Seismic Performance and Modeling of Reinforced Concrete and Post-Tensioned Precast Concrete Shear Walls
Past earthquakes have shown examples of unsatisfactory performance of buildings using reinforced concrete structural walls as the primary lateral-force-resisting system. In the 1994 Northridge earthquake, examples can be found where walls possessed too much overstrength, leading to unintended failure of collectors and floor systems, including precast and post-tensioned construction. In the 2010 Maule Chile earthquake, many structural wall buildings sustained severe damage. Although Chilean design standards result in different reinforcement detailing than is common in U.S. walls, the failure patterns raise concerns about how well conventionally reinforced structural walls in U.S. buildings will perform during the next earthquake. Alternative wall design philosophies that offer more predictable response, with better damage control, should be investigated.
After the Mw 8.8 Chile earthquake, the 15-story Alto Rio building in Concepción sustained failures near the base, overturned, and came to rest on its side. The collapse of the Alto Rio building was significant because it was designed using the Chilean Building Code NCh433.Of96, which requires the use of ACI 318-95 for design of reinforced concrete structural elements intended to resist design seismic forces. The failure of the Alto Rio building is significant for many reasons. It is the first modern shear wall building of its type to collapse by overturning during an earthquake. The building is studied using forensic data and structural models of the framing system subjected to earthquake shaking. The study identifies the likely failure mechanism and suggests areas for which design and detailing practices could be improved. The capabilities and shortcomings of the analyses to identify details of the failure mechanism are themselves important outcomes of the study.
A second study explores the behavior of structural wall buildings using unbonded post-tensioned structural walls. Such walls offer the opportunity to better control yielding mechanisms and promote self-centering behavior. The study focuses on the measured responses of a full-scale, four-story building model tested on the E-Defense shaking table in Japan. The seismic force-resisting system of the test building comprised two post-tensioned (PT) precast frames in one direction and two unbonded PT precast walls in the other direction. The building was designed using the latest code requirements and design recommendations available both in Japan and the U.S., including the ACI ITG-5.2-09. The test building was subjected to several earthquake ground motions, ranging from serviceability level to near collapse. Analytical studies were carried out to test the capability of the structural models to replicate behaviors important to structural engineers, and to assess whether available analysis tools are sufficient to model dynamic behavior that results when a full-scale building is subjected to realistic earthquake ground shaking. Measured response data from such an outstanding test provides an opportunity to fully understand the response characteristics of PT walls and assess the ability of nonlinear analytical models to reproduce important global and local responses, including three-dimensional system interactions, both prior to and after loss of significant lateral strength. Moreover, this study to assess behavior and system interaction of PT walls leads to improvements of the current design ideas and performance expectations.
The present study examines both the collapse of the Alto Rio building in Chile and the shaking table tests of the unbonded post-tensioned wall building in Japan. The collapse study suggests areas of improvement in current design and detailing practice. The shaking table study suggests an alternative approach to design of shear walls in buildings. Both studies demonstrate the use of modern structural analysis tools to interpret building responses to earthquake shaking. Taken together, the studies provide added confidence in earthquake simulation capabilities and demonstrate alternatives for designing earthquake-resistant buildings that use structural walls.