Since the ensemble of variables that must be considered makes the determination of the most susceptible fatigue location difficult with current specifications, an intuitive design method was developed.
• For the determination of stress concentration factor (SCF) values, actual design guide formulas cannot be extrapolated to low g  values, such as g  £ 12, typical to bridges. For these cases, new SCF values were derived from FE analysis.
• There is a strong tendency for SCF values to decrease with decreasing g values. A low g value, as commonly seen in existing bridges, is therefore a desirable geometric characteristic.
• The recommended minimum SCF at critical joint locations in current design specifications, SCF = 2.0, is highly penalising if applied to welded CHS bridge K-joints. When g  £ 12, the recommended minimum SCF can be taken as SCF = 1.0 for CHS K-joints.
• The recommended fatigue strength curve for welded joints corresponds to Detail Category 80 (Eurocode 3), with a size correction factor taken as (T/20)0.25 for T > 20 mm. This strength curve must be used in conjunction with hot-spot stress ranges calculated according to the present study.
• It is highly recommended to design directly welded joints with fully penetrated welds.
• Post weld treatment using needle peening resulted in at least a 60% improvement in fatigue strength, or in terms of fatigue life, an improvement of over 4 times.
• The DIN 1690-2 quality levels of cast steel nodes can be less than actually employed in existing tubular bridges (S1V1 for stub ends and S3V3 for the centre part of the node). However, more precise information will be given once the study on the quantification of allowable defect sizes in cast nodes is completed.
• The recommended fatigue strength curve for girth butt welds between cast stubs and tubular members is FAT 60 without backing bar and FAT 90 with backing bar. The SCF due to differences in thickness and diameter of the tubular members and cast stubs should be taken into account.

Fabrication:

• For welded joints, the use of a backing ring to ensure full weld penetration does not affect the fatigue resistance (in comparison to joints with fully penetrated welds without backing rings), but it ensures good fit and ease of welding to produce satisfactory welds.
• For cast steel nodes, at connections between stubs and CHS members, fully penetrated welds are absolutely necessary for adequate fatigue resistance. The use of backing rings is therefore highly recommended to ensure good penetration.