1998 Research Reports No R762-R780

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.

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.

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. 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 sheet steels available today.

A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) details the basic material behaviour of G550 sheet steels. The authors 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 that were 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.

This document reports on the failure mechanisms, as well as the fracture properties of G550 sheet steels tested in tension. Descriptions of failure surfaces, which were observed through a scanning electron microscope are provided. The failure behaviour of perforated and solid coupon specimens is documented, the fracture resistance of G550 sheet steels is measured for a range of temperatures and a numerical study of the effect of cracks on structural performance in the elastic load range is completed using the FRANC2D finite element computer program.

Abstract
This report describes tests on various types of knee joints for portal frames constructed from cold-formed rectangular hollow sections (RHS). Welded stiffened and unstiffened knee joints, bolted knee joints with end plates, and connections with a fabricated internal sleeve, were included in the experimental investigation. Most connections tested under opening moment failed by fracture in the heat affected zone of the RHS near the weld. The connections tested under closing moment failed by web local buckling which occurred near the connection.

While the stiffened and unstiffened welded connections satisfied the strength interaction requirements in the available design guides, the connections did not maintain the plastic moment for sufficiently large rotation to be considered suitable for a plastic hinge location. The unstiffened welded joints were not able to reach the plastic moment.

The use of an internal sleeve moved the plastic hinge in the connection away from the connection centre-line and reduced the stress on the weld between the legs of the connection. It was found that sleeve connections were capable of sustaining the plastic moment for large rotations considered suitable for plastic design.

Keywords
Cold-formed steel, hollow sections, RHS, welded connections, bolted connections, joints, plastic design, bending, rotation capacity, local buckling, fracture, heat affected zone, internal sleeve.