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BS 8110: Part 1 gaves minimum concrete covers and minimum structural dimensions for various fire durations; these are applicable to typical fires in buildings. Part 2 of the code gives more detailed information on design for fire resistance. It identifies that lightweight aggregate concrete has a better fire performance than dense concrete; no other indication of the relative strengths and weaknesses of materials is given. This standard has now been withdrawn and superceded by Eurocode 2, BS EN 1992-1-2 Eurocode 2: Design of concrete structures. General rules - Structural fire design
The code suggests that the behaviour of elements in fire (flexural elements) is largely determined by the strength of materials at elevated temperatures. Design curves are given for the variation of concrete strength with temperature. For dense concrete (aggregate type not specified) the strength is assumed to be unchanged until the temperature reaches 3500C and then drops linearly to 20% of its value under ambient conditions at a temperature of 8000C.
Lightweight aggregates are assumed to be stable to 5000C and then drop to 40% of their strength at 8000C. Under fire conditions, the partial safety factors applied to the concrete strength and to the loads are reduced.
It should be noted that the design approach does not take into account the effects of differential thermal expansions, spalling etc. or the moisture condition of the concrete.The report Fire safety of concrete structures: Background to BS 8110 fire design reviews the data used to developing the tabulated approach in BS 8110 and the evidence from performance in real fires over a number of years. It shows that the provisions are in many cases conservative and that further research would potentially result in even greater construction and cost economies for concrete structures.
Acknowledgement: The Concrete Society
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