New Bolted Connection Design Formulae for G550 and G300 Sheet Steels Less Than 1.0 mm Thick (No. R769)

Abstract

Cold formed structural members are fabricated from sheet steels which must meet the material requirements prescribed in applicable national design standards. The Australian / New Zealand standard for cold-formed steel structures (AS/NZS 4600) allows for the use of thin (t < 0.9 mm), high strength ( fy = 550 MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels which are cold reduced to thickness the engineer must use a yield stress and ultimate strength reduced to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Specification further limits the use of thin, high strength steels to roofing, siding and floor decking panels. Sheet steels are required to have a minimum elongation capability to ensure that members and connections can undergo small displacements without a loss in structural performance, and to reduce the harmful effects of stress concentrations. A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) detailed the basic material behaviour of G550 sheet steels. It was concluded that the ability of G550 sheet steels to undergo deformation is dependent on the direction of load within the material, where transverse specimens exhibit the least amount of overall, local and uniform elongation. A research project on the behaviour of bolted connections composed of 0.42 and 0.60 mm G550 and G300 sheet steels has been completed, and the results are contained in the following research report; Bolted Connection Tests of Thin G550 and G300 Sheet Steels (No. R749). The authors of this report concluded that the low ductility measured in coupon tests did not influence the net section fracture mode of failure, although a modification to the bearing coefficient provisions for thin G550 and G300 sheet steels is required 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). Additional bolted connection specimens composed of 0.80 and 1.00 mm G550 and G300 sheet steels have been tested and the results are incorporated into this report. They have been used to develop a gradated bearing coefficient method which is dependent on the thickness of the connected materials and the size of the bolt(s) used in the connection. The measured variation in bearing resistance between the thin 0.42 mm G550 sheet steels and the typical 1.0 mm and thicker sheet steels is incorporated into an all encompassing bearing formulation. The additional bolted connection specimens were dimensioned such that only bearing failure would occur, with test specimens milled from the longitudinal, transverse and diagonal directions of the sheet.