Thinking big for fire safety
Fire safety systems are an essential part of any building, but the hazards and risks associated with high-rise buildings in particular must be addressed in a pragmatic way throughout the design process. Dean McGrail, associate director, WSP Fire talks to MEP Middle East about the issues and possible solutions.
When assessing best practice in implementing fire suppression systems ultimately safety is the first priority. Some of the issues which need to be considered include: the type of evacuation plan that should be enforced in case of an emergency; smoke control measures; facilities that are incorporated into the building to aid fire-fighting operations; and ensuring that the structure is of sufficient fire resistance to withstand the effects of a fire.
There are various different factors involved in ensuring that people can get out of a building in a safe and efficient manner. There are a number of fire and life safety codes and regulations that are enforced depending on the area of the world in which the building is being constructed, such as the National Fire Protection Association (NFPA), International Building Code (IBC), British Standards (BS) and local Civil Defence Codes.
These fundamentally have the same principles and the vast majority include a performance-based design option. You don't actually have to follow the prescriptive guidance given, instead a performance-based solution can be used, and in a lot of the iconic buildings currently under construction a performance-based solution is a fundamental aspect of the design.
Before deciding what fire systems to include in a tall building it must first be determined: what is a tall building? It's interesting to note that a lot of the codes define tall buildings as anything over 23m in height. The NFPA 5000, for example, states that a tall building is: 'a building where the floor of an occupiable storey is greater than 23m above the lowest level of fire department vehicle access'. We at WSP Fire think of tall buildings as those from 80-450m; super tall buildings as being up to 750m; and those above 750m as hyper-tall buildings.
With towers growing in height, in the future there will be buildings that are 1000m plus. Can we really apply codes that are applicable to buildings of 23m to something that is potentially 1,000m? Whilst the codes form a good design basis, we should not rely on them implicitly to design the fire services for tall buildings in the same way as we would use them for a building that is 23m tall.
There are other issues that must be considered when designing and installing fire systems in a tall building. These include the environmental challenges and the building use. But is there an inherent fire risk associated by a building being tall? In reality high-rise buildings have traditionally been significantly safer than their low-rise equivalents. But with tall buildings, the combination of height and fire can potentially be very costly.
If a fire occurs in a low-rise building the potential losses are lower; when something goes wrong in a high-rise building there's the potential to lose five or six storeys, or even the entire building.
This means that we cannot be complacent. We must understand fire and how it impacts on people and buildings so that we can safeguard against human and commercial losses. By studying and understanding the causes, modes of fire, smoke spread and impact of fires that have occurred around the world we can learn from them and incorporate systems effectively in buildings that detect fire, protect the property and, most importantly, protect the lives of the building occupants and firefighters.
Likewise, to protect against fire in a tall building we need to understand the elements at work, such as longer evacuation times; the reliance on phased evacuation; access; the potential impact from vertical fire spread; and the cost implications of services integration.
The construction phase of a building is one of the most dangerous in terms of a fire event occurring and its potential impact. The fire protection systems are often not installed or not commissioned, escape stairs may be blocked and/or unenclosed, systems incorporated to prevent smoke spread up the stair shafts may not be operational. Add to that the storage and use of flammable materials together with hot works and you have all the components necessary to cause the ignition of a fire, with little or no means of controlling its development or the spread of smoke.
It is not yet a legal requirement in the UAE to look at fire safety during the construction period. Construction phase fire safety is also not included in the prescriptive life safety codes, but this must be addressed both by building designers and legislators to reduce the possible occurence and subsequent impact of fires.
Training and management are imperative to reduce the probability of a fire occurring, whilst ensuring that people are aware of how to raise the alarm and evacuate the building. Fire engineering comes into place during construction as well as after completion. But what is fire engineering?
Approved Document B of the UK Building Regulations provides the following definition: "Fire safety engineering is a recognised method of achieving adequate fire safety in a building. It takes into account the entire fire safety engineering package and is sometimes the only viable method of achieving a satisfactory standard of fire safety in large or complex buildings."
Basically, a tall building can be considered as a complex building. But applying prescriptive codes may not necessarily result in the best solution. You need to take into account all the fire safety features included in the building such as fire detection systems, sprinklers, compartmentation, and the hazards and risks should be addressed on a case-by-case basis according to their design and use.
Fundamentally fire engineering is about understanding how a fire and smoke spreads during the key stages of construction, plus the response of people and structures to fire. While statistically fire alarms are installed at various times during the building's construction, a key issue at all stages is the ability to ensure that people can safely evacuate the building.
Traditionally the means of escape is down to the design of the stair cores and a phased evacuation plan for tall buildings which are complete. Generally the evacuation will be restricted to the fire floor and the floor above initially, while the fire's development is controlled by means of sprinklers and compartmentation. An extreme event may affect the whole building and is a completely different scenario to a contained fire; in this event a phased evacuation may not be feasible.
Also, as buildings become taller, a more effective way to evacuate people quickly and safely must be found. The next stage of evacuation proposes the use of elevators. They've improved in technology and, under British Standards, firefighters use elevators to reach the upper levels of buildings. Firefighting elevators are well protected, well insulated and have a back-up power supply connection available, so why not use them as we would when there is no fire?
By using elevators for evacuation you remove the need for separate systems for disabled and able-bodied people. The new double-decker lifts also help in terms of their capacity to evacuate tall buildings successfully. We need to change the culture of non-use that people have been previously asked to follow and develop the standard for lifts to allow their use during fires as standard.
We need to look at each building and increase fire protection where it is needed. By using latent technology and fire suppression rather than traditional extinguishing methods, the added benefit of removing the fire problem is gained. You can include such systems in the MEP design on every single floor and create different levels of redundancy throughout the building if required.
- the type of evacuation plan that should be enforced in the event of an emergency;
- smoke control measures;
- what facilities to include to assist fire-fighting operations;
- the fire resistance of the structure should be sufficient to withstand a fire.