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Fire resistance - Beating the heat

New Concrete Engineering - Recent research has shown that the fire resistance figures given in the current concrete code are generally conservative. Anna Scothern reports.

The BRE report Fire safety of concrete structures: background to BS8110 fire design published earlier this year, investigated the background to methods for establishing the fire resistance of concrete structures specified to BS8110. In particular, BRE examined and revisited the original research and test results that underpin the tabulated data.

The research was carried out by BRE at the request of The Concrete Centre and the British Cement Association. It brings together a body of information that covers test results and research carried out over a number of years.

With the passing of time there was a danger that much of the important work supporting the development of codes and standards could have been lost.

There was a need to collate and assess all the relevant information to ensure that the important lessons from the past were recorded and used to help define the strategy for a new generation of codes and standards.

To this end, the research focused on the original research and tests underpinning the tabulated date in BS8110 in order to assess the relevance of the prescriptive approach to modern concrete construction.

Its most important findings were that the experimental results used as data for developing the tabulated approach to BS8110 fully supported the provisions of the code in relation to assumed periods of fire resistance. Furthermore, the research found that these provisions are in many cases very conservative as they are based on the assumption that structural elements are fully stressed at the fire limit state and take into account the spalling characteristics of concrete.

For example, with reinforced concrete beams, some tests measured the fire performance of the beams at a load three times greater than the figure laid out in the table. The research therefore concluded that the load ratio (the load on the beam divided by the strength of the beam) had no significant impact on the fire performance of the element.

A different conclusion was reached in the case of concrete columns. A number of column specimens failed to achieve the assumed fire resistance because of high load ratios. However, BRE reported that this was not an area of concern because in reality concrete would never be exposed to loads this high.

For 381mm diameter columns, six of the 13 columns failed to achieve the prescribed level of fire resistance. However, the load ratios of concrete columns tested ranged from 0.77 to 0.96, which is far above the applied loads typically found in real buildings.

The BRE report clearly demonstrates that evidence from the concrete performance in real fires over a number of years proves that the tabular approach has been effective. It also suggests that the conservatism of the existing data means that further research would potentially result in even greater construction and cost economies for concrete structures.

For example, in general, the provision of BS8110 in terms of minimum cover to the reinforcement are based on limiting the temperature rise, and therefore the reduction in strength, to values of 5,500'C and 4,500'C for reinforcing bars and prestressing tendons for the prescribed period of fire resistance. The assumption is that the elements are supporting the full design load at the fire limit state. This is a conservative assumption and may lead to the inefficient use of concrete in buildings.

Despite the increasing adoption of the whole building performance based approach of the Eurocodes, the prescriptive approach of individual elements of BS8110 will continue to be popular.

The research carried out by BRE is important not only because it fully validates the fire resistance of concrete by highlighting the conservatism of the prescriptive approach but also because by doing so it proves today's relevance of the historic tabulated data.

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