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  • Writer's pictureVarun Garg

Seismic Isolation in Bridge Design #2: Bridge Configuration and Modeling

In structural engineering, designing, and analyzing bridges require a deep understanding of structural dynamics and a nuanced approach to modeling these complex systems. This post focuses on the configuration and modeling aspects of a continuous multi-span bridge subjected to seismic forces, taken as our case study. We will examine the specifics of the bridge's seismic design, the simplifying assumptions made for analysis, and how the MIDAS Civil software is utilized to model and analyze the structure.

For a background on this study review Post #1 here.

Bridge Configuration

Our case study centers on a four-span continuous girder bridge, with equal spans of 30 meters for a total length of 120 meters.

  • Superstructure: The bridge employs a single cell Box Girder with a uniform cross-section of 6.00 sqm and a depth of 2.0m. The girder's center of gravity is located 0.74m from the top of the box. The bridge employs two bearings at each abutment/pier, spaced 4m apart in the transverse direction.

  • Material Properties: The superstructure has normal weight concrete with a strength of 45MPa. The piers use a higher grade of concrete to support the superstructure effectively.


The analytical model of the bridge is developed using MIDAS Civil, a state-of-the-art bridge engineering software known for its ease and flexibility in modeling complex structures.

  • Idealization: The superstructure and piers are modeled in detail to accurately reflect the actual physical properties of the bridge. The software allows for detailed input of material characteristics, geometric configurations, and boundary conditions, ensuring a realistic representation of the bridge. By allowing for detailed input and sophisticated analysis, MIDAS enables engineers to simulate the behavior of bridges under seismic loads accurately, identifying potential issues and optimizing the design for both safety and performance.

  • Loading Conditions: The model incorporates various loading conditions, including the self-weight of the bridge, superimposed dead loads (SIDL), and live loads, ensuring a comprehensive analysis of the structure under different scenarios including wind and thermal effects.

  • Assumptions for Simplification: To simplify the analysis, certain simplifications are made:

    • The contribution of pier caps to stiffness is ignored, focusing analysis on the primary structural elements.

    • The base of the piers is fixed on an open foundation, with foundation stiffness not considered in the model, simplifying the interaction between the superstructure and substructure.

Figures 1 to 3 below help visualize to understand the bridge's cross-section, span configuration, and the idealization of both the bridge structure in the model.


The detailed configuration and modeling of the bridge provide a solid foundation for the subsequent analysis of seismic forces and the evaluation of seismic isolation devices. By understanding the bridge's design and the assumptions made for its analysis, we can appreciate the complexity and precision required in the engineering of structures capable of withstanding seismic events. In the next posts, we will explore the loading and seismic design parameters, followed by the seismic analysis of the bridge, shedding light on the effectiveness of seismic isolation in enhancing the resilience of bridge foundations.

This series is brought to you by Varun Garg, based on a paper he co-authored with Mr. Rajiv Ahuja for the Structural Engineering Digest, Quarterly Journal for the Indian Association of Structural Engineers in March 2021. The paper can be downloaded by clicking the link below.

Effect of Seismic Isolation on Design of Bridge SED - January-March, 2021
Download PDF • 18.87MB

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