One-year stress relaxation of timber joints assembled with pretensioned bolts |
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Authors: | Ali Awaludin Takuro Hirai Toshiro Hayashikawa Yoshihisa Sasaki Akio Oikawa |
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Institution: | (1) Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan;(2) Graduate School of Agriculture, Hokkaido University, Kita 9 Nishi 9, Kita-ku, Sapporo 060-8589, Japan;(3) Present address: Civil and Environmental Engineering Department, Gadjah Mada University, Yogyakarta, 55281, Indonesia |
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Abstract: | In our previous study, great increases of hysteretic damping and initial slip resistance of timber joints were attained by
applying axial pretension to the steel fasteners. To evaluate the effectiveness of this method, 1-year stress-relaxation measurement
was carried out. Nine prestressed joints were prepared and three of them were restressed after 3 and then 6 months after the
initial prestressing. All joints were exposed to indoor conditions, and relaxation of the pretension was regularly measured
from time-dependent decreases of axial strain of the bolts. After measurement, the joints were subjected to cyclic and monotonic
loading tests until failure. The average ratio of residual stress to the initial prestress after 1 year was about 0.23 and
0.66, respectively, for joints without restressing and those with restressing. A simulated stress-relaxation curve developed
from the four-element relaxation model predicted 3% of the initial stress after 5 years. Without a regular restressing program,
the initial prestressing effect therefore must be considered negligible. However, about 20% of the pre-stress level can be
reasonably assumed if restressing is carried out annually. This small residual stress was found to introduce suffi cient frictional
damping to signifi cantly increase the equivalent viscous damping ratio of the joints.
Part of this study was presented at the 10th World Conference on Timber Engineering, Miyazaki, June 2008 |
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Keywords: | Bolted joint Frictional damping Pretension Residual stress Stress relaxation |
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