چكيده به لاتين
Self-centering systems have been developed to sustain lateral loads with the following behavioral characteristics: 1) providing restoring force mechanism, and 2) providing Energy Dissipation (ED) mechanism. These two mechanisms result in a flag shaped force-drift curve under cyclic lateral loads. Among the self-centering systems, the rocking reinforced concrete wall has recently attracted a lot of attention. Rocking walls, as one of the common SC systems, composed of both restoring force and ED mechanisms. The restoring force mechanism in the system is provided by Post-Tensioning (PT) the cables between the wall and the foundation or the rocking blocks, and after the lateral force overcomes the restoring force, the gap opening in this system is always provided by rotating the wall relative to the foundation or the wall blocks relative to each other. ED fuses in rocking systems, including plain bar, butterfly-shaped fuse, and viscous damper, which are usually used in gap openings or around the wall in the rotation of the wall, can be employed in different rocking core. In high rise base-rocking walls, the higher mode effects created additional moment and shear demands in the middle of the height. In this study, bi-rocking walls and multiple rocking walls have been proposed to reduce the higher mode effects. The seismic studies of structures in three parts include; (1) base-rocking walls, (2) bi-rocking walls, and (3) multiple rocking walls. For this purpose, 4, 8, 12, 16 and 20 stories structures were evaluated seismically in three types of structures. In all three types of structures, seismic analysis was performed by scaling the ground motions to the spectrum of ASCE 7 code. Furthermore, probabilistic analysis of the first two types of structures has also been performed. The nonlinear time-history analyses were conducted in two directions using OpenSees software under three sets of seismic records including 22 Far-Field ground motions and 28 Near-Field ground motions that half of which are Pulse-like.
In the first part, the base-rocking walls are examined. The moment and shear demand created by the effects of higher modes on the base-rocking walls increase with increasing height. The moment demands created by FF and NF-no pulse seismic recordes and the shear created by FF and NF-pulse seismic recordes were more significant. The increased moment of wall of a 20-story structure subjected to FF and NF-no Pulse ground motions at the level of CP performance is 46 and 39 percent, respectively. According to the results of the analyses, the residual inter-story drift values in self-centering rocking walls are negligible. In this respect, the maximum amount of residual inter-story drifts in the 20-story structure under far-field ground motions was about 0.0011 in CP performance level. Finally, the NF-Pulse ground motions created more stress ratio than other seismic records in prestressed tendons.
To determine the appropriate location of rocking section in bi-rocking walls, one-quarter (R2-M1), one-half (R2-M2), and three-quarter (R2-M3) models were examined. The second part has two parts, which in the first part are analyzed under scaled records. The obtained results revealed that R2-M3 model is not efficient in reducing the effects of higher modes. However, R2-M2 model in high-rise buildings under FF and NF-no-pulse records could be effective in decreasing the moment by a maximum of nearly 41% and the shears by a maximum of 25% and 18%, respectively. Furthermore, the results showed that bi-rocking walls could not be effective in reducing the influence of higher modes under NF-pulse ground motions. Generally, the residual drifts were negligible in all the rocking systems under study. In the second part, probabilistic analysis was performed. To assess the seismic vulnerability of the models, a probabilistic approach based on the concept of fragility curves is utilized. To determine the optimum design, a utility coefficient is defined as the average of shear and moment reduction coefficients. The results showed that R2-M2 walls are effective under FF and NF-No pulse records, while R2-M1 walls show their best performance under NF-Pulse records.
In the third part, two sections introduced to study the behavior of multiple rocking walls. These sections included double and quad rocking walls. In both sections, seismic records have been scaled to design spectrum of ASCE7. In the first section, based on the area of prestressing cables, three types of double-rocking walls are considered and compared with the base-rocking and the fixed base walls. The results showed that generally, the double-rocking walls provides higher desirability coefficients than the other considered systems. Furthermore, the double-rocking walls by reducing the cable area in the bottom block (R2-H1) are more effective in reducing the effects of higher modes. In the second section, the results showed that the effects of higher modes (shear and moment demand) increase with the increase in the height of the rocking structures. Furthermore, NFnP and FF records produced higher mode effects in rocking systems as compared to NFP records. The results suggested that the quad-rocking walls with the reduced tendon area in the first block (R4-S1) are highly effective in reducing the effects of higher modes in NFP and NFnP records. They showed maximum utility coefficients of 67% and 65%, respectively. Also, quad-rocking walls with the reduced tendon area in the third block (R4-S3) were more effective under FF records and their maximum utility coefficient was 65%. The residual roof drift of rocking walls was so negligible that its maximum value in the 8-story structure under NFP records was 0.0008.
