Research On Design Indexesof High-strength Structural Steels

Due to the excellent mechanical properties and weldability, high-strength steel(HSS) has begun to be applied in building and bridge structures gradually. It has been proven in recent studies that, the behaviours of HSS structures and members are quite different from those of the conventional mild steel, which results in the differences of design methods. The new version of Code for design of steel structuresin China has proposed the partial factors for resistance and the design strength values of the steel with the nominal yield strength 460 MPa. However, the design index of the steel with the nominal yield strength more than 460 MPa is still blank. In the mean time, though domesticGJ steels have also been used in someconstructions, the design index in design codes has just covered Q345 GJ. On the basis of Specification for design of high strength steel structurescodification, this thesis analysed the reliability and calculated the design indexes of HSS and GJ steels with different strength grades through the experiments, numerical simulation and theoretical analysis. The constitutive model of HSS under monotonic loading has also been studied. The research works in this thesis are as follows:(1) Mechanical properties were collected, including 11638 groups of HSS(Q500, Q550 and Q690) and 2912 groups of GJ steels(Q390GJ, Q420 GJ and Q460GJ). 79 tensile coupons of HSS and GJ steels were tested. The influences of the loading rate and compliance of the testing machine on the measured yield strength of HSS were also investigated. Utilising the mathematical statistical method and taking the collected and test data above into consideration, the statistical parameters of material property uncertainties of HSS and GJ steels were obtained.(2) The finite element model was established in the general finite element software ABAQUS. After verified by the test data, this model was used to calculate the ultimate loading capacity of 880 members for combined axial compression and bending, and 460 flexural members of HSS(Q500, Q550, Q620 and Q690).The differences between the existing data, finite elementanalysis values and code values were compared and then the statistical parameters of the model uncertainties of HSS members for axial compression, bending, and combined axial compression and bending were calculated.(3) On the basis of the results above and the existingresearch data, the suggested partial factors for resistance of HSS and GJ steels were proposed by applying the first-order second-moment reliability method which was programmed by the mathematic software Matlab. Furthermore, these factors were verified by the reliability analysis.(4) By the means of summarising the HSS experimental results and utilisingthe tensile test data above, the parameters of the multi-linear constitutive model of HSS were updated. Then a nonlinear constitutive model based on the Ramberg-Osgood model and a revised multi-linear constitutive model was presented.