Screwed Connection Tests of Thin G550 and G300 Sheet Steels (No. R761)

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 Design Standard for cold formed steel structures (AS/NZS 4600) allows for the use of thin (t < 0.9mm), high strength (fy = 550MPa) 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 limited to 75% of the minimum specified values. 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 thin high strength G550 steels available today. 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. Furthermore, the G550 sheet steels tested for this project do not meet the Dhalla and Winter material requirements regardless of direction, except for the uniform elongation of longitudinal coupon specimens. An additional research report entitled Bolted Connection Tests of Thin G550 and G300 Sheet Steels (No. R749) concluded that a modification of 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 document reports on the testing of single overlap screwed connections concentrically loaded in shear, and fabricated with multiple point fasteners using G550 and G300 sheet steels which range in base metal thickness from 0.42-1.0mm. Test specimens were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on connection capacity and failure type. All specimens tested for this report failed in the combined bearing/tilting mode. The results of additional screwed connection specimens, which were mainly composed of single point fasteners, and tested by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) Division of Building, Construction and Engineering, were also included as data for this report. The results of all tests included in this report indicate that the current connection provisions set out in the CSA-S136 and Eurocode 3 Design Standards can be used to predict the bearing/tilting failure mode of screwed connections fabricated from thin G550 and G300 sheet steels. The AS/NZS 4600 and AISI Design Standards could be used to predict the bearing/tilting failure mode of screwed connections if the stress reduction factor provision was removed from the net section fracture formulation. The Eurocode 3 Design Standard provides conservative estimates of the load carrying capacity of screwed connections using G550 sheet steels, whereas the AS/NZS 4600, CSA-S136 and AISI Design Standards provide accurate load predictions when the two connected sheet steels are of a similar thickness. However, when the two connected sheet steels are of a different thickness the calculated connection capacity using the AS/NZS 4600, CSA-S136 and AISI Design Standards becomes unconservative. It is necessary to reduce the bearing coefficient contained in these standards to accurately design thin G550 and G300 sheet steel screwed connections. Furthermore, all of the design standards are reliable when the 0.75¦ u material reduction requirement is used, although when the full material properties are used in design the reliability of the CSA-S136 and Eurocode 3 Design Standards decreases. It is also noted that the thread profile of screw fasteners has a significant effect on the load carrying ability of connections, where screws with a coarse thread pattern outperform their fine threaded equivalents. The proposed method of analysis for screwed connections loaded in shear can be used to improve the accuracy of predicted load resistance when two different thickness sheet steels are joined. It is recommended that the variable bearing coefficient formulation and the unreduced net section resistance be used in the design of screwed connections.