FIRE ENGINEERING was used in the design of a three-storey school building with an atrium, to achieve a high standard of fire safety without compromising on architectural and functional design.

The main fire safety issue at the school – situated in the town of Banagher, County Offaly – is that escape from the classrooms on the upper levels is only possible using a balcony. UK guidance set out in the Department for Children, Schools and Families publication, Building Bulletin 100: Designing for fire safety in schools (BB 100), would permit this arrangement with the provision of sprinklers. However, in Ireland, it is often not necessary to follow the recommendations for sprinklers in BB 100 because the risk of arson in the country’s schools is not as serious as in the UK. In this case, an alternative fire engineering design was therefore adopted.

Building design

 

The building is a large, three-storey school with a projected enrolment for 550 pupils. It is part of a project involving a total of four schools, being developed by MacQuarie Partnership for Ireland in conjunction with Pierse Contracting. The architects are A&D Wejchert and Partners, Dublin. Currently under construction, the school is due to open in September 2010.

The main architectural feature of the building is a 100m long central atrium linking ground, first and second floors. At ground floor, the classrooms open onto the atrium base. On the upper levels, the classrooms are accessed from balconies within the atrium. There are various bridge links connecting either side of the atrium. The school is an example of a modern, energy-efficient building, with the inclusion of the atrium allowing for efficient lighting, heating and ventilation.

Guidance documents

 

The Technical Guidance Document Part B (TGD B) is the equivalent guidance document in Ireland to Approved Document B in England and Wales, and sets out recommendations to meet Building Regulations. TGD B does not contain specific guidance for schools, so it is also normal practice in Ireland to refer to guidance in BB 100, the UK guidance document specific to schools.

Many of the recommendations in BB 100 have been written to address the serious arson problem in the UK. However, this problem is not as serious in Ireland and therefore some of the recommendations are not appropriate, for example, those relating to sprinklers and compartment floors.

The Building Regulations in Ireland are functional and permit alternative fire engineering approaches to be adopted. As in other jurisdictions, a fire engineering solution needs to demonstrate that the standard of safety is at least as good as a theoretical building designed strictly in line with the prescriptive codes. The design of the fire engineering solution developed for the school in Banagher is based on the principles of BS 7974: Application of fire safety engineering principles to the design of buildings.

Balcony escape

 

The main issue of non-compliance in the school was escape solely along the atrium balconies. BB 100 recommends that in an unsprinklered school, where there is a void connecting floor levels, escape in the event of fire should not be towards or within 4.5m of the void edge (TGD B recommends 5m). Since the only means of reaching the protected stairs at upper levels is along the atrium balconies, this is clearly not achieved.

The inherent benefit of a large volume space, such as an atrium, is that it will take substantially longer for the space to fill with smoke when compared to a low-ceiling, single-storey corridor. Smoke rising through the atrium space will also mix with the air in the atrium, making it cooler, more dilute and less toxic. Visibility through the smoke will also be better when compared to a conventional low-ceiling corridor.

 

Natural smoke venting

 

Following discussions with the local authorities, it was agreed that the fire strategy should ensure that travel distances on the balconies beneath a smoke layer should be limited to 18m. Travel distances from all the classrooms to a storey exit are within the 18m on the balconies. However, there are offices at first and second floors, where travel distances are up to 28m.

To address this, a natural smoke venting system was proposed for the atrium to ensure that, in the event of fire, a smoke-clear layer on the second-floor balcony can be maintained. This will divide the atrium roof into two smoke reservoirs. Careful location of smoke channelling screens beneath the balconies at ground and first-floor level will direct smoke into one reservoir. The location of the smoke reservoir screen ensures that travel distances from the offices beneath the smoke layer do not exceed 18m.

Further to this, the main fire load in a school is located in the classrooms, rather than the circulation spaces, which will generally be kept clear for pupils to circulate though. The classrooms at ground and first-floor levels are therefore fire-separated from the atrium. This measure will contain the fire within the room of origin and limit the potential for smoke flow into the atrium.

In addition to smoke venting system and containment, enhanced fire detection and alarm is also employed. When considering the evacuation of a building, travel distances to protected escape routes are limited so that occupants can reach a place of safety in a reasonable period of time. However, prescriptive codes do not consider the time taken for occupants to be alerted to a fire and begin moving towards the exits. This period can make up as much as two-thirds of the total evacuation time. To reduce the pre-movement time of the evacuation, a full voice alarm system, compliant with BS 5839-8: Fire detection and fire alarm systems for buildings. Code of practice for the design, installation, commissioning and maintenance of voice alarm systems, was also adopted.

 

Computational fluid dynamics

 

A computational fluid dynamics (CFD) study was also carried out using Fire Dynamics Simulator (FDS) – a model of fire-driven fluid flow – to validate the design and provide a visual representation of various fire scenarios in the school. Figure 1 shows a three-dimensional image of the model in the Pyrosim window.

The CFD analysis examined several deterministic fire scenarios, including a comparative analysis, to compare conditions in the proposed building to a theoretical code-compliant school building with traditional low-ceiling corridors.

As discussed, the fire-resisting enclosure to the accommodation on the ground and first floors should prevent fire and smoke from spreading into the atrium void. However, a sensitivity analysis was applied to look at the situation assuming a fire door was propped open. Also, while circulation needs dictated that the potential for a fire load on the majority of the atrium base was limited, in some areas small displays, such as Christmas trees, could be located.

Two examples of the fire scenarios modelled were:

  • classroom fire with door propped open at ground floor
  • a Christmas tree fire within the atrium base

The Christmas tree fire scenario involved a dry, medium-sized Nordic pine tree, and had an overall burn time of 75 seconds. The fire reached a peak heat release rate of just over 1,817kW at 35 seconds. This rate of heat release is recommended by BS 7974 (PD 1: Initiation and development of fire within the enclosure of origin). The analysis showed that, despite the very rapid fire growth, conditions on the upper levels were not adversely affected, primarily due to large amounts of air entrained into the rising smoke plume, cooling and diluting it.

In all the fire scenarios modelled, a smoke-clear layer was maintained on the second floor. It can be seen in the screenshots in Figures 2 and 3 that the smoke is well retained within a single reservoir, so that travel distance to a smoke-clear area beneath the adjacent reservoir is within 18m. In addition to temperature and carbon monoxide concentrations, visibility levels on the upper floors were also assessed.

Summary

 

The inclusion of voids and atria in schools is becoming more common in the UK and Ireland, particularly with the growth in new school projects under the Government’s Academy and Building Schools for the Future programmes. However, the inclusion of an atrium often causes unwarranted concern for the safety of occupants that is rarely justified. While Banagher is in Ireland and BB 100 does not have the same weight there, this case study shows there are fire engineering solutions that can deal with concerns and ensure a safe building, even when BB 100 recommendations are not met

 

Luke Goldsmith is a senior engineer in Jeremy Gardner Associates, Dublin office