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The winner of the 2006 fire engineering category in the FSE Design Awards demonstrates how making the most of prescriptive elements in some areas of a building, can lead to innovative and cost-saving solutions in other parts.
ANATOMY OF AMETHYST HOUSE
Arup Fire was appointed to develop a fire engineering strategy, primarily for Building Regulations compliance with Part B (fire safety). An intricate fire engineered approach was adopted which involved innovative design approaches. The building is a city centre office development with a central atrium running through the full building height. The use of sprinklers was key to a large number of fire engineered areas. The main fire engineering points are:
– Sprinklers: A cost benefit analysis was undertaken to demonstrate to the client the benefit sprinklers would provide in areas other than those prescriptively required.
– Atrium: The use of sprinklers and a temperature control system allowed toughened glazing to be used, rather than fire rated glazing.
– Structure: Sprinklers and a time equivalence analysis allowed the application of 60 minutes fire protection to achieve the required 120 minute performance.
– Means of Escape: Compartment floors allowed a phased means of escape philosophy to be used, which reduced stair core sizes.
– Firefighting A dual entry firefighting lift was used with fire and smoke curtains and a mechanical 0.7m2 smoke shaft was used.
– External fire spread: Sprinklers allowed reduced areas of facade protection, while drenchers on toughened glazing were used instead of fire rated glazing on some elements.
Developments in city centres have become increasingly problematic due to congested sites, which in turn introduces increased efficiency pressure on new buildings. With tight city centre sites and the desire for large floor to floor heights, office innovation is typically stifled by prescriptive guidance.
So fire engineering approaches have taken strides to allow maximised efficiency and best value to the client, while maintaining levels of safety. A cost-benefit analysis as well as a fire engineered approach can help clients understand the roles engineered approaches take. For example, a system may have a high initial capital cost outlay and appear expensive, but when this is balanced against the gains and long term return, value may outweigh cost.
Arup Fire was appointed early in the development of Amethyst House in Manchester, so this enabled the development of a holistic fire engineering design. Such techniques were incorporated extensively throughout the building, which enabled innovative architecture and maximum value for the client. The fire engineering elements interact with each other providing an innovative and holistic design.
Amethyst House is a nine storey building with two basement levels in Spring Gardens, near to the city’s main shopping area. The congested site is bound by Spring Gardens, Marriot Walk and Brown Court with an existing building on the remaining facade.
The reception area is a double height entrance foyer, and a light well passes through each of the upper floor plates from the ground floor. Retail and office accommodation is provided at ground level with office units from the 1st to 8th floor. The light well increases in area at each level and so the office floor plate decreases in size. The total floor area including the light well at each floor is a little over 1000m2, while the light well itself increases from 80m2 to almost 100m2 as it passes through the building. Vertical circulation is provided by a bank of four lifts which open into the atrium. Balconies over the atrium provide horizontal circulation from the lift bank, and two stair enclosures serve the building.
The design created the following issues:
– The height of the building required sprinklers, compartment floors and 120 minutes fire protection.
– The atrium passed through compartment floors.
– Phased evacuation – which was needed in order to reduce stair core sizes – meant that fully fire rated facades to the atrium were required.
– External fire spread was an issue.
– 120 minutes’ structural fire protection was required.
– A firefighting core was provided with a dual entry lift and mechanical smoke shaft.
– Ventilation of the basement car park was needed.
Sprinkler benefits
Sprinklers, designed to the life safety standard in BS 5306, were incorporated early in the design. These helped by permitting a smaller design fire size and so helping in the design of the following areas :
– the building height being in excess
of 30m
– use of toughened rather than fire resistant glazing in the atrium
– the temperature control system
– rationalisation in structural fire resistance
– firefighting shaft ventilation
– external facade protection
The payback offered in each of these areas was demonstrated in a cost benefit analysis to the client. Sprinklers were omitted from the stair enclosures, toilets and basement areas, providing further value to the client.
The prescriptive requirement (BS 5588: Part 7) for fire rated glazing to allow for phased evacuation was rationalised by using a temperature control system, and having a sprinkler system enabled the assumption of a far reduced fire size. Through ventilation provisions, it could be demonstrated that a fire breaking into the atrium could not break back into upper floor levels. So people on non-fire rated floors awaiting evacuation were safe without fire rated glazing.
Means of escape
City centre sites often create issues with means of escape, due to the prescribed size of stair cores. To enable them to be smaller than those typically associated with a simultaneous evacuation procedure, a phased evacuation philosophy is used. This reduction in stair core sizes directly increases the nett area of the lettable floor space.
The provision of a voice alarm system – and comprehensive analogue addressable detection to an L1 coverage – ensures early detection of fire and a clear, concise set of instructions to building occupants. Travel distances within office levels are marginally over those recommended in guidance documents, but the detection system provided results in reduced pre-movement times and allows evacuation to be initiated sooner.
Planning conditions were strict and so the only means to provide protection to the facade would have been through the use of internal screens or fire rated glazing. The combination of sprinklers and compartment floors ensured these were not required. Due to the high levels of glazing, the upper levels needed fire protection. Instead, however, of using costly fire rated glazing, a drencher system is deployed onto toughened glazing to provide a protected facade. This was shown to provide a level of performance equivalent to that of fire rated glazing.
Fire spread
As it is over 30m high, the building would require 120 minutes fire protection as well as compartment floors and sprinklers, if prescriptive guidance were followed.
So a time equivalence analysis following the Eurocode approach was undertaken. Fire resistance scenarios were assessed and compared to how steel elements are tested under BS 476 for fire resistance. This enabled 60 minutes’ fire protection to be installed, but which actually achieves the required 120 minutes fire resistance performance. Compartment floors were kept to aid the phased means of escape philosophy.
Land-locked firefighting shafts are recommended to be ventilated using 3m2 smoke shafts, but instead a mechanical smoke shaft system was used. The system was designed on the basis of a clear layer smoke management system. The smoke shaft size was reduced to 0.7m2 which includes the inlet air path and again, the sprinkler provision helped justify this decision as far as controlling the anticipated fire size.
But depressurisation in this lobby would prevent the door from being opened from the stairs and vice versa, so an air relief path is provided by splitting the smoke shaft into two. The ‘A’ value of 50Pa pressure difference between the stair and lobby is used to ensure the door is always openable. A fire size was assumed with smoke entering the lobby area at a calculated velocity. Smoke is extracted mechanically into the smoke shaft to ensure a clear layer is maintained in the firefighting lobby. The solution allows an increased lettable floor area, and a performance in excess of the guidance documents.
Because the lift opens onto an atrium, a separate firefighting lift would normally be required under BS 5588: Part 5. But the only firefighting lift provided is dual entry. So to ensure a fire in the office floor areas does not affect other levels of the firefighting shaft, a two hour fire and smoke curtain is provided over the atrium side of the lift openings.
Smoke control on the car park levels was provided by a specialist contractor by means of tunnel fans. Sprinklers were not provided at these levels.
Conclusions
With the combination of sprinklers, compartmentation, temperature control systems, means of escape and firefighting provisions, the fire engineering at Amethyst House provides an innovative combination of techniques which complement each other. Maximum value for the client was attained, while achieving the functional performance of Building Regulations, and so achieving adequate levels of life safety.
This article is based on the entry from Adam Monaghan and Simon Goodhead of Arup Fire, which won the fire engineering category of the FSE Design Awards 2006.