When does an open market become a closed space? That was one of the questions faced by fire engineers, Warringtonfire Consulting, in the redevelopment of Spitalfields Market in London, the winning entry in the small-to-medium sized category of the FSE Design Awards.
Spitalfields market near liverpool street in London dates from the 1680s and is an essential part of the historic infrastructure of London. The market is surrounded on three sides by three-storey listed buildings (known as the Horner Buildings). On the fourth side it faces, over a road, a new office block designed by Foster & Partners.
Owners Ballymore Properties undertook a refurbishment project, which includes new two-storey retail and restaurant pavilions within the covered market place. In addition, changes were proposed to the roof, which also impacted on the surrounding buildings.
Because the market was all at a single storey and the roof has a large number of openings, the agreement with the authorities was that it was treated as an external space. However, with the proposed works it was not certain whether this approach would still be valid. In addition, the managers of the market needed to be sure that the proposals would be acceptable under new risk-based fire safety legislation.
In order to solve these problems, Warringtonfire Consulting was appointed to assess the fire safety risks in the existing market, and to deal with the changes that would occur as a result of the redevelopment.
Overall strategy
A highly fire engineered approach was taken, based on an overall assessment of the fire risks and close liaison with the designers and client. This helped ensure that the design achieved the client’s objectives, maintained the architectural concept and minimised the cost.
One of the concerns at an early stage of the design was that restrictive codes, such as BS5588: Part 10, might be applied to the site. As an existing market, it would not be possible to comply with these codes, so it was important to take a more risk-based approach.
One of the main areas where fire engineering principles were used was in analysing whether the market’s existing smoke ventilation provisions would be sufficient to deal with a large scale, unsprinklered fire inside a pavilion. The existing arrangements would need to be able to provide safe, tenable means of escape for people in the market as a whole, so allowing it to continue being treated as an ‘open space’.
It was also necessary to deal with the Horner Buildings, which surround three sides of the market. These contain retail units at ground and basement levels, and residential apartments on the first and second floors. The residential apartments currently have secondary escape routes which lead out onto a walkway and which go around the perimeter of the roof.
The main aspects of the fire strategy, which was developed liaising closely with the building control authority, the fire and rescue service and all members of the design team, were:
– A means of escape analysis for the main market area, combined with a computational fluid dynamics (CFD) smoke flow analysis for a fire within a pavilion.
– A risk assessment for the Horner Buildings and the impact the new proposals would have on these buildings, including an external fire spread analysis to determine the effects of a pavilion fire on the surrounding listed buildings.
Smoke flow CFD analysis
There were two main issues relating to smoke flow within the market:
– The position of a steady state smoke layer and how this would effect the ability of people in the market to escape directly to the outside.
– The time taken for smoke to fill to a level that would compromise escape from the upper levels of the pavilions.
A CFD smoke flow analysis was carried out in order to solve these issues. Warringtonfire Consulting developed fire scenarios which were selected to be the most onerous for smoke build-up within the space (this included a model of the surrounding buildings to determine whether there would be any detrimental effects caused by unusual wind conditions). These were discussed and agreed with all relevant parties (especially Building Control and the fire brigade) and then modelled.
The results of the analysis showed that there would be more than sufficient time for all occupants of the upper levels of the pavilions to escape down to ground level – before untenable conditions occurred. They also showed that a stead- state smoke layer was maintained well above head height in the market, indicating that once occupants were at ground level, there would be safe evacuation conditions for a substantial amount of time. This demonstrated that the introduction of two-storey structures into the market (which was originally only on a single level) should be safe. The analysis also helped to show that tenable conditions would be maintained for fire-fighting operations.
The risk assessment carried out was aimed at minimising the effects of the market proposals on the neighbouring developments, which included the secondary escape routes from the residential buildings. Part of this assessment was a survey of the site to identify any potential risks to both the market and the existing buildings. The other part of the assessment was an in-depth analysis of external fire spread.
Due to the width of the pavilions in some areas, restrictive and expensive facades to the pavilions would have been needed if a ‘code compliant’ enclosure method were adopted. So a computer programme, developed by Warringtonfire and which uses the fundamentals of radiation analysis, was used to calculate the amount of radiation that would be received on the adjacent buildings – in the event of a fire that involved the largest pavilion on the site.
The analysis showed that the total radiation inflicted on adjacent buildings would be below that which could cause the fire to spread, and so demonstrated that certain pavilion facades would not need to be fire rated.
Achievements
The main achievements of the fire engineering approach were:
– The historic market could continue to operate as a vibrant and exciting part of London’s heritage.
– The architectural design for the pavilions was safe and acceptable to all parties (this would not have been achievable by a ‘code compliant’ approach).
– It was not necessary to include prohibitively expensive fire safety systems, such as sprinklers.
– Existing smoke extract provisions were used, so there was no need for additional ventilation provisions.
– Open deck retail and restaurant spaces were permitted at high level within the market.
– The glazed facades to the pavilions did not need expensive fire rated glass.
The fire engineering strategy developed for the building was considered by all parties to be highly successful. It allowed the architectural concept to be achieved while ensuring that the design was as safe and cost effective as possible. By working closely with the team and having regular liaison with building control and the fire brigade, the design was developed and implemented efficiently.
