Innovative application of post-tensioned slabs to meet complex structural challenges

Space House was originally constructed in the late 1960’s with a structural engineering design produced by Pell Frischmann. The original structure was made up of Kingsway House, 7 storeys, and the tower block 16 stores. The structures shared 2 basement levels.

The portion of the refurbishment which involved Interspan was the 2-storey extension of the tower. A 345mm thick new portion of slab for level 15, a new 300mm thick PT slab at level 16, and a 350mm thick transfer slab at level 17.

The intricacies of the project were critical; the floorplates at levels 16 and 17 were circular. To mitigate the impact of tendons on a radius, Interspan adopted circular duct. The stressing sequence at each tendon was important to avoid locking individual strands, and higher than normal friction losses needed to be considered.

For the level 15 infill slab, the connection to existing concrete is to be considered as a pin. There was also a release required between the existing and new slabs to avoid cracking due to differential shrinkage strain. This was in the form of sleeved dowel bars at 300 centres into the existing slab, which were grouted after 56 days.

All levels of the structure included a perimeter pour strip, which primarily isolated the slabs from the existing façade for the initial drying shrinkage to take place, but also served a secondary purpose of providing edge stressing access.

Space House in Holborn, London, showcases the innovative application of post-tensioned slabs to meet complex structural challenges while preserving the architectural heritage of a Grade II listed building.

This project is not only a testament to engineering ingenuity but also a stunning addition to London’s skyline, a harmonious blend of the old and the new.

Space House truly represents the future of sustainable, efficient, and visually captivating construction in the heart of the city.