CROSS Safety Report
Steelwork splices do not match design calculations
Remedial works had to be carried out after it was found that steelwork splices on site did not match calculations which were retrospectively provided by a contractor.
Key Learning Outcomes
For civil and structural design engineers:
Highlight to contractors the importance of receiving subcontractor designs and drawings well in advance of fabrication and installation and the safety issues that may arise if they are not received
When checking structural designs from contractors:
work with them to ensure that the design is checked before it is built
if the design is already built, ensure that you have adequate information and evidence to verify that the design is safe
If possible, attend site and inspect the installation of safety critical elements and connections
If you are unable to attend site, consider asking the contractor for site photos of the installation of critical structural elements
For the construction team:
Proper planning and allowing adequate time can ensure all subcontractor designs are checked and approved prior to construction on site
Competent subcontractors should be appointed to undertake and deliver the project brief
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This report covers events occurring during the refurbishment of a substantial 1930s steel framed building. Part of the project involving removal of mechanical equipment in the lower levels was awarded to a demolition contractor. This task included infilling the resulting voids plus an existing lightwell on the upper floors. The infills were to become permanent structures and consisted of hot rolled steel beams supporting lightweight cold-rolled joists with a plywood deck above.
Inadequate connection design submissions
Fabrication drawings were received from the contractor for the upper level infills and comments were provided by the designer regarding connection loads at the beam ends. A drawing for a beam splice was then submitted by the contractor but without calculations or any reference to a location on plan.
Comments were provided accordingly requesting proposed location and calculations to justify design. There then followed a series of similar issues with unsatisfactory submissions from the contractor. Calculations were eventually received when work was underway, but these contained gross errors and proposals did not match the details constructed on site.
Calculations were eventually received when work was underway, but these contained gross errors and proposals did not match the details constructed on site.
Construction did not match design intent
In one case, the calculations were for a splice detail with x number bolts in each flange on either side of the joint and with these, connection capacities met the required loads. However, no drawings had been seen showing the x bolts. Moreover, the engineers could not recall seeing this detail on site, and review of site photos showed no connections of this type. By then the areas had been covered up with floors and ceilings.
The contractor maintained the x bolt variant was the detail that had been built and so the reporter requested that the areas in question be opened up to prove this was the case. All connections exposed revealed that the splices had just over 50% of the required number of bolts. Remedial works had to be carried out.
Expert Panel Comments
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An underlying comment is that it seems to be frequently difficult to procure adequate proposals for construction details if this task is sublet. Since structural capacity is most likely to be governed by details/connections, the policy on procurement (and what is to be included) should be given careful consideration at the outset.
Since structural capacity is most likely to be governed by details/connections, the policy on procurement (and what is to be included) should be given careful consideration at the outset
Programme demands may lead to corners being cut by main contractors and subcontractors to the extent that time pressures are allowed to outweigh the importance of the technical standards required. Indeed, programme pressures can mean work proceeds on site prior to issue of construction drawings or before subcontractor’s information has been reviewed.
Site photos and record keeping
A recurring theme in CROSS reports is the inability to evidence what has actually been constructed. Nowadays there is no reason why photographic evidence cannot be taken, and it would be wise to include such a demand within the contract documents especially for parts with key structural safety significance.
For a contractor to retrospectively submit calculations that did match what had been built is evidence of extremely poor practice or a lack of competence. If such a step had been taken deliberately and then had then been a collapse, the consequences for those involved would have been severe. Those who carried out the checks and discovered the problem demonstrated professional vigilance.
The need for appropriate supervision
How many similar events go unchecked? The industry needs to invest in appropriate engineering supervision to ensure that such latent defects do not happen in the first place, and hence reduce the risk of distress or failure and more serious unintended consequences.
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