CROSS Safety Report
Steel canopy collapse during building completion works
This report is over 2 years old
During the construction of a new school, a long span steel truss failed, resulting in the collapse of a canopy.
Key Learning Outcomes
For civil and structural design engineers:
Designs should be robust enough to ensure failure of one element will not lead to disproportionate collapse of the structure as seen in this report
For construction professionals and steel fabricators:
Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
If alterations are required due to constructability issues, these should be approved by the design engineer
Welding standards specify that 100% visual inspection is to be carried out before, during and on completion of welding to ensure that production quality is being maintained
They also specify that the weld sizes should be checked by a welding inspector and an additional visual inspection should be carried out by a qualified non-destructive testing (NDT) technician
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During the construction of a major new school facility, a 57m single span steel truss failed, resulting in the catastrophic collapse of a steel framed canopy supported by the truss.
At the time of failure five workers were on the top of the canopy, some 15m above ground level. All received significant injuries but survived. The immediate cause of the failure was associated with the fracture of a number of sub-size fillet welds joining paired load bearing tie bars at nodal points. The design of the ‘T’ shaped joint required the end of some connection plates to be cut with a bevel angle of 80o.
However, to simplify production the plates were cut at 90o on the contact edge. This resulted in a gap of approximately 4.5mm to one side of the joint (refer to Figure 1 of intact cross section recovered from the collapse). This gap resulted in a reduction in the effective weld leg length and throat dimension. What should have been a 10.6mm throat dimension was found to be between 4-7mm.
Single connection failure causes zip like failure
Failure had occurred through the weld material at numerous nodes and assessment showed that failure of one connection at one node would have been sufficient to unzip the truss. In other words, a single inadequate connection would make collapse highly likely - meaning that the design was overly sensitive.
There were additional issues with the design that made installation difficult and led to installers cutting and rewelding connections to the building. Such site alterations are often implicated in collapse incidents albeit that, in this case, the root cause lay elsewhere.
Addressing complex situations
Legal requirements and published guidance and standards are based on ensuring that the designer provides a structural design that is sound and straightforward to build. More complex situations need sequence and assembly instructions to be provided. This is to ensure that:
The fabricator can understand which connections are safety critical
Components are correctly cut and prepared to achieve the specified fit
Fit-up tolerances can be achieved both during welding and during site installation
Difficult fabrication can be carried out under controlled shop-floor conditions and not on site
Quality assurance checks can see what is being achieved to minimise opportunity for hidden defects
Welding standards and inspections
Welding standards for structural steelwork specify that 100% visual inspection is to be carried out before, during and on completion of welding to ensure that production quality is being maintained. They also specify that the weld size should be checked by a welding inspector and an additional visual inspection should be carried out by a qualified non-destructive testing (NDT) technician. Concern at any stage should be referred back to the designer.
They also specify that the weld size should be checked by a welding inspector and an additional visual inspection should be carried out by a qualified non-destructive testing (NDT) technician. Concern at any stage should be referred back to the designer
Since 2014, it has been a legal requirement under the Construction Products Regulations 2013 that the fabrication of building permanent works in structural steel and/or aluminium is carried out in accordance with an accredited quality scheme based on the specification given in BS EN 1090. Projects that use sensible quality assurance practices including early and regular discussion and cooperation between designers, fabricators and installers will avoid the pitfalls outlined above.
Expert Panel Comments
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This is a classic example of the intimate relationship between design, workmanship, and safety. The failure also illustrates a theme that has repeated in a number of recent CROSS reports which is of poor workmanship leading to disaster.
It appears that this event occurred through inadequate workmanship and inadequate quality control. As with the failures in Edinburgh schools (Inquiry into the construction of Edinburgh Schools - February 2017), one aspect is that the poor workmanship in this case could not be detected by post fabrication inspection. Rather, to assure quality, inspection ‘before covering up’ should have been carried out.
Rather, to assure quality, inspection ‘before covering up’ should have been carried out
Over the years, CROSS has received reports of failure involving tie rods of various types. A fundamental cause has been that tie rods have no effective ductility unless special attention is paid to their end connections. In this failure, with such weak end welds, it is apparent that no reliance at all could be placed on tie rod ductility to assist in re-distributing overload.
As is said in the report, from 2014, the requirements of the Construction Products Regulations and BS EN 1090 Part 1 makes it a legal requirement for CE marking of all fabricated structural steelwork (with some exceptions) and with Execution Class specified to BS EN 1090 Part 2. It is essential that client requirements and specifications accord with this.
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