A numerical investigation of the structural behaviour of bolted connections with a staggered bolt pattern

Depending on the connection details, the design of a bolted tension member may sometimes require the use of a staggered bolt pattern. The current rule to account for the effects of bolt stagger, s2/4g, is based mainly on the work of Cochrane in 1922. This rule was developed using the maximum stress theory of failure which was subsequently discredited for its applicability in predicting plastic flow of ductile materials. In addition, it has also been found that there was an appreciable difference between the ultimate strengths of some riveted joints with a staggered pattern as determined by tests and the strengths of the same joints as predicted with the s2/4g rule. Moreover, the experimental investigations of the stagger pattern were mainly based on riveted connections, whereas the test data of bolted connections with bolt stagger were very limited. Therefore, the use of the Cochrane rule for structural steels commonly used nowadays was uncertain. Hence, a research project consisting of both experimental and numerical study was initiated to examine the applicability of the Cochrane rule. This paper presents a numerical study on the bolted connections with a staggered bolt pattern using the finite element method. The commercial finite element program, ABAQUS, was used to develop the finite element models and conduct the analyses. The models were calibrated with existing test data of hot rolled steel angles with bolted connections. In general, the numerical results did not predict the increase of the connection strength due to the increase in the pitch. This result contradicted the predictions by the s2/4g rule. It is believed that the increase in the pitch might cause both an increase in the connection strength and an increase in the non-uniformity of the stress distribution across the zigzag net section. These two effects might have counteracted each other such that there was no significant improvement in the connection strength due to an increase in the pitch. Numerical and experimental work has been planned for further study of this area.