CROSS Safety Report
Lack of lateral stability in steel frame
This report is over 2 years old
Concerns were raised after the steel frame of a building was found to have a lack of horizontal and vertical bracing.
Key Learning Outcomes
For all built environment professionals:
If you come across or are aware of a live or urgent safety issue:
Your first step should be to raise this with the organisations concerned if possible
If applicable, you should speak to your line manager
If this does not resolve the issue, or if the response you receive is inadequate, then you should inform the appropriate regulator
A fundamental principle of structural work is that all buildings have a viable stability system, that is a viable load path to transmit horizontal loads back to ground
Consider appointing a single entity (or Chartered Engineer) to have overall control of the stability system to ensure situations such as this report do not occur
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This concerns three similar buildings. The roof of building No. 1 was a concrete composite deck supported on lattice trusses and it collapsed whilst carrying a moderate imposed load. The report relates to the findings on one of the other similar buildings designated building No. 2. The reporter's firm was appointed to review the structure of building No. 2 and the initial scope was to determine if the roof was structurally adequate to carry the same level of imposed load as building No. 1.
The reporter's firm reviewed the 'for tender' documentation and the steel fabricator's 'for construction' drawings. The original tender documentation showed a steel framed building with bracing in both vertical and horizontal planes and with the shear studs along the cellular steel beams. The 'for construction' documentation, produced by another firm, shows a steel frame building with no vertical or horizontal bracing, no shear studs, and the cellular beams had been changed to fabricated lattice trusses.
The 'for construction' documentation, produced by another firm, shows a steel frame building with no vertical or horizontal bracing, no shear studs, and the cellular beams had been changed to fabricated lattice trusses
All the steel connections had been designed for vertical shear forces only. There were no moment connections and inadequate tie forces. The columns did not have enough capacity to act as a wind frame. In effect building No. 2 had no form of stability system. The owners of the three buildings have been informed of the situation by the reporter’s firm. However, the general concern of the reporter is that other buildings, similar to these, may not have adequate stability systems.
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This case involves three apparently similar buildings whose owners have been informed of the circumstances by the reporter. There seems to have been a breakdown in communication between the designer and the steelwork fabricator with significant differences between the tender scheme and the constructed building.
The risk of repetitive problems
More generally, repetitive problems are always difficult because a single defect may be repeated; something that mass producers are keen to avoid. When a generic problem is detected, and other buildings of the same type are identified then steps can be taken to resolve the situation. Past examples include Ronan Point type slab and wall buildings, some early box girder bridges, and floor slabs containing high alumina cement. Those cases involved strengthening or replacement on a large scale.
The requirement for a viable stability system
A fundamental principle of structural work is that all buildings have a viable stability system, that is a viable load path to transmit horizontal loads back to ground. The party to assign this system is the main designer whose obligation is also to document and explain the stability system (and any assumptions) so that the whole team understands what holds what up at all stages of construction.
Within the documentation, say on a drawing, it may be desirable to state the basic stability requirements. There is a risk that post- design modifications may be carried out by other parties, either during fabrication of during a retrofit, who misunderstand the stability principles. The main designer should then always verify that component design matches overall design intent.
There is a risk that post- design modifications may be carried out by other parties, either during fabrication of during a retrofit, who misunderstand the stability principles
A safe design will ensure adequate provision of information to allow the connection designer to design adequate connections, ensure that assumptions are verified, and ensure the contractor can enable stability at all times during construction and when complete. The reporter has advised the owners of similar buildings about what was found, and this information should undoubtedly be shared with the steelwork fabricator.