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City goes long on frame analysis

A large City of London commercial development has been a proving ground for new fire engineering techniques using finite element frame analysis. Nick Barrett reports from a Framed in Steel seminar which heard how it was done.

FACT FILE: The British Land

Client: The British Land Company plc
Architect and Structural Engineer: Arup Associates
Fire Engineering: Arup Fire
Steelwork Contractor: William Hare

A groundbreaking application of the developing science of structural fire engineering has delivered a major new high quality office development on a prime site at the heart of the City of London, on the corner of Eastcheap and Mincing Lane.

Plantation Place South is the second of two adjoining buildings which together constitute one of the City’s largest developments. Other innovative aspects of the 160,000 sq ft building’s design include the use of load-bearing stone rather than simply stone cladding for the envelope. But attention focuses on the use of finite element analysis based software to produce a rigorous analysis of what will happen to a structure in the event of a real world fire – making some conservative assumptions such as that the sprinkler system is not working – and to calculate where extra fire protection could usefully be applied, and where none is in fact needed.

Architects from Arup Associates told the seminar they chose steel frame as a safe and reliable method for the main framework, using a concrete slipform core.  Comparisons were made with concrete alternatives but the small number of bays and consequent lack of repetition took concrete multi-bay construction out of consideration. Concrete would also have created  foundation loads 20% higher than for steel. The congested nature of the site meant there would be little room for storing formwork for concreting; steel offered off site fabrication and just in time delivery. Overall, for the same floor depth steel offered a 6% saving in frame costs, 10% on foundations and about 5% in programme benefits.

The ambition was not just to save money on fire protection, stresses Dr Barbara Lane of Arup Fire: “We wanted to demonstrate that a building as complex as this one could be safely and robustly designed using the latest structural fire engineering techniques.” Nonetheless, a significant saving was achieved on fire protection.

Dr Lane explained that the traditional fire engineering approach is based substantially on looking up a table and making sure a design complies with it. This regime assumes that all fires are the same, and that a fire in an office is comparable to one at an airport. “Building regulations are derived from old buildings,” Dr Lane said.

“They ignore facts such as that whereas an unrestrained beam will deflect at 450°C, a restrained beam will not deflect until 800°C is reached.” A frame also has a different response to fire than the traditional furnace tests suggest. Dr Lane and her colleagues drew on the known behaviour of steel in fires from the Broadgate fire and Cardington tests.

She explained: “We calculated what happens in a real fire, using real temperatures. We then calculated the heat transfer through all the structural materials and then we were able to calculate the mechanical response of the whole frame. We then know where fire protection is needed, and also where it is not needed.”

At Plantation Place South all secondary beams have been left unprotected. Core connection protection was also modified to accommodate thermal effects. Dr Lane said: ”Because of the increased emphasis on structural design we were able to demonstrate that the stability and compartmentation requirement would be met with the reduced level of fire proofing.” Armed with this in-depth understanding of structural behaviour in fires, Arup was able to satisfy all other interested parties that the solution was robust – including the client, the building control authority which was the Corporation of London, and the building insurers.

For further information see Framed in Steel: Plantation Place South available from Corus Literature Line
01724 404400.

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