Section 3
BS 5588-11:1997
3) Thus both stairs need to be not less than 1 400 mm wide (this width is also adequate for a storey exit serving 155 persons). 4) If three stairs were provided, each would need to have a capacity of 155/3 = 52 persons, i.e. each needs to be not less than 1 000 mm wide. NOTE At least one of the stairs would also need to serve as a firefighting stair. Therefore if three stairs are provided, the firefighting stair(s) would need to be at least 1 100 mm wide to be in accordance with BS 5588-5.
9.4.3.3 Using Table 5 (phased evacuation) Example C Use of building: office. The height of building, number of floors and total population is assumed to be as in example B. a) Every stair needs to be approached through a protected lobby or protected corridor [see 9.6.2f)]. Therefore all stairs can be used to evacuate the building [see 9.4.2a)]. b) Number of persons per storey is 1 240/8 = 155 persons. c) If only two stairs are provided, then both stairs need to be able to accommodate half the population of one storey (i.e. 155/2 = 78 persons). d) The width of one stair to accommodate 78 persons is 1 000 mm (maximum capacity 100 persons). e) Thus both stairs need to be not less than 1 000 mm wide. However, this needs to be increased to 1 100 mm to be in accordance with Table 3 for 155 persons. f) If three stairs were provided, then each would need to be not less than 1 000 mm wide (i.e. the minimum width given in Table 5). NOTE At least one of the stairs would also need to serve as a firefighting stair. Therefore if three stairs were provided, the firefighting stair(s) would need to be at least 1 100 mm wide to be in accordance with BS 5588-5.
9.4.3.4 Comments on examples In each of the examples the population was assumed to be evenly distributed on each floor. However, where this is not the case the following apply. a) If using Table 4, then the width of the stair: 1) at every storey level should be not less than that of the storey exit at that level; and 2) should not narrow in the direction of escape [see 9.4.2b)1)]. b) If using Table 5, then the floor with the greatest population should be used. 9.5 Basement stairs 9.5.1 Commentary Areas below ground level, especially if used for storage, are more likely to become completely filled with smoke and heat from a fire than are the ground and upper storeys. There is, therefore, a greater risk that a stair in a basement will become obstructed by smoke and heat, particularly in a fully developed fire. For this reason, it is preferable that all stairs to basements be entered at ground floor level from the open air and only from such positions that smoke from any basement fire will not obstruct any exit serving the ground and upper storeys of the building. However, in buildings having two or more stairs available for escape from the upper storeys, no objection is seen to one or more stairs continuing down to the basement provided each such storey is protected from ingress of smoke from the basement and at least one stair serving the upper floors of the building (or part of the building) is terminated at ground level.
Š BSI 8 December 2004
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