Establishment Of Constitutive Model At Elevated Temperatures For High Strength Bolt And Study On Mechanical Behavior Of T-stub Joint At Elevated Temperatures

Steel frame joints are generally composed of independent components such as beams,columns,bolts,etc.,and they are combined to bear loads according to design requirements in some form.According to the actual force response of the nodes,the nodes can be divided into hinged joints,rigid joints,and semi-rigid joints.And it is more economical to design the node as a semi-rigid node.In fire,the properties of the steel will change due to temperature,which is mainly reflected in the attenuation of strength and stiffness.Therefore,it is necessary to study the high-temperature characteristics of the individual components,high-strength bolts are an important part of the node,grade 8.8 and grade 10.9 high-strength are mainly used in constructions.With the increasing height of the structure and the increasing span of bridges,the use of grade 12.9 high-strength bolts is more common.There is no unified research and analysis on their properties at elevated temperatures,and there is a lack of research on the properties of grade 12.9 high strength bolts at elevated temperatures.When the research is transferred from components to nodes,the T-stub joint is one of the basic components in beam to column end plate connections.It reflects the stress characteristics of the tension zone for the end plate connections.There are many researches on T-stub joints at ambient temperature,but there is a lack of research for elevated temperature.When the full-scale test of the beam to column joints at elevated temperature is limited,it is economical and feasible to study the performance of the T-stub joint at elevated temperature,and there is almost no domestic test and analysis for T-stub joints at elevated temperatures,therefore,it is increasingly important to strengthen its research on the properties at elevated temperatures.This project is based on the National Natural Science Foundation of China(No.51578092).It has completed the study of the properties of high-strength bolts at elevated temperatures and proposed a constitutive model for high-strength bolts at elevated temperatures.The properties of the T-stub joints at elevated temperatures was further studied by utilizing the proposed constitutive model for high-temperature.The content of this article is summarized as follows:(1)The material tests for grade 8.8,10.9,and 12.9 high strength bolts at elevated temperatures.The stress-strain curve and the various mechanical indices at different temperature were obtained through the test.Besides,the corresponding reduction factor model was given.The reduction factor model is applicable to any type of high-strength bolt.(2)Study on the constitutive model of Grade 8.8,10.9 and 12.9 High Strength Bolts at elevated temperatures.Based on the Ramberg-Osgood model,a stress-strain constitutive model which is suitable for different grades of high-strength bolts was obtained by fitting tensile test data at elevated temperatures.(3)The constitutive model for high-strength bolts at elevated temperature and the mechanical reduction factor model were used in the analysis of T-stub joints.Based on the experiments of T-stub joints at elevated temperature,42 finite-element models of T-stub joints with different geometrical dimensions and temperatures were established.The effectiveness,applicability,and rationality of the proposed constitutive model are verified,and the variation of the bearing capacity and initial stiffness of the T-stub joint at high temperatures were also presented.(4)Based on constitutive model for elevated temperature,and the validated finite element model,a large number of parametric analysis of T-stub joints at elevated temperature was performed,including a total of 353 finite element models for steady state analysis and transient analysis at elevated temperature.Different factors have been considered in the evaluation of T-stub joints at elevated temperature.The innovations in this article are as follows:(1)For the first time,the mechanical indexes at elevated temperature for the whole series of high-strength bolts(grade 8.8,grade 10.9,and grade 12.9)have been obtained through experimental research,and a mutual reduction factor model have been proposed for three types of high-strength bolts.(2)For the first time,the constitutive model for three grades of high-strength bolts(grade 8.8,grade 10.9,and grade 12.9)at elevated temperature was established,which was used in the actual joint analysis at elevated temperature.It verified the applicability and effectiveness of the constitutive model.(3)For the first time,through finite element numerical analysis the mechanical properties of T-stub joints at elevated temperature were investigated in detail,including steady-state performance and transient performance at elevated temperature.And it provided some suggestions for fire-resistant design of T-stub joints.