Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
The rate of heating of a given section is related to its section factor which is the
ratio of the surface perimeter exposed to radiation and convection and the mass,
which is directly related to cross-sectional area:
Am
exposed surface area of section per unit length m
section factor =
2
V cross-sectional area of the section per unit length m
=
( )
( )
A member with a low Am/V value will heat up at a slower rate than one with a high
Am/V value and will require less insulation (fire protection) to achieve the same fireresistance
rating. Standard tables are available listing Am/V ratios for structural sections
(see the Appendix Section factors for UBs, UCs, CHSs and RHSs). These
factors are calculated as indicated in Fig. 34.1.
Sections at the heavy end of the structural range have such low Am/V ratios, and
therefore such slow heating rates, that failure does not occur within 1 /2 hour under
standard BS 476 heating conditions even when they are unprotected.
Limiting section factors for various structural elements are given in Fig. 34.2 (corresponding
to a load ratio of 0.6).
B
DLI
profile protection
50 69
column blocked-in
web
Standards and building regulations 1015
box protection
3-sided protection
4-sided protection
90
beam
A2D+B
A2D+2B
Fig. 34.1 Some different forms of fire protection to I-section members
Fig. 34.2 Maximum ratios of Am/V (m -1 ) of exposed steel to give 30 minutes fire resistance