Failure Modes of Bolted Sheet Steel Connections Loaded in Shear (No. R772)

Abstract

Cold formed structural members may be joined with bolted connections, which are designed with the aid of applicable national design standards. The ultimate load carrying capacity of a connection will be governed by one of many failure modes including; bearing, end pull-out, net section fracture, bolt shear, block shear rupture, etc.. A research project in which the behaviour of bolted connections composed of 0.42 to 1.0 mm G550 and G300 sheet steels is investigated has been completed, and the results are contained in the following research reports; Bolted Connection Tests of Thin G550 and G300 Sheet Steels (No. R749) and New Bolted Connection Design Formulae for G550 and G300 Sheet Steels Less Than 1.0 mm Thick (No. R769). It was concluded in these reports that there are a number of problems with the existing load capacity formulations contained in the current cold formed steel design standards, based on observations made during the testing of thin bolted connection specimens. Both the Australian / New Zealand (AS/NZS 4600) and the American Iron and Steel Institute (AISI) design standards cannot be used to accurately predict the failure mode of thin sheet steel bolted connections loaded in shear. Typically, net section fracture is predicted when test results reveal that bearing distress in the sheet steel is the controlling mode of failure. A modification to the bearing coefficient provisions for thin G550 and G300 sheet steels is necessary to account for the reduced bearing resistance of the connected materials. This reduction in bearing resistance is related more to the steel thickness than to the steel grade (G550 vs. G300) and a proposed gradated bearing coefficient method is presented in research report No. R769. A revision of the net section fracture design method is also required. Furthermore, a detailed analysis of the procedure used to identify the cause of failure in bolted connections is needed to ensure that accurate failure mode assessments are made, and ultimately to ensure that accurate design equations are formulated. Misidentification of failure modes and the misuse of data can lead to serious errors in the accuracy and applicability of design equations. This research report details the behaviour associated with bearing and net section failure of bolted connections loaded in shear. Bearing behaviour includes piling of the sheet material in front of the bolts, as well as the material tearing associated with out-of plane sheet distortion. Recommendations concerning the procedure used to identify the net section fracture and bearing failure modes are made. In addition, a detailed discussion of the test data used in the development of the current AS/NZS 4600 and AISI design equations for net section fracture at connections is completed.