Duration Of Load Effect And Creep Performance On Hybrid CLT Fabricated With SPF And COSB

The wooden structural members will gradually increase in deformation and decrease in strength due to the duration of load(DOL)effect with the duration of the load increasing.The orthogonal lay-up and rolling shear characteristics in transverse layer of cross-laminated timber(CLT)make its DOL effect more complicated.Hybrid cross-laminated timber(HCLT)based on the development of domestic structural boards is an important direction for the development of CLT materials.The development of long-term mechanical performance research on HCLT will be contributed to promoting the application and improvement of CLT for wooden structure buildings in our country and relevant design standards.The research object is CLT/HCLT with different lay-up structure made of SPF and domestic construction oriented strand board(COSB).Low-cycle fatigue cycle test and long-term creep test were carried out to study the DOL effect and creep performance of CLT/HCLT.The main research methods are as follows:(1)Low-cycle fatigue performance research: CLT/HCLT specimens with three-layer and five-layer lay-up structures were subjected to low-cycle fatigue cyclic loading shear and bending tests,respectively.The failure modes and failure cycles of the specimens were recorded.to establish the stress-based damage accumulation model and then obtain the stress ratio-failure time logarithmic relationship curve.the CLT/HCLT DOL adjustment coefficient was obtained based on the stress ratio evaluation method to analyze the difference of long-term fatigue performance for different groups about structural strength.The test results showed that the fatigue specimens have almost the same failure mode compared with the short-term test.The predicted value of the calibrated damage accumulation model,whose average is about 14 percentage,can be better consistent with the test data.For shear specimens,3BS,3CS and 5BS groups whose transverse layer is COSB board,showed better fatigue resistance.The fatigue performance of 5-layer specimens is generally better than that of 3-layer specimens.For bending specimens,the fatigue performance of 3AL(common structure CLT)group is the worst,and that of 3CL group(full COSB panels CLT)is second to 3BL group.In the case of 30 years of long-term constant load,the DOL adjustment coefficient of the 3AS group(common structure CLT)is the smallest,which is 0.519.The value is lower than the strength correction factor recommended in the CLT manual without considering the conditions of use.The DOL adjustment coefficients of the 3BS group and 3CS group specimens are 0.745 and 0.775,respectively,showing good long-term strength.Among the 5-layer shear specimens,the DOL adjustment coefficient(0.75)of the 5AS group is also lower than that of the 5BS group(0.816)which is hybrid structure.For the bending specimens,the DOL adjustment coefficients are very close,which are between 0.81 and 0.83.In general,the use of COSB placed in the transverse layer to prepare CLT components can effectively improve the long-term strength performance of CLT.(2)Creep performance study: A 94-day shear and bending constant load test was carried out on three-layer CLT/HCLT specimens with different lay-up structures to analyze the creep development law of different span-to-height ratios and different lay-up structures.The power law model and the five-parameter creep model are used to fit the test data to predict the long-term creep coefficient,and analyze the long-term creep performance of CLT/HCLT.The test results showed that the shear and bending specimens with common structure,such as 3AS and 3AL groups,have large fluctuations in creep deformation affected by temperature and humidity changes due to the completely solid wood composition.The relative creep value of the shear specimens is generally higher than that of bending specimens.There is no obvious difference in creep deformation among different shear specimens groups.For bending specimens,the 3BL group(HCLT)with COSB in the transverse layer shows the most stable creep curve and the minimum creep deformation value.Compared with the power law model,the five-parameter creep model can better fit the test data.The five-parameter creep model predicts that the 50-year creep coefficient of the bending specimen is less than 2.The 3BL group has the minimum creep coefficient(1.299)).The 50-year creep coefficients of the shear specimens are all greater than 2,and the minimum one is the 3CS group(2.013).