CROSS Safety Report
Propping of post-tensioned slabs during construction
This report is over 2 years old
A reporter raises their concerns after an infill slab strip was not poured in time which resulted in overloading of the slab below during a concrete pour.
Key Learning Outcomes
For construction professionals:
Ensure pour strips are filled in and have achieved adequate strength if they are to temporarily prop floor levels above
Detailed method statements breaking down the sequencing of the proposed works can prevent unsafe conditions such as slab overloading scenarios from occurring
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When casting in-situ concrete slabs, they are typically propped and supported by the two floors below. For a concrete framed building that a reporter was involved with, this was how the contractor constructed the floors. The slab was post-tensioned, and the contractor had left strips out to allow tensioning of the tendons. Once the slab had been tensioned, the small infill strips of slab were to be cast.
During the construction at one level, the infill slab strip at the level below had not yet been poured. Therefore, the weight of the slab being cast, and the weight of the floor below was being taken solely by the floor two levels below (Figure 1). Fortunately, this slab was designed for high superimposed loads and finishes that were not acting on the slab yet.
If this had not been the case, the slab could have failed, says the reporter. Lessons learnt are that the contractor needs to ensure that post-tensioning strips are always cast and have achieved the required design strength if they are to be used to prop the slabs above.
Expert Panel Comments
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The broader lesson is the wider one of the interactions between design and construction. In reality, the two phases are intimately linked, and it is not at all uncommon for critical design cases to occur during the construction phase. It is therefore vital that the design and construction teams co-operate to assure that designs can be safely built and thereafter be safe in service.
it is not at all uncommon for critical design cases to occur during the construction phase
Once a safe method of construction has been designed, it is essential that this method of construction is followed. In this case, it was fortunate that the lower slab was strong enough to withstand the wrong sequence - this would not always be the case.
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