Report No.: CCEER-13-12
Title: ASSESSMENT OF FOUNDATION ROCKING BEHAVIOR IN REDUCING THE SEISMIC DEMAND ON HORIZONTALLY CURVED BRIDGES
Authors: Saad, A., Sanders, D. and Buckle, I.
Date: June 2013
Sponsoring Agency: Federal Highway Administration (FHWA)
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557
It is critical to find ways to mitigate earthquake damage in bridges. One of the ways that has been proposed to achieve this is to permit the foundations to rock. In order to investigate the system effects of rocking foundations, a two-fifth scale, three-span horizontally curved steel plate girder bridge experiment was conducted on the shake table array in the Large-Scale Structures Laboratory at the University of Nevada, Reno. The bridge consisted of two abutments and two single column hammer-head piers. In the study, the two single columns were allowed to rock on their foundations. Neoprene pads were used underneath the footings to simulate the effect of the soil. Rocking foundations have been shown to have self-centering capabilities in addition to adding damping to the system provided competent soil condition exist. Previous research conducted experiments on the effect of rocking foundations on column damage through single pier testing without including the system effects.
In this study, the system effect of rocking foundations was studied at different earthquake levels. The rocking behavior considerably limited the damage in the columns under different earthquakes intensities. At earthquake levels equivalent to the Maximum Considered Earthquake (MCE), only cracks at the plastic hinge areas of the columns were observed without spalling of concrete. The rotation of the pier under the lateral loading was distributed between the footing rotation and the column rotation. This resulted in an essentially elastic behavior of the columns with minor damage. For higher earthquake levels, up to three times the design earthquake, the beneficial effect of rocking diminished, allowing damage to occur in the plastic hinge regions, similar to fixed-base column behavior. However the overall damage was still less than in a fixed-base column and the system was stable. The decrease in the rocking effect and the increase in the column damage at high earthquake levels results in less soil stresses and visible damage in the column that can easily be repaired after major earthquakes. This result for rocking foundations is promising plus rocking foundations have lower cost compared to other damage mitigating techniques (e.g. seismic isolation and damping devices).
This report also presents the results of analytical investigations using SAP2000, which starts with comparing the analytical results of nonlinear response history analyses to the experimental results. The analytical model provided good agreement with the experimental results at low level earthquakes (up to the MCE). For higher earthquake levels, agreement was less, mainly because second-order effects were not incorporated in the analytical model.
Parametric studies were also performed. This included steel girder bridges with different horizontal curvature and footing sizes. It was found that horizontal curvature has a limited effect on the behavior of steel girder bridges with rocking foundations. Footing sizes within the range of 3.5 to 4 times the column diameter were found to be the most efficient on the overall behavior of the bridge. Orientation of footings with the bridge deck was also found to have an effect on the column damage state and the maximum soil stresses for horizontally curved bridges restrained in the radial directions. A study of the effect of rocking on columns with different ductility capacities concluded that bridges designed with rocking foundations have less ductility demand than bridges with conventional columns. This gives the ability to reduce the transverse reinforcement ratios in the columns and still sustain high levels of earthquake without reaching the ductility capacity of the column.
The overall conclusion of the project was that rocking foundations provide a very good option for mitigating the damage of bridges due to earthquake effects.