CROSS Safety Report
Problem when launching a prestressed beam
When lowered onto its elastomeric bearings, a 40m span post-tensioned ‘I’ beam started to move towards the outer edge of the pier cap.
Key Learning Outcomes
For civil and structural design engineers:
Be mindful of lateral instability during construction
Consider putting warning messages on drawings to highlight potential instability of components in their temporary condition
Liaise with the contractor when possible during construction
For the construction team:
The temporary works designer needs to consider issues of temporary stability, including an adequate margin for unknowns
Be aware that flexible bearings can introduce eccentric loading and hence instability in temporary construction stages
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A reporter’s attention was drawn to an issue with a post-tensioned ‘I’ beam spanning around 40m. This was one of a number forming a road crossing.
When lowered onto its elastomeric bearings, the beam had started to move towards the outer edge of the pier cap. If the crew had removed the crane slings, it would have fallen onto the carriageway below.
Bowing caused by prestressing
On careful examination, it was discovered that the beam had a slight bow, the bottom flange being about 60mm displaced from straight (approximately 1:600). This was probably because the initial stressing tendons had been disturbed during casting; when the initial prestress was applied the tendon naturally straightened. It meant that the centre of gravity was off centre, so the beam when lifted from each end adopted an imperceptible slant between the points of support. When landing on the bearings, the outer edge compressed more than on the inner edge and the beam started to move.
It meant that the centre of gravity was off centre, so the beam when lifted from each end adopted an imperceptible slant between the points of support. When landing on the bearings, the outer edge compressed more than on the inner edge and the beam started to move.
It was the small curvature that had a disproportionate effect. It was hard to spot the lateral curvature and the site team knew immediately that the beam couldn’t be landed on the bearings – they just couldn’t see the reason.
Solution and lessons learned
The cure was to erect the other beams first and then use a steel comb to hold the top flanges in place and the webs vertical until the diaphragms were cast. As a further safeguard, and probably the most significant, the bearings were temporarily replaced with rigid packs, which removed the problem completely.
The lesson learned was to not presume that a prestressed beam is straight; check it, and if it is slightly bowed, land it on hard packs and replace these with a bearing after construction of the diaphragms. Had the beam fallen it would have been very hard to diagnose the reason; it was only apparent by close examination with a high powered torch.
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Expert Panel Comments
An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.
Lateral stability in the erection of bridge beams (or indeed any beam) has been an issue for decades and all designers should be aware of the erection processes for beams in their temporary state. Early failures at the UK Barton Bridge on the M6 in the 1960s, when fabricated steelwork girders collapsed with fatal results, were a wake-up call to the industry.
There are known stability issues with long slender precast beams during installation. Not only is there a risk of the beam itself falling over and collapsing, there is also a risk of it destabilising adjacent beams in a domino type effect. The need to lock (or wedge) bearings when installing such beams is a useful technique.
Who is responsible for ensuring stability during construction?
Conformity with tolerance requirements would normally be checked by the producer, the risks associated with temporary stability noted by the designer, and the issue of temporary stability reviewed by the temporary works designer including an adequate margin for unknowns.
Conformity with tolerance requirements would normally be checked by the producer, the risks associated with temporary stability noted by the designer, and the issue of temporary stability reviewed by the temporary works designer including an adequate margin for unknowns
More generally the need to consider temporary stability and the responsibilities around ensuring this is done have been the subject of a number of reports for example CROSS reports 642 Toppling of precast concrete unit during lifting causes serious injury and 505 Failure of precast concrete cover slab to circular shaft and is a key learning point. There are also reports on bridge construction including 781 Quality of design and construction of a major bridge structure, and 333 Falsework support to a bridge – a near miss
Raising awareness of stability issues on drawings
The message on maintaining stability during construction needs to be reinforced. One way of doing this is to have warning messages provided on drawings that draw the reader’s attention to the potential instability of components in their temporary condition, with the need to assure stability as part of the construction methodology and temporary works sequencing.