28 January 2002
Magna Fire Engineering
Nick Troth Bsc(Hons) Fire Safety, Dip Fire Eng, MBEng, Associate at Buro Happold, explains the cohesive fire strategy behind the award-winning attraction.
The fire engineered approach enables an appropriate package of fire safety measures to be adopted for a particular building rather than the rigid or generic application of inappropriate contemporary guidance. It is often the only viable option to enable innovative designs to progress satisfactorily while achieving accepted levels of safety. Such an approach can take account of, and enable full quantification of, the use of modern materials, not currently addressed by contemporary fire safety guidance.
Complex analysis tools provide an increase in design confidence. Zone models are easy to apply and enable early identification of areas of uncertainty within the design process. The use of more complex CFD modelling techniques can then be adopted more effectively where necessary, resulting in economic savings. Furthermore, the use of a time-based approach to means of escape design is more appropriate than the use of generic distances and allows full quantification of the risks posed by a particular design.
Magna, the new science and adventure centre recently awarded the RIBA Stirling Prize, is housed within the vast former Templeborough Steelworks in Rotherham. These ‘supersheds’ are 350m long and rise to a height of 40m in parts. The concept and design for this colossal space was developed by Wilkinson Eyre Architects, who used this backdrop to create a unique experience where visitors explore four exhibition areas based on the elements, and created by exhibition designers Event Communications. Throughout, the emphasis has been on bringing art and science together, Magna makes science fun and appealing.
Buro Happold’s Fire Engineering Design and Risk Assessment Group (FEDRA) was responsible for the development of the fire safety strategy for this unique building, where visitors move around the exhibitions on high level walkways, which serve to direct movement and offer protection. The rigid and generic application of contemporary fire safety guidance such as that contained within Approved Document B to the Building Regulations and the British Standard BS5588 series would have been restrictive in the realisation of the client and architectural aspirations for the project. FEDRA appreciated that the solution was the fire safety engineering approach to fire safety design.
Developing the fire strategy
Buro Happold FEDRA worked closely with the Wilkinson Eyre architectural team and the project design team to develop a cohesive fire strategy. This involved early meetings with officers from both Rotherham Borough Council Building Control and South Yorkshire Fire and Rescue Service. In fire safety terms the building is seen as five main buildings within a large weather protected envelope. The distinct areas are the four exhibition Pavilions and the main entrance and adjacent exhibition hall.
The fire safety strategy was developed in such a way that the design of each of the areas could be progressed independently of the others. Each space formed its own specific “fire zone”. Although the means of escape from the Pavilions relied upon escape through the main sheds, the locations of alternative exit routes were positioned to limit the time required to travel through this area. The means of escape was designed to accommodate a simultaneous evacuation of each area.
Fire and smoke modelling
One of the most important considerations in the design was the need to ensure that the environment within the main shed remained tenable throughout the escape period. FEDRA assessed the risks that would exist and determined a number of fire scenarios. Initial zone model calculations were undertaken for each of these to establish the worst possible fire scenario. In the event of a fire in the Earth Pavilion the occupants at the most immediate risk were those on the suspended walkways and in the Air Pavilion. This scenario would set the parameters for the smoke control design.
A further challenge for FEDRA was the series of rooflights located within the lid of the Earth Pavilion. Some of the rooflights were located below the footprint of the Air Pavilion, suspended some 10m above. The analyses undertaken needed to consider a fire developing within the Earth Pavilion and the risk this posed to occupants of the Air Pavilion above.
Fire assessments
FEDRA used a series of engineering calculations and computer modelling techniques to determine the Available Safe Egress Time (ASET) for occupants of the Air Pavilion in the event of a fire in the Earth Pavilion below. It is crucial that the ASET is greater than the actual time required by the occupants to vacate the building.
FEDRA undertook calculations and assessments to enable appropriate predictions of
the growth of fire in the Earth Pavilion as a function of time
the time at which the Earth Pavilion glazing failed
the time at which the Air Pavilion envelope would be destroyed
the time at which the escape routes from the Air Pavilion would become untenable for escape.
Engineering calculations enable predictions of the height of flames from various fires along with the rate at which the fire is likely to spread through a room or space. The volume of fire effluent products, smoke and hot gases, can also be predicted, along with temperatures and toxicity.
Smoke assessment
Magna’s overall internal volume provided a substantial smoke reservoir. Large volume spaces can often provide additional factors of safety for the fire engineer due to the fact that:
in the event of a fire it would take an extended time to fill with smoke compared to a smaller volume.
fire loads within exhibition spaces are typically low compared to other buildings
fires in the open air often grow more slowly than in confined enclosures and low ceiling spaces.
Fire engineers have a number of tools to determine the rate and extent of fire development and establish the consequences to inform a design. Fire growth and smoke modelling are frequently used to establish safe designs and value.
FEDRA used zone models to undertake initial assessments of fire growth and smoke movement within Magna. These models are developed from standard equations, such as those found in DD240 and CIBSE Guide E. the models allow predictions of smoke layer depth, smoke temperature, fire temperature, fire size and visibility through smoke
Such models allow rapid sensitivity analysis to be easily carried. However, zone models are limited in that they can often be stretched beyond their limits of applicability in large volume spaces and they take no account of either complex geometry’s or internal or external environmental flows.
As all these factors applied to Magna FEDRA used CFD analysis, which allows the flow of fluids (air) through a volume to be accurately predicted. The models utilise first principles from the fundamental laws of thermo-dynamics. The volume to be modelled is broken down into a series of small cells and can accommodate very complex geometries. CFD analyses require large and powerful computers. FEDRA utilised Buro Happold’s in house Computational and Simulation Analysis team (CoSA) to develop a model for Magna using the CFX CFD program. The resulting model took account of the internal environmental conditions of the sheds and the typical airflow’s and thermal properties that existed both within the main sheds and the Pavilions. These could be informed at early stages in the design from the mechanical and electrical service design, also developed by Buro Happold. A fire in the Earth Pavilion could then be placed within the model. The fire size and growth rates were ascertained from the initial zone model studies.
The results of the CFD analysis were represented graphically, allowing assessment of the effects of a smoke plume breaking out of the Earth Pavilion and its interaction with the Air Pavilion. The CFD model established that smoke remained high within the main sheds and the loss of buoyancy was not a concern; the existing natural ventilation extract louver’s in the roof would be effective in removing smoke and would be assisted by internal environmental flows; the ETFE enclosure of the Air Pavilion would not be destroyed; the use of fire resisting glazing to the Earth Pavilion rooflights was unnecessary; the smoke would render one of the external walkways from the air Pavilion untenable for escape after approximately 20 minutes, making this the ASET.
After establishing the ASET, the overall Escape Time could be determined using fire engineering techniques. In Magna’s case this is the overall time that the occupants within the Air Pavilion would need to reach a place of safety in the event of a fire in the Earth Pavilion below, and is influenced by the occupancy profile. The prediction took full account of the time for the fire to be detected within the Earth Pavilion.
Other factors were taken into account, such as: the time taken for the alarm to be raised once the fire had been detected, either by automatic means or staff intervention; the time taken for the occupants in the Air Pavilion to be aware that a fire is developing in a remote area of the building, which they can neither see, feel nor smell; the time taken before the occupants in the Air Pavilion decide to commence moving towards an exit as a result of their awareness of an impending emergency; the time taken for them to travel along the exit routes out of the Air Pavilion to a safe place. Computer evacuation modelling using building EXODUS was also used to further test the actual travel time within the Pavilion.
Time predictions, based upon current research and historical fire scene data, could be established. These escape periods were factored to build in additional factors of safety and then compared to the established Available Safe Egress Time. Predictions indicated that a maximum escape time of eight minutes was likely. This overall time is less than the determined ASET of 20 minutes.
Following this analysis FEDRA was able to validate that the means of escape was satisfactory for the fire scenarios considered and the appropriate approvals were attained.
Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976 offering civil and structural engineering, mechanical and electrical engineering, quantity surveying, building services and environmental engineering, infrastructure and traffic engineering, geotechnical engineering, facade engineering, fire engineering, computational fluid dynamics analysis, access consultancy, project management, urban design and a range of specialist CAD services.
