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<p>This study aimed to clarify the performance of high-strength bolted joints for thin-walled glass-fiber- reinforced polymer (GFRP) members by conducting slip tests and long-term relaxation tests. The parameters of the slip test were the FRP surface treatment, bolt axial force, and bolt hole diameter. Relaxation characteristics might also be affected by variations in fiber content based on differences in production lots. Hence, samples from different production lots were taken. However, in these tests, the influence of all parameters was relatively minimal. One year after tightening, the axial force reduction gradually subsided and tended toward convergence. However, because it is difficult to determine convergence based on temperature changes, long-term measurements will continue. In the slip tests, the highest slip coefficient was obtained when the GFRP was coated with fluoroplastic and the connecting plates were treated with phosphate. This study proposes a design slip coefficient for GFRP high-strength bolted friction joints.</p>