Investigation Of The Tension Strength Perpendicular To Grain Of Wood And Duration Of Load Effect

The perpendicular to grain tension strength of wood is very low and tension perpendicular to grain should be avoided in design of wood structures.But in some special geometrical shapes of the structural members like the curved and pitchedtapered beams or in the vicinity of some mechanical and adhesive joints,the local tension perpendicular to grain will happen inevitably when the structure is loaded.The Duration of Load(DO L)effect of wood is one of the characteristics that distinguish wood and wood products from other building materials.The DO L effect is that the strength of wood loaded will be reduced with load time.The scientific treatment of DOL effect has been going on for more than 200 years.Much effort has since been devoted to DOL research,and much new knowledge has been gained.Even so,the understanding and modelling of the duration of load behavior of wood and wood products is still far from complete;because so far little work has been done on the wood strength perpendicular to grain.Usually,the research on the DO L effect concentrates on the member under bending parallel to grain;Therefore,the research on mechanical performance of members in tension perpendicular to grain for short term and DO L effect perpendicular to grain for long term is very significant and practical.Firstly,the tension strength perpendicular to grain of wood was studied;this describing some of the basics mechanical properties of the wood perpendicular to the grain,based on the structural scales of the wood,in order to study the tensile strength of the wood perpendicular to the grain.The global tensile strength of the wood perpendicular to grain based on the Weibull weakest link theory was also described,and some factors affecting the strength of wood.Detail was given on the tensile strength perpendicular to grain,and measures for strength improvement were introduced.The objectives were to investigate the duration of load behaviour of wood in tension perpendicular to the grain over time periods,to investigate the effect of applied stress level upon the duration of load,to get information on how deformations are affected by time,i.e.creep behaviour.To attend these objectives,short-term tests of tension strength of wood perpendicular to grain of load effects in tension perpendicular to grain and long-term test of duration of load effect in tension perpendicular to grain were conducted.The specimens of Pinus sylvestris species for tension perpendicular to grain were cut fro m a glulam beam,and tests were conducted on the specimens subjected.It was found out that distribution of the perpendicular to grain tension strength of the specimens was in accordance with the two-parameter Weibull distribution.Based on the Matched Sample Technique,the matched specimens were subjected to constant tension load perpendicular to grain for longterm testing and the data of the long-term strength and creep were acquired.The long-term investigations were done at constant climate cond itions of 20°C and 65% relative humidity and during four months(from January 09 to May 31 2017).As a major general result it can be stated that that: the duration of load effect appears to be dependent upon the applied stress level,the DOL effect for tension perpendicular to grain is obviously slighter than the widely used Madison curve,and similar to the DOL effect for bending parallel to grain and the duration of load effect is less than that predicted by the Madison curve used in building codes for loads lasting less than 1 year.The explanation for this material-specimen behaviour consists in the effect of cylindrical anisotropy which results in a pronounced stress redistribution within the specimen.The methods to calculate the DO L factor or the DO L adjustment factor in some foreign codes for design of timber structures were summarized.The DOL adjustment factor for tension perpendicular to grain was shown according to the Canadian design specification,then the curve reflecting the relation between the res istance factor and the reliability index with the DO L effect was shown based on the result of the DOL test.A detailed discussion of the modelling of the duration of load effects based on the three common models used: the Gerhards model,the Foschi and Yao model,and the Neilsen model were introduced.