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Earthquakes and Structures Volume 16, Number 2, February 2019 , pages 185-196 DOI: https://doi.org/10.12989/eas.2019.16.2.185 |
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Response modification factors of concrete bridges with different bearing conditions |
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Seyed Mehdi Zahrai, Amir Khorraminejad and Parshan Sedaghati
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| Abstract | ||
| One of the shortcomings of seismic bridge design codes is the lack of clarity in defining the role of different seismic isolation systems with linear or nonlinear behavior in terms of R-factor. For example, based on AASHTO guide specifications for seismic isolation design, R-factor for all substructure elements of isolated bridges should be half of those expressed in the AASHTO standard specifications for highway bridges (i.e., R=3 for single columns and R=5 for multiple column bent) but not less than 1.50. However, no distinction is made between two commonly used types of seismic isolation devices, i.e., elastomeric rubber bearing (ERB) with linear behavior, and lead rubber bearing (LRB) with nonlinear behavior. In this paper, five existing bridges located in Iran with two types of deck-pier connection including ERB and LRB isolators, and two bridge models with monolithic deck-pier connection are developed and their R-factor values are assessed based on the Uang\'s method. The average R-factors for the bridges with ERB isolators are calculated as 3.89 and 4.91 in the longitudinal and transverse directions, respectively, which are not in consonance with the AASHTO guide specifications for seismic isolation design (i.e., R=3/2=1.5 for the longitudinal direction and R=5/2=2.5 for the transverse direction). This is a clear indicator that the code-prescribed Rfactors are conservative for typical bridges with ERB isolators. Also for the bridges with LRB isolators, the average computed Rfactors equal 1.652 and 2.232 in the longitudinal and transverse directions, respectively, which are in a good agreement with the code-specified R-factor values. Moreover, in the bridges with monolithic deck-pier connection, the average R-factor in the longitudinal direction is obtained as 2.92 which is close to the specified R-factor in the bridge design codes (i.e., 3), and in the transverse direction is obtained as 2.41 which is about half of the corresponding R-factor value in the specifications (i.e., 5). | ||
| Key Words | ||
| response modification factor; concrete bridge; nonlinear static analysis; nonlinear time-history analysis; seismic isolator; ductility; seismic design | ||
| Address | ||
| Seyed Mehdi Zahrai: School of Civil Engineering, College of Engineering, The University of Tehran, Tehran, Iran Amir Khorraminejad: Department of Civil Engineering, Shahid Beheshti University, Tehran, Iran Parshan Sedaghati: Department of Civil Engineering, Semnan University, Semnan, Iran | ||