Understanding the stay put or defend-in-place evacuation strategy is crucial to understanding fire safety in purpose built blocks of flats in the UK. This post explains, what a defend in place evacuation strategy is, what is necessary for the strategy to work, and the circumstances in which a change in strategy might be necessary.
Defend in Place
Defend in place has been a central component of the design of apartment blocks since the post-war reconstruction era. The concept is quite simple, buildings are designed so that each compartment, or residential unit is designed to contain a fire within the compartment of origin, and thereby away from the means of escape and other residential units for at least 30 minutes. The residential units are provided with individual detection and alarm systems, the common areas of the building are not. In the event of a fire, the occupants in the compartment or apartment of original evacuate the building, closing the doors behind them, raise the alarm, and the fire and rescue service attend, extinguishing the fire before it can reach other parts of the building or compromise the means of escape.
Why Defend in Place?
So thats the theory behind defend in place, but what is the rationale behind it? The answer is really quite simple, and lies in the design of the buildings themselves. The dominance of single stair core designs for apartment blocks or blocks of flats in the UK presents a conflict in the event of fire between the need for firefighters to scale the staircase with their equipment, and for residents to evacuation down it. A defend-in place strategy resolves that conflict by holding residents unaffected by fire in their apartments, and reserving the staircase primarily for firefighters to reach the areas of the building affected by fire.
What is needed for a defend in place strategy to work?
For a defend in place evacuation strategy to work, the compartmentation to the building needs to be adequate. There are two components to that - the compartmentation needs to be adequately maintained, and resident behaviour conducive, to prevent the spread of heat, fire and smoke earlier than intended.
My earlier article on compartmentation covered off what compartmentation is, and how the compartmentation to a building can be compromised through the life of a building. So we will not cover that ground here other than reminding of the need for service penetrations to be appropriately fire-stopped by competent contractors when works take place, and that effective periodic inspection regimes for the passive protection, fire stopping and fire doors within a building should ensure that compartmentation should remain adequate to support a defend in place strategy.
External Wall Systems
Another way in which the compartmentation to the building can be compromised, rendering the defend in place strategy ineffective, is where an external wall system has been fitted to the exterior of the building. If materials with the wall system are combustible, and can be ignited from sources of heat within the apartment, then fire may reach other compartments sooner than anticipated through the external wall system and window apertures. The fires at Grenfell Tower, and before that Lakanal House show this in practice.
A building can have the best compartmentation in the world, but human behaviour can override it too. The defend in place strategy depends upon retaining fire in the compartment of original, and maintaining the escape stair clear of heat and smoke.
Resident behaviour can impact the effectiveness of the compartmentation strategy in a couple of ways:
Fire doors need to be maintained effectively self closing at all times. In some scenarios doors may be held open on automatic release devices, though that is unlikely in a building with a defend in place evacuation strategy - where these buildings have common detection systems they are usually only used to activate ventilation systems to the means of escape. So holding open fire doors is significant in two ways. First, it damages doors and self-closers impacting the efficacy of the doors in the event of fire. Second - fire doors wedged open at the time of fire may allow fire heat and smoke to penetrate the means of escape far earlier than intended - hindering firefighting, and reducing the window for evacuation.
The same principles apply to the apartment front doors. Where self closers have been removed, doors will remain open unless manually closed, and closing front doors in evacuating buildings is the least of the residents' worries. So ensuring that the door furniture is maintained, and that doors will self-close automatically in the event of evacuation is again important to ensuring the ongoing viability of the means of escape.
Change to Simultaneous Evacuation
Occasionally a building will need to transition on a temporary or permanent basis to a simultaneous evacuation strategy because it has been shown that the compartmentation is inadequate to support the defend in place strategy.
This has been a fairly common transition for many high rise accommodation blocks fitted with external wall systems found to contain combustible materials that could support the spread of fire.
We’ll cover simultaneous evacuation in a separate article, though a simultaneous evacuation strategy has prerequisites that buildings designed for defend-in-place do not. In particular simultaneous evacuation requires early detection of fire and warning to other residents.
The National Fire Chiefs Council produced the guidance document Guidance to support a temporary change to a simultaneous evacuation strategy in purpose-built blocks of flats to provide guidance to dutyholders in making the switch.
Given the absence of warning and detection systems, a waking watch is likely necessary to provide early detection and warning in the first instance. We’ll talk about waking watch more in a subsequent article, but a waking watch consists of the manual patrol and surveillance of a building looking for the early signs of fire. Its far from ideal or perfect, and very expensive. For that reason the guidance recommends the installation of temporary or wireless detection and warning systems where the switch is likely to be for a prolonged period.