During strong earthquakes, brittle fracture occurred to the welded joints of steel frame, which caused severely structural damage. The traditional models, such as bar model, solid model, fine model, multi-scale model, and spring-bar model, are unable to simulate and predict the bearing capacity degradation and stiffness degradation of steel components and joints accurately and efficiently at the same time. Traditional models may underestimate the structure deformation and overestimate the structural collapse capacity; it is a potential safety hazard. For the bolt welded joints, which are common of steel frames, one theoretical model was proposed to simulate damage degradation of welded steel frame with enough accuracy and efficiency. Dynamic elastic-plastic time history analysis was conducted, considering the joint rotation characteristic of steel frame through the finite element program, to develop an effective method of seismic vulnerability analysis. The dissertation mainly includes the following five parts.
(1) The comparison and analysis is performed about the advantages, disadvantages, and the applicability among the common finite element models, such as bar model, solid model, fine model, multi-scale model and spring-bar model. A new type spring-bar model considering the rotation performance of bolt welded joint was proposed in this research; this model was validated by the strong axis and weak axis joints test, pseudo-static experiment, and shaking table test of steel frame. By comparing the dynamic characteristics (period and vibration mode) of finite element (FE) models from ANSYS and SAP2000 programs, the feasibility and practicability of simulation was verified. (Chapter 2)
(2) The similarities and differences between performance-based seismic design specifications of China and the United States are studied in terms of seismic fortification levels, performance level, and performance goal. Based on the existing literatures about steel frame, a series of performance-based parameters about performance level, damage grade, and performance indicator are selected. The Pushover analysis method is used to perform static elastoplastic analysis and determine the eigenvalues of the performance indicators of the plane and space steel frame. (Chapter 3)
(3) The seismic demand model of steel frame structure was established by taking earthquake peak ground acceleration (PGA) as the intensity parameters of earthquake ground motion, with the seismic demand parameters of the maximum inter-story displacement angle (ISDA) and the maximum roof displacement angle (RDA). According to the site characteristics and seismic intensity, 22 seismic ground motion records were selected from the Pacific seismic database. The ground motion intensity are divided into 9 groups including three seismic fortification levels. The incremental dynamic analysis (IDA) method is applied to study the unidirectional and multidirectional seismic time history analysis of the plane model and the space model of steel frame. The influence of joint rotation, space effect, floor-slab action, and the multidirectional seismic waves on the seismic response of the steel frame were discussed and analyzed. (Chapter 4)
(4) Considering 12 structural uncertainties and 1 ground motion uncertainty, latin hypercube sampling was used to study the seismic demand analysis of 100 "Structure - ground motion" samples of steel frame. Therefore, the influence of the uncertainty about structure and ground motions on structural response were further studied. (Chapter 5)
(5) The unidirectional and multidirectional seismic vulnerability analysis were conducted for plane model and space model of steel frame. Based on the method of probability, the structure failure probability were researched under a given ground motion intensity. The seismic vulnerability curves of the model were obtained in terms of the maximum ISDA and the maximum RDA to study the influence of joint rotation, space effect, floor-slab action, and the multidirectional seismic waves on the seismic fragility of the steel frame. The exceeding probability of the various performance levels were derived from the seismic vulnerability curves and compared to the seismic code. Finally, recommendations were made about the seismic analysis of steel frame and are beneficial for the structural seismic safety assessment. (Chapter 6)