Re-use of fire damaged steel

An often quoted general rule for fire affected hot rolled structural steels is that if the steel is straight and there are no obvious distortions then the steel is probably still fit for use. At 600ºC the yield strength of steel is equal to about 40% of its room temperature value; it follows therefore that any steel still remaining straight after the fire and which had been carrying an appreciable load was probably not heated beyond 600ºC, will not have undergone any metallurgical changes and will probably be fit for re-use.

However, where the load in the fire was less than the full design load, and also with high strength steels, this cannot always be held to be true. In such cases it is recommended that hardness tests are carried out on the affected steel. In practice it is recommended that, in all instances, some hardness tests should be carried out. For grade S275 steel, if the ultimate tensile strength resulting from the tests are within the range specified in the table below,  then the steel is reusable.

For grade S355 steel additional tensile test coupons should be taken from fire affected high strength steel members when hardness tests show that:

1 – there is more than 10% difference in hardness compared to non-fire affected steelwork, or

2 – hardness test results indicate that the strength is within 10% of the specified minimum.

Where deflections are visible, general guidelines on the maximum permissible levels of deflection to ensure satisfactory performance are difficult to specify. The amount of deflection or distortion must be checked so that its effect under load can be calculated to ensure that permissible stresses are not exceeded and the functioning of the building is not impaired. Therefore every building should be considered as a separate case and the structural engineer involved in the reinstatement exercise must decide what level is acceptable to satisfy the relevant Codes.