Note for Guidance

The Fire Safety Advice Centre

Basic Means of escape from Fire

Introduction

The purpose of this page is to allow you to understand the basics of means of escape and not how to design a means of escape from fire. There are many considerations, not covered here, in planning means of escape, but it will give insight in how a means of escapes are designed.

Definition of Means of Escape from Fire

Means of escape from fire is, at best, an inexact science with only a few rules and formulae. However, the following definition is widely acceptable: -

Means of Escape is a structural means, whereby a safe route is provided for persons to escape in case of fire, from any point in a building to a place of safety, clear of the building, without outside assistance

Information required to design a means of escape from Fire

You need to know the following information when designing a means of escape from fire in a premises or building,

• Time of evacuation
• Travel distances
• Number of Occupants
• Calculation of Exit Widths
• Calculation of Minimum Number of Exits

Time of evacuation

Time of evacuation is dependent on on the following factors;

• Building construction
• Occupancy

The construction of buildings are divided into three basic types,

• Class A - complete noncombustible construction, i.e. elements of structure, floors, and walls. Supporting structure of brick or concrete;
• Class B - traditional construction, i.e. noncombustible walls with combustible floors;
• Class C - combustible construction, i.e. timber floors and walls.

Based on these classes arbitrary evacuation times were decided upon and the times that are generally accepted as -;

• Class 'A' construction - 3 minutes
• Class 'B' construction - 2.5 minutes
• Class 'C' construction - 2 minutes

These are not hard and fast times, and can be extended or reduced according to the particular circumstances

Number of Occupants

You must consider the people normally in the building, and the those who may use the building.

Number of Occupants

The number of occupants for an existing building with a reasonably fixed population may be ascertained by questioning the responsible person who owns or occupies the building. For buildings such as theatres or cinemas, the number of seats provided should be counted.

Density Factor

In an unoccupied premises, to calculate the maximum numbers of people permissible in any given occupancy you must refer to density factors. The density factor may be defined as "the available floor space per person" For example, the Offices, Shops and Railway Premises Act 1963 made reference to 1 person per 4 m2 of floor space.

Design codes for new buildings lay down specific density factors, which will vary, dependent upon the intended use of the floor space, i.e. lounge, restaurant, etc, check out the following documants,

• Guide to Fire Precautions in Existing Places of Work that Require a Fire Certificate
• Guide to Fire Precautions in Existing Places of Entertainment and Like Premises
• Guide to Safety at Sports Grounds
• Building Regulations 1991 - Approved Document B
• BS 5588 Part 6: 1991
• BS 5588 Part 8: 1999
• BS 5588 Part 10: 1991
• BS 5588: Part 11: 1997

To ascertain the maximum numbers of people, you need to calculate the floor space, delete the area of permanent features, i.e. stairs, toilets, lifts, escalators, corridors and other circulation spaces. What is left is the usable floor space and this is divided by the density factor giving you the number of person able to occupy that area.

Persons who may use the building

Occasional visitors to the building must be considered and in the case of shops and departmental stores these people will out number the people normally occupying the premises. In the case of retail premises density factors will usually give the numbers other situations a fair estimate must be made.

Calculating the Number of Occupants

Consider a retail premises consisting of a sales floor canteen and offices. As different parts of the building are put to a different uses therefore it is necessary to use different density factors. To calculate the number of people, it is necessary to divide the floor area by the floor space allowed per person.

Number of People = area of room or storey / floor space per person

If density factors for the above example are:

1. Shop floor (main sales) - 2 m2 per person;
2. Offices - 7 m2 per person;
3. Canteen - 1 m2 per person;

and the relevant floor areas are:

1. Shop floor - 20 m x 30 m = 600 m2
2. Offices space - 8 m x 8 m = 64 m2
3. Canteen - 8 m x l0 m = 80 m2.

This is the total floor area - not the usable floor area. It is therefore necessary now to deduct the floor area of the permanent features in the area where they are situated

1. W.C. In shop = 10 m2
2. Stairs in shop = 15 m2 x 2 = 30 m2
3. W.C. In canteen = 10 m2.

1. Shop floor - 600 m2 - 40 m2 = 560 m2
2. Offices space - 64 m2 - 8 m = 64 m2
3. Canteen - 80 m2 x l0 m2 = 70 m2.

Therefore, the total number of people for which escape routes must be designed is,

1. Shop = 560/2 = 280
2. Offices = 64/7 = 9
3. Canteen = 70/1 = 70

TOTAL = 359 people

Therefore the number of exits should, be adequate for 359 people.

Travel Distance

Travel distance is measured from any point in the building to a place of safety (i.e. relative or ultimate). Travel distance will depend on how quickly people will react and make their escape and how long it will be before the fire prevents that movement to the escape route.

Research done after the second world war came up with findings that people in smoke, escaping to a place, clear of smoke traveled 40 feet per minute therefore with an escape times of 2 minutes a person could travel 80 feet, 2.5 minutes 100 feet and 3 minutes 120 feet. It also showed that if the exit allowed one person to Pass to 40 persons could escape in a minute. These figures were accepted for sometime until codes of practice appeared giving various travel distances for deferent occupancies however they do reflect the post war studies.

Places of Relative Safety

It is often necessary to devise a temporary place of safety, such as when evacuating high buildings. This may be defined as a place of comparative safety and includes any place, which puts an effective barrier (normally 30 minutes fire resistance) between the person escaping and the fire. Examples are as follows;

1. A storey exit into a protected stairway or to the lobby of a lobby approach stairway;
2. A door in a compartment wall or separating wall leading to an alternative exit;
3. A door which leads directly to a protected stair or a final exit via a protected corridor..

A staircase, which is enclosed throughout its height by fire resisting structure and doors, can usually be considered to be a place of comparative safety. In these cases, the staircase can be known as "a protected route". However, the degree of protection to staircases to enable them to be considered as a place of comparative safety varies for differing building types, and is normally defined in the relevant codes of practice.

BS 5588: Part 8: gives guidance on means of escape for the disabled and allows the creation of "refuges" in which they can await for assistance in escape.

Place of Ultimate Safety

Ideally this should be in the open air where dispersal, away from the building, can be achieved. Escape routes should never discharge finally into enclosed areas or yards unless the dispersal area is large enough to permit all the occupants to proceed to a safe distance away. Total dispersal in the open air therefore constitutes ultimate safety. When inspecting any building, it is important to always follow the escape route to its ultimate place of safety;

Travel Distance in one direction only

If a room or compartment has two or more exits preferably diametrical opposed, (Escape in more than one direction) then a person escaping can turn their back on the fire. If there is only one exit then a person may have to travel towards the fire (Escape in one direction only) consequently the travel distance is reduced up to 50%

Calculation of Exit Widths

The width of exits required, depends on the number of occupants, rate of flow and the 'flow time' and is expressed by the formula

U = N / (40 x T)

Where: -
U = number of units required;
N = number of occupants;
40 = standard rate of flow - constant;
T = Flow time (i.e. 3 mins for Class 'A', 2.5 mins for Class 'B' and 2 mins for Class 'C')

The resulting number of units may well result in less than a whole number. If any fraction is greater than or equal to 0.3 it should be rounded up.

It should be noted that it is not normal for doors to be supplied in sizes compatible with the width of individual units, e.g. a 750 mm door can only be regarded as 1 unit wide.

Calculation of Minimum Number of Exits

The minimum number of exits depends upon the number of units of exit width required and the maximum size of any particular exit, and is expressed by the formula

E = U / 4 + 1

E = number of exits
U = number of units of exit width (from exit width formula);
4 = size of largest exit permitted.
1 added to ensure there would always be at least one unit.

The result of this formula may well work out as a fraction, and if this fraction is greater than or equal to 0.75, it should be rounded up. Again, the + 1 in this formula is included to ensure that, in the event of the U/4 element being less than 0.75, then at least one exit would be provided to the room.

In most codes of practice, however, it is the norm for 2 exits to be required situations in which rooms are occupied by more than 50 or 60 people, the reason being that the one exit required by the above formula could well - in the worst possible scenario - become blocked by the fire.

Therefore, in addition to the formula, an additional exit equivalent to the largest size should be added; but where this is physically impracticable, the numbers permitted must be reduced.

Example

A traditionally constructed room has been designed to accommodate 710 people. How many units of exit are required, and what is the minimum number of exits.

a) U = N / (40 x T) = 720 / (40 x 2.5) = 710 / 100 = 7.1
This fraction is less than 0.3 therefore it is rounded up to 7 units

(b) E = U / 4 + 1 = 7 / 4 + 1 = 2.75
This fraction is equal to 0.75 therefore it is rounded up to 3 exits

Therefore, the answer is that a minimum of 3 exits should be provided totaling 7 units, e.g. 2 x 2 units each + 1 x 3 units
However, we must consider the possibility of one of these exits being obstructed by the fire, with the worst case being one of the largest being lost. In our example, this would leave us with 2 exits of 2 units each leaving just 4 units to discharge 710 people.

Therefore, keeping the exits as they are, these remaining exits would need to be increased in size to accommodate our 710 people, and in this case we would need to increase each exit to be 4 units wide (i.e. 1950 mm each) in order to ensure the evacuation of our 710 people.

Therefore, by now providing 3 exits of 4 units width each, we have allowed for one exit being blocked but still leave 8 units available for the evacuation, which is now a greater capacity than is required. A further adjustment could allow for one of the units to be a 3-unit exit that would leave us with 7 units for evacuation - which is satisfactory.

We can transpose the formula for the number of units to N = U x 40 x T in order to arrive at the maximum number of occupants allowed.

The distribution of alternative exits is important so as to ensure that they can be effectively used in case one is blocked due to a fire nearby. This is achieved by ensuring that the alternatives are further apart than an angle of 45° from the farthest point in the room from the exit.

DESIGNING A MEANS OF ESCAPE FROM FIRE

You need to know the following information when designing a means of escape from fire in a premises or building,

• Time of evacuation
• Travel distances
• Number of Occupants
• Calculation of Exit Widths
• Calculation of Minimum Number of Exits

Whilst these factors are present in all cases, it is not so obvious that their importance will vary according to the circumstances. Sometimes one, sometimes another, will assume greater importance in your assessments and subsequent solution.

For example, a precast concrete manufacturer housed in a single storey building constructed entirely of noncombustible materials. Because of the construction, your considerations will probably centre upon maximum travel distance to an exit within a reasonable period of time.

Now consider a manufacturer using highly flammable materials, you would be looking for a rapid evacuation in a very short time and the fact that the building is completely noncombustible has little, effect.

In new building, and the occupancy is unknown the differences required between the above two situations are considerable, and this can cause great difficulty in planning means of escape. In such cases, the means of escape must be designed to cater for the worst possible case.

Having considered the factors that will influence escape, and having seen how these can be related to building type, it is important to look at the stages in the process of escape and the maximum distances people can be expected to travel.

Escape is generally considered in four distinct 'Stages' as follows

Stage 1 - escape from the room or area of fire origin

Stage 2 - escape from the compartment of origin via the circulation route to a protected stairway or an adjoining compartment offering refuge

Stage 3 - escape from the floor of origin to the ground level

Stage 4 - escape at ground level away from the building.

You need to study each floor plan and consider each room or compartment, deciding if the travel distance from the furthest point of that room or compartment to a place of comparative or ultimate safety is less than the maximum travel distance.

You need to then calculate the number of occupants and ascertain if the exits from the room or compartment to a place of comparative or ultimate safety is adequate.

This handout provides a general overview and basic information on this topic. It may not apply to everyone, consequently to find out if this guide applies to you and to get more information on this subject, study all the relevant legislation, guides and British/European Standards. Also you should seek advice from an expert on the subject or your local Fire Safety Officer or Fire Safety Professional.

Merseyside Fire Liaison Panel. Permission is granted to print and photocopy this material for nonprofit educational uses.

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This page was last checked and modified on Saturday, 22 March, 2008 22:41