CROSS Safety Report
Design deficiencies in long span steelwork portal
This report is over 2 years old
A firm commissioned to design steel joints on a large portal frame structure noticed that there was lack of restraint to a series of long span rafters.
Key Learning Outcomes
For civil and structural design engineers:
- A quality assurance system within your organisation, that includes the internal checking of calculations, can help prevent safety issues like this from occurring
- Competent supervision of design by experienced personnel can allow less experienced engineers to develop a feel for what is the right solution
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A firm was commissioned to design steel joints on a large, single-storey supermarket structure for a fabricator. Construction was underway and steelwork was due on site in a matter of weeks. Whilst carrying out a review of the design drawings the reporter’s consulting firm identified two areas of serious concern and brought them to the notice of the frame designer.
Additional steelwork was then added to the design to resolve the problems. The main problem was lack of restraint to a series of long span portal rafters. The frame is more than 50m wide twin span propped portal supported at the apex by a mixture of internal columns and ridge beams, hence each rafter spans over 25m.
Lack of rafter restraints
On one side of the structure there is a north-light roof, which comprises secondary rafters propped from the main rafters, this north-light continues the line of the opposite roof slope, projecting upwards from the ridge. The presence of the north-light resulted in the primary rafter having no purlins or any other restraining steelwork over a length of 12m.
The section, a 533 x 210 UB, was clearly excessively slender for such a long unrestrained length and had not been designed for this situation. To make matters potentially worse, the frame could probably have been erected and supported its self-weight and the weight of the roofing, allowing the structure to be completed and potentially left in an unsafe condition.
The section, a 533 x 210 UB, was clearly excessively slender for such a long unrestrained length and had not been designed for this situation.
The application of, say, design snow load would have almost certainly led to collapse. A further, albeit less serious problem with lack of lateral restraint to columns was also highlighted by the reporter’s firm and steelwork added as a result. The project had been checked and independently certified by a firm employing experienced engineers. This is the latest and most striking in a regular diet of defective design identified by the reporter’s firm whilst carrying out connection design.
Expert Panel Comments
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From the limited information available it is difficult to identify the cause of this. The main problem here is that the original designer did not consider lateral-torsional buckling of the very slender 12m long unrestrained 533 x 210 UB section. They similarly did not correctly consider buckling of the columns.
Guidance on portal frame design
Both BS 5950 and Eurocode 3 give guidance on design for lateral-torsional buckling. Also, the Steel Construction Institute has published a guide on ‘Design of Single-Span Steel Portal Frames to BS 5950-1:2000’ which considers in-plane stability, rafter design and stability and column design and stability and contains a number of worked examples.
BCSA, Tata Steel and SCI are currently preparing a similar publication for the design of portal frames to Eurocode 3. Member strength capacities are only justified if the assumptions used in calculation are clearly understood and verified in detailing. In steelwork, this applies to connection capacity and stiffness, and matters of restraint on beams and columns: that is both global restraint (to limit sway) and local restraint to prevent buckling.
Stability of portal frames
Portals have complex conditions where flanges are in compression: along the top edge of rafters and on the inside of their eaves haunch/column interface. Stability also needs to be considered during erection when necessarily permanent restraints such as purlins are absent. A number of failures have occurred through lack of restraint; FC Twente Stadium in Holland during erection (2011) and on Hartford Civic Centre Roof Failure (1978) (purlins not in the same plane as the compression boom and therefore ineffective).
Stability also needs to be considered during erection when necessarily permanent restraints such as purlins are absent.
This example illustrates the importance of thorough design checks and the value of a ‘review’ undertaken by a more senior person who has the ability, and time, to stand back and ask the searching questions.
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