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1. CAPACITY AND DAMAGE INVESTIGATION OF PRECAST CONCRETE SELF-CENTERING SHEAR WALLS

   Basereh, Sina; Sharma, Sumedh; Baque, Paulette; Blaggan, Esha; Okumus, Pinar; Aaleti, Sriram (January 2021, 2021 PCI/NBC Convention)

   null (Ed.)

   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. 

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2. A study on Residual Drift and Concrete Strains in Unbonded Post-Tensioned Precast Rocking Walls

   Sharma, Sumedh; Aaleti, Sriram (June 2019, Proceedings of the ... Canadian Conference on Earthquake Engineering)

   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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3. Lateral response of post-tensioned pendulum shear walls

   https://doi.org/10.1016/j.istruc.2024.106987  

   Silva, PF; Lagler, C; Burgueño, R (July 2024, Structures)

   This paper presents an experimental evaluation of a new interface configuration for unbonded post-tensioned shear (UPTS) walls. In the proposed configuration, the wall-base interface is shaped in a circular profile. This circular profile presents a major difference from the currently used flat profile at the wall-base interface of rocking UPTS walls. During lateral response, this circular profile induces the wall to predominantly rotate as a rigid body about a fixed point without uplift, and the system dissipates energy through the contact friction that develops at the wall-base interface. This rigid body motion resembles that of a pendulum, thereby designating this system as a pendulum UPTS wall. At this stage, research has demonstrated the many advantages of this system by proof-of-concept testing of a pendulum light-frame wood (LiF) UPTS wall specimen under increasing levels of post-tensioning force. Compared with rocking UPTS walls, experimental results demonstrate that besides performing damage-free when subjected to high drift levels, the proof-of-concept pendulum LiF-UPTS wall offers the following promising outcomes: (1) insignificant to no wall uplift, (2) insignificant to no wall base shear sliding, (3) reduced stress concentrations at the wall toes because contact stresses are distributed along the wall-base interface over a larger region, (4) nearly constant post-tensioning forces under high drift levels, which limits post-tensioning losses due to yielding of prestressing bars or tendons, (5) increase in energy dissipation capacity of the system through friction, and (6) decrease in the damping reduction factor and thus, reduction in the lateral displacement and force demands in pendulum LiF-UPTS walls. These research outcomes are likely to translate positively to other shear wall types, namely reinforced concrete (RC) precast pendulum UPTS walls. 

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4. Lateral response of post-tensioned pendulum shear walls

   Silva, Pedro; Lagler, Carmen; Burgueño, Rigoberto (September 2024, Structures)

   Leroy, Gardner (Ed.)

   This paper presents an experimental evaluation of a new interface configuration for unbonded post-tensioned shear (UPTS) walls. In the proposed configuration, the wall-base interface is shaped in a circular profile. This circular profile presents a major difference from the currently used flat profile at the wall-base interface of rocking UPTS walls. During lateral response, this circular profile induces the wall to predominantly rotate as a rigid body about a fixed point without uplift, and the system dissipates energy through the contact friction that develops at the wall-base interface. This rigid body motion resembles that of a pendulum, thereby designating this system as a pendulum UPTS wall. At this stage, research has demonstrated the many advantages of this system by proof-of-concept testing of a pendulum light-frame wood (LiF) UPTS wall specimen under increasing levels of post-tensioning force. Compared with rocking UPTS walls, experimental results demonstrate that besides performing dam-age-free when subjected to high drift levels, the proof-of-concept pendulum LiF-UPTS wall offers the following promising outcomes: (1) insignificant to no wall uplift, (2) insignificant to no wall base shear sliding, (3) reduced stress concentrations at the wall toes because contact stresses are distributed along the wall-base interface over a larger region, (4) nearly constant post-tensioning forces under high drift levels, which limits post-tensioning losses due to yielding of prestressing bars or tendons, (5) increase in energy dissipation capacity of the system through friction, and (6) decrease in the damping reduction factor and thus, reduction in the lateral displacement and force demands in pendulum LiF-UPTS walls. These research outcomes are likely to translate positively to other shear wall types, namely reinforced concrete (RC) precast pendulum UPTS walls. 

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5. Seismic Retrofit of RC Shear Walls Using Selective Weakening and Self- Centering

   Sharma, Sumedh; Aaleti, Sriram; Okumus, Pinar (July 2022, 12th National Conference on Earthquake Engineering)

   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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