Unbonded post-tensioned rocking walls for seismic resilient structures

• Principal investigator: Sri Sritharan, Iowa State University
• Co-PIs: Catherine French, Eric Musselman, Andrea Schokker
• Sponsored by: NSF-NEESR (1041650)

Project summary

The damage caused by earthquakes and subsequent economic losses underscore the need for designers and researchers to focus on developing seismic resilient buildings.

One method of achieving resilient buildings is through the use of self-centering structural systems that are typically designed with lightly pre-stressed unbonded post-tensioning tendons.

The concept was investigated in precast walls nearly a decade ago through the NSF PREcast Seismic Structural Systems (PRESSS) program, but the implementation of the only wall system from this study has been limited due to several deficiencies.

To overcome these deficiencies, PI Sritharan, and his students developed a cost-effective alternative known as PreWEC, which was formed by joining a Precast Wall with two End Columns using easily, replaceable energy dissipating elements.

The PreWEC system has been proven analytically and experimentally to have the potential to provide an excellent seismic resilient system, resulting in minimal structural damage under simulated earthquake effects. In these developments, the potential energy loss caused by the wall impacting the foundation during rocking motions was not given consideration although significant evidence suggests this mechanism alone may be sufficient to dissipate the seismic energy. The reason for neglecting the impact energy loss is due to lack of fundamental knowledge.

Furthermore, the resilience of a building containing rocking walls with or without hysteric energy dissipation capability is also dependent on the behavior of surrounding structure, especially floors and gravity columns, and their interactions with the self-centering systems. Past tests that have included floor systems typically minimized these interactions. To ensure a fully resilient structure, these interactions should be addressed by understanding the wall-floor connection responses. Finally, to ensure the reliability of these systems, critical research is necessary with regard to investigating tendon anchorages as past research, though not addressed many of the critical variables, has led to conflicting results.

With strong collaboration opportunities with E-Defense and NEES@Auckland, the proposed project focuses on the development of seismic resilient building solutions utilizing the fundamental characteristics of seismic rocking of both single walls (SRW) and PreWEC. In addition to developing new knowledge in the areas identified above, significant effort will be placed on the identification of different energy dissipation sources of rocking walls such as impact (or radiation damping), viscous damping and hysteretic damping, and the influence of hysteretic damping on the impact energy loss.

Involving an international, cross-disciplinary team of experts and utilizing two NEES equipment portfolios, the project team will complete the following objectives:

1. Understand the fundamental characteristics of seismic rocking of self-centering walls through NEES/international tests
2. Develop suitable connections between rocking walls and floors, and quantify the wall-floor-column interactions using large-scale tests
3. Design seismic resilient structures
4. Improve numerical simulation of buildings designed with rocking walls and different floor systems
5. Formulate guidelines
6. Educate students, practitioners, and others (e.g., policy makers) on the significance of the proposed study

The shake table tests will focus on characterizing the dynamic characteristics of SRW and PreWEC subjected to in-plane earthquake loading, thereby quantifying the energy loss from different sources and the parameters affecting the amount of energy loss due to impact. The scope of the proposed test plan is to test six SRWs and two PreWEC systems at NEES@UNR using one biaxial shake table. It is planned on carry out the experimental program on two phases; each consists of four walls.