Experimental research on slip-resistant bolted connections after fire

• Slip factor after fire increases with increasing exposed temperature.
• Bolt pre-tension force after fire decreases with increasing exposed temperature.
• Tri-linear model is proposed to calculate slip factor after fire.
• Tri-linear model is proposed to calculate pre-tension force in 10.9 bolts after fire.
• Bolts can be re-used when the maximum exposure temperature is no more than 300 °C.

Slip factor and bolt pre-tension force for slip-resistant bolted connections after fire were investigated experimentally. 74 connections made up of four plates and four class 10.9 bolts were first heated to specified temperature levels, and then cooled to ambient temperature and tested. Slip load tests were conducted to obtain the slip factor and bolt pre-tension force for the post-fire connections. In case of slip factor tests the bolts in connections after fire were replaced by new bolts and the pre-tension in the new bolts was measured, so the slip factor could be determined from the post-fire slip load. While in case of bolt pre-tension tests the old bolts were kept and the residual pre-tension was calculated based on the slip factor data obtained from accompanying tests. Two friction surface treatment methods were considered which were blast-cleaning (Class A) and inorganic zincs paint coated after blast-cleaning (Class B). Nine temperature levels from 200 °C to 700 °C were considered. Test results show that heating–cooling process has significant effects on both slip factor and bolt pre-tension force. The slip factor after fire increases with increasing temperature level, and residual bolt pre-tension force decreases with increasing temperature level. The increase in slip factor for Class A friction surface is much greater than that for Class B friction surface. Tri-linear models are proposed to calculate the normalized slip factor and bolt residual pre-tension force for slip-resistant 10.9 bolted connections after fire. New suggestions are proposed for post fire checking of slip-resistant bolted connections.