Worked Example: Stair Width Calculation for 12-Storey Office Building with Even Population Distribution

A building comprises 12 storeys of offices (ground + 11). What is the minimum width needed for the stairs serving the office floors with a population of 1200 people (excluding the ground floor population which does not use the stairs), using simultaneous evacuation? Two stairs satisfy the travel distance limitations. The population is distributed evenly. As the top office storey is at a height greater than 18 m, both stairs need added protection (see paragraph 2.69). Therefore, if both stairs are entered at each level via a protected lobby, then both stairs can be assumed to be available (see paragraph 2.64). P = 1200/2 = 600, n = 11 From the formula – 600 = 200w + 50(w - 0.3)(11 - 1) 600 = 200w + (50w - 15)(10) 600 = 200w + 500w - 150 750 = 700w w = 1.07 m (1070 mm) Therefore, both stairs should be at least 1070 mm wide. But this needs to be increased to 1100 mm as the formula applies to stairs 1100 mm or wider (see paragraph 2.65). This width should also be adequate to comply with paragraph 2.63 when one storey exit is discounted in accordance with paragraph 2.52 (i.e. the stair widths are not less than the minimum widths needed for 109 people in Table 2.4).