1996 Research Reports No R720-R735

Abstract

Cold formed structural members are fabricated from sheet steels which must meet various material requirements prescribed in applicable national design standards. The Australian / New Zealand Design Standard allows for the use of thin (t < 0.9mm), high strength ( fy = 550MPa) sheet steels in all structural sections. However, in design the engineer must use a yield stress and ultimate strength reduced to 75% of the minimum specified values, due to the lack of ductility exhibited by sheet steels which are cold reduced to thickness. The American Iron and Steel Institute (AISI) Design 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. The ductility criterion specified in the Australian / New Zealand and North American Design Standards is based on an investigation of sheet steels by Dhalla and Winter, which did not include the high strength G550 sheet steels currently used in construction.

This report details the findings of the first phase of a research project into the tensile behaviour of G550 sheet steels which range in base metal thickness from 0.40 to 0.60mm. Tensile coupons were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on material properties, including ductility. Measurements of overall, local and uniform elongation were completed using a fine gauge length grid (2.5mm c/c) marked on the surface of the tensile coupons. Perforated specimens were tested to determine the ability of the G550 sheet steels to redistribute stress concentrations throughout the cross-section, and to calculate resistance (capacity) and partial safety factors for use in limit states tensile design.

Test results indicate 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. The G550 sheet steels included in this report do not meet the Dhalla and Winter elongation and ultimate strength to yield stress ratio requirements regardless of direction, except for uniform elongation of longitudinal test specimens. However, perforated specimens are able to develop the full net section capacity under tensile load. The current resistance (capacity) and safety factors specified by the Australian / New Zealand, North American and European Design Standards are adequate for use in the prediction of tensile loads for concentrically loaded coupon specimens. The second and third phases of this project will consist of the testing of concentric and eccentric bolted and screwed single lap connections, as well as additional finite element analysis.