Performance Of Curved Glulam Beams Under Load And Moisture Variations

Glued laminated timber(Glulam), as an engineered wood product, is widely used in timber structures due to its beautiful natural grain and good mechanical performance. Tensile stress perpendicular to grain can be induced in curved Glulam beams by the bending. Once the stress exceeds the strength perpendicular to grain which is very low in nature, failure of member is due to happen. During the service life of buildings, curved Glulam beams will be exposed to air humidity variations due to climate change or human activities. Desorption and absorption of moisture will induce differential moisture distribution in Glulam member, which gives rise the moisture-induced stress. The combined actions of load and moisture variations will cause combined stress effect, which affects the long-term performance of the Glulam beams and even lower their load capacity. Therefore, it is useful to investigate the long-term performance of curved Glulam beams under the combined actions both the theory aspect and engineering practice.Firstly, tests of full-size curved Glulam beams made from Northeast China larch in four-point bending were conducted. The load-carrying capability and the brittle failure mode was caused by the stress perpendicular to grain of the beams were revealed. Secondly, long-term tests of the curved Glulam beams under combined actions of moisture variations and load were conducted in an artificial climate room. The tests showed that the stress under load and moisture variation lower the load capacity of the curved Glulam beams. Moreover, mechanical parameters of Northeast China larch, such as the fracture toughness of Modes Ⅰ and Ⅱ, tension strength perpendicular to grain, tension strength parallel to grain, shear strength parallel to grain and the elastic constants were also determined by experiments, which offered a database for further numerical simulations.The constitutive model of wood, taking thermal expansion, swelling and shrinkage due to moisture content change, viscoelastic creep, mechano-sorptive creep, failure modes, anisotropic damage and effect of duration of load(DOL) into consideration, was established. Luikov’s heat transfer model and fick’s second law of diffussion were employed as the governing equations for the heat and moisture transfer. Viscoelastic creep of wood was described with the Kelvin Chain Model. Mechano-sorptive compliance matrix, which is related to rate of moisture change, was employed to describe the Mechano-sorptive behavior of wood. Orthotropic failure criteria were developed. The strain softening model of wood was established by introducing the 9 damage factors to simulate the brittle failure under tension and shear and plastic deformation under compression. A user defined sub-routine UMAT was encoded in Fortran to include the material constitutive model of wood into ABAQUS.FE models incorporating ABAQUS were developed to simulate the short-term test in four-point bending and long-term tests of curved Glulam beams under combined actions of moisture variations and load. The magnitude and distribution of moisture-induced stress and the failure modes of the tested curved Glulam beam were revealed via the FE simulations. Failure of curved Glulam beams under combined actions of moisture variations and load were simulated successfully and the time-to-failure was predicted fairly accurately. XFEM based models incorporating ABAQUS were developed to simulate the short-term tested curved Glulam beams subjected to four-point bending and to investigate crack initiation and propagation in curved Glulam beams.Integration of the stress perpendicular to grain over the distributed width was developed to express the magnitude of the moisture-induced stress. The moisture-variation-affected depth was defined to indicate the distance that reflects the significant effect of moisture variation deep into the member, or on the other hands, the limit that the effect of moisture vatiation decreases to virtually insignificant. Effects of the cross-sectional width, period and amplitude of RH variations and number of cycles of RH variations on the moisture-induced stress perpendicular to grain were investigated. A method of predicting failure of the curved Glulam beams due to tension stress perpendicular to grain was introduced. Moisture-induced stress perpendicular to grain under the RH vatiations in typical climate zones of China was investigated. The maximum stress perpendicular to grain in Glulam members in service conditions of some representative citites was obtained, providing useful reference for design of Glulam members in different servive conditi ons.