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    Bridge

    Influence Line & Surface Analysis on Bridges?

    BLOG BRIDGE INSIGHT

    Influence line & surface analysis are moving load analysis techniques for bridges used in many structural analysis programs. In this post, we will try to understand the basic contents of these two interpretation techniques.

     

    The moving load analysis function in midas Civil is used to statically analyze and design bridge structures for vehicle moving loads.

     

    Important features are included as follows:

     

    • Generation of influence line and influence surface for displacements, member forces, and reactions due to moving loads
    • Calculation of maximum/minimum nodal displacements, member forces, and support reactions for a given moving vehicle load using the generated influence line and influence surface

    Moving load analysis of a bridge structure entails a series of analyses for all loading conditions created along the entire moving load path to find the maximum and minimum values, which are used as the results of the moving load case.

     

    In order to carry out a moving load analysis, we define vehicle loads, traffic lanes, or traffic surface lanes and the method of applying the vehicle loads, and then we apply a unit load at various points to traffic lanes or traffic surface lanes to calculate influence line or influence surface.

     

    An Influence line is presented on the traffic lane and represents a specific component of analysis results obtained from static analyses of a bridge structure subjected to a unit load moving along the traffic lane. An influence surface represents a specific component of analysis results obtained from static analyses of the traffic lane plane of a bridge structure subjected to a unit load located at the plate element nodes and is presented on the points of load application. The components of results that can be calculated for influence lines or influence surfaces include nodal displacements of the structural model, member forces for truss, beam and plate elements, and support reactions.

     

    An analysis procedure for a vehicle moving load using influence lines or influence surfaces can be summarized as follows:

     

    1. Define vehicle loads, method of applying the moving loads and traffic lanes or traffic lane surfaces.

     

    2. Calculate influence lines or influence surfaces for each component by performing static analyses for unit loads that are generated by the traffic lane or traffic surface lane.

     

    3. Produce the analysis results due to the vehicle movement using influence line or influence surface according to the moving load application method.

     

     


    Influence line diagram
    Figure 1: Influence line diagram
     

     

    An influence line is calculated by applying a unit load (vertical load or torsional moment) along the traffic lane. Influence lines can be produced for nodal displacements, member forces, and reactions of all nodes, truss, beam and plate elements, and supports included in the model.

     

    An influence surface is calculated from the analysis in which a unit load (vertical load) is applied to the nodes constituting plate elements in a traffic surface lane.

     

     

    Features and applications of influence line and influence surface analyses

    Table 1. Features and applications of influence line and influence surface analyses

     


     

    Influence Line modeling and analysis results

     

     

    Modeling procedure

     
     
    Input girder bridge as a beam element
    Step 1. Input girder bridge as a beam element
     
     
     
     
     

    Step 1. Input girder bridge as a beam elementStep 2. Input Bridge support boundary conditions
     
     
     
     
     
     
    Input Influence line by specifying a lane
     
    Step 3. Input Influence line by specifying a lane
     
     
     
     
     
     
    Input other load conditions
    Step 4. Input other load conditions
     
     
     
     
     
     
     
    Completed Girder bridge model
    Step 5. Completed Girder bridge model

     

     


     
    Analysis results

     
    Influence line on the deflection of the first central span
    fig. Influence line on the deflection of the first central span
     
     
     
     

    Influence line for the moment (My) of the first central spanfig. Influence line for the moment (My) of the first central span
     
     
     
     
    Maximum displacement due to vehicle moving loads
     
    fig. Maximum displacement due to vehicle moving loads
     
     
     
     
     
    Maximum moment (My) due to vehicle moving loads  
    fig. Maximum moment (My) due to vehicle moving loads
     
     
     

     

    Influence Surface modeling and analysis results

     

     

    Modeling procedure

     
     
    Step 1. Input Slab bridge as a plate element
    Step 1. Input Slab bridge as a plate element
     
     
     
     
     

    Step 2. Input Bridge support boundary conditions-1Step 2. Input Bridge support boundary conditions
     
     
     
     
     
    Step 3. Input Influence surface by specifying a lane.
     
    Step 3. Input Influence surface by specifying a lane
     
     
     
     
     
     
    Step 4. Input other load conditions-1
    Step 4. Input other load conditions
     
     
     
     
     
    Step 5. Completed Slab Bridge ModelStep 5. Completed Slab Bridge Model
     


     

    Analysis results

     
     
    Influence line on the deflection of the first central span-2
    fig. Influence line on the deflection of the first central span-2
     
     
     
     
     

    Influence line on the deflection of the second central spanfig. Influence line on the deflection of the second central span
     
     
     
     
     
    Influence line on the moment (Mxx) of the second central span
     
    Influence line on the moment (Mxx) of the second central span
     
     
    Maximum displacement due to vehicle moving loads-2
    fig. Maximum displacement due to vehicle moving loads
     
    Maximum moment (Mxx) due to vehicle moving loads
    fig. Maximum moment (Mxx) due to vehicle moving loads
     
     
     
     

    The above influence lines or influence surfaces are drawn on the screen or printed out through the Post-Processing Mode.

     

    In Midas Civil, the influence lines or surfaces are used to calculate the structural response due to vehicle moving loads, and the linear interpolation is used for the interval between the load points 

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    About the Author
    TaeGook Kim | Technical Engineer | MIDAS IT

    TaeGook Kim has over 3 years of experience with technical and marketing strategies for MIDAS programs. He is currently working as a civil engineer within MIDAS IT’s Global Technical Planning Team.

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