Soil Category Effects, on the Efficiency of Additional Isolated Upper Floors (AIUF), in Tall Structures

Current research is carried out on Tall Steel Structures with welded connections, illustrating the efficiency of using Seismic Base Isolation System under the top stories. For this purpose, 8 two dimensional 20~60 stories braced Tall Steel Structures, equipped with belt trusses are modeled and designed due to 1st, 2nd and 3rd type of soil categories, based on Iranian 2800 seismic code and for Sa=0.25g Sa=0.35g spectral accelerations. Then, all designed models are equipped with Additional Isolated Upper Floor, behaving as a Soft Story on the top of the structure. After tuning the frequency and the damping ratios of these secondary systems, Nonlinear Time History Analyses are carried out for preliminary and secondary systems, using accelerograms recorded on mentioned soil categories and spectral acceleration levels. Finally by, studying the lateral base shear forces and lateral displacements of all models for all mentioned records, the efficiency of using AIUF in tall structures are discussed.