Lateral and Torsional Seismic Vibration Control for Torsionally Irregular Buildings
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During strong earthquakes or wind gusts, it is likely that buildings with torsional irregularity in the plan have an can be seriously damaged, partially collapsed or fully collapsed. This is because Torsionally Irregular Buildings (TIBs) may have significant aerodynamic torsion loads that increase the eccentricity between the center of mass and the center of rigidity, especially in dominant torsion modes. For this reason, torsion leads to excessive increase in lateral motions when dynamic loads excite the buildings. Torsional irregularity is one of the main failure causes during strong dynamic excitations due to earthquakes or wind gusts. Ignoring torsional irregularity in seismic design analysis can cause unexpected damages and losses. To enhance the safety and performance of buildings, most of the current seismic provisions address this irregularity in two main ways. The first is computing torsional moment at each floor by using equations provided in various current seismic code provisions. After they are applied on each floor, the seismic analysis will be performed. The second is shifting the center of mass (CM) or stiffness (CS) to eliminate the eccentricity by putting additional masses or structural components such as braced frame systems on buildings. This research developed and validated a new torsionally effective control system for the purpose of enhancing the performance/safety and mitigating structural failure in Torsionally Irregular Buildings (TIBs) under bidirectional strong earthquake loads. It introduces the new integrated control system (ICS) applied to a benchmark 9-story steel building developed for the SAC project in California to suppress the undesirable lateral and torsional coupling effects due to eccentricity. The dynamic responses of the system were evaluated under N-S and W-E components of the real earthquake excitations of the El Centro (1940), Loma Prieta (1989) and Kocaeli (1999) earthquakes. First the traditional method (cross-braced frame systems) was implemented in the benchmark building with different pre-determined placement layouts. The most effective placement was determined and the benchmark building was analyzed with that for comparison purpose. Secondly, tuned mass dampers (TMDs) were designed and applied to start from the center of mass (CM) through two translational directions under bi-directional seismic loads such as N-S and E-W components of selected ground motions. Then the performance evaluation for TMDs was determined. The effectiveness of the TMD system was evaluated in terms of energy analyses and performance evaluation criteria including maximum floor displacement, maximum drift, and maximum floor acceleration. Based on these comparisons, there is a substantial reduction of the amplitudes of the frequency response validated the effectiveness of the ICS in controlling the seismic responses for two-way eccentric elastic buildings. Unlike traditional TMDs placed in two orthogonal directions, the ICS is more comprehended to control not only two orthogonal (x- and y-) directions, but also effectively control rotational (θ-) direction. By means of the proposed system configuration, the structures first-three dominants modes can effectively be controlled by the ICS regardless of any external energy sources. The ICS is also more robust in restricting the inter-story drift ratio as compared with TMDs. It sufficiently mitigates the RMS and peak displacement on the top floor of the Benchmark building. Thus, the ICS has a better performance than the TMDs and the CFs placement in terms of response reductions. According to the performance evaluation criteria, there are substantial reductions for both the tuning case and the detuning case. For both cases, the performance indexes are overall less than the bare Benchmark building and its respective application with the TMDs.