Study on the Seismic Response Variations of Tall Hybrid Hramed Tube Skeletons in Near-Fault Sites

This research investigates the trend of changes in seismic response of tall hybrid framed tube skeletons according to the obtained analytical results through conducting nonlinear dynamic response history analyses (NL-RH analyses) under three components near-field earthquake records. For this purpose, three 30-story structural models with framed tube resistant skeletons were selected and designed. The first resistant skeleton is classified as the basic model with a framed tube structural system. The second and third models are introduced by embedding of multi-level configurations of large scale zipper elements on the basic model, which connected to one or two columns in the first story. The existence of a designed multi-level arrangement of large scale zipper elements prevents the formation of expanded plastic mechanism and also relatively blocks the occurrence of any possible buckling in the lower-stories columns. The connection of the large scale zipper elements to the columns was defined rigid. The studied structures were loaded and designed in accordance with the notified provisions recommended by the Iranian national building codes (divisions six and ten) as well as the standard 2800 (fourth edition) [1-3]. The assumed hysteresis loops related to the possible formation of plastic hinges in structural elements have been adapted from the FEMA 356 [4]. These notifications were described to clarify the assigned non-linear behavior of the elements of each studied structure. All of the analyses were conducted through SAP 2000 software [5]. To perform nonlinear dynamic response history analyses, an ensemble of five earthquake records including one far-field and four near-field ground motions contain forward-directivity effects, were selected and scaled according to the fourth edition of the Standard 2800. The main criterion in choosing near-field records is the existence of distinct coherent pulses caused by the strong rupture directivity effects, which are emerged in the ground velocity time history [6-7]. In this research, a comprehensive numerical assessment was accomplished on the seismic response parameters of the studied structural models. The analytical evaluations are focused on the maximum inter-story drift ratios, the maximum relative velocity and absolute acceleration of the floors (defined at the center of mass CM), maximum axial and shear force resultants, the upper bound of flexural and torsional moment of the columns, and also the maximum rotation of the formed plastic hinges. By comparing the configuration of the plastic hinges formed in columns and beams, it is resulted that the presence of the large-scale zipper elements in the lower four stories of the structure relatively causes less damages as well as a greater time domain of dynamic stability. The use of these elements in the perimeter bays of tall framed tube structures results a more uniform distribution of the axial and shear forces, as well as bending and torsion moments in the peripheral columns. It is also resulted a noticeable reduction for the maximum inter story drift ratio of floors, the maximum relative velocity and absolute acceleration of floor levels. Moreover, by comparing the total weight of studied models, it is clear that the architectural embedding of the large-scale zipper elements would cause a slight increase for this factor while reducing the average relative displacement near to 15% as well. REFERENCES 1- Iranian National Building Code (2014) Design Loads for Buildings - Division 6. Tehran, Iran (in Persian). 2- Iranian National Building Code (2014) Steel Structures - Division 10. Tehran, Iran (in Persian). 3- Iranian Standard No. 2800 (2014) Iranian Code of Practice for Seismic Resistant Design of Buildings. Fourth edition. Tehran, Iran (in Persian). 4- FEMA (1998) Prestandard and Commentary for the Seismic Rehabilitation of Buildings, FEMA 356. Federal Energy Management Agency. 5- SAP 2000, Integrated Software for Structural Analysis and Design. Computers & Structures, Inc., Berkeley, California. 6- Mukhopadhyay, S., and Gupta, V.K. (2013) Directivity pulses in near-fault ground motions—I: Identification, extraction and modeling. Soil Dynamics and Earthquake Engineering, 50, 1-15. 7- Mukhopadhyay, S., and Guptaa, V.K. (2013) Directivity pulses in near-fault ground motions—II: Estimation of pulse parameters. Soil Dynamics and Earthquake Engineering, 50, 38-52.