The Behaviour of High-strength Grade 8.8 Bolts in Fire

摘要

Full component tests on high-strength Grade 8.8 bolts in tension and double shear at temperatures up to 800℃ 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 highlighted the possible premature failure due to thread stripping. This mechanism was found to be controlled by the degree of fit between the two components. Practical measures are suggested to enable the full capacity of the bolts to be utilised. Conventional hot tensile tests were carried out using machined specimens from bolts produced by different manufacturing processes. These were found to exhibit similar values of 0.2% proof stress over the temperature region where the normal design stresses coincide with the onset of plastic deformation. The data are also compared with those given for structural steel in the draft Eurocodes. For post-fire evaluation, the influence of the temperatures attained on the residual hardness of high-strength bolts has highlighted the sensitivity of their mechanical properties to overheating. Use has been made of the metallurgical changes to develop a technique for identifying the maximum temperature bolts may have achieved in a fire. This information can assist in the investigation of fire-damaged buildings.