Shear resistance of glulam beams under varying humidity conditions

The research programme of this study included tests of 104 glulam beams. The aim of the experiments was to find out whether the variation of humidity affects the shear strength of large size glulam sections. It is known that moisture gradients are induced in dynamic humidity variations, and these produce internal stress distributions to wood members. The target of this study was to find out if these stress distributions have an effect on the shear strength of a glulam beam. The beams were divided into five test series and they were stored for different time periods in specific moisture conditions before testing. Beams were first conditioned in moist air (RH 90%) and then in dry air (RH 30%). The effect of different coatings was also studied since coatings may affect the moisture variations of a wood member caused by varying humidity. The results showed that moisture gradients caused by varying humidity conditions had no effect on the shear strength. Solely the actual mean moisture content of the beam seemed to affect the shear strength. The coatings applied were discovered to have only minor effects on the induced moisture gradients and no effect at all to the shear strength of the beams. The high initial moisture scatter between the material provided by one producer (out of four) disturbed the interpretation of test results especially when considering moisture content effects. The deviation in the moisture content measurement results was significant even when the moisture scatter from one producer was not taken into account. The differences in shear strengths between manufacturers had also an effect to the analysis of the results. The effects of moisture induced stresses could have been easier to observe if the material had been more homogenous. Finite element calculations were conducted to evaluate the moisture induced stresses in the beam. Calculations supported the obtained results that moisture gradients and moisture induced stresses do not decrease the shear strength of glulam beams. The calculated moisture induced stresses were small in those directions that had highest stresses in the actual loading. The visual examination of failure planes together with finite element calculations suggested that shear failure occurred because shear strengths in Trl and Ttl directions of the local wood coordinate system were exceeded. Calculated moisture induced shear stresses were however small and local in those directions.