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Modern seismic resistant design has been focusing on development of cost effective structural systems which experience minimal damage during an earthquake. Unbonded post-tensioned precast concrete walls provide a suitable solution due to their self-centering behavior and their ability to undergo large nonlinear deformation with minimal damage. Several experimental and analytical investigation focusing on lateral load resisting behavior of unbonded post-tensioned precast walls has been carried out in the past two decades. These investigations have primarily focused on lateral load resistance, self-centering capacity, energy dissipation and extent of damage in confined concrete region of the wall system. Past experimental results have shown that self-centering capacity of the wall system decreases at higher lateral drifts. Particularly, rocking walls with higher energy dissipation capacity, sustain considerable residual displacement. This residual displacement in the wall system may affect the ability of the entire structure to re-center. Though increasing initial prestressing force helps in reducing residual drift, it also subjects concrete to increased axial compressive stress which may lead to premature strength degradation of confined concrete in rocking corners. Accurate prediction of expected concrete strains in confined regions during increasing drift cycle is critical in design of such wall systems. Simplified design procedures available in literature assume different values for plastic hinge length to estimate critical concrete strain values. The results from the experimental tests available in literature were analyzed, to understand the effects of energy dissipating elements on residual drift and to examine the accuracy of simplified design procedures in predicting critical concrete strain. Based on the findings, recommendations are made on design of energy dissipating elements and plastic hinge length for unbonded post-tensioned precast rocking walls.
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Seismic Retrofit of RC Shear Walls Using Selective Weakening and Self- Centering
Traditional retrofit methods typically focus on increasing strength/stiffness of the structure. This may increase seismic demand on the structure and could lead to excessive damage during a seismic event. This paper presents an alternative retrofit method which integrates concepts from selective weakening and self-centering (rocking) to achieve low seismic damage for substandard reinforced concrete shear walls. The proposed method involves converting traditional cast-in-place built shear walls into rocking walls, which softens the structure, while allowing re-centering. Laboratory tests were performed to validate the retrofit concept on a benchmark wall specimen designed to pre-1970s standards. Observations from the test showed minimized damage and excellent recentering in the retrofitted wall. Additional testing was carried out to verify a novel anchorage scheme for post-tensioning elements, required to implement the proposed retrofit. Ultra-High-Performance Concrete (UHPC) was judiciously used to minimize damage and optimize the retrofit process.
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- Award ID(s):
- 1662963
- PAR ID:
- 10382804
- Date Published:
- Journal Name:
- 12th National Conference on Earthquake Engineering
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Precast concrete shear walls with unbonded post-tensioning, which resist seismic loads have attracted the attention of researchers over the past 20 years. This study provides a database of a special subset of precast concrete shear walls tested under monotonic or cyclic loading: rocking walls, hybrid walls, and walls with end columns. These shear walls experience joint opening, undergo rocking motion over the foundation, and utilize unbonded post-tensioning to self-center after load removal. Seismic energy is dissipated in distinct ways that vary from nonlinearity of concrete and post-tensioning strands (rocking walls) to yielding of mild steel reinforcement or external energy dissipaters (hybrid walls and walls with end columns). The experimental drift capacity, strength, and damage sequence of walls from the literature were compiled. Onsets of cover concrete spalling, yielding of energy dissipaters, yielding of post-tensioning strands, fracture of energy dissipaters, and crushing of confined concrete were reported. ACI guidance on shear walls were evaluated by comparing the lateral drift and strength measured by testing and predicted by ACI.more » « less
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