Research On Standard Of Wood Member Safety Grade Appraisal Based On Reliability Level

The current"Standard for Reliability Evaluation of Civil Buildings"classifies the structural safety assessment into four grades.However,in the inspection items of safety assessment of single component,only three boundary values between four grades are specified for the bearing capacity items,while in other items,only the boundary values between a_u,b_u and c_u,d_u are specified.In this paper,the reliability theory is used to study three safety appraisal testing items of wood component:bearing capacity,ultimate deformation and fire resistance limit.The general idea is to deduce the corresponding function from the resistance and effect calculation formula,then determine the target reliability index?of each level on the basis of current standard,and finally get the specific classification standard of the detection items matching the target reliability index.The research contents and results of the three testing projects are as follows:At present,the ultimate deformations item only gives the boundary of whether reinforcement is needed,which origins from"Technical Specification for Maintenance and Reinforcement of Ancient Building Wood Structures"and other data,which is equivalent to only two grades.This paper fully affirms the rationality of the current standard,and uses the function of long-term deformation considering creep to find out the implied reliability index of the existing grade limit.The target reliability index of the three grade limits is composed by adding or subtracting0.25 on the base of the implied reliability index,and the classification standard is listed.In addition,when calculating the reliability index,the load combination checked is:constant load,constant load adding live load of constant load,residential floor,live load of office floor,wind load and snow load,and the change of the ratio?equaling live load to constant load from 0 to4.0 is taken into account,so it is suitable for lumbps,log and modern light wood structures in terms of load combination.Fire-resistance limit classification of wood components is a new test item proposed in this paper firstly.After determining the fire-resistance limit of existing wood structural components by testing and calculating,according to the classification standard obtained in this paper,the fire-resistance limit of existing wood structural components can be classified,and then whether additional fire-resistance measures of the components aer needed can be determined.Taking the residual section bearing capacity of components after burning as long as T_d,the function containing fire resistance limits of different grades is established.Provided that components of grade a_u meet the fire resistance limit of the design code,the target reliability index of other grades is determined by subtracting 0.25 each grade down.According to the corresponding relationship between the ratio of geometric parameters of residual section‘a'and the reliability index,the value‘a'of each grade is obtained.Finally,the fire resistance limit classification criteria related to size is given for beams and columns with circular and rectangular sections.The theory of bearing capacity testing project has been updated.The target reliability indexes of each grade of general components are determined as?₀,[?]and[?]-0.25 respectively according to“the Unified Standard”,which are different from the current standards.This paper compares the reliability indicators of 50,20 and 10 years of service life in the future,and finds that with the decrease of service life,the overall reliability indicators improves,but have little impact on classification.The calculated grade is lower than the standard grading standard.