Connections and foundations

The tensile strength reduction for grade 4.6 bolts is similar to that for S275 steel. For grade 8.8 bolts, which are heat treated in manufacture, the residual strength reduction is more marked if the material temperature has exceeded 450ºC. The residual strength of these bolts falls to 80% and 60% after reaching temperatures of 600ºC and 800ºC respectively.

To err on the side of caution it is recommended that bolts should be replaced if they show any sign of having been heated e.g. blistered paint, smooth grey scaled surface.

Contraction of heated members after the fire can cause distortion of connections. When carrying out an inspection of a fire damaged building it is recommended that special care is taken in inspecting the connections for cracking of welds, end plate damage, bolt failure etc. A number of bolts should be removed to inspect for distortion. Similar care should be taken when inspecting foundations for bolt failure, concrete cracking etc.