| Seasonal and daily changes in relative humidity (RH) and temperature of the surrounding air lead to fluctuations in moisture contents (MCs) of wood-based materials. So the moisture content of wood in light frame houses is always changing from 10% to 18% due to the temperature and the relative humidity of atmosphere is fluctuating all the time. Some connection problems caused by moisture content will appear. Therefore, a lot of researchers abroad did investigate the effects of moisture content on metal plate connection properties. So far, there isn't any research report about such subject in China. But such research is very significant for wood house safety, house life-span prolonging, effects of moisture content on metal plate connection properties understanding, and connection properties of MPC (metal plate connector) joints improving.In this paper, the MPC joints connection properties of Chinese larch and North American SPF under both unalterable moisture and moisture cycling conditions were investigated. The variable conditions were changing between 90%RH and 40%RH (90%RH-40%RH), 90%RH and 80%RH (90%RH-80%RH), 40%RH and 80%RH (40%RH-80%RH), and unchangeable condition, 40%RH (40%RH), while all changes were under 21℃and four cycles for each moisture content cycle level were designed, 0, 1, 2, and 4. The connection properties were including ultimate tension load (UTL), deflection, initial stiffness, stiffness at the design load, stiffness at critical slip, failure mode and load-deflection curve. The effects of MPC size, MPC specimen orientation and treatment time on MPC joint properties under ulterable moisture content and the changing of MPC backout, basic mechanical properties, the relationship of backout and properties under four variable moisture conditions were all investigated. Then the load-deflection curves were fitting by Foschi three- parameter equation, and the action mechanism of wood fiber-MPC tooth and the effect mechanism were analyzed. The main achievements of this study were summarized as follows:(1) The UTL and initial stiffness of Chinese Dahurian larch (Larix gmelinii) MPC joints were all higher than that of southern pine. Both of them had the same failure modes, tooth withdraw, metal plate broken and wood tear.(2) AA orientation MPC specimens had the highest UTL. The UTL value was increasing as the width, length, and area of MPC increasing, and the width addition was more effective to UTL increasing than length addition. Considering UTL and deflection at UTL, the suitable treatment time of gaining good properties was between 7 and 21 days.(3) The gap between metal plate and wood surface had big effect on UTL and initial stiffness of MPC joints. The smaller gap the UTL and initial stiffness higher was. There were good correlations between UTL, initial stiffness and MPC gaps, and the coefficients of determine were 0.99 and 0.96. The decreasing degree of initial stiffness was a little lower than that of ultimate tension load as the MPC gaps increasing.(4) After moisture content cycling treating, the density, moisture content, UTL, deflection at UTL, stiffness at critical slip of MPC joint were separately among 0.35-0.50 g/cm3, 7.90%-15.28%, 13.88-28.52KN, 0.63-2.78mm, and 25.19-53.43KN/mm. The average UTL value under zero moisture cycling cycle was 23.01 KN, which was a little lower than Chinese Dahurian larch MPC joints, 27.09KN. A good linear correlation was between UTL and deflection, which the coefficient, R was0.69. Tooth withdraw was the main failure mode for SPF MPC joints.(5) After moisture content cycling treating, the tooth backout of MPC joint was increasing to an invariable value as the cycle increasing, and the zero moisture cycling cycle had nearly no effect on the tooth backout value; the order from big to small was 90%RH-80%RH, 40%RH-80%RH, 90%RH-40%RH and 40%RH. The tooth backout value was determined mainly by the first two cycles. It was seen the tooth backout was caused by wood fibers moving during swelling and drying. Moisture absorption was the main reason for such phenomenon, that the tooth backout value was turn to an invariable value. And the passivation of wood fibers which generated from moisture content changing made the backout not to increase as the cycling cycle increasing.(6) After moisture content cycling treating, all the UTL values of MPC joints droped, but the degrees of droping were different at four moisture cycling levels. The order of decreasing from big to small was 90%RH-40%RH, 40%RH-80%RH, 40%RH, and 90%RH-80%RH. Higher scope of moisture cycling and more cycle times had more weaken action on UTL. (7) After moisture content cycling treating, the stiffness at critical slip of MPC joints decreased, but the degrees of decreasing were different at four moisture cycling levels. The order of decreasing from big to small was 40%RH-80%RH, 40%RH, 90%RH-80%RH, and 90%RH-40%RH. Moisture cycling had weaken action on stiffness st critical slip, and the more cycle of moisture cycling the more weaken was.(8) After moisture content cycling treating (except the forth cycle in 21℃, 40%RH condition), the initial stiffness was decreasing as the cycle increasing, the order from big to small was 40%RH-80%RH, 40%RH, 90%RH-80%RH, and 90%RH-40%RH. The initial stiffness value was decreasing fast after the treating of the first cycle of moisture cycling, and then increasing a little. Both the initial stiffness and the stiffness at critical slip had the same trends.(9) After moisture content cycling treating, both the UTL and the initial stiffness of MPC joints were declining as the backout increasing. It had a good correlation with R2, 0.89 between UTL and backout under group C moisture cycling condition.(10) The load-deflection curves of both Chinese Dahurian larch MPC joints and North American SPF MPC joints were all fitted very well by Foschi equation. The initial stiffness can be gained from the fitting equations. X ray was able to be a method for measuring the backout of MPC joints quickly.
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