Delayed destressing – an enhanced demolition strategy for a 29-storey office building with mega transfer beams in Singapore

This article presents the demolition strategy adopted for a 29-storey office tower in a highly built-up city area in Singapore. The building to be demolished had three major transfer beams, each 7.5m deep, spanning about 30m and supporting 15 storeys above them. Each weighed 1000 tons and was designed to support an estimated dead load of 3000 tons from the tower structure above. The transfer beams comprised post-tensioned strands, cast in reinforced concrete which encased steel trusses designed to take the massive load of the high-rise tower. Detailed study of the demolition sequence, involving an assessment of the overall stability of the supporting core structure, transfer beams and the temporary works, was carried out. Instead of adopting the conventional method of reversing the construction sequence entirely and destressing the post-tensioned transfer beams first during the demolition, a delayed destressing approach was used. This approach had two significant advantages: ensuring the integrity of the transfer beams by not destressing while the upper floors were supported; and optimising the need for temporary works to support the transfer beams. Upon removal of the floors above, destressing of the transfer beams was carried out in stages, effectively supporting the self-weight and efficiently using the temporary propping.