Seismic Response of Bridges with Integral Deck-Abutments:Ambient Vibration Testing

Abstract eng:
Integral abutment bridges are single or multiple span structures consisting of a continuous concrete deck integrated with abutments and a flexible support system. These structures have become very popular due to the elimination of costly and maintenance prone expansion joints and bearings. However, the exclusion of joints causes the superstructure to interact more closely with the soil embankments, making the evaluation of vibration properties and seismic demands a process that involves many uncertainties. The purpose of this paper is to contribute to the better understanding of this type of bridges by investigating in detail their vibration characteristics at low levels of excitation. The paper presents the identification of modal vibration properties of six bridges with integral deck-abutments using ambient vibration tests. Results indicate that the motion of the embankments causes the superstructures of some single-span bridges to deflect with an operational mode similar to the first vertical natural mode of vibration. This behavior suggests that the seismic response of the bridges in the vertical direction may be strongly influenced by the seismic response of the embankments and the abutments. Steel bridges have developed thick cracks at the end of the approach slabs in the embankments. These bridges have the lowest fundamental frequencies of all the tested bridges. The presence of these cracks suggests that the abutment foundation may be too flexible for this type of superstructures. The identification of predominant frequencies of vibration of the embankments is done with a proposed technique named “cross response spectrum”. This technique is useful when the identification of predominant frequencies can not be easily determined by more traditional identification techniques such as the power spectral density and the H/V ratio.

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