Characterizing the Behavior of Rocking Systems
The significant damage and permanent deformations sustained by structures during earthquakes have motivated researchers to investigate the use of precast concrete and masonry walls with jointed connections at the wall-to-foundation interface (rocking walls). These walls resist the seismic lateral loads by rocking motion, which minimizes damage and re-centers the full structure effectively, providing seismic resilience.
Nevertheless, there is lack of understanding of their seismic behavior. This includes energy dissipation by the impacts during rocking motion, which is a key source of energy dissipation in the walls. Impact events were inhibited in most experimental research studies, because they employed quasi-static tests. Even when dynamic tests were used, little focus was given to characterize the individual energy dissipation components in the walls, including hysteretic, impact, and other continuous energy dissipating mechanisms. Apart from the lack in experimental insight, the use of simplified models that emulate those used for monolithic walls have been able to capture only the global seismic behavior of walls with jointed connections, but not the local responses.
Our research combined experimental and analytical investigations of precast concrete and masonry walls with jointed connections to improve understanding of their seismic responses. Their quasi-static behavior was investigated first to characterize their hysteretic and force-displacement responses. At this stage, our research focused on masonry walls because of their more intricate behavior than that of precast concrete walls, which involves three deformation mechanisms, confinement effects due to lateral friction at the wall-to-foundation interface, and increased hysteretic action. An iterative procedure was developed to estimate the envelope responses of masonry walls, using monotonic sectional analysis at the wall base. This procedure captures the deformations at the critical wall regions and accounts for the confinement in masonry due to lateral friction at the wall-to-foundation interface. To enable a methodology that can be implemented in design, a simplified procedure was also developed.
Next, these procedures were extended to capture the hysteretic behavior in the walls using fiber-element sectional analysis at the wall base. The dynamics of walls with jointed connections was also investigated to capture their impact energy dissipation. Free vibration tests of carefully monitored precast concrete units were employed for this purpose. It was found that rocking takes place over a contact length and that the rotation center of rocking members migrates from one bottom toe to the other as a function of their base rotation. These observations were used to develop an expression for energy losses during impacts. The accuracy of this expression was tested further using shake-table tests of a large-scale precast concrete wall system, which was part of an NSF funded project. To conclude this part of the investigation, a generalized dynamic model for walls with jointed connections was developed. The model integrates impact, hysteretic, and inherent energy dissipation, rocking and flexural deformations in the walls. Its accuracy was verified using experimental data that captures a broad range of material and geometric characteristics.
Finally, recognizing that inadequate damping is available in walls with jointed connections when used in seismic regions, an investigation was used to improve their damping performance and minimize their damage during seismic motions. Elastomeric pads were strategically employed at the wall-to-foundation interface to a) increase damping in the walls; and b) minimize the strain demands on concrete and masonry by shifting most of the hysteretic action into the pads. Combining analytical and experimental means, it was shown that appropriate design of the wall-to-foundation interface allows the elastomeric pads to effectively dissipate the energy imparted to the walls through lateral seismic loads.
