Planning principles governing the number and location of exit stairways

The following review is restricted to 'required' stairs: that is to say, stairs used exclusively for the evacuation of a floor to the open air at ground level, contained within a fire-rated enclosure.

The location and number of legally required exit stairs from any floor is determined by the total occupancy of the floor, the distance of travel from any point on the floor to the nearest stair, and the basic principle that stairs should be remote one from another.

The occupancy of the floor is calculated by referring to a table that sets out the maximum floor area allowance per occupant for a wide range of uses. The allowance for a usage not specifically included in the table may be determined by negotiation with the building official.

Once the occupancy has been determined the minimum of stairways is calculated as in Figure 10.6. The second factor determining the number of stairs required is the limitation of the distance of travel (measured as an exact path of travel) from any point on the floor nearest a stair or fire-rated corridor leading to a stair. This distance varies in accordance with usage and whether or not the space is served by sprinklers. For example, in such general usages as assembly, business, mercantile or educational, the maximum path of travel distance is 250 ft (76 m) if the space has sprinklers and 200 ft (61m) if it has not. Hospitals are more restricted; warehouses less. The final planning

Minimum number of

Occupancy load

stairs

500 or less

2

501-1000

3

Over 1000

4

Figure 1O.6 Calculation of minimum number of stairways

Figure 1O.6 Calculation of minimum number of stairways restraint is the doctrine of 'remoteness' which governs the location of stairs in relation to one another. The rule states that where only two stairs are required they should be no closer (when measured in a straight line) than half of a straight line connecting the two most remote corners of the space. The distance of separation may be halved if the space is sprinklered; a concession, it should be noted, that is not allowed in the Uniform Building Code.

In large floors where more than two stairs are required the rule governing path of travel ensures that stairs are well separated, but in any case the code inspector will insist on a reasonable distribution of stairs.

Width of stairs

Having calculated the number of stairs it is now necessary to calculate the total stair width. This is, of course, directly related to the occupancy of the floor. The method of calculation, however, varies radically between BOCA and UBC and I will therefore review both.

Stair width calculation, BOCA Code

The total stair width is calculated by using a unit width multiplied by the number of occupants. The unit width varies from use to use and is significantly less when the space is sprinklered. For example:

a For general use such as assembly, business, educational, mercantile, residential, storage or manufacture the unit width is 0.2 in (5 mm) if the space is sprinklered, and 0.3 in (7.5 mm) if not.

b In hospitals the unit width is 0.3 in (7.5 mm) if sprinklered and 1 in (25 mm) if not.

Exit stair width is calculated for the occupancy of the floor only, that is to say, the stair 'load' is not cumulative.

Stair width calculations using UBC

Total stair width is calculated by dividing the total occupancy by 50, expressing the result in feet. No distinction is made between use groups and no concessions are granted for sprinklered floors.

The major difference between BOCA and UBC however is that UBC calculates the stair loading in a multi-storey building on a cumulative basis. The rule is that the stair must accommodate 50 per cent of the load of the floor immediately above plus 25 per cent of load of the floor above that.

It is therefore immediately clear that in a building of more than two floors UBC is a much stricter code than BOCA. As a practical matter, however, the different ways of calculating stair width do not become a design issue until the floor occupancy reaches a little over 200; as for example, in a multi-storey office building with a uniform single floor area of 20 000 ft2 (1860 m2). In a floor of this size BOCA requires a total stair width of 60 in (1524 mm) while UBC (bearing in mind that the load is cumulative) requires a width of 84 in (2134 mm). Both are therefore still below the mandated minimum requirement of two stairs of 44 in width each, i.e. 88 in (2 235 mm).

The UBC requirements do become onerous, however, in buildings such as department stores where an upper floor occupancy is often as high as 1 500 persons.

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