Modified Elastic Dynamic Analysis (EDA) for Seismic Demand on In-Span Hinge Shear Keys in Multi-Frame Bridges

Long concrete box-girder bridges are typically constructed in multiple frames that are separated by in-span hinges. Shear keys, located at the in-span hinges, help preserve the transverse integrity of the bridge frames. To date, no reliable method other than nonlinear time history analysis exists to estimate the seismic force demands on in-span shear keys. Methods such as pushover and elastic dynamic analysis (EDA) do not provide accurate estimations. In this study, a rational and reliable analysis method was developed for obtaining the seismic demand on in-span shear keys of multi-frame bridges. A large number of time history analyses were performed on two- to five-frame bridge models with single- and two-column bents that were designed according to California Department of Transportation (Caltrans) Seismic Design Criteria. The results show that high-frequency modes of vibration of the superstructure significantly contribute to the shear key force demands. These modes may also cause transverse yielding in columns. It was established that a modified EDA method may be used to approximate the shear key force demands. In the proposed modified method, the modal forces are reduced separately by the corresponding modal displacement ductility before performing modal combination. This method accounts for the nonlinear response under high-frequency modes of vibration.