THE BEHAVIOR OF HIGH-STRENGTH GRADE 8.8 BOLTS IN FIRE

Full component tests on high-strength Grade 8.8 bolts in tension and d ouble shear at temperatures up to 800-degrees-C have shown the present UK guidelines for designing at the Fire Limit State, BS5950: Part 8, are, for the most part, conservative. New guidelines are proposed. The tests in tension, in combination with Grade 8.8 nuts, have highlighte d the possible premature failure due to thread stripping. This mechani sm was found to be controlled by the degree of fit between the two com ponents. Practical measures are suggested to enable the full capacity of the bolts to the utilised. Conventional hot tensile tests were carr ied out using mechined specimens from bolts produced by different manu facturing processes. These were found to exhibit similar values of 0.2 % proof stress over the temperature region where the normal design str esses coincide with the onset of plastic deformation. The data are als o compared with those given for structural steel in the draft Eurocode s. For post-fire evaluation, the influence of the temperatures attaine d on the residual hardness of high-strength bolts has highlighted the sensitivity of their mechanical properties to overheating. Use has bee n made of the metallurgical changes to develop a technique for identif ying the maximum temperature bolts may have achieved in a fire. This i nformation can assist in the investigation of fire-damaged buildings.