Sheldon Square Diagrid Tower
A 32-storey office building carried entirely on its facade, so that not one internal column interrupts the floorplates.
The brief
The site is narrow, wedged between a rail cutting and an existing basement that could not be disturbed. A conventional column grid would have landed loads exactly where the ground was least able to take them, and would have cut the lettable floorplates into awkward strips.
Moving the structure to the perimeter solved both problems at once. A triangulated steel exoskeleton carries gravity and wind load down four corner nodes onto piles founded clear of the existing basement, leaving 900m² of uninterrupted floor on every level.
Because the diagrid resists lateral load in its own plane, the concrete core could be reduced to lift and stair enclosure only, a saving of 2,100 tonnes of concrete and the single largest carbon reduction on the project.
Three problems worth describing
Node fabrication
Every intersection of the diagrid is a six-way welded node. We reduced 340 unique geometries to eleven repeating types, which took the fabrication programme from 22 weeks to 13.
Thermal movement
An exposed steel frame moves with the weather. STAAD.Pro models across a 60°C range set the tolerance for the facade brackets, all of which were fabricated before the frame topped out.
Erection sequence
A diagrid is only stable once a full band is closed. We designed and checked a temporary propping scheme for each of the eight lift stages, over a live rail cutting.
“Forty years of judgement, delivered with modern analysis. They cut 18% of the embodied carbon out of our frame and it still came in under budget.”
Development Director, Commercial Office Developer
Project questions
Asked most often about this scheme.
Is a diagrid more expensive to build?+
The steelwork rate is higher per tonne because of the node fabrication, but you use fewer tonnes and a much smaller core. On this project the frame came in 6% below the budget for a conventional scheme.
How is fire handled on an exposed frame?+
The diagrid sits outside the thermal line, where temperatures during a compartment fire stay far lower. Intumescent coating was still applied to the four corner nodes as a redundancy measure.
Could this approach work at a smaller scale?+
The principle does: put the structure at the perimeter and free the plan. We have used it on four-storey buildings and on the single-storey pavilion at Carpenters Road.