All safe escape routes should be protected from the effects of fire by passive measures such as fire compartmented corridors or stairs or active systems such as smoke control systems or a combination of both. Tenable conditions can be defined in terms of visibility and heat.

In terms of visibility limits, the escape routes should maintain a smoke clear layer above eye level of at least 2.5m above floor level or ensure that a minimum visibility of 10m through any smoke is maintained for occupants to see the exits and evacuate without undue hindrance.

In terms of heat, the smoke layer temperature should not exceed 200 degrees Celsius which is equivalent to a radiant heat flux of 2.5 kW.m^-2. Above such temperature, any escape below such hot smoke layer would be deemed untenable.

ii. Smoke extraction in the common corridor or circulation space

Smoke is spilled from the room of fire origin into the circulation spaces for extraction. In this case, the common circulation space serves as a smoke reservoir for the ESCS or SHEVS.

For design purposes, the design fire size may be categorised into:

• Growing fire
• Steady-state fire

It is commonly assumed that the fire stops growing when the sprinklers are activated. The design fire size is estimated to be the size the fire has grown to at the moment of sprinkler activation. For design purposes, the sprinkler activation time may be taken as the time for the activation of the 2^nd ring of sprinklers.

A fire will progress through various stages from ignition to fully developed unless action is taken to suppress or extinguish it. Up to flashover condition, the rate of growth of a fire can be restricted by controlling flame spread, heat release and ease of ignition. However, once flashover has occurred, it is necessary to restrict the spread of fire by containment. Such containment of fire is known as compartmentation.

The ability of sprinklers to control fires means that the sprinklers have been used to justify the increase of the size of compartmentation and / or reducing the fire resistance requirements of the enclosing boundaries. Prescriptively, compartment sizes have been allowed to be doubled with the provision of sprinklers. However, for fire safety engineering solution, the effective relationship between compartmentation and sprinklers should be developed accordingly.

In general, the use of a fixed fire size is deemed as the conservative solution (usually adopted in prescriptive fire codes). The design fire sizes can be determined by the active fire protection system that is installed.

(1) If automatic sprinkler system is installed, fire size can be determined based on 2^nd ring of sprinkler activation

(2) If alternative water suppression systems are installed in lieu of sprinklers, fire size can be determined based on the following:

• Pre-action sprinklers
• Deluge system
• Water monitor system

Widely used computer programs known as DETACT-T2 and FPETool are able to predict sprinkler response times and corresponding fire sizes at sprinkler operation by combining the effects of fire growth, sprinkler location and sprinkler sensitivity.

Prescriptive codes require minimum exit widths and maximum travel distances with no mention of the time required to escape. In fact, during a fire scenario, the escape process is very time-related. For any escape design to be successful, the time available for evacuation before untenable conditions occur must be greater than the minimum time required for escape. In other words, ASET > RSET

ASET : Available Safe Escape Time [the time from fire ignition to start of untenable conditions]

RSET : Required Safe Escape Time [the time upon fire ignition till complete evacuation of the occupants to a place of safety]

The general methodology to estimate the total required safe escape time (RSET) is illustrated in Fig.1.

Very often, when a window breaks in a fire compartment during a fire situation, the effects of the wind can cause unpredictable and undesirable smoke movement in the building. For example, if the window on the windward side of the building breaks, the positive wind pressure can drive the smoke backwards into the building throughout the fire floor and even to the other floors, causing smoke logging and obstruction for the occupants to escape.

Therefore, wind analysis and studies are essential to ensure that the smoke control designs are functional and reliable under the effects of wind overpressures.

In any case that the adverse wind pressures cannot be overcome, a mechanical smoke extraction system may have to be used.