Published: 05 April, 2011
A safe outcome in High-rise response relies heavily on the fire resisting performance of interior compartments in tall buildings, writes Paul Grimwood, Principal Fire Safety Advisor for Kent Fire and Rescue Service.
We are constantly reminded just how difficult and dangerous fighting a fire in a high-rise building can be, and over the past decade there have been several serious fires in high-rise buildings. But when analysed statistically, the risks of death, injury and property damage may appear lower in high-rise buildings than in other buildings of the same occupancy type.
However, it is becoming clear that the time needed to establish a safe and effective firefighting intervention on the upper levels of a tall building places great reliance on the fire resisting performance of interior compartments. After the fire has been located and the forward command bridgehead is being established in a safe location – 2-3 floors below the fire floor– any building occupants that may be at immediate risk by subsequent firefighting actions must first be evacuated to safety before intervention occurs.
These are time-dependant operations that require adequate resources based on a critical-task analysis. The numbers of firefighters needed to establish the basic evacuation of primary risk zones; confine the fire; and implement an intervention are wholly dependent on the levels of horizontal and vertical fire resistance within the structure – and any fire suppression systems that will limit fire spread.
Most recently, several high-rise fires have demonstrated how high fire loads, coupled with a trend to provide greater expanses of glazed outer walls, can lead to faster growing fires and vertical exterior fire spread that causes immense problems for firefighters working inside the building. When this occurs, the provision of a high standard of interior fire resisting compartmentalisation is absolutely crucial to firefighter and occupant safety. In such cases the primary tactical objective may be to contain the fire as much as possible, within fire resisting sections to a controlled percentage of the building. If fire has spread beyond the floor of origin, the overriding objective is to maintain escape routes for occupants clear of smoke where resources, staffing or available flow-rate at upper levels are compromised by such rapid fire growth.
Fire resisting compartments and/or sprinklers
Where fire resisting compartmentalisation is reduced or removed from key areas within tall buildings (such as that provided in occupant escape or firefighter access routes), there should be adequate compensatory provisions in the form of sprinklers to counter the risk of unconfined fire spread. Without such provisions in both residential and commercial high-rise then the potential for trapped occupants and multi-floor fires is clearly an increasing threat.
q In a five-room 70m2 apartment fire the ventilation controlled heat release throughout the space can reach 15 MW within less than a minute following a single room flashover.
q The spread of fire in large open-plan commercial floors is primarily dependent on fire load and ventilation. The quantity of water needed to deal with such fire spread is easily calculated using established engineering principles. In general, water flow-rates of at least 5 lpm per m2 of fire involvement are needed for effective suppression. This means a 500 lpm hose-line can, in general, deal with 100 m2 of fire during steady growth or steady state burning rates (not necessarily during the flashover stage), or a 1,000 lpm hose-line up to 200 m2 of fire.
q In an open-plan floor area containing office work-stations, a fire can develop so rapidly that it will be beyond the control of a firefighter’s hose-line within 8 to 10 minutes of surpassing the incipient stage of fire development.
q Commercial floor space should ideally be protected by sprinkler provisions but where in excess of 100 m2 this should be legislated. An alternative approach might suggest internal compartmentalisation providing 60-minute FR to >100m2 compartments, which may compensate where sprinklers are not provided.
q Where corridor sub-dividing smoke or fire doors are removed to enhance the effects of smoke ventilation systems, and where alternative exits and stairs are non-existent due to reliance on such systems, any rapid spread of fire and smoke can soon cut occupants off from their only means of egress and make it very difficult for firefighters to reach them internally.
High-rise firefighting tactics
When faced with a fire on an upper level in a high-rise building it is important that the fire service promptly responds with adequate staffing, resources, and weight of attack. It is equally as important that an effective chain of command with clear communication channels between each sector and any location is established at key points, namely marshalling; lobby; staging; bridgehead (forward command); and search commands. Beyond these assignments, key functional and support officers will be needed to support ongoing operations.
The initial tactical objectives will be to carry out reconnaissance in order to locate the fire and stabilise the fire floor by taking control and clearing occupants from the most immediate area of risk. This may also entail evacuating any occupants from the area immediately above the fire (primary risk zones).
From this point the incident commander must assess the situation and decide on the most appropriate tactical approach to be taken:
Tactical-decision-tool – ICE
It can be seen (Figure 1) that following the reconnaissance and assessment stage there are three main tactical options open to the incident commander. The decision-tree approach should be based upon the stage of fire development, construction, location and status of known remaining occupants, resources available, likelihood of success and the existence of compartmentalisation and/or sprinklers. It is important that safe systems of work are utilised at all times.
In confined fire situations the intervention option may appear the primary solution. However, it is critical that before any decision is made to open the fire compartment or involved fire area, that the effect of wind direction and velocity is taken into account. If a wind of any great extent (>5 mph at 10m) is hitting the face of the building where the fire compartment is situated, then consideration must be given to delaying, or restricting/preventing an entry for intervention purposes. An exterior wind – likely to become stronger at heights greater than 10 metres – may create some rapid fire development that will be totally unmanageable at the fire floor. Recent research by NIST in the USA demonstrated that untenable conditions for firefighters were created where exterior winds pushed fire directly at firefighters in such circumstances. In effect, unless they were able to find some immediate point of refuge, firefighters remaining in the air-flow path filled by the blow-torching of flames would be rapidly overcome by the heat and fail to escape. Even temperatures in the corridor serving the apartments were untenable to firefighters during NIST tests.
Therefore, unless there is an immediate and over-powering requirement for an intervention to locate and rescue confirmed saveable life, the tactical priority should be that of evacuation of primary risk zones, or possibly containment.
Where a rapid intervention for saveable life is undertaken, it is crucial that local procedure is followed, based on clear guidance or directives. The UK National GRA 3.2 for high-rise building fires provides guidance that compartment fire entry should be implemented with at least one charged hose-line as protection. It goes on to advise that where warning signs of rapid fire development exist, a secondary back-up hose-line should be provided prior to opening the fire compartment.
The evacuation of the primary risk zones is an early tactical solution that will ensure – where fire does spread out (either externally or internally) from the compartment of origin – that all occupants have been removed from adjacent zones. These areas include all apartments/compartments that are not protected by corridor sub dividing doors and the apartment/compartment directly above the involved fire area.
Corridor sub divisions that have been removed to enhance ventilation of the corridors via smoke shafts, and secondary exit stairs that have been removed from the prescriptive requirement, may create long corridor runs with apartment doors either side that require evacuation. This can be a time-dependant, resource-hungry operation. Where sub-dividing doors are provided in the corridors this may halve or even quarter the size of the primary evacuation zone and allow firefighters to rapidly evacuate much smaller areas with less resources deployed.
In situations where single exit stairs may become compromised by smoke as firefighters open up the fire compartment, the evacuation of the entire building above the fire floor, and possibly below it, may need to be prioritised. This will depend on a careful assessment of risk, based on anticipated fire conditions, exterior wind conditions and building layout. Where a code compliant sprinkler system is installed then any fire development will be limited in size, although toxic fire gases may still be emitted prior to firefighters completing suppression.
Two or more exit stairs may provide alternative escape options for occupants but without effective fire separation in the form of corridor sub dividing doors, these alternative exits may become unusable if large quantities of smoke and heat were to transport into the corridor.
Secondary risk zones are areas where occupants may not be at immediate risk but this could change and such areas may become primary risk zones at a later stage. For example, escape stairs can become compromised by smoke and heat, necessitating top to bottom searches by firefighters where occupants might remain in the stairs (for whatever reason). A fire in a 37-storey office tower in Chicago in 2003 saw five occupants lose their lives when they became trapped in the stairs. In some situations smoke can fill a stair shaft top to bottom, even below the fire floor, where air-flows into the stairs are driven by external wind effects.
Total building evacuation (either before or after intervention) occurs may become an option in producing the best outcome and should always be a tactical consideration. However, key features in the building design will play a part in how such an option may be undertaken safely and effectively.
Where a fire is threatening to spread out from the compartment of origin to affect the floor above then an early containment option may provide the best outcome. It should be remembered that early access to the area above the fire may have been achieved during any evacuation of primary risk zones. If this is the case then the deployment of a charged hose-line onto the floor above the fire and into the compartment directly above the fire compartment may be a tactical consideration.
In this situation it is critical to ensure that the fire remains contained and that no attempt is made to open and enter the fire compartment for intervention. The tactical objective is to prevent or halt the vertical spread of fire up the face of the building and with a well-placed hose-line above the fire floor, this is very achievable.
Kent Fire & Rescue Service (UK) has developed some innovative high-rise firefighting procedures and ICS 2/3 training programmes following two very bad fires in 2001. The basic model for these tactical approaches has been adopted by nine fire and rescue services in the SE CFOA region. However the ability of firefighters to operate effectively against fast-spreading fires on upper floors of tall buildings is directly dependent on adequate compartmentalisation, particularly in escape corridors, maximum sized fire resisting compartments to >100m2 and/or sprinklers in all high-rise over 18m.