Recent Developments in the Australian/New Zealand Standard AS/NZS 4600 for Cold-Formed Steel Structures

The Australian/New Zealand Standard AS/NZS 4600 is currently under revision based in part on the latest edition of the North American Specification AISI S100:2012 and partly based on the latest research in Australia and New Zealand. The Direct Strength Method (DSM) of design has undergone substantial research since the 2005 edition of AS/NZS 4600 and this research is now incorporated in the revised edition. The new areas in the DSM include shear, combined bending and shear, combined bending and compression, sections with holes and inelastic reserve capacity. Further, the prequalified sections now include most sections with longitudinal web and flange stiffeners based in part on Australian research on high strength sections with multiple stiffeners. New areas in the Australian/New Zealand Standard include extension of Section 8 Testing to design based on testing, Section 9 Design for Fire, Appendix B Methods of Analysis including advanced analysis, and Appendix D Buckling moments and stresses for local, distortional and global buckling. Revisions of design rules for net section tension and block shear rupture at bolted connections based on Australian research, inclusion of oversize and slotted holes, and screwed connections in tension and shear now are also included. The paper includes the research basis of the latest revisions with the supporting references. The Australian Buildings Code Board (ABCB), which regulates buildings in Australia by way of the National Construction Code (NCC 2015), has recently changed the loading data for wind, snow and earthquake from 50 year to annual probability of occurrence. This has the effect of increasing the target safety indices. The paper describes the recalibration process for test based design “using the revised loading data

Sway Stability Testing of High Rise Rack Sub-assemblages

The stability in the down-aisle direction of high rise storage racks is usually provided by the beam-to-column and base-plate connections. In order to evaluate the effect of the beam-to-column connections, a complex test rig has been designed and developed at the University of Sydney to test rack sub-assemblages which are the lower levels of high rise rack systems. The rig is designed to apply up to 500 kN vertical load via gravity load simulators. Pins at the top and bottom of the uprights under test remove the effect of the base-plates on the lateral stability. To apply horizontal loads, a down-aisle controlled displacement is introduced at the top of the frame and a load cell is used to measured the force required to obtain this displacement. The corresponding vertical loads are then applied via 2 hydraulic actuators mounted in the gravity load simulators. The rig allows both the loading and unloading in the post-ultimate range to be followed. The paper describes the development of this test rig, and preliminary testing performed using the test rig. The ability of the rig to follow the unloading curve is demonstrated

Compression Tests of Cold-reduced High Strength Steel Stub Columns

This paper describes a series of compression tests performed on stub columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550 MPa. (80 ksi) The steel is classified as G550 to Australia Standard AS 1397. The test results presented in this paper are the first stage of an Australian Research Council research project entitled Compression Stability of High Strength Steel Sections with Low Strain-Hardening . The tests include lipped-square and hexagonal sections, including 94 box-shaped fix-ended stub columns. The purpose of these tests was to determine the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. The results of the successful stub column tests have been compared with the design procedures in the Australian/New Zealand Standard for Cold-Formed Steel Structures and recent (1999) Amendments to the American Iron and Steel Institute Specification. As expected, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. This is contrary to recent design proposals in the USA where it was proposed that the more slender sections would be more greatly influenced by low strain hardening. A simple proposal for improved design capacity is given in the paper

Direct Strength Method for the Design of Purlins

The Direct Strength Method (DSM) has recently been developed by Schafer and Pekoz for the design of cold-formed steel structural members. What is now required is the calibration of the method against existing design methodologies for common structural systems such as roof and wall systems. The paper firstly explains the application of the DSM for the design of simply supported and continuous purlins. Some generalizations, such as how to handle combined bending and shear at the ends of laps, have had to be made to implement the method for continuous purlin systems. The method is then applied to study a range of section sizes in C- and Z-sections and a range of spans for simply supported, continuous and continuous lapped purlins. The results are compared with purlin design capacities to the Australian/New Zealand Standard AS/NZS 4600. This standard is similar to the AISI Specification except that it includes design rules for distortional buckling. Some modifications have had to be made to the strength equations in the DSM to achieve an accurate and reliable comparison. These modifications are included in the paper

Comparison of a Non-linear Purlin Model with Tests

A non-linear elastic analysis has been developed for determining the lateral deflections of, and stresses in, the unconnected flanges of simply-supported and continuous channel and Z-section purlins screw-fastened to sheeting and subject to either wind uplift or gravity loading. The analysis incorporates a model, based on a combination of those developed by Pekoz and Soroushian, and Thomasson, which depicts the unconnected purlin flange as a beam-column. The restraint against lateral deflection is provided primarily by the sheeting, and is represented by a linear extensional spring of stiffness k located at the level of the unconnected purlin flange. To verify the model, lateral deflections and failure stresses obtained from vacuum rig tests can be compared with those determined by the non-linear analysis. In this paper, results from vacuum rig tests on continuous Z-section purlins screw-fastened to sheeting and subject to simulated wind uplift and gravity loading are compared

Purlin Design to AISI LRFD Using Rational Buckling Analysis

The latest edition of the American Iron and Steel Institute (AISI) Specification for the Design of Cold-Formed Steel Structural Members was published in 1996 (AISI, 1996). Design rules are presented in both allowable stress design (ASD) and load and resistance factor design (LRFD) formats. The LRFD rules of the latest AISI Specification form the basis of the Australian/New Zealand Standard AS/NZS 4600:1996 Cold-Formed Steel Structures (SA/SNZ, 1996), which was published in late 1996 and supersedes the corresponding Australian permissible stress Standard AS 1538-1988. One of the main applications of cold-formed steel is purlins and girts in metal roof and wall systems. The design rules for these structural members have been refined over the years and procedures are now available which allow the effects of the sheeting restraint, lapped regions, and height of load application to be incorporated. Nevertheless, unlike AS/NZS 4600: 1996, the AISI Specification does not explicitly allow the use of advanced numerical techniques such as rational elastic buckling analyses within Clause C3 .1.2 to improve the accuracy and reliability of the design procedures. This paper summarises the existing two approaches to puriin design (herein termed the C-factor approach and the R-factor approach) in the AISI Specification, and presents a third approach based on the use of elastic rational buckling analysis to determine the lateral buckling strength of the purlin system. The relative merits and drawbacks of each approach are discussed. The importance of distortional buckling as a failure mode to be considered (currently neglected in the AISI Specification but included in AS/NZS 4600:1996) is also highlighted. The ultimate load capacities computed using the various design models are compared with test results obtained from vacuum rig testing at the University of Sydney over a period of more than 10 years. The use of rational elastic buckling analysis in conjunction with the existing AISI beam strength curve is found to be effective as a means of assessing the lateral buckling strength of puriin systems