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The clamping force of a high-strength bolt reduces within a certain time period after the initial clamping force. When special treatments are used on a faying surface, the clamping force is relaxed severely. Tests were conducted for slip critical joints subjected to various faying surface parameters. Relaxation occurred for slip resistant joints with an uncoated surface that had been shot-blasted, cleaned, milled or rusted. It was observed that the initial clamping force dropped from 6.2% to 8.0% after 744 hours. For joints with a 5 mil thick zinc coating, the clamping force of the bolts decreased by 8.37%, and in the case of 4.9 mil thick red lead painted treatment, the relaxation ratio was 24.6%. The standard deviation of the slip coefficients between the time immediately after tightening and 1,000 hours after tightening was 0.223. It is particularly noteworthy that, for red lead painted treatment, the slip coefficient dropped from 2.5 times to 2.8 times when compared with the slip coefficient before relaxation. Joints with inorganic zinc should not have thickness exceeding 5 mils, and joints with a red lead painted surface should not be allowed regardless of the thickness of coating.

The torque shear type high strength bolt is clamped in general when pin-tail of shank is cut off. However the clamping loads of high strength bolt did not often meet the required tension due to variation of torque coefficient. This is why the viscosity of lubricant on bolt shank affects the torque coefficient by temperature, dust and debris. In this study, the clamping tests of various high strength bolts were performed independently both at indoor condition and at construction sites for six years. The temperature of construction sites was recorded with the range from -11℃ to 34℃.. The temperature at indoor condition was set up to compose from -10℃ to 50℃ at each 10℃ interval. At construction sites, the clamping load of high strength bolt was varied from 159 to 210 kN and the torque value was varied from 405 to 556 Nm. The torque coefficients at construction sites were analyzed from 0.126 to 0.158 whereas tensions were measured from 179 to 192 kN by using tension meter. The torque coefficients at construction sites were analyzed to range from 0.118 to 0.152. From these tests, the diverse equations on torque coefficient, tension subjected to temperature were taken by statistic regressive analysis.

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