Duration Of Load Effect And Reliability Analysis Of Timber Structures

Duration of load(DOL)effect of wood is that with time increasing the strength of wood loaded will decrease while the deformations of wood will increase.The DOL effect of wood is an important problem in design of wood structures,which significantly affects the determination of the strength design value of wood products,as well as stability evaluation of wood members and other aspects in design of wood structures.Nowadays,Chinese and foreign codes commonly use reliability methods to determine the strength design value of wood products for which the treatments of DOL effect is one of the key issues.Firstly,this paper studied the test methods of DOL effect.The existing testing methods for DOL effect were studied in terms of the experiment materials,loading mode and sample matching methods.Professor Madsen's controversial test results are analyzed by the theory of damage models.Several kinds of prediction models for wood long-term strength and the underlying theories were introduced including experience model,Gerhards model,Foschi model and creep-rupture model.The DOL adjustment factors in Chinese code and some foreign codes were summarized and all found close to Madsion curve.Through short-term and long-term tests,the perpendicular-to-grain tensile strength of larches in Northeast China was obtained and the influence of the DOL effect on the strength and elastic modulus of wood perpendicular to grain was investigated.It was found that the DOL effect for tension perpendicular to grain was less than that predicted by Madison curve,and such DOL effect follows a was convex trend line.According to the test data of this paper,the DOL effect for tension perpendicular to grain did not affected its elastic modulus.The parameters of several DOL model were calibrated using test data and DOL factors were given proposed for use in Chinese code.This paper studied the characteristics and differences of reliability analysis methods in Chinese,Canadian and European design codes of wood structures,especially the treatment methods of wood DOL effect in reliability analysis.The reliability analysis of bending wood members under different load combinations with the Chinese performence function.Canada reliability analysis methods adopted damage theories to calculate the damage of wood in design time and calculate the reliability index.Matlab programs were made to calculate the reliability index with Chiese methods and Canada methods.The influences of the wood strength variability and load ratio to the reliability analysis results were studied.