Toranzo et al. 2009
In this project, the concept of rocking walls was implemented on confined masonry walls. This system was built with low-cost hysteretic EDDs. The main purpose of using confined-masonry rocking walls is to minimize structural damage and residual drifts. This article presented and discussed the results of a shake-table test and the effect of the energy dissipation devices on the dynamic response of the structure was highlighted. First, behavioral mechanisms of rocking walls with supplementary damping were explained. Next, the prototype building and test specimens were described. After providing information about input ground motions, results of experiments were explained in parts.
System Concept
Rocking walls have several advantages and disadvantages. The walls have re-centering mechanisms and a high capacity to sustain large lateral displacements without damage. Unfortunately, they have low energy dissipation capacity, potentially large impact actions and unpredictable seismic response. These negative features are caused by the lack of a reliable source of energy dissipation within the system. EDDs could provide a dependable source of supplementary energy dissipation and decrease the effects caused by impact. Mild steel bars exhibiting a hysteretic response were anchored at the base of the walls to serve as the EDD. This system exhibits large initial stiffness; however, it would be difficult and expensive to replace the system after damage since the EDDs are cast in concrete. The EDDs could be placed at the toes of a rocking wall to dissipate most of the energy through flexure. In this configuration, these devices could also transfer the wall shear force into the foundation, reducing the reliance on friction for shear transfer.
Four EDDs were connected to the base of the wall to be tested. The EDDs transfer forces to the lower corners of the rocking wall; the corners were detailed to transfer these forces to the confining columns. Steel tubes were placed at the base of the confining columns to receive the force from the EDDs by means of pins. The tubes were surrounded by the longitudinal reinforcement of the columns and the base beam. The other end of the EDDs was fully-fixed to the foundation to allow the EDDs to behave like cantilevers. A bracket was attached to the back end of the EDDs. Then the bracket was bolted to the steel plate and anchored to the foundation.
Experimental Study, Results, and Discussion
The shake table tests were performed on a 2/5 scale model of a segment of a prototype 3-story school building. A performance-based design methodology was used in the design of the prototype building system. The design methodology emphasizes four important goals of the structure: prevention of structural damage up to the design earthquake, enforcement of a prescribed mechanism, control of the extent of lateral drift of the structure, and prevention of any residual displacement.
Five historical ground motions were chosen and sixty dynamic tests were run. Several of the tests were used to determine the natural period of structure. The other tests were used to reproduce the seismic demand at different design levels and act as trials to check the instrumentation.
Limited damage and zero residual displacements were observed in any of the tests. The natural period of the structure before propagation was found to be 0.14 s; this lengthened to 0.22 s after propagation. A maximum roof drift ratio was observed as 2.5% without visible damage. Moreover, this system showed the ability to self-center. The tests also validated the expected ability of the EDDs to provide the system with significant supplemental energy dissipation capacity.
Reference
Toranzo, L.A., Restrepo, J.I., Mander, J.B., and Carr, A.J. (2009). “Shake-Table Tests of Confined-Masonry Rocking Walls with Supplementary Hysteretic Damping,” Journal of Earthquake Engineering, 13:882–898, pp. 882-900.