Researchers at the University of Bath have load-tested the first physical prototype of a vaulted cross-laminated timber floor, a curved system that carried seven to eight times the load it was designed for and cuts embodied carbon by up to 53 per cent compared with a flat CLT slab. That is according to Shane Hossell, lead researcher at the University of Bath’s Department of Architecture and Civil Engineering, who set out the prototype results in the RIBA Journal.
Conventional CLT slabs resist load in bending, and the tensile stresses in the bottom lamella govern their capacity, which forces designers to add depth rather than span. Timber resists compression better, and the vault geometry turns the floor into a compressive membrane, shifting the governing criterion from deflection to strength.
“Concrete is very good at forming membrane shells, but it has high embodied carbon,” Hossell said of the thin-shell concrete work at ETH Zürich and on the UK’s ACORN project that the Bath team set out to better by substituting timber for cement.
The team now puts the carbon saving at 40 per cent over a 6 metre span and 53 per cent over 18 metres, with cost running 20 to 45 per cent below an equivalent flat slab across the same range. Two years ago, the same researchers had modelled savings of 54 to 63 per cent before ribs, granular infill, and fabrication demands were introduced into the design.
A secondary grid of ribs above the shell carries a level floor surface, and the voids between them are filled with granular material, such as demolition waste, adding mass and damping to meet the demands of footfall vibration. Modelling of an 8 metre vault without that fill returned a fundamental frequency of 10.5 hertz and a response factor slightly above the limit set for office floors, Wood Central understands.
Vaulted floors held real weight for a century and a half before timber entered the argument, with William Strutt turning brick vaults between beams into the first mill sold as fireproof in 1790s Derby, and the jack arch floors that followed spanning brick between iron joists with tie rods holding the thrust. Builders levelled those vaults with concrete and rubble exactly as Bath now proposes to level its own, and the system reached New South Wales by the mid-1800s, where mills, warehouses, and sugar factories took it up.
Timber shells have already proved the geometry at building scale, with the segmented BUGA Wood Pavilion at Heilbronn spanning more than 28 metres on hollow cassettes robotically fabricated at the University of Stuttgart. Every completed shell of that kind stands over people as a roof, and a floor must meet fire, noise, and footfall demands that no garden pavilion ever faces; the gap the Bath prototype is built to close.
Where a flat CLT slab reaches a maximum of 8 metres, the researchers claim the vault could span 12 to 16 metres, widening the column grid and opening the floor plates that commercial tenants want. Hossell said those spans are what would let the system compete with steel and concrete, and the team is now targeting commercial and office buildings.
It comes as England’s building safety regulator closed a consultation on 17 June on guidance that would ban structural timber above 11 metres, a proposal architects called flawed even as engineers flagged the sector’s thin fire-test record. Thin vaults lack the sacrificial charring layer that protects a conventional CLT floor, and no building code addresses how diagonal seams, column supports, and steel ties would behave in a fire.
Acoustic testing has yet to be undertaken, and Hossell named the apex as the point of concern, where the vault carries no infill material to damp the footfall load. Longer spans built from thicker vaults may prove the more viable route, since added depth restores some of the fire protection a thin shell gives away.
The university is now in talks with fabricators and searching for further funding for a system aimed at the heaviest element in a building, with Hossell and his co-authors putting construction’s share of global energy-related emissions above 10 per cent and the world’s building floor area on course to double by 2060.
For further information: Hossell, S., Hawkins, W., Darby, A. & Ibell, T. Timber groin vault floor system for low embodied carbon buildings. Proceedings of the IASS 2024 Symposium: Timber and Bio-based Spatial Structures, Zurich, August 2024. https://app.iass2024.org/files/IASS_2024_Paper_461.pdf
