Improving Seismic Behavior of Irregular Buildings by Whirling Tuned Mass Dampers

Buildings should be designed to resist earthquake-induced deflections and internal forces. The experience of recent earthquakes illustrates that the amount and extent of damage in irregular buildings are far more significant than the others. Irregularities in the structural system may amplify structural response leading to significantly more severe damage compared to regular structures. In fact, when irregular structures are subjected to lateral seismic loads, they will experience lateral motion accompanied by torsional rotations, which is due to an eccentricity between the center of mass and the center of stiffness. In other words, structural irregularities decrease the seismic performance of buildings significantly, and they will be heavily damaged as a result of torsional effects on structural elements. Many studies have been conducted on reducing torsional effects on structures. One of the approaches is to apply control systems. In this study, efficiencies of some passive damping controls are investigated to reduce the torsional irregularities in building structures. One type of passive control system is tuned mass dampers (TMD), which usually have a significant mass. Having a great mass can be a drawback for these types of systems and limits their application in practice. Therefore, to eliminate this issue, a new type of tuned mass damper called Whirling Tuned Mass Damper (W-TMD) has been recently introduced in the literature. This type of tuned mass dampers has a smaller mass compared with ordinary TMDs. In the present study, the seismic performance and behavior of ordinary TMD and W-TMD have been investigated and compared. For this purpose, the seismic behavior of three similar buildings, with different controlling systems, having five story steel moment resisting steel structures are compared. The first building does not have any controlling system; however, the last two ones are equipped with TMD or W-TMD. Nonlinear time history analysis results of these buildings under five earthquake records are compared. The applied records are for Northridge, Loma Prieta, Kobe, Imperial Valley, and Chi-Chi earthquakes. The obtained results show that buildings with controlling systems are much better; however, W-TMD has a better performance in reducing the story drift and structural torsional modes, compared to TMD.
Moreover, a sensitivity analysis is carried out on the properties of a W-TMD by changing the method of supplying the required inertia. Two different methods are chosen: the first one has a solid disk but the second has a ring section. The results showed that when W-TMD is fitted with the ring cross-section, not only it has a smaller mass, but also it has a better performance in decreasing the irregularity response of the structure. To be exact, the higher the ratio of the radius of the inner circle to the outer circle of the ring, the greater the amount of inertia will be, and therefore W-TMD requires less mass. Since a W-TMD applies less mass to the structure, it can be an excellent alternative for TMD. If the W-TMD is equipped with a disk section, it has 66% the mass of TMD, while using ring section, it can have 42% of the TMD mass. The obtained results of the sensitivity analysis of W-TMD confirm that the damper mass can be reduced up to 50% without significantly reducing its efficiency. Some damage indices, including drift story, torsional rotation of floor and torsional irregularities coefficient, are considered for evaluating the performance of each model equipped with TMD and W-TMD. It can be concluded that the model equipped with a W-TMD has a much better performance in reducing all damage indices.