Do wood-frame buildings need to account for thermal movement?

普遍的共识是，木结构建筑的设计师不需要考虑热运动, 由于温度升高和水分流失，木材的收缩抵消了热膨胀，这是设计师需要考虑的.

而混凝土和钢结构建筑通常设计有伸缩缝，以考虑由于环境温度波动引起的热运动, wood has a significantly lower coefficient of thermal expansion. Wood can experience dimensional movement with temperature changes. As noted in Chapter 4 of the USDA Forest Products Lab’s Wood Handbook, “The thermal expansion coefficients of completely dry wood are positive in all directions; that is, wood expands on heating and contracts on cooling.“然而，湿度波动同时影响木材的尺寸运动.

木材具有吸湿性，这意味着它具有吸收和释放水分的能力. 当这种情况发生时，它也有可能在维度上发生变化. 请注意，以上摘自《皇冠2登录welcome》的节选引用了完全烘干的木材(0%水分含量), or MC). 用于建筑结构的木材在施工期间甚至施工后都不会完全干燥. As wood experiences an increase in temperature, it may experience some thermal expansion, but there is also a loss of MC. The shrinkage due to reduced MC is more significant than the expansion due to increased temperature; therefore, the net result is shrinkage. 以下是《皇冠2登录welcome》第四章的部分内容:

“含有水分的木材对不同温度的反应与几乎在烤箱中干燥的木材不同. When moist wood is heated, 它由于正常的热膨胀而膨胀，由于水分含量的损失而收缩. 除非木材最初非常干燥(可能是3%或4%或更少的水分含量), 加热时因水分损失引起的收缩会大于热膨胀, so the net dimensional change on heating will be negative. 木材在中等湿度水平(约8%至20%)会膨胀，当第一次加热, 然后随着木材在加热条件下逐渐失水，逐渐缩小到比初始体积更小的体积.”

木材的收缩/膨胀最明显地发生在垂直于纹理的方向上, 这意味着一个实心锯木钉或地板托梁将改变宽度和深度. 由于湿度变化引起的纵向尺寸变化可以忽略不计, 这意味着螺柱或地板托梁的长度将基本保持不变. As the Wood Handbook notes:

“Even in the longitudinal (grain) direction, 湿度变化引起的尺寸变化很小的地方, 除非木材最初非常干燥，否则这种变化仍将比热膨胀引起的相应尺寸变化占主导地位. For wood at usual moisture levels, 在长时间加热后，净尺寸变化通常为负.” 

Section 4.美国木材协会木结构建筑国家设计规范®(NDS®) Manual also provides commentary on this subject, 并包括一些木材种类的热膨胀系数.

而适应热运动通常被认为是不必要的, 建议木结构建筑的设计师在施工期间考虑到由于增加的水分暴露而引起的膨胀. 考虑墙体、地板和屋顶护套的膨胀尤为重要. Because panel products start at a low moisture content (~8-12% MC) and are directly exposed to the elements during construction (in many cases increasing to >19% MC), the expansion of these products is likely to be more pronounced. Panel buckling, which occurs when there is no room for panel expansion, 按照APA在技术说明D481N ?中的建议，在所有护套板边缘和端接头之间使用标准的1/8英寸空间来防止 Minimizing Buckling of Wood Structural Panels. In larger buildings (over 80ˈ in length), 建议增加面板之间的间隙，并采取额外的施工顺序预防措施，以避免面板屈曲. The WoodWorks paper, Accommodating Shrinkage in Multi-Story Wood-Frame Structures, discusses this, as does technical note U425 Temporary Expansion Joints for Large Buildings from APA.