BS5950 Part 8 was published in August 1990 and brings together in one document all of the methods of achieving fire resistance for structural steelwork. Although it is based on evaluation of performance of structural steel members in the BS476 Part 20 standard fire it may also be used in fire engineering assessments when natural fire temperatures are derived by calculation.
BS5950 Part 8 also includes design information and guidance for design of portal frames, hollow sections, external steelwork, composite slabs and beams and calculation of protection thicknesses based on limiting temperatures.
The code contains two basic approaches to assessment of fire resistance:-
- From Tests - in accordance with BS476 Part 21.
- By Calculation - in accordance with either :-
- the limiting temperature method
- the moment capacity method
A commentary to the standard giving more detailed information and worked examples has been published by the Steel Construction Institute.
Fire resistance derived from tests
All approved protection materials have been tested in accordance with BS476 and the required thickness of each product has been evaluated with regard to fire resistance period and section factor. Recommendations based on these evaluations are given in simple design tables in the “Yellow Book” published jointly by the Association of Specialist Fire Protection (ASFP) and the Steel Construction Institute.
Limiting temperature method
The limiting temperature method allows the designer to assess the need, or otherwise, for fire protection by comparing the temperature at which the member will fail (the limiting temperature) with the temperature of the hottest part of the section at the required fire resistance time (the design temperature). In BS5950 Part 8 this is done via a set of prepared tables and is illustrated graphically. If the limiting temperature exceeds the design temperature no protection is necessary.
Moment capacity method
This calculation method allows the designer the opportunity to assess the fire resistance of a beam by calculating its moment capacity using the temperature profile at the required fire resistance time. If the applied moment is less than the moment capacity of the beam the member is deemed to have adequate fire resistance without fire protection.
The method is only applicable for beams with webs which satisfy the requirements for a plastic or compact section as defined in BS5950 Part 1. It is best suited for use with shelf angle floor beams. Appendix E of BS5950 Part 8 gives all the information required to calculate the moment capacity of shelf angle floor beams at 30, 60 and 90 minutes and a more detailed treatment is given in the appropriate Steel Construction Institute publication.
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