Investigation Of Duration Of Load Effect Of Wood

The strength of wood decreases and the deformation of wood increases under constant load,this phenomenon is known as the duration of load effect(DOL effect)of wood.It is important for the design of timber structures to correctly reflect the DOL.However,there is still a lack of agreement on some important issues,such as whether the DOL of structural timber is the same as that of clear wood,and whether load duration affects the modulus of elasticity of wood.Meanwhile,there is still a lack of explanations to the decrease of wood strength caused by the DOL.Hence,in this paper,a systematic and in-depth study was conducted on the DOL of wood.The methods for the DOL test were discussed and investigated.The accuracy of the two strength estimation methods,pair matching and sample matching,was compared.The DOL models for predicting the long-term strength of wood based on damage accumulation theory and fracture mechanics theory were introduced.The damage processes of wood subjected to long-term tests using the constant loading method and the stepwise loading method in DOL tests were calculated based on the damage accumulation model.And the DOL curves based on the two loading methods were compared in terms of the decrease of wood strength caused by DOL.The methods for determining the DOL factors in different national design standards for timber structures were presented and analysed,and a method for calculating the DOL factors that took into account the load history during the design reference period was proposed.An experimental investigation was conducted on the DOL of structural timber and clear wood,in which the DOL of structural timber was reflected through the bending tests of Grade No.2 and No.3 dimension lumber,and the DOL of clear wood was reflected through the bending tests of small clear specimens.The short-term and long-term tests were conducted at constant temperature and relative humidity.The sample matching method was used to estimate the short-term strength of dimension lumber specimens and small clear specimens in the long-term tests.And the DOL curve,which reflected the decrease of wood strength with load duration,was obtained.The test results were compared with the Madison Curve,Wood curve and Madsen and Barrett’s test results,and the reasons for explaining the differences and similarities of the comparisons were analysed.The stress ratio threshold for wood damage was verified based on the sample matching results,and the suggested value of the stress ratio threshold was proposed.An experimental investigation was conducted on the DOL of clear wood loaded parallel to grain and perpendicular to grain.A loading rig was designed for the long-term tensile test of the small clear specimens.The constant loading on the specimens came from the amplified load from the weight of the steel cubes by the use of the lever.The relationship between the strength decrease and the load duration time was obtained through the long-term tests.The DOL models were calibrated based on the DOL test data to predict the strength of wood beyond the long-term loading period.An experimental investigation was conducted on the effect of load duration on wood deformation(creep)and modulus of elasticity.The creep power-law model was calibrated using the creep data.The deformation factors corresponding to a design reference period of 50 years were given based on the calibrated creep power-law models.The long-term modulus of elasticity of the survived specimens from the long-term loading was measured and calculated,and the decrease of the modulus of elasticity of wood caused by the load duration was obtained.As a viscoelastic material,wood has the properties of creep and stress relaxation.In this paper,the decrease of the wood strength caused by the DOL was explained in terms of viscoelastic theory.A constitutive relationship based on viscoelastic theory was proposed to reflect the strength decrease and creep of wood.The concept of stress transfer between parallel viscoelastic elements,the concept of time-dependent viscoelastic parameters and the concept of fractional derivative dashpot element were introduced into the proposed constitutive relationship model.The test results were used to calibrate the proposed constitutive relationship model,such that the calibrated model could reflect the effects of load duration on wood strength,modulus of elasticity and deformation simultaneously.