Experimental and numerical study on net section resistance of high strength steel staggered bolted connections

This paper investigates the structural behaviour and ultimate strength of high strength steel (HSS) staggered bolted connections under tension. A total of 33 HSS (Q690 and Q960) and 15 Q345 staggered bolted connection specimens were tested. All specimens failed in net section fracture through the staggered path. The predicted net section resistance based on the s²/4g rule was achieved by most of the HSS specimens. For the specimens with a small bolt pitch to gauge (s/g) ratio, the connection efficiency (i.e., the ratio between the test ultimate load and the calculated net section resistance) exceeded 1.0 and decreased gradually with an increasing s/g ratio. Compared with the mild steel (MS) specimens, in general the HSS specimens have lower connection efficiency and overall deformation capability. Subsequently, the structural behaviour of the test specimens was investigated using finite element (FE) analysis to further interpret the test observations. The results of the test and the FE analysis indicated that for the HSS specimens, effective stress redistribution along the staggered net section was observed although the ductility and the ratio of tensile strength to yield strength (f_u/f_y) of HSS material are relatively lower than those of the MS material. In addition, it was also observed that the biaxial state of stress that existed in the plate area between the staggered bolt holes was influenced by the s/g ratio, which would affect the ultimate strength of the test specimens. A reliability analysis of the HSS specimens test results was carried out to re-evaluate the partial factor adopted in the design equation of the net section resistance of HSS (up to S700) in EN 1993-1-12. The analysis results of the limited test data showed that the net section resistance of HSS staggered bolted connections was predicted conservatively by the current design equation with a partial factor of 1.25.