Fatigue strength of truss girders made of Very High Strength Steel

An effective application of Very High Strength Steel (VHSS) in civil engineering structures is expected in stiff, truss like structures, typically made of Circular Hollow Sections (CHS). Use of castings in combination with CHS could be promising for the design of highly fatigue resistant joints. Cast steel and CHS are more and more applied in fatigue loaded bridge and offshore structures. Choosing optimal shapes of the cast steel parts could result in joints with low stress concentration, while shifting welds out of the most severe stress location. The welded connection in between the cast steel part and the CHS are of great importance for the fatigue strength of the joint. The fatigue strength of the welded connections and the cast joints are to be investigated. Two truss girders, made of welded CHS and K-joint cast members with yield strength of 690 MPa and 890 MPa, are tested under static and fatigue loading conditions at the Stevin Laboratory of the Delft University of Technology. The test frame consists of a 10,000 kN cylinder, pull bars and H-girders. Strain gauges are used to monitor the strain distribution and the cycles until crack initiation. In the first part of the test program, presented in the current paper, measurements of static loads are compared to a FEM model to verify the stress distribution in the CHS and cast joint parts.