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Good vibrations

Cover story Steel

A Chichester hospital's pioneering treatment centre is getting maximum benefit from a new design guide on vibration in health sector buildings.

Treatment centres are key to the government meeting its health care targets. They are effectively mini hospitals where routine, programmed surgery can be carried out free from interruption by accident and emergency.

Chichester's St Richard's Hospital has one of the first centres which is now under development. Its structure is going up quickly, efficiently and, somewhat surprisingly, in steel.

That this form of construction has been chosen has much to do with a new document to be formally published next week.

The Design guide on the vibration of floors in hospitals sets out to confront long established reluctance to use steel frames and floors for operating theatres and precision laboratories.

The common perception is that the prospect of vibration is too high for steel to be used for 'quiet environments'. Research underpinning the new guide clearly indicates this perception is wrong and the guide itself, applied in draft form, helped reduce the amount of steel to be used in the Chichester centre's floors by a staggering 40%. This kept costs down to a manageable level and is allowing all the advantages of using steel, such as M&E design flexibility (see box), to be exploited.

But what exactly are the hospital vibration design guide's credentials? Commissioned by supplier Corus, the guide is being published by the Steel Construction Institute (SCI) and is co-authored by Dr Stephen Hicks, a leading authority on vibration effects. 'It draws on five years' work done at the institute, ' he says, 'plus collaboration with Arup Advanced Technology Group, to update the 1989 floor vibration 'bible', SCI Publication 076. We also spent over a year looking specifically at hospitals.'

That specific look included unprecedented insitu dynamic testing of three existing hospital floors with a variety of structural arrangements based on steel.

Walking is one of the prime causes of noticeable vibration in a building. Impact tests - with dynamic property calculations - were used to determine the walking pace which produced the biggest floor response.

Controlled walking tests were then carried out to find the principal response factors.

These were processed for comparison with acceptance levels in the National Health Service performance standard for hospitals, Health technical memorandum 2045. The steel composite floors came well within the minimum vibration requirements, 'performing much better than even we thought they might', says Hicks.

Having proved the floors' stability, the research team could then retrace its steps, refining the initial numerical modelling to assess the floors' dynamic properties. This information was used to develop the design guide. Also built into the new document is the knowledge that the traditional practice of checking the natural frequency of one component of a building, and proceeding if this produces a good enough result, is inadequate. It became clear that the floor plate as a whole has to be assessed.

'The new guide has taken a while, ' says Hicks, 'but we now know and can prove that steel floors and beyond them steel frames can be designed and built with vibration characteristics suitable for the most demanding health environments.'

Self help Structural engineer for St Richard's Hospital Treatment Centre is Gyoury Self. The firm has designed the Chichester centre with Slimdek floors - a proprietary system involving asymmetric steel beams whose bottom flanges are wider than those at the top. The bottom flanges support galvanised steel trough decking, with cast insitu concrete infilling and creating the top surface (NCE 14 May 1998).

'The system's advantages include minimal structural depth, no downstands, allowing flexibility to M&E designers and installers, virtual elimination of fire protection and rapid construction, ' says Gyoury Self associate partner Brian McCarey.

'And it lends itself to fast track construction. When you factor in speed and the saving possible in M&E design and installation costs, the price of Slimdek is on a par with flat slab concrete.'

The Chichester centre is being built for Royal West Sussex NHS Trust, which wants it in a hurry. It is three storeys high, including a plant level, and is arranged on a horseshoe plan. Its atrium is partially covered by a Texlon transparent cushion roof.

Slimdek had never before been used for a substantial hospital building and belt and braces measures were initially taken to ensure no vibration occurred. This ratcheted up both the amount of steel in the structure, and the cost.

Gyoury Self turned to the Steel Construction Institute for advice just as the institute was finalising its work on a new design guide.

Gyoury Self's design was analysed using numerical modelling techniques employed to develop the guide, as the method of analysis had been confirmed by practical testing. The consultant was told that it could remove a considerable quantity of the steelwork from its design and still have a safe, vibration free structure.

'It means our client can enjoy the benefits of steel without suffering a cost penalty, ' McCarey says.

Who's who

Client: Royal West Sussex NHS Trust

Engineer: Gyoury Self Partnership

Architect: Nightingale Associates

Main contractor: Henry Jones, Kier Group

Steelwork contractor: FH Dale

There's more. . .

Steel company Corus Construction & Industrial is holding a series of free seminars during February on hospital design and construction, where the new design guide will be launched. Details from www. corusconstruction.com/hospitals.

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