Steven Morgan discusses how a strategic approach to fire engineering can meet duty of care, while still achieving the architect’s vision
A BUILDING’s fire strategy could be complicated by the characteristics or complexity of the design. From a fire safety design perspective, there may be a number of reasons why a building may require a more intricate approach, from the size of the building and number or type of occupants to the layout or surroundings of the property.
Regardless of the challenges, every building needs to meet the required level of safety and comply with the standards and regulations. However, for many large or complex buildings, a strict application of Approved Document B (ADB) of the Building Regulations, or what is referred to as a ‘code compliant solution’, will not be possible.
This is because the guidelines concerning elements such as travel distances and escape routes are based on the assumption that a set of standard solutions will be used. For example, the smoke control solution recommended in ADB is a natural ventilation system. This is relatively simple and cost effective to implement, but relies on the natural movement of air to extract the smoke.
The alternative to a completely code compliant solution is to apply a degree of fire engineering. This requires specialist fire engineers to look at the proposed plans for the building to identify where adjustments to the standard strategy can be made, to create a tailored plan. This allows the internal layout to be optimised and provides much greater design flexibility.
Taking smoke ventilation as an example, selecting a mechanical smoke ventilation system (MSVS) with a greater capacity and efficiency can allow for longer travel distances and, in some instances, the removal of staircases other than those recommended in the regulations while maintaining or even improving safety. ADB also references these systems as part of an engineered approach, pointing to BS EN 12101-6 for guidance.
The internal layout and the design intention of the building will have an effect on how complex the fire requirements are. For example, increasing the amount of natural light within a building is progressively becoming a focus in modern building design. It has been demonstrated to improve the productivity and lower the absence rates of office workers.
In schools, increased levels of natural light have been shown to improve learning, and for hospitals, research suggests this can speed up patient recovery. However, to achieve this, especially in internal areas of the buildings, it is often necessary to create large, open plan spaces and openings between storeys.
The issue with open plan spaces is that in the event of a fire, the flames and smoke can spread more quickly than in a smaller area where they can be contained. The standard approach to building design under the building regulations is to create compartmentation within the building to prevent this.
However, the correct use of fire suppression systems (such as sprinklers) within the space can provide a solution to help stop the spread of the fire, and make the larger spaces safe. Early detection of a fire is also crucial to minimise the risk to occupants, so often an aspirating smoke detection system will be specified to achieve this. This fire engineered solution to the problem has been applied to a range of different building types, from offices and retail environments, to open plan flats and residences.
Several types of fire detection are available. Certain types of buildings may be complicated by the specific requirements for their usage.
An example of this is airports, where the air side and land side division in the building needs to be considered when assessing the fire safety requirements. If a terminal building had to be completely evacuated due to a fire in the land side area, this would cause serious disruption to the departure of flights.
In a full scale evacuation, the pre and post security passengers need to be kept separate, otherwise those who had already been cleared by security would need to be rechecked when returning to the area. As a result, it is far more effective for people in unaffected areas to remain where they are. For this to be done safely, a robust compartmentation strategy must be in place to prevent the spread of smoke and fire.
Also influencing the fire safety requirements of a building are how the building will be used and by whom. Hospitals, which by their nature are often large buildings with a complex layout, also have the added complication that many of the occupants have limited mobility. This consideration will dictate much of the fire strategy, and makes direct evacuation in the event of a fire impractical.
Therefore, providing safe areas within the building for patients to be moved into is the most common approach. This is referred to as a progressive horizontal evacuation (PHE), and it involves subdividing any patient care areas into protected sections that are separated from each other by fire resistant walls and doors.
The aim of any PHE plan is to quickly remove the occupants from immediate danger to an area from which they can then be evacuated without immediate time pressure. Furthermore, if the nature of the building means that the occupants are likely to be relatively unfamiliar with the layout and escape routes, this needs to be accounted for.
Included in this are public buildings and those with multiple uses, such as educational organisations whose facilities are also used by the general public. In these situations, it is vital to ensure that the evacuation routes are clear of smoke, and install detection systems that provide early warning when a fire breaks out.
When refurbishing an existing building, the fire strategy may be complicated by restrictions caused by the original layout or features. This is particularly common in older buildings that were constructed with lesser standards of fire safety. Similarly, the renovation of historical buildings could be further complicated by legal restrictions if the building in question has a listed status.
Regardless of the age, any restoration or updates to the building will probably need to comply with all modern standards, so the design process must consider how the original features and aesthetics can be protected, while also delivering complete safety. Some of the common issues faced with these types of building include fewer, or narrower staircases than recommended, longer travel distances within the building and lack of sufficient compartmentation or refuge areas.
Case study – 80 Pall Mall, London
WHEN THE 17th century building at 80 Pall Mall was renovated, FDS Consult was brought in to provide guidance on a fire safety strategy. Originally built in the 1690s as the residence of the third Duke of Schomberg, the building was converted into offices in the 1950s.
The latest renovation included a plan to introduce glass fronted meeting rooms around the periphery of the ground and first floors, leading to an open atrium with a central open staircase.
Other challenges of the building’s original layout included an original staircase that was narrower than the regulations stipulate, and a light well on one side of the structure which did not meet current standards. To overcome these challenges, the consultant team recommended introducing fire curtains linked to the smoke detection and fire alarm system to ensure compartmention of the offices in the event of a fire.
They also analysed and calculated the evacuation times in order to demonstrate that the number of occupants could safely leave with minimal delay, despite the narrower staircase.
Fire service access
One further factor that needs to be considered for large or complex buildings is access by the fire services. Effective smoke control is vital to ensure that firefighters can easily progress through the building. For this reason, mechanical smoke ventilation systems will be recommended to provide a higher level of smoke clearance.
Another issue when fighting a fire in a large building is access to water. Running the charged hoses through the building or across multiple storeys is often impractical. An effective solution to this issue is dry or wet risers – a system of pipework within the building to provide access to water. These are either filled with water (wet risers), or left dry for the fire service to connect the water supply. These are mandatory in buildings over 18m, with wet risers mandatory for buildings over 50m.
Testing a solution
Finally, an important part of any fire engineered solution is demonstrating how the proposed systems will work in practice. Techniques such as computational fluid dynamics (CFD) modelling allow the systems to be tested virtually to ensure they achieve the required level of safety.
Using CFD modelling, it is possible to simulate the fluid dynamics (movement) of the smoke and fire within a digital model of the building over a set time period. This allows the physical behaviours, such as smoke rising, to be observed as it unfolds. It shows how the proposed layout and features of the building will affect the spread of the smoke, enabling the creation of an informed fire safety strategy and then evaluation of how it will work in practice.
The computer model also highlights any areas of concern, such as where smoke is hotter or denser, or where fire may spread more quickly. In addition to this, evacuation modelling and time equivalence modelling allow the simulation of an evacuation to demonstrate that the proposed layout allows the occupants to leave in a timely and safe manner.
A code compliant approach to fire strategy is often restrictive or impractical, especially for buildings that are complex either by their design, intended use or inherent characteristics. However, engaging with experienced fire design consultants early in the process will allow a bespoke, engineered solution to be formulated and tested to meet both the required level of fire safety and the needs of the occupants.
Steven Morgan is associate director of FDS Consult. For more information, view page 5