Seismic Behavior Of Bolted End-plate Connections For Composite Steel And Concrete Structures

One of the most significant lessons learned from the 1994 Northridge earthquake and the 1995 Kobe earthquake was the premature cracking and brittle failure of welded steel special moment resisting frame (SMRF) connections. Research after the earthquakes indicated that the possible reasons resulting in the premature fractures at the bottom flanges were mainly attributed to the presence of floor slabs and the potential weld defects due to the discontinuities of in-situ welding. Therefore, it is urgent to find improved details such as reduced beam sections in an effort to protect the welds and study the effect of composite action of concrete floor slabs. Composite steel and concrete moment resisting frame systems integrating structural steel beams with reinforced concrete columns or composite columns gradually gain acceptance as cost-effective and efficient alternatives to conventional steel and reinforced concrete structures in regions of middle or high seismic zones and are widely applied in the United States, Japan, China and etc in recent years. In order to improve the constructability and meanwhile ensure excellent seismic behavior, an innovative type of connection details for composite structures consisting of steel beams and reinforced concrete columns (RCS) or concrete filled circular steel tube columns (CFT) was proposed. The proposed composite connection details involve post-tensioning the shop-welded endplates of the steel beams to the reinforced concrete or concrete filled steel tube columns using high-strength steel rods. The proposed joint details require no field welding, eliminating the problems of the welded steel connections and improving the speed of construction. In addition, the contribution of concrete in panel zones on shear resistance could be mobilized by both endplates and through-column bolts. As a new type of connection details, a rational design procedure was suggested to assure a ductile behavior, and both experimental and analytical research was conducted to evaluate the seismic behavior of bolted endplate connections. The main achievements in the dissertation can be summarized as follows:1. To evaluate the seismic behavior of bolted endplate connections and investigate the effects of beam type, reduced beam sections and concrete floor slabs on joints, a total of seven full-scale exterior RCS and CFT joint models were designed and tested under simulated seismic loading. The experimental results indicated that composite RCS and CFT joints with good welding exhibited good ductility and energy- dissipation ability. The bolted endplate connections could provide sufficient strength and stiffness for moment resisting frames under inelastic cyclic loading. The proposed design method could result in a ductile failure pattern for the connections, particularly along with the use of a steel beam with reduced section near the critical moment end. The presence of floor slabs contributed to the strength of joints significantly and reduced beam sections were effective in moving the buckling zone away from the welds.2. Three-dimensional finite element models for proposed bolted endplate connections were developed using general-purpose finite element program ANSYS and the models were verified by comparing the behavior of models with experimental results. The nonlinearity of geometry, materials and surface-to-surface contacts was included in the models. Using the program ANSYS, some parameters such as initial pre-tension force of bolts, details of bolted endplate connections, axial force ratio on columns, and strength of concrete as well as ratios of steel bars in the floor slabs affecting the seismic behavior of joints were analyzed in detail. The analytical studies demonstrated that the initial pre-tension force had no apparent effect on the global performance of the joint specimens but local effect. The thickness of endplate has some influence on the stiffness of connections when the flexural strength of the endplate was not higher than that of the steel beam. Owing to the contribution of concrete floor slabs, both the sagging and hogging strength of composite joints increased. The varied strength of concrete had little effect on the sagging strength for the weakness of concrete in tension but the hogging strength of joints was enhanced significantly with the increase of concrete strength. The reinforcement ratio of floor slabs had obvious influence on the sagging strength of joints but it didn't affect the hogging strength apparently. To ensure beam hinging according to the concept of"strong column–weak beam", the effect of floor slab should be taken into account in calculation of the strength ratio between columns and beams.3. An analytical model for bolted endplate connections was proposed using the platform of OPENSEES and the seismic performance of composite frames with bolted endplate connections were investigated by nonlinear static and time-history analysis of twelve case-study frames. The influence of composite action of concrete floor slabs and reduced beam sections on the seismic behavior of composite frames was studied in detail. The analytical studies show that the proposed analytical model for bolted endplate connections can simulate the test results well. Composite steel and concrete frames with proper design exhibited good seismic behavior, meeting the requirements of current seismic codes. Comparative studies of frames with or without considering the contribution of concrete floor slabs to the strength of beams indicate that the composite action of concrete floor slabs enhance the strength of frames and may change the failure pattern of frames. The composite frames designed under control of elastic deformation result in over-strength of frames. The reduced beam sections can reduce the ratio of over-strength of frames with slight reduction on stiffness. The reduced beam sections designed according to FEMA350 can result in the reduction of stiffness of frames from about 5% to 7%.Based on the above-mentioned experimental and analytical results, the proposed bolted endplate connections and composite frames with bolted endplate connections exhibit good seismic performance, meeting the requirements of current codes for seismic resistant frames. The suggestions for the design and analysis of bolted endplate connections can be applied in engineering practice and revision of future codes.