CROSS Safety Report
Light steel truss issues
This report is over 2 years old
A reporter believes that greater awareness is needed of deficiencies in the light steel truss industry (trusses made from cold-formed steel sections).
This report draws attention to poor design and/or fabrication practices relating to connections in light steel trusses, limitations in software when dealing with eccentric loads, and potential problems when scaling up design from low-rise to medium-rise buildings.
Key Learning Outcomes
For structural and civil engineers:
Awareness of potential fabrication issues with light steel members used to form trusses
Awareness of limitations of design software when dealing with eccentric loads
Need to have designs for connections in light steel trusses checked by a competent structural engineer before fabrication, particularly where members are to be cropped short of their connection points
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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.
Design and software short-comings
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.
Lack of guidance on eccentric load paths
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.
Need for thorough review of light steel truss designs
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.
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Re light cold formed trusses, Load path eccentricities are common. I suggest that some full scale loading to failure might point the way to acceptable detailing.
I teach cold formed steel design at the University of Auckland and have been a technical advisor to NASH New Zealand since its formation in 1989. Cold formed steel framing is an incredibly versatile material to work with and, properly designed and detailed, it delivers dependable structural solutions with a strength to weight ratio far superior to other structural materials. However, it is also a challenging material to design, especially at a system level and one in which the load transfer through connections must be very well understood and applied. This is especially the case where elements in a member cross section are removed to allow one member to connect into another. It is extremely important that the loads to be transferred through the connections are accurately quantified and the load paths well understood. Only then can the designer be certain that the connection components carrying the loads can transfer these as intended. This is vital in all steel construction and especially in multi-member connections in light steel framed systems. It is not something that can or should be covered in AS/NZS 4600 but it relies on the experience of the design engineer to both perform an adequate design and then to document this to the fabricator so it gets built as the designer intended. The fabricator must then build the system in accordance with the drawings. When this is done, the cold formed steel system will deliver excellent structural performance. When it is not done correctly, there are many ways in which the performance can be compromised.
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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).
Poor design and/or fabrication of connections
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. As eccentric connections are common with these types of trusses, it is surprising that available software does not take this into account.
Warning: Effects of scaling without understanding implications
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.