Research On The Properties Of Hclt In Compression Perpendicular To Grain

When used as a floor component,Cross-laminated timber(CLT)bears the vertical load imposed by the column or wall and even destroys.Therefore,the design and application of CLT products require parameters of modulus of elasticity and strength in compression perpendicular to grain.How to improve the properties of CLT in compression perpendicular to grain,improve the overall structural strength and building safety factor have become the focus of current CLT research.Previous studies have shown that hybrid cross-laminated timber(HCLT)can improve the bending and shearing performance of CLT.Therefore,this paper was aiming to evaluate the properties of compression perpendicular to grain of HCLT and expand the application of HCLT.In this paper,the full surface,partial point load and line load of HCLT in compression perpendicular to grain made with spruce-pine-fir(SPF)and laminated strand lumber(LSL)were studied.Three ratios of the thickness of LSL to the total thickness of specimen,i.e.(the thickness of LSL)were design and the influences of thickness ratio on the compression properties perpendicular to grain were evaluated Furthermore,the effects of different loading methods(two-sides load,one-side load)and loading positions(center,corner,edge parallel or vertical)on the partial compression perpendicular to grain of HCLT were also analyzed in this paper.Four calculate methods were used to analyze the adjustment coefficient of compression properties perpendicular to grain.The method one to four used were do not distinguish HCLT structure and do not correspond to full surface and partial loading area,distinguish HCLT structure and do not correspond to full surface and partial loading area,distinguish HCLT structure and correspond to full surface and partial loading area,and stress diffusion model with different HCLT structure,respectively.Finally,the digital image correlation(DIC)method was used to measure the strain development process and analyze the failure modes.(1)Full surface compression properties perpendicular to grain of CLT/HCLT.The test results show that under the condition of full surface loading,HCLT significantly improves the compressive strength of the CLT.With the increase of the ratio of the thickness of the LSL to the thickness of the HCLT,the strength increases linearly.The highest compression strength perpendicular to grain of HCLT is 35.71% higher than that of regular CLT.The compression modulus perpendicular to grain of HCLT decreased slightly compared to regular CLT.(2)Partial point load properties of compression perpendicular to grain of HCLT.With the increase of the proportion of LSL thickness in the thickness of HCLT,the properties of point load compression perpendicular to grain was significantly improved.In the case of one-side loaded in the corner position,the compression modulus and strength of the specimen(LSL-SPF-LSL)was 39.42% and 36.31% higher than those of the specimen(SPF-LSL-SPF),respectively.The bearing capacity of one-side loading was significantly higher than that of two-sides loading,the SPF-LSL-SPF and LSL-SPF-LSL specimen groups had higher strength of one-side loading in the center than two-sides loading,itwas 13.93% and 36.47%,and the modulus was 49.09% and 28.25% higher,respectively.The strength and modulus of the HCLT specimen loaded at the center were the largest,and the minimum at the corner.The modulus and strength of HCLT in the direction perpendicular to the surface texture were higher than those in the direction parallel to the surface texture.(3)Partial line load properties of compression perpendicular to grain of HCLT.With the increase of the proportion of LSL thickness in the thickness of HCLT,the strength of line load compression perpendicular to grain was significantly improved,LSL-SPF-LSL group had better properties than SPF-LSL-SPF group.When loaded in the center position,the modulus of LSL-SPF-LSL group was lower than that of SPF-LSL-SPF group,no matter it was one-side or two-sides loading,15.50% and 4.85% lower respectively.When loaded at the center,the bearing capacity of partial line load was better than that at the edge.Under the center loading,the compressive strength of SPF-LSL-SPF and LSL-SPF-LSL increased by 24.75% and 19.53%,respectively and the modulus increased by 31.07% and 44.37%,respectively.(4)Adjustment coefficient of compression properties perpendicular to grain.The adjustment coefficients of the two-sides loading of the point and line loads were smaller than those of the one-side loading,and the point load of the corner position and the line load of the edge position were lower than the adjustment coefficients when the center position was loaded.The adjustment coefficient of vertical surface texture direction loading was higher than that of parallel surface texture direction loading.The adjustment coefficient of modulus was lower than the adjustment coefficient of strength.The adjustment coefficient of the strength of the specimen group with the blank assembly method LSL-SPF-LSL was higher than that of the SPF-LSL-SPF specimen group.The range of point and line load strength adjustment coefficients calculated according to method one are 1,21 to 1.99 and 0.99 to 1.48,respectively.The adjustment coefficients calculated by method two,method three,and method four in the SPF-LSL-SPF specimen group were relatively closed,while for the LSL-SPF-LSL specimens,the adjustment coefficients obtained by the method three were significant high,while in addition,the adjustment coefficients calculated by method two and method four were very closed,and the adjustment coefficients calculated by the HCLT stress dispersion model established by method four were in good agreement with the adjustment coefficients calculated by method two.(5)DIC analysis and failure modes.The results show that the CLT/HCLT specimens under the stress level of the maximum load,the high strain area of the horizontal strain field appeared at the unglued side of the SPF,the high strain area of the vertical strain field mainly appeared in the LSL,and the high strain area of the in-plane shear strain field was mainly concentrated in the SPF.Vertical strain field of HCLT specimens under different stress levels can be seen that the destruction of the material under the loaded rigid plate was partialized,the high strain area was concentrated in the position below the loading end,and the impact of other wood areas not directly compressed was not obvious.The main failure modes was that there were cracks in the pulp center of SPF,or along the direction of rings and wood rays,the fiber ofLSL material would break when it was partially loaded,and the rubber layer cracked in the area without direct compression was more serious.According to the strain distribution field of HCLT specimen,the strain distribution was consistent with the stress dispersion model of HCLT,and the longitudinal and transverse stress dispersion angles were about 45°and 15°,respectively.