Barrett Byrd Associates

A Chichester hospital’s pioneering Treatment Centre is obtaining maximum benefit from a crucial new design guide on vibration in health sector buildings.

Treatment Centres (TCs) are the key to 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 of the 10 or so currently under development around Britain and 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’ has as an underlying objective the reversal of prejudice against steel frames and floors being used where operating theatres and precision laboratories are planned.

The common perception is that vibration is too much of a possibility 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 steel to be used in the Chichester TC’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 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 of work done at the institute, plus collaboration with Arup Advanced Technology Group, to update the 1989 floor vibration ‘bible’, SCI Publication 076; plus more than a year looking specifically at hospitals,” he says.

That specific look included unprecedented insitu dynamic testing of three existing hospital floors of a variety of structural arrangements based on steel. Walking is one of the prime causes of noticeable vibration in a building and the impact tests were used to determine – via dynamic property calculations – the worst walking pace for the biggest floor response.

Controlled walking tests were then carried out at that pace to find out the greatest response factors, which were subsequently 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”.

Having proved the stability of the floors, the research team could then retrace its steps to refine the numerical modelling carried out initially to assess the floors’ dynamic properties. This information was then used to develop the design guide. Also built in to 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 produced a good enough result, is not adequate enough. It became clear that the floor plate as a whole has to be looked at to calculate overall response.

“The new design guide is a good one,” Hicks says. “It has taken a while, 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 of health environments.”

BOX
...giving excitations
Structural engineer for St Richard’s Hospital Treatment Centre is Gyoury Self, a firm which favours steel construction and in particular Slimdek floors. Slimdek is a proprietary system involving asymmetric steel beams whose bottom flanges are wider than their top. The bottom flanges support galvanised steel trough decking which sits on the supporting beams’ wider bottom flanges; with concrete, cast insitu, infilling and creating the top surface.

“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’s lead structural engineer Brian McCarey. “ Moreover, it lends itself to fast tracking. 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 that of flat slab concrete. It also offers a client tremendous flexibility in the future, should changes be necessary within the building.”

McCarey’s firm has good experience of Slimdek and considers it ideal for the Chichester TC which is on a very tight programme. The building is three stories high including a plant level, is horseshoe on plan with the atrium partially covered by a Texlon transparent cushion roof and has a client – Royal West Sussex NHS Trust – that wants it in a hurry.

The problem was that Slimdek had never before been used for a substantial hospital building and taking belt and braces measures to ensure no vibration occurred ratcheted up the amount of steel to be used, 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 on vibration in hospitals.

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

“It means our client can enjoy the benefits of steel without suffering a cost penalty,” McCarey says. “Everyone is happy with the result.”

PROJECT BOX
Client: Royal West Sussex NHS Trust
Engineer: Gyoury Self Partnership
Architect: Nightingale Associates
Main contractor: Henry Jones, Kier Group
Steelwork contractor: FH Dale

BOX
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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