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Mode Shape and Natural Frequency Identification of a Prestressed Box Girder Bridge in Nonthaburi, Bangkok

Sanjoy Kumar Bhowmik 1, * Md. Monirul Islam1, Anisuzzam Khan1, Mohammad Osman Ghone1
1Department of Civil Engineering, IUBAT-International University of Business Agriculture and Technology, Dhaka, Bangladesh
*Corresponding author: E-mail:

ABSTRACT: Gradual deterioration of structures leads the engineers towards the Structural Health Monitoring system to confirm the existing serviceability scenario, where modal properties are the indispensable part of the process. Modal properties like modal frequency and mode shape from the service providing structure are essential to adjust the numerical finite element model to make the model more like the existing structure. This study built up the mode shape and computed the natural frequency of a precast box girder bridge to compare with numerical model. The selected bridge for this study is situated close to Bangkok city known as canal crossing bridge. A finite element model was developed based on as built drawing, and then, the model was updated with the existing structure’s modulus of elasticity. The ultrasonic pulse velocity test was conducted to obtain modulus of elasticity data. Then accelerometers data were collected under ambient vibration situation from each predefine station to find the modal properties. The study found that mode shape and natural frequency for first mode are similar for both calculated from acceleration data and numerical model.


KEYWORDS: structural health monitoring, modal parameters, mode shape, natural frequency.

Bridges, the horizontal structures, are inseparable part of transportation system, and residential or commercial buildings, the vertical structures, have significantly ameliorated the people living standard on which society vigorously relies upon (Medhi, Dandautiya, & Raheja, 2019). Deterioration on serviceability, performance and durability of bridges start just after the construction due to continuous variable traffic loading, outrageous weather conditions, and effects from earthquake and wind. Moreover, deterioration process starts during the construction phases as well, while structural integrity needs to be confirmed between design numerical model and as-built condition, where assessment of the structural behaviors become obvious (Lantsoght, van der Veen, de Boer, & Hordijk, 2017). So, Structural Health Monitoring (SHM) system is highly needed to confirm the level of performance and durability of the bridge structures (Wenzel, 2009), where modal properties need to extract from the real structures.

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