Effects of bolt hole clearance on sway deflections of cold-formed steel portal frames

This paper presents test results from a series of monotonic tests on full-scale cold-formed steel (CFS) moment resisting frames. The structural capacities are obtained from pushover failure tests and frames horizontal stiffnesses under characteristic loads are obtained from both monotonic and cyclic tests. A series of seven monotonic pushover tests and six cyclic tests were performed on three different section sizes of CFS portal frame. Two types of bolted connections were used, one with hole diameters the same as bolts diameter (true fit joints) and second with holes of standard tolerances (standard joints). Results demonstrated that joint type had negligible effect on the frame overall resistance under the
horizontal load. All of the frames failed due to bucking of the column web at the location where the maximum compressive stresses were expected. It was also demonstrated that the friction between steel galvanised plates can significantly improve joint rotational stiffness thus reduce frame deflection. Frames with true fit joints have demonstrated horizontal stiffnesses which were on average 28% greater than stiffnesses of frames with standard joints. The slip of bolts in standard tolerance holes contributes largely to horizontal deflections of frames with standard joints. It was shown in cyclic tests that only one out of six frames delivered a required residual deflection below 20% after the load was removed. It was also recorded that frames with standard bolt hole tolerances had residual deflections between 11% and 46% greater than frames with true fit bolts. In this paper, accuracy of published analysis methods for simulating sway behaviour of CFS portal frames with ‘bearing type’ connections was compared against test data. In terms of three analytical methods for predicting joint rotational stiffness, mixed accuracy was obtained. The ‘EC3’ model matched closely load-deflection curves for low-resistance frames but under predicted deflections of medium and high-resistance frames. The joint rotational stiffness model proposed in this paper, was found to represent the lower bound of frame stiffness (conservative solution). In comparison with some test results however, frames deflections were largely overestimated by the proposed analysis model.