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Steel Designers' Manual - 6th Edition (2003)
660 Composite columns
The axial load Pf in fire conditions is determined for load factors of unity. The concrete
strength of 0.83fcu is the cylinder strength of concrete, which can be developed
in both circular and rectangular sections in fire. The material factors for concrete
and for reinforcement are taken as 1.0 in this formula.
The term K1 is determined as in section 22.2.1 or as in Table 10 of BS 5950 Part
8. 10 It excludes the contribution of the steel hollow section and includes the
enhanced strength of the concrete. The reinforcing bars are assumed to be fully
effective, provided they are located at the appropriate cover. 8 A method of including
the effect of applied moment is given in BS 5950: Part 8, 10 which is described
more fully in Reference 11.
Fire resistance periods that can be achieved for plain (or bar-reinforced) or fibrereinforced
concrete-filled hollow sections are presented in Table 22.2.
Where enhanced periods of fire resistance or greater load resistance are required,
external fire protection may be provided. Normally, the effect of concrete filling
offers a considerable reduction in the thickness of fire protection material that
would be required for an unfilled hollow section. 9
An important practical requirement for the use of concrete-filled sections is the
provision of vent holes at the top and bottom of each column. These are to prevent
dangerous build-up of steam pressure inside the columns in the event of a fire. They
also permit seepage of any excess moisture in the concrete after construction. Two
12 mm diameter holes placed diametrically opposite each other at the top and
bottom of each storey height have been used in testing and proved to be adequate.
Further guidance on fire resistance of concrete-encased and concrete-filled
columns is given in ENV 1994-1-2 (Eurocode 4 Part 1.2) and in Reference 13.
References to Chapter 22
Table 22.2 Fire-resistance of concrete-filled
hollow sections as a function of load
ratio 10
Load ratio, h
Fire-resistance Plain 5% fibre
(min) concrete concrete
30 – –
60 0.51 0.67
90 0.40 0.53
120 0.36 0.49
1. British Standards Institution (1979) Steel, concrete and composite bridges. Part
5: Code of practice for design of composite bridges. BS 5400, BSI, London.
2. British Standards Institution (1969) Specification for the use of structural steelwork
in building. BS 449, BSI, London.