THE EFFECTS OF INITIAL POST TENSIONING FORCE ON SEISMIC BEHAVIOR OF STEEL MOMENT RESISTING FRAMES BY POST - TENSIONED CONNECTIONS

After Northridge earthquake in 1994, different details were recommended for constructing new moment-resisting connections to attain a ductile behavior under earthquake excitations. The main aim of these aforementioned details, was just to avoid the occurrence of failure in the weld and transmit the inelastic deformation of beams in the interior span of the beam rather than the joint connection area. As a substitution instead of welded moment-resisting connections, researchers recommended the utilization of post-tensioned moment connections in moment-resisting frames to resist against earthquake loading. This study is to investigate the seismic behavior of the connections. Post-tensioned connections are composed of high-strength steel cables, which are erected parallel to the beam webs, and then are crossed through the column, and finally would be fixed at the column's flange face. Firstly, steel cables create compression between beam and column flanges; secondly, they provide the resisting-moment of connections against applied service loads, and eventually these cables would cause self-centering of columns, which means that the columns would get back to their initial position after they are tilted by earthquake excitation. Moreover, two steel angle sections are added at the top and bottom surfaces of these connections, in order to dissipate the earthquake energy in the joint area. Results of previous research indicate that the use of post-tensioned connections in steel moment-resisting frames reduces seismic demand of structure in comparison with moment-resisting frames with welded rigid connections. Moreover, post-tensioned connections modeled with the OPENSEES software and accuracy of modeling were compared with that of experimental results. The seismic demand of post-tensioned steel moment-resisting frames by a change in initial post-tensioning force for 4 ,7 and 10 story frames was evaluated under static and dynamic nonlinear analyses. The results indicated that with increasing initial post- tensioning force drift angle, residual drift angle was decreased, but by increasing the excessive initial force, leading to increase of base shear, energy dissipation in connections and development of local buckling in beam flange were redued.