摘要
木结构房屋坐落在自然环境中,其木材含水率总处于10%-18%的动态平衡状态中,大气水分含量的不断变化影响木结构连接性能。为此,国外很多学者相继关注齿板连接性能的水分效应,纷纷对其进行研究,我国关于齿板连接水分效应方面的研究尚处于空白。研究轻型木结构桁架齿板连接性能与水分循环的关系对提高轻型木结构房屋使用安全性、延长使用寿命、改进连接质量具有重要意义。
本论文通过对恒定水分状态及四种水分循环变化状态下(温度均为21°C,相对湿度分别在90%RH与40%RH,90%RH与80%RH,40%RH与80%RH循环状态和40%RH恒定状态四种水分循环水平变换,循环周期分别为0、1、2、4四种)节点连接性能的测试,分析齿板尺寸、齿板构型、放置时间和水分循环状态下水分循环水平、循环周期、齿板嵌入深度、齿板拔出量对齿板节点极限拉伸载荷、位移、刚度和破坏形式等力学性质的影响,利用Foschi三参数模型拟合载荷-位移曲线,并对木材与板齿作用机理和齿板拔出量对连接性能影响机理进行分析和探讨。
主要结论如下:
1.国产兴安落叶松(Larix gmelinii)齿板节点拉伸强度和初始刚度均高于南方松(Southern Pine)齿板节点,齿板拔出、齿板断裂和木材破坏是主要的破坏形式,两树种齿板节点的破坏形式基本相同。
2.AA方向(载荷方向、齿板长度以及木材纹理走向一致)是四种齿板构型中承受拉伸载荷能力最好的方式。齿板节点极限拉伸载荷随着齿板长度、齿板宽度和齿板面积的增加而增加,齿板宽度增加对提高齿板节点极限拉伸载荷更为有效。综合载荷与位移两个主要指标,齿板试件压制后适宜进行力学测试的放置时间为7至21天。
3.齿板嵌入深度对SPF规格材齿板节点的极限拉伸载荷和初始刚度都具有重要影响,嵌入深度越深,节点的极限拉伸载荷和初始刚度越高,但初始刚度随齿板-木材间隙不断增大而降低的幅度略低于极限拉伸载荷降低的幅度;齿板嵌入深度分别与节点的极限拉伸载荷、初始刚度间具有很好的相关性,决定系数分别为0.99和0.96。
4.经过水分循环作用后,拉伸测试时北美SPF(Spruce-Pine-Fir)规格材齿板试件密度为0.35-0.50g/cm3,含水率为7.90%-15.28%,极限拉伸载荷在13.88-28.52KN之间,0循环极限拉伸载荷平均为23.10KN,略低于国产落叶松齿板节点的极限拉伸载荷,27.09KN;最大拉伸载荷时位移0.63-2.78mm,临界滑移刚度25.19-53.43KN/mm。极限拉伸载荷与位移呈现较好的线性相关性,相关系数R为0.69,齿板拔出为北美SPF规格材齿板节点主要破坏模式。
5.经过水分水分循环作用后,齿板拔出量均呈现出增加趋势,增加幅度从大到小依次为90%RH至80%RH循环水平>40%RH至80%RH循环水平>90%RH至40%RH循环水平>40%RH恒定水平,但0循环对齿板拔出量基本没有影响,且随着循环周期的增加齿板拔出量趋于恒定,齿板拔出量大小主要受前两个循环的影响比较大。木材自身干缩湿胀、木纤维相对齿板发生移动是产生齿板拔出现象的主要原因;经过两个周期水分循环作用后齿板拔出量趋于恒定的现象主要与木材自身的吸湿滞后性造成的木材纤维上的自由羟基数量降低,吸纳水分子的空间减小,纤维对水分变化的反应灵敏性被钝化,木纤维受水分循环变化作用下相对位移增加的可能性越来越小有关。
6.经过四种水分循环水平作用后,齿板节点的极限拉伸载荷均呈现出下降趋势,但下降幅度各不相同,从大到小依次为:90%RH至40%RH循环水平>40%RH至80%RH循环水平>40%RH恒定水平>90%RH至80%RH循环水平;水分循环变化幅度越大、循环次数越多对节点的极限拉伸载荷削弱作用越大。
7.经过四种水分循环水平作用后,齿板节点的临界滑移刚度均呈有所下降,下降幅度各不相同,从大到小依次为:40%RH至80%RH循环水平>40%RH恒定水平>90%RH至80%RH循环水平>90%RH至40%RH循环水平。水分循环作用对节点临界滑移刚度具有削弱作用,水分循环次数越多对齿板试件的临界滑移刚度削弱程度越大。
8.经过水分循环作用后(除21℃,40%RH恒定状态第4循环外),齿板节点的初始刚度均随着循环周期的增长而降低,在第1个水分循环周期作用后下降的比较大,之后均又略有所恢复,下降幅度及变化趋势与临界滑移刚度变化趋势一致。
9.经过水分循环作用后,齿板节点的极限拉伸载荷与初始刚度均随着齿板拔出量的增加而降低,其中极限拉伸载荷与齿板拔出量间存在较高的相关性,R2为0.89。
10.Foschi三参数非线性模型可以很好拟合国产落叶松和北美SPF规格材齿板节点的拉伸载荷-位移实验曲线,并可得到齿板节点的初始刚度值;X射线可以快速准确地确定齿板拔出量的大小。
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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