CROSS Safety Report
Steel truss failure during lifting operation
The lifting operation for a 40m long span steel truss had to be abandoned after the localised failure of a joint.
Key Learning Outcomes
Ensure all stages of construction including fabrication and lifting are considered
Stress states during transportation and erection differ from those in the permanent condition
For the construction team:
Discuss lifting and erection stages with the designer before starting operations
Check stability and strength during lifting
Keep track of fabrication and assess strength and stability at each stage
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A correspondent discusses a project where they designed steel trusses with a span of around 40m. To facilitate the construction, the trusses were spliced and the connections checked. However, both the correspondent and the fabrication team failed to review the lifting operation fully and reassess the forces in the truss during lifting.
The truss was lifted in such a way that it was laid flat in its weak axis and the force in the weak axis of the splice connection was in excess of the weak axis capacity of the connection. This resulted in a localised failure of the joint and the lift had to be abandoned.
Following this failure, the lifting operation was then fully considered, and the joints were designed for the required forces with the works progressing successfully.
The correspondent believes that a cause of this safety issue was a lack of clarity over who was responsible for checking the truss during the large lifting operation. They go on to say that going forward, they will not design a spliced element or connection without fully agreeing the lift sequence step by step.
The correspondent believes that a cause of this safety issue was a lack of clarity over who was responsible for checking the truss during the large lifting operation.
As a Chartered Structural Engineer with good experience in designing steelwork structures and connections, they acknowledge that they missed the process of lifting the truss from laid flat to vertical and did not account for this out of plane force. Whilst the consequence of this was minor in this instance, they say that it could have been significant.
They conclude by saying that this was a worthwhile learning experience for themselves and others.
Expert Panel Comments
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The reporter has been refreshingly candid and open and is to be commended on such a valuable report. It is a useful reminder of the importance of communication, clarity on responsibility, and consideration of temporary works: a classic case demonstrating the need for the temporary works and permanent works designers to liaise.
Communication and collaboration between temporary and permanent works
The designer of a long span structure needs to know transportation limits to enable an understanding of the structure in the temporary condition and the need for appropriate connections and temporary lifting points. It is interesting to note that the permanent works designer was asked to consider the construction sequence. Did the designer provide a detailed method statement so that the contractor was aware of the conditions at all stages?
Single controlling mind required for all stages
Designers, therefore, need to give thought to "how could this go wrong?" during either the construction or deconstruction process and engage in discussion with all parties to ensure all scenarios are addressed. There should be a single controlling mind to take this through from design, through fabrication, transport and erection to ensure that all is well.
The general cases for erection are:
1) any frame will not be in its (permanent) stable condition and loading cases during erection may be the critical ones,
2) any member will have stress states during its erection (e.g. during lifting) that differ from the stress states when that member is fully incorporated into a frame. The stress states depend on how the member is lifted and where the attachment loads are located.
Checks should be made to include stability of the compression boom under self-weight, which may be critical even if the truss is lifted vertically because, during lifting, the compression boom will not have any of the lateral bracing that might exist in the permanent condition. This is of especial concern when the lifting process is such that the normal 'tension boom' has to temporarily act as the compression boom (depends on the truss attachment points).
Lesson learned could be expanded to include the point that it could have been the section members in the truss itself, as well as the splices that might have failed during the lifting operation. For some trusses a support frame may be used to aid the lift.
Early contractor involvement
The report highlights the need for early contractor involvement and the need to refer to Relevant Good Practice for Temporary Works contained in BS 5975:2019 Code of practice for temporary works procedures and the permissible stress design of falsework.
The only way to assure that such incidents are avoided is to track the process of assembly/erection in detail and assess strength and stability in each stage.
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