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Steel Designers' Manual - 6th Edition (2003)
In a fire test columns are exposed to fire on all four sides and axially loaded vertically,
whereas beams are loaded horizontally in bending and are exposed on three
sides, the upper flange being in contact with the floor slab, which also acts as the
furnace roof.
The stability limit is deemed to have been reached for columns when ‘run-away’
deflection occurs. For beams this is more accurately specified when the deflection
rate reaches L 2 /9000d (where L = beam span, d = beam depth), within a deflection
range of L/30 min to L/20 max.
34.7 Fire engineering
Fire engineering 1027
There are many special building forms which can take advantage of a rational
approach, called fire engineering, 12,13 which takes account of the temperatures developed
in a real fire, as opposed to a standard fire (see Fig. 34.11).
Essentially the fire engineering design method can be divided into two main steps:
(1) Determination of the fire load
The fire load of a compartment is the maximum heat that can theoretically
be generated by the combustible items of contents and structure i.e. weight ¥
calorific value per unit weight.
Fire load is usually expressed in relation to floor area, sometimes as MJ/m 2
or Mcal/m 2 but often converted to an equivalent weight of wood and expressed
as ‘kg wood/m 2 ’ (1 kg wood ∫ 18 MJ). Standard data tables giving fire loads of
different materials are available. 12,13
Examples of typical fire loads (wood equivalent) are:
schools 15 kg/m 2
hospital wards 20 kg/m 2
hotels 25 kg/m 2
offices 35 kg/m 2
department stores 35 kg/m 2
textile warehouses >200 kg/m 2
(2) Prediction of maximum compartment temperature
The heat that is retained in the burning compartment depends upon the
thermal characteristics of the wall, floor and ceiling materials and the degree
of ventilation. Sheet steel walls will dissipate heat by conduction and radiation
while blockwork will retain heat in the compartment and lead to higher
temperatures.
It is assumed that window glass breaks in fire conditions, and calculations take
into account the size and position of such ventilation. Openings close to the
ceiling level of a compartment (or disintegrating roofing materials) will tend to
dissipate heat whereas openings close to the floor will provide oxygen to feed
the fire.