3-D NONLINEAR ANALYSIS for SEISMIC ASSESSMENT of VERTICALLY MIXED STRUCTURES
Mixed structures, which consist of two different structural systems in height, don’t show good seismic response. Use of reinforced concrete frames for lower stories and steel frames for upper stories is an example of hybrid buildings and Under the earthquake loads, steel frames may slip on lower concrete parts. In this paper 5, 10 and 15 story buildings that have R.C frames with shear walls in lower stories and steel frames with bracings in upper stories are investigated. The linear behavior of these buildings (lateral displacement, internal forces in transition level,…) is investigated with ETABS 2000 software. Moreover Push-over analysis is done and capacity diagrams are presented too. Also nonlinear behavior of these buildings under the earthquake loads (ductility, ultimate lateral displacement, ultimate strength, arrangement and position of plastic hinges, hystersis loop diagrams,…) is investigated with RAM PERFORM-3D software according to FEMA-273 and FEMA-356. Analytical results show that with using “TRANSITION STORY” between two different parts of hybrid structures, the seismic behavior of these buildings is improved. Transition story, which is located in the transition level, is a composite (steel- concrete ) story with composite columns, Shear walls and steel bracings. Number of transition floors is dependent on total height of hybrid building and applied loads. More details about this story presented in this paper. The best position of this story is in one-third of the height of hybrid building from above. Moreover with using transition story in hybrid structures, axial and shear forces at transition level decrease and ductility increases.