2.21ii Guidance – Emergency Exit Calculations
This appendix is reproduced from the Home Office “Guide to Fire Precautions in Places of Entertainment and Like Premises” with the permission of Her Majesty’s Stationery Office.
Table: Exit calculations – relevant factors.
| One unit of exit width | 525 mm |
| Rate of discharge per minute through one unit | 40 persons |
| Maximum permissible calculated evacuation time (in Class C buildings) | 2 minutes |
| Occupant load factor | see table in MAXIMUM OCCUPANCY CALCULATIONS section |
| Floor area in square metres |
Number of persons = floor area in square metres divided by occupant load factor
With these factors it is possible to calculate the number of units of exit width and subsequently the number and width of exits required for a given number of persons:
| Number of units of exit width: | Number of exits: |
| U = N / (40 x T) Where: N = Number of persons T = Time factor in minutes (2 for marquees) U = Number of exit units requiredWhere a decimal of 0.3 or over results, the next whole number is used. |
E = (U / 4) + 1 Where: E = Number of exits or stairs requiredWhere a decimal of 0.75 or over results, the next whole number is used. |
Note: It is assumed that one exit will not be available for an evacuation.
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EXAMPLE: A marquee being used as a dance hall Dance floor area of 420 sq M. Using the Table: showing occupancy load factor. Dance floor will accommodate = 420 / 0.55 = 763 people MAXIMUM TOTAL OCCUPANCY = 913 people Number of Units (U) of exit width required: U = N / (40 x T) = 990 / (40 x 2) = 11.42 units Round up if over 0.30 Therefore total units of exit width (U) = 12 Number of exits (E) required: E = (U / 4) + 1 = (12 / 4) + 1 = 4.0 Round up if over 0.75 Therefore number of exits required (E) = 4 These may be implemented by having: 3 Exits of 3 Units width each and 1 Exit of 4 Units width OR 2 Exits of 4 Units width each and 1 Exit of 3 Units width and 1 Exit of 2 Units width. |