CROSS Safety Report
Light gauge steel fabrication
Referring to Report AUS-4, Light steel truss issues, this reporter has concerns about the low ductility of the high tensile steel, in particular with respect to connection design and capacity.
Close adherence to industry guidelines and standards that have been verified by testing should be followed, and any deviation from this guidance should be verified by appropriate testing.
Key Learning Outcomes
For structural and civil engineers:
Be aware of effects of dynamic, cyclic, or fluctuating loads (e.g., wind loads) on low ductility thin gauge high tensile steel used in truss construction
Consider the increased likelihood of fatigue as a possible failure mechanism when using thin gauge high tensile steel
Pay attention to increased risk of lack of warning of failure at connections in trusses made using thin gauge high tensile steel
Be aware that, in fire conditions, thin gauge high tensile steel has very different behaviour to mild steel
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A reporter refers to Report AUS-4 published in CROSS-AUS Newsletter 3 of Feb 2020 entitled Light steel truss issues. The reporter, who has many years’ experience working with light gauge steel as a designer, has been looking into thin gauge high tensile steel, as used in the prefabrication industry to manufacture trusses. In addition to the points raised in the article, the reporter also has concern about the low ductility of the high tensile steel, with respect to connection design and capacity.
Low ductility and increased likelihood of fracture at joints
The reporter's experience is that it is not uncommon for these steels to have an elongation less than 2%, and for certain gauges AS1397 does not specify a minimum elongation. Thus, fracture of the joints is a serious concern, particularly as most testing would only consider short term loading scenarios. The reporter agrees with the concerns around the points raised in Report AUS-4, and that these members are likely to fail at a very localised area around the heel if not designed accordingly.
Expert Panel Comments
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This report is a good example of the need for designers to understand the properties of the materials they are dealing with and the environment in which they will be used. When materials with low ductility, such as thin gauge high tensile steel, are being used there will be less warning of failure as the ultimate load is approached and with cyclic or fluctuating loads such as wind loading, fatigue failure becomes a critical issue especially at connections.
Industry guidelines and standards
Thus it is important that industry guidelines and standards that have been verified by testing are closely adhered to and any departure from these either in the material being used or the loading being applied should be verified by further testing that replicates the actual situation. It is our understanding that some high strength light gauge steel truss systems have been tested under cyclic loading representative of wind loading from tropical cyclones and reference should be made to the specialist suppliers of these systems or to the Cyclone Testing Station for further advice in this area.
Behaviour in fire conditions
Behaviour in fire is also an important consideration as thin gauge high tensile steel exhibits very different behaviour than mild steel sections and if it is to be used in a fire rated assembly, there should be test evidence available to allow the engineer, and the regulatory authorities, to assess its suitability.
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