CROSS Safety Report
Light gauge steel fabrication
This report is over 2 years old
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.
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In the report AUS-22 Light Gauge Steel Truss Fabrication of August 2020, it is stated that “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 purpose of this submission is to give the background to the ductility requirements of AS1397 Continuous Hot-Dipped Metallic Coated Steel and Strip for steels with an elongation less than 2% and the design of bolted and screwed connections in these steels to AS/NZS 4600 Cold-Formed Steel Structures. This submission is written by me as Chair of Committees MT1 of Standards Australia which produces AS1397, and BD/82 which produces AS/NZS4600 to give the basis for the selection of the design rules. I am also one of the principal authors of the research publications used as the basis for the design rules.
Considerable research was performed in the late 1990s on the AS1397 G550 Steel in thin gauges down to 0.42mm to confirm its applicability for structural design. The design rules were included in the 2005 edition of AS/NZS 4600 and are in the current 2018 edition. Five important papers form the basis of these rules. They are:
"Failure Modes of Bolted-Sheet-Steel Connections loaded in Shear", Journal of Structural Engineering, ASCE, Vol 126, No 3, 2000, pp 288-296 (CA Rogers and GJ Hancock)
"Fracture Toughness of G550 Sheet Steels subjected to Tension", Journal of Constructional Steel Research, Vol 57, No.2, 2000, pp 71-89 (CA Rogers and GJ Hancock)
“Screwed Connection Tests of Thin G550 and G300 Sheet Steels”, Journal of Structural Engineering, ASCE, Vol 125, No. 2, 1999, pp 128-136, (CA Rogers and GJ Hancock)
“Bolted Connection Tests of Thin G550 and G300 Sheet Steels”, Journal of Structural Engineering, ASCE, Vol 124, No 7, 1998, pp 798-808 (CA Rogers and GJ Hancock)
“Ductility of G550 Sheet Steels in Tension”, Journal of Structural Engineering, Vol. 123 (12), 1997, pp 1586-1594 (CA Rogers and GJ Hancock)
Firstly, AS1397:2011 includes Table 2.2 where G550 steel down to a thickness of 0.6mm is required to have an elongation of 2% in a 50mm or 80mm gauge length. Below this length, it is not specified as stated in Note 2. This aligns with the CROSS-AUS 22 quotation above. Secondly, AS/NZS 4600:2018 Section 220.127.116.11 Applicable Steels Item (b) states that “Steels conforming to AS1397, Grade 550, less than 0.9mm thick may be used provided- (i) The yield stress (fy) used in design in Sections 2, 3, 4 and 7, and the tensile strength (fu) used in design in Section 5 are taken at 90% of the corresponding specified values or 495MPa, whichever is the lesser, and for steel less than 0.6mm in thickness, the yield stress (fy) used in design in Sections 2, 3, 4 and 7, and the tensile strength (fu) used in design in Section 5 are taken at 75% of the corresponding specified values or 410MPa, whichever is the lesser.”
Consequently, the AS1397 G550 Steels with unspecified uniform elongation less than 0.6mm thick are limited to 75% of their yield or tensile strength in design. This requirement has been validated by the research in the 5 papers cited above which are available from the author on request (email@example.com). One important factor to note is that the steels do have low uniform elongation in a 50mm gauge length but substantial local elongation in a 7.5mm gauge length, even transverse specimens with lower ductility than longitudinal specimens. Professor Gregory Hancock, Emeritus Professor and Professorial Research Fellow, University of Sydney
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.