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FIRE CASE STUDY

FOCUS ON : FIRE SAFETY

One Bishops Square in east London, designed by Foster and Partners, is a redevelopment of the historic Spitalfields Market site, designed to create a large offi ce development, writes Miller Hannah. It consists of ground to 11th-floor offi ce space, with separate commercial developments on the ground and lower-ground floors, while the 12th floor is used for plant.

All office accommodation is connected via three atria - the 'central' atrium and two others, known here as 'A' and 'B'. The atria are separated from the office accommodation at all levels by 60-minute firerated construction (laminated glass provided with sprinkler protection), with the exception of the ground and first fl oors.

The fire strategy was developed by Hoare Lea Fire Engineering. It contains several alternative solutions that allow for extended travel distances and reduced fire protection to elements of the structure. The strategy also includes a unique method of providing smoke clearance to the atria.

Part of the design concept was to create accessible and interesting terrace gardens, but the code-compliant solution of mechanical ventilation input and extract would have been unsympathetic to this aspiration. Just imagine two big fans on a beautifully landscaped terrace. Natural ventilation was not allowed, because of the height of the atrium.

The gauntlet was laid down. How do we achieve an acceptable level of smoke clearance without locating fans on the external terrace or, God forbid, taking up net lettable offi ce space?

The architect felt that natural vents in the roof could be accommodated without compromising the look and feel of the terraces. So could we provide mechanical input (ie. blow air in) and 'push' the smoke up and out of the natural vents?

We compared a prescribed mechanical ventilation system (four air changes per hour - hence, code compliant), with a natural ventilation system with 18m 2 vents. The mechanical/ natural system had to provide comparable, if not better, results than the code-compliant design in terms of temperature, smoke concentration and over-pressure.

The systems were analysed and compared by the use of Computational Fluid Dynamics (CFD). This is the analysis of fluid flow, heat transfer and related phenomena using advanced software and numerical techniques. CFD works by dividing the region of interest into a large number of cells or control volumes.

In each of these cells the partial differential equations describing the fluid flow (the Navier-Stokes equations) are rewritten as algebraic equations that relate the temperature, pressure, velocity and other variables. These equations are then solved numerically to give a complete picture of the flow.

The design fire was assumed to be 1.5MW and several sensitive analyses were conducted in the atrium base, and in open office areas with the atrium enclosure removed. The results indicated that the naturally ventilated smoke-extraction system (18m 2 vents, with mechanical air input) was comparable with the mechanical smokeextraction system.

Smaller natural vent sizes were also assessed, but proved to cause over-pressurisation of the atrium, and were therefore not considered compliant.

MECHANICALVENTILATION OF THE ATRIUM The results in Figure 1 and Figure 2 are for a BS 5588 Part 7-compliant mechanical extraction system that provides four air changes per hour to the atrium. They show that the peak temperature rise within the atrium is approximately 40ºC at the head of the atrium, with the majority of the atrium varying between 35ºC and 40ºC from one-third of the atrium's height and above.

The static pressure profiles for the mechanical system indicate that the neutral plane within the atrium is located at about two-thirds of the atrium's height, with a peak of 10 pascals at the atrium's head.

NATURAL VENTILATION OF THE ATRIUM The results in Figures 3 and 4 are for a naturally ventilated system (18m 2 vents) with mechanical, low-level air-input.

The static temperature profiles show that the smokelayer temperatures for the mixed natural and mechanical system are comparable with that of the conventional mechanical system, peaking at approximately 35ºC. The temperature profile is uniform throughout the upper half of the atrium's height.

The static pressure profile for the mixed natural and mechanical system indicates that the neutral pressure plane for the atrium is located at approximately half of the atrium's height, with a peak pressure of 8 pascals at the head of the atrium. This indicates that smoke will not be spread through associated areas, but ventilated via the atrium as per with the design intent.

It is our conclusion that a mechanically assisted naturally ventilated smoke-extraction system (ie. with mechanical air input) is just as efficient as a solely mechanically ventilated (with mechanical input) smokeextraction system designed in accordance with the guidance of BS 5588 Part 7.

Miller Hannah is principal of Hoare Lea Fire Engineering

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