Date of Award
A practical retrofitting method for enhancing the inelastic performance of existing semi-rigid steel connections is introduced and verified in this study. The method entails adding high-strength steel strands parallel to the beam, passing them through the column, and anchoring and post-tensioning them properly. To verify the proposed retrofitting system, firstly, a series of semi-rigid bolted connections was experimentally tested under monotonic and cyclic loadings. Then, the post-tensioning system was applied on the connections with the same geometry, and the cyclic responses were studied under different post-tensioning parameters. Following the recommended setup explained in this study, a locally post-tensioned (PT) structural frame will experience enhancement in the lateral loading responses in the formed of a self-centering capability and increases in the stiffness and strength. In the experimental approach, the test subassembly was represented by an exterior beam-to-column connection and was constructed with connection angles with three different thicknesses. The specimens were loaded monotonically and cyclically to investigate the complex interaction between the connection components, particularly the bolts and the angle column leg interaction. A special effort was made to study the effect of the slip between the angle beam legs and the beam flanges on the nonlinear responses of the beam-to-column connections. According to the observed deformation patterns, analytical equations predicting the moment-rotation behavior of bolted angle connections were presented and compared to the monolithically loaded test results. Generally, steel frames with semi-rigid connections experience high residual connection rotations and story drifts following an earthquake that cause significant repair expenses or require demolishing the whole structure. The research studies on PT steel connections showed a self-centering capability that eliminates or reduces these residual deformations after cyclic loading. The general analytical equations for evaluating the effect of post-tensioning on the moment-rotation response of steel connections are presented and modified according to the locally PT system represented in this study. The proposed retrofitting method of existing semi-rigid connections was experimentally tested by adding PT strands to the subassembly setup. This retrofitting setup is applicable to be added to both interior and exterior connections. To anchor the high-strength steel strands, stiffener plates were welded to the beam at the specific length from the column face. The strands were then passed through the column flanges and the stiffener plates and were anchored against them. Five PT exterior connections with different PT strand lengths, initial post-tensioning forces, and angle thicknesses were tested. Comparing the test results of the PT specimens to the semi-rigid connections without post-tensioning showed the cyclic response improvement. The post-tensioning approach decreased the residual rotation and increased the stiffness, strength, and hysteretic energy dissipation capacity of the connections. Shorter strands provided higher increases in the stiffness and strength; however, the effect of the strand length on the energy dissipation capacity should further be studied. The tension force loss in the PT strands highly reduced the self-centering capability and was more significant in the shorter strands. In the last chapter of this dissertation, the presented analytical equation for predicting the effect of PT strands on the bending stiffness of a PT connection was verified and later modified using the experimental test results.
dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Movaghati, Sepehr, "Cyclic Performance Improvement of Existing Steel Semi-Rigid Connections Using Post-Tensioning" (2018). Electronic Theses and Dissertations. 1954.