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Ductile steel panels with shear link-brace system to enhance the seismic performance of reinforced concrete buildings
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In this research, a seismic retrofitting method for chevron-braced frames (CBFs) is proposed. The key idea here is to prevent the buckling of the chevron braces via a conventional construction technique that involves a hysteretic energy-dissipating element installed between the braces and the connected beam. The energy-dissipating element is designed to yield prior to buckling of the braces, thereby preventing the lateral stiffness and strength degradation of the CBF caused by buckling, while effectively dissipating the earthquake input energy. Nonlinear static pushover, time history and damage analyses of the CBF and retrofitted CBF (RCBF) are conducted to assess the performance of the RCBF compared with that of the CBF. The results of the analyses reveal that the proposed retrofitting method can efficiently alleviate the detrimental effects of earthquakes on the CBF. The RCBF has a more stable lateral force–deformation behavior with enhanced energy dissipation capability than the CBF. For small-to-moderate intensity ground motions, the maximum interstory drift of the RCBF is close to that of the CBF. But, for high intensity ground motions, it is considerably smaller than that of the CBF. Compared with the CBF under medium-to-large intensity ground motions, the RCBF experiences significantly less damage due to prevention of buckling of the braces.
For a full-scale seismic upgrade of the U.S. Court of Appeals building, nonlinear dynamic time-history analyses were performed to evaluate three isolation systems: high damping rubber; lead rubber bearings; and a friction-pendulum system, recently developed in the U.S., that depends on pendulum motion and friction to reduce earthquake forces. The FPS was finally chosen on the basis of highest technical rating and lowest cost. (FPS bearings provide flexibility in installation and maximum headroom without the expense of additional foundation work). The FPS implementation and construction are described here.
Four reinforced concrete column units were tested subjected to simulated seismic loading to investigate repair and strengthening techniques. The as-built columns were 350 mm (13.8 in.) square and contained low quantities of transverse reinforcement as was typical of building columns designed and constructed prior to 1970. The column units represented the column region between the midheights of successive stories. A stub was present at the mid-height of each unit to represent a portion of the two-way beams and slab at the beam-column joint. Two column units were tested, repaired, and strengthened by jacketing and retested. The other two column units were strengthened by jacketing and tested. The jacketing consisted of a 100-mm (3.94-in.) thickness of added reinforced concrete. The new longitudinal reinforcement was placed through the floor slab. Two arrangements of transverse reinforcement in the jacket were investigated. The as-built columns displayed low available ductility and significant degradation of strength during testing, whereas the jacketed columns behaved in a ductile manner with higher strength and much reduced strength degradation. The retrofit of columns using reinforced concrete jackets was found to be successful but labor-intensive.
In this paper an experimental analysis is presented for the mechanical characteristics of multi-layer elastomeric isolation bearings where the reinforcing element—normally steel plates—are replaced by a fiber reinforcement. The fiber reinforced elastomeric isolator (FREI), in contrast to the steel reinforced elastomeric isolator (SREI) which is assumed to be rigid both in extension and flexure, is assumed to be flexible in extension, but completely lacking flexural rigidity. The FREI is designed and fabricated for evaluation of the performance on seismic isolation. Experiments are carried out to evaluate and compare the performances of fiber reinforcement with performance of steel reinforcement, and the differences in performance among different kinds of fiber reinforcements. From the experiments, the performance of the FREI is shown to be superior to that of the SREI in view of horizontal stiffness and vertical stiffness of the isolator. Therefore, it is possible to produce an FREI that matches the behavior of an SREI. Consequently, the FREI could replace the conventional SREI for seismic isolation with low-cost manufacturing and lightweight installation.
Prestandard and commentary for the seismic rehabilitation of buildings
FEMA 356. 2000 Prestandard and commentary for the seismic rehabilitation of buildings. Federal
Emergency Management Agency. Building Seismic Safety Council, Washington, D.C.