Quick Search
AUS-4 Light steel truss issues
Report ID: 817
Report Overview
A reporter believes that we need to raise awareness of some deficiencies in the light steel truss industry (trusses made from cold-formed steel sections).
Report Content
A reporter believes that we need to raise awareness of some deficiencies in the light steel truss industry (trusses made from cold-formed steel sections). The reporter's experience is that the deficiencies are representative of some fabricators but not all of them.
Figure 1 shows a connection where the members have been cropped well beyond the connection point. Figure 2 shows another example of the fabrication of trusses resulting in an eccentric load pattern. According to the reporter, there are limitations with some software packages that are commonly used to design light steel trusses in that they do not allow for non-concentric loading of members as shown in Figure 2.
Another example of non-concentric loading noted by the reporter is when the top chord of a truss is stiffened by the fabricator doubling up, or boxing, the member with an additional member fastened alongside the top chord. While the software output may give the number of fasteners required, it does not indicate a connection technique/arrangement.
According to the reporter, AS4600 (Cold-formed steel structures) does not provide any guidance on eccentric truss load paths and the only option for industry is to design to the North American Standard for Cold-Formed Steel Structural Framing, AISI S240-2015. However, in this instance, the reporter notes that eccentricity appears to have been ignored in the design and that there is a disconnect between the software, the fabricator and the certifying engineer. The reporter suggests that a simple solution could be to make certifying engineers more aware of the limitations of some of the software packages being used. The reporter has seen many certifying engineers rely solely on the fabricator to input support locations correctly into the software and has seen this performed poorly by the fabricator.
The reporter believes it is unlikely that there are many issues in the domestic market where truss design is simpler with smaller spans. However, the reporter is aware that light steel trusses are being increasingly used in the commercial/refurbishment market and would recommend that all engineers thoroughly review light steel truss designs for any structure, paying close attention to any areas requiring transfers, boxed members or additional connections.
Comments
Images
Cropped members
Eccentric connection
Previous page
Email Updates
How to Report
Online submission:
Submit by post:
Feedback
Submit comments and questions regarding CROSS-AUS and this website.
According to the Australian Steel Institute (ASI) the use of cold-formed light gauge steel is a growth area in Australia. Our country arguably leads the world in the development and manufacturing of leading-edge, cost-effective, high-performance roofing and walling systems.
The ASI further notes that there is increasing use of cold-formed light gauge steel in mid-rise construction and gives examples of some recent developments of mid-rise (up to 7–8 storeys) commercial and residential buildings with cold-formed light gauge steel used for the majority of the primary structural framing.
However most of the technical information available in Australia relates to low-rise residential construction from organisations such as the National Association of Steel-Framed Housing (NASH).
The reporter highlights several issues that can arise with this form of construction and one could add fastener edge distances for tension members and potential buckling at the cropped ends of compression members. And as eccentric connections are common with these types of trusses it is surprising that the available software does not take this into account.
This would appear to be another example of the effects of scale as highlighted in SCOSS Alert of November 2018, where we take a system that is well established at one level (in this case low-rise residential), and extend it to another level that requires a more fundamental re-appraisal of the application in terms of the design, fabrication and construction.
A further matter raised by the reporter is that the software being used may not give the complete picture. Concerns with software generally have previously been expressed to CROSS and engineers must always be aware that it is they, and not the software, who are responsible for any subsequent problems. Software cannot replace engineering judgement and working from first principles.