Stress Concentration in Built-Up Steel Members

摘要

Older steel truss bridges are often made of built-up members with riveted connections. These fasteners usually develop a low and unreliable level of pretension such that the joints behave like bearing type connections, i.e. the service loads place the bolts in bearing. Under cyclic loading, and depending on the stress concentration around the fastener holes, fatigue failure can occur at nominal stresses significantly lower than in members with no stress concentration. Current design standards account for this by calculating the stress range on the net section and using fatigue category B for assessment of the fatigue life. The net area calculation used for stress calculation is based on the procedure proposed by Cochrane (the s~2/4g rule), which is adequate for shear type rupture. Tests have shown, however, that Cochrane's approach does not apply for fatigue failure since rupture does not take place in a ductile shear mode. Josi et al. (2004) investigated the effect of fastener hole stagger on the fatigue resistance of shear splices with flat plates, but indicated that the validity of their findings for built-up tension members needs to be investigated. DiBattista and Kulak (1995) carried out fatigue tests on riveted built-up tension members connected to gusset plates with staggered rivet holes and observed that fracture took place on a plane perpendicular to the axis of the member, thus invalidating a stress calculation based on the s~2/4g rule. It was suggested that the close proximity of holes to the fatigue fracture surface for members with staggered holes may increase the effective stress on the failure plane. It was suggested that the effect of hole stagger be investigated. An investigation into the effect of connection size and hole pattern on the fatigue resistance of built-up I section to gusset plate bearing type connections was carried out. A design equation that considers the effect of hole stagger on the stress concentration factor, hence the fatigue resistance, is proposed. The appropriate fatigue category for these members is also recommended.