Steel for living

Two-storey houses don't need a steel or concrete frame – bricks and blocks or timber frame hold them up perfectly well. But in the push for higher density housing, two storeys have turned into multistoreys requiring structural frames. For the major contractors that dominate the largely steel-framed office market, this offers the prospect of exploiting their steel expertise by transferring it to residential buildings. And as a 17-storey speculative development in Leeds shows, steel frame can meet the acoustic requirements with ease.

According to Alan Todd of steel supplier Corus, 35% of the 180,000 residential units built in the UK each year are now in blocks five storeys or taller, compared with around 10% five years ago. In that time steel's share of the market has quadrupled to 40% - the same as that of concrete.

Like Todd, Peter Haines, director for consulting engineer Terence Dudley Associates, dismisses steel's acoustic shortcomings. "Although sound travels through a steel frame faster and better than concrete," he says, "the trick is not to allow sound to reach the frame in the first place."

Sound barrier
Haines has pulled off this trick on White Hall Quay, a 17-storey block of 195 flats in Leeds. He kept sound out of the floor steels by using a slimdeck floor to encase them in 200mm-thick precast concrete slabs, with a resilient layer of foam and a floating screed on top and a suspended ceiling below. He has also wrapped the vertical steel beams in mineral wool and insulation boards, and these measures have worked: in situ acoustic testing proved that White Hall Quay outperforms Part E.

"Steel meant more fiddling around," admits Haines, "but it's part of the fit-out and not on the critical path. The key to an economic building is getting the frame up, the floors in and the roof on. Steel let us do that far quicker."

With no fear about the acoustics issues, Haines reckons steel for White Hall Quay was a no-brainer. Haines performed a side-by-side comparison with concrete for developer Linfoot, and the savings from steel's speed of construction proved decisive. The lead-in for precast concrete was 16-20 weeks compared with eight for steel, while in situ concrete involves long waits for the concrete to gain strength before striking the formwork and pouring the next level.

"Steel reduced time on site by 16 weeks, " says Haines. "That meant big savings on Linfoot's borrowing for the £20m project." The shorter programme also cut construction spend on plant, access, skilled labour and management. And although steel, unlike concrete, needs fire protection, Haines says the price of intumescent coatings has fallen heavily and that painting the steel takes very little time.

It was the suggestion of the design and build contractor Barr Construction to employ sheetpiled steel for the two-storey underground carpark. This choice of material allowed Haines to reduce fire protection in other areas, because heat conducted through the frame of the building can pass through the basement's steel walls and into the surrounding groundwater and soil. The steel basement was also much cheaper than using interlocking concrete piles, and where rough concrete would have had to be drylined the steel basement just needed painting.

White Hall Quay is a bankside building, and Haines says his biggest challenge was keeping water out. The solution came in three parts: welding the steel joints on their detail with the ground slab, casting bars into the concrete slab that expand on contact with damp, and putting a membrane under the ground slab.

"Don't get me wrong," says Haines, "I'm a fan of concrete. It's particularly good for fancy, unusual or awkward shapes. Concrete can give a client everything they want – at the cost of time and money. It's seen as a straightforward way to get acoustic performance, but I once did a nightclub in concrete and had terrible problems getting the bass beat out of the structure."