CROSS Safety Report
Painted faying surfaces leads to connections with insufficient load capacity
Painted, rather than the required unpainted connection faying surfaces, were identified by the resident engineer of a reporter's firm during the erection of a primary frame for a large project.
The painted surfaces resulted in the connections not having sufficient load carrying capacity and necessitated remedial works.
Key Learning Outcomes
For civil and structural design engineers:
- If there are any unusual features of a design, these should be made known to all parties and emphasised both in the specification and with notes on drawings
- Good communication between designers, contractors, and site supervisors is essential and there should be a suitably qualified and experienced person responsible for coordinating this
- The importance of finishes on faying surfaces on joints should be more widely recognised
For steel fabricators and contractors:
- Where there are large connections with faying surfaces ensure that the specification for finishes is agreed with the designer
- Where special conditions are specified, ensure that all parties know what these are and can comply with them and that adequate quality control measures are in place
- Pay particular attention to inspections of painted/unpainted areas and the requirements of the specification
- Steel fabricators should not make unilateral changes to the engineer's specification or design without their approval
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Faying surfaces are the surfaces or faces placed in tight contact to form a joint. During the erection of a primary frame for a large project, the resident engineer (RE) of a reporter's firm identified the presence of painted faying surfaces, rather than the required unpainted ones.
The issue was identified by the RE when the steel was delivered to site and prior to it being erected. The error was brought to the attention of the contractor, who took the decision to erect the steelwork despite it being non-compliant with the design or with the specification requirements.
The connections in question were significant and were present in the main long span roof trusses of a major structure with some connections carrying very high loads. The long span nature of the roof structure meant that slip in the connections, arising from movements made possible by bolts in clearance holes, would cause considerable deflection of the trusses. The designer’s specification therefore called for no slip in the connections at ultimate limit state (ULS).
The reporter continues that the connection design submitted by the subcontractor complied with this requirement and assumed fully prepared unpainted faying surfaces at the connections for maximum friction, together with tension control bolts to ensure that the correct clamping forces were achieved.
should slip occur in the joints suddenly, at or close to serviceability limit state (SLS) loads, significant additional dynamic forces could be generated
The reporter was also of the view that, should slip occur in the joints suddenly, at or close to serviceability limit state (SLS) loads, significant additional dynamic forces could be generated because of the very substantial weight of the roof dropping suddenly. Once it was found that the faying surfaces had been painted, and so therefore the design assumptions had not been met, the reporter’s firm undertook an independent review of the implications and laboratory testing. This review showed that the slip factor was dramatically reduced, and that the connection could slip at ULS.
In the reporter's view, this case calls into question the quality and checking regimes on some major sites where there are Consequence Class 3 buildings
The reporter considers the issues to be significant due to the scale and mass of the roof structure involved and the risk of joints slipping under serviceability design loads. In the reporter's view, this case calls into question the quality and checking regimes on some major sites where there are Consequence Class 3 buildings (as referred to in Approved Document A). Furthermore, the reporter cites that in accordance with BS EN 1990:2002+A1:2005 Table B5 - Inspection levels (IL), such buildings should have been subject to extended inspection including third party inspection.
The reporter says remedial works, including welding and plating, were subsequently undertaken. These were however, in the opinion of the reporter, not completed without frustration.
The reporter has highlighted the issues above, to emphasise the risks of inadequate site supervision, the potentially dangerous impact of changes to design, and the safety risk that may arise from any lack of ownership and responsibility to rectify identified defects.
Expert Panel Comments
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This is an interesting report which raises several issues. Years ago, when high strength friction grip (HSFG) bolts were widely used, there were many technical articles about them. There was a good deal of discussion about how to guarantee the preload and what the surface friction values were, largely because their introduction was to replace rivets. It then became accepted that modern high strength bolts didn't need to be preloaded (at least for standard applications), however US practice at that time was always to torque the bolts up. Much of this knowledge of prestressing bolts was forgotten and it is unusual nowadays to look for no slip connections in building structures.
Bridge designers however, sometimes require no slippage at ULS so it may be that the designer used a bridge connection design approach. If so, bridge fabricators, with their associated higher design and workmanship standards might have been familiar with this condition.
Were the implications of painting of the steelwork highlighted on the drawings and in the project plan? If so, was there a communication issue between the designers and fabricators?
It is certainly true that slip in truss joints will potentially add to truss deflection and requiring a nonslip joint is one strategy. However, it is questionable whether it can be achieved. This follows because at high loads, plates in tension will thin and that thinning will lead to loss of preload and so earlier slip. The basic philosophy of such joints in Codes has always been nonslip at SLS, with slip at ULS, but with limits on plate thicknesses to assure that shear capacity and in bearing capacity is achieved. This means that strength capacity is there even if the joint slips.
Detailed information on the subject can be found in the Guide to Design Criteria for Bolted and Riveted Joints, 2nd Edition from the American Institute of Steel Construction.
The basic philosophy of such joints in Codes has always been nonslip at SLS, with slip at ULS, but with limits on plate thicknesses to assure that shear capacity and in bearing capacity is achieved
There are two significant issues in this report. The first and most obvious is that painting the faying surfaces was in contravention to the specification. The second issue is that while the design team may have departed from normal practice by requiring no slip at ULS, the contractor should not have proceeded with the works once the issue was raised. If something unusual is being proposed, then it becomes very important to ensure that everyone concerned, including the fabrication team, understands the implications.
In this case, were the implications of painting of the steelwork highlighted on the drawings and in the project plan? If so, was there a communication issue between the designers and fabricators?
The panel agrees with the reporter about the safety critical importance of communications between designers and fabricators, and the recognition of the importance of changes.
As has been said before, a robust specification supported by robust inspection and test plans, combined with adequate supervision are good precautions against points of difference between designers and fabricators.
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