Design model for bolted angle members in compression including joint stiffness

Knowledge of type and size of rotational restraints at the member's ends (provided by the adjacent structure) is crucial for the prediction of the compression member capacity of bolted angles. This was highlighted by means of experimental and numerical investigations in the past. Due to the eccentric connection on only one angle leg, additional bending moments are acting on the member, leading to a complex load carrying behaviour with flexural and/or lateral torsional buckling phenomena.
Detailed analytical models for the estimation of appropriate spring stiffness values have been developed for several practical applications in buildings with two-bolt connections at both member's ends. The investigated connection details comprise a simply fixed gusset plate (e.g. as attachment to concrete walls), a joint to the flange of an I-shaped section and a joint to the web of an I-shaped section via a gusset plate. Based on that, a design model was developed for bolted angle members with rotational spring restraints at the member's ends and an eccentric compression loading N, in order to calculate the compression capacity NR. The thereby determined resistances NR,model are compared with the results of sophisticated 3D nonlinear finite element calculations NR,FE that consider the detailed joint configuration and the eccentric load introduction as well as equivalent geometric imperfections. It is shown that the resistances based on the proposed design procedure are slightly conservative compared to the real behaviour of the angle member. In addition, the large increase in capacity (compared to the simply supported reference case) is highlighted.