Abstract: [Objective] Environmental temperature and relative humidity (RH)play important roles in dimension deformation and physical mechanical properties of bamboo culm, the investigation of strain property of bamboo culm can provide a basis for dimensional stability and environmental adaptability of bamboo culm utilization in original status.[Method] Annual mean temperature and RH of six cities in China were selected and analyzed to research hygrothermal strain behavior of bamboo culm (Phyllostachys edulis). Bamboo culms with similar diameter (96.52 mm±1.46 mm)and different lengths (500,400,300 and 200 mm)were placed in test chamber undergoing the temperature and humidity environment which were successively changed with six stages (5℃,66%; 13℃,50%; 20℃,70%; 20℃,80%; 30℃,80%; 9℃,36%). The circumferential strain value of the top and middle parts and the axial strain value of bamboo culm were collected by static strain indicator, and the influence of temperature, humidity and length on bamboo culm response behavior were analyzed and discussed.[Result] The initial value of strain was set as zero, and the bamboo culm was shrinkage from laboratory environment to test chamber, the strain value was negative. Subsequently, the bamboo culm showed swelling with the increase of temperature and humidity for moisture adsorption, and the strain value was positive. Swelling strain value was declined quickly and even indicated shrinkage with the sharp decrease of temperature and humidity. Circumferential strain value range was -500-3 000 με,while axial strain value ranged from -50 to 225 με. Especially, the circumferential strain of top part displayed more sensitive to the temperature and humidity change, while the strain of middle part was hysteresis. The max strain difference between top and middle parts of bamboo culm with length of 500 and 400 mm was significantly larger than that of bamboo culm with length of 300 and 200 mm. The difference values of bamboo culm with length of 300 and 200 mm changed from positive to negative, and the strain displayed wane and wax.[Conclusion] Bamboo culm could make immediately response to environment changes, temperature, humidity and length had significant impacts on the swelling and shrinkage strain behaviors of bamboo culm and the influences of temperature were always reflected by humidity which was associated to moisture content of bamboo culm. Circumferential strain behavior was similar with that of axial strain, while its value was obviously greater than axial strain value, top part of bamboo culm was more sensitive to temperature and humidity, and the longer length means more strain difference between top and middle parts. The stress unequilibrium in longer bamboo culm induced by moisture element adsorption and release is the main reason for bamboo culm deformation and rupture.

Key words: bamboo culm, hygrothermal environment, annular strain, axial strain, stress distribution