Doing our level best with polystyrene blocks
Different floor heights in a new private hospital were levelled using polystyrene and the original timber frame. Elaine Knutt reports.
When it comes to “repurposing” old buildings, the constraints imposed by the existing structural frame can either work in favour of the new design proposal, or consign it to the recycling bin. Or, as at Brighton and Hove’s new £35m Montefiore Hospital, the project team could find a third way — an ingenious solution whereby the timber frame of the former Victorian warehouse is pressed into service in the construction of its replacement.
The building, which had latterly been converted to offices, appealed to client Spire Healthcare as a city-centre hospital with enough charm to compete with anonymous “anywhere” health facilities. But when architect IBI Nightingale undertook a viability study, it immediately hit a stumbling block. The building had two wings, joined by a staircase, where the floors were at different storey heights. Not an issue if you’re converting to flats, but a big problem if you need to transport patients and trolleys around the building, plus ensure safe evacuation routes.
Demolition of the existing timber floors to cast new slabs to unify the two wings would be technically possible, but also a major undertaking. The new floors would have to be supported by a small forest of props, which would also render the rest of the building a no-go area for other trades for weeks on end. And with the floors removed, there would also have to be temporary lateral bracing to restrain the walls from inward collapse.
The floor heights of the first and second floors in the original building (top) were raised to match the floor levels in the north wing (right hand side of section). In the remodelled structure (above), part of the third floor was removed to create a high-ceilinged operating theatre.
“The conventional solution is also a drastic one — in effect, you’d end up with a retained facade solution,” explains Richard Ager, associate director at IBI Nightingale.
But architect and structural engineer Gyoury Self Associates came up with an innovative idea. Utilising the structural capacity of the original building, the proposal was to pour the concrete slabs over the original timber floors, with thick polystyrene blocks laid on top to raise the level to match the adjacent wing. “There was confidence the floors could take the weight — Victorian warehouses were designed to take heavy loads. And when we checked, we found that was the case,” says Ager.
Alan Pike, senior structural engineer at Gyoury Self, had previously specified similar over-sized polystyrene blocks as bulk fill under road embankments, where there was a risk to the sub-surface from heavier materials, but no need to take a bearing load. “The contractor [Mansell] did look at alternatives, but in the end came round to the original proposal. The biggest problem was disposing of the [waste] polystyrene afterwards, although it was eventually resolved.”
Layer on layer
Polystyrene blocks, up to 1m thick in places, were laid directly on to the timber floor, topped by a separating membrane. Rebar and structural steelwork was laid on top, then the concrete was poured. After curing, the timber beams and floorboards were removed, and a screed finish laid.
At the junction with the original walls, the slabs were simply bolted to the wall with steel cleats. Meanwhile, the floor was also connected into new concrete columns via reinforcement, creating in effect a new concrete frame within the warehouse walls.
The project also involved demolition of part of the third-floor slab to create a void large enough to accommodate an operating theatre with extensive ceiling-height servicing. The north wing of the building also had a new steel frame inserted to meet Building Regulation requirements on preventing disproportionate collapse.
The design team is proud of its innovation, but mindful that the building itself played a vital part. “It’s always interesting to a do a bit of innovation,” says Ager. “But it’s quite rare to find a building where there was the potential to do this, because it was designed as a warehouse.”