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Steel Designers' Manual - 6th Edition (2003)
1014 Fire protection and fire engineering
The fire-resistance requirements of Document B apply only to structural elements
used in:
(a) buildings, or parts of buildings, of more than one storey,
(b) single-storey buildings that are built close to a property boundary.
The degree of fire resistance required of a structural member is governed by the
building function (office, shop, factory, etc.), by the building height, by the compartment
size in which the member is located, and by whether or not sprinklers are
installed.
Fire resistance provisions are expressed in units of time: 1 /2, 1,1 1 /2 and 2 hours. It
is important to realize that these times are not allowable escape times for building
occupants or even survival times for the structure.They are simply a convenient way
of grading different categories of buildings by fire load, from those in which a fire
is likely to be relatively small, such as low-rise offices, to those in which a fire might
result in a major conflagration, such as a library. Fire-resistance recommendations
for structural elements are given in Reference 2.
34.2.2 BS 5950: Part 8
BS 5950: Part 8 2 permits two methods of assessing the fire resistance of bare steel
members. The first, the load ratio method, consists of comparing the design temperature,
which is defined as the temperature reached by an unprotected member
in the required fire-resistance time, with the limiting temperature, which is the temperature
at which it will fail. The load ratio is defined as:
load carried at the fire limit state
load ratio =
load capacity at 20∞C If the limiting temperature exceeds the design temperature no protection is necessary.
The method permits designers to make use of reduced loads and higherstrength
steels to achieve improved fire-resistance times in unprotected sections.
The second method, which is applicable to beams only, gives benefits when
members are partially exposed and when the temperature distribution is known. It
consists of comparing the calculated moment capacity at the required fire-resistance
time with the applied moment. When the moment capacity exceeds the applied
moment no protection is necessary. This method of design is used for unusual structural
forms such as ‘shelf-angle’ floor beams. Some examples of the use of the
moment capacity method are given in the handbook to BS 5950: Part 8.
Limiting temperatures for various structural members are presented in the
Appendix Limiting temperatures. These ‘failure’ temperatures are independent of
the form or amount of fire protection. Beams supporting concrete floors fail at a
much higher limiting temperature than columns, for example.