Pushover and Response Spectrum Analysis
This guide focuses on performing Pushover Analysis and Response Spectrum Analysis (RSA) for seismic design of bridge substructures. It is assumed that you are already familiar with creating Column objects. This guide will demonstrate how to:
Define Moment-Curvature Reports for column sections
Create Hinge Properties and assign them to columns
Set up Pushover Analysis cases
Configure Response Spectrum Analysis cases
Run analyses and generate pushover capacity curves
Prerequisites
Step 1: Define Moment-Curvature Report
The Moment-Curvature Report calculates the nonlinear behavior of reinforced concrete sections under various axial loads. This report provides the hinge stiffness data needed for pushover analysis.
Navigate to Properties > Pushover and create a new Moment-Curvature Report object. In the Section tab, define your column section geometry including Section Type (Circle, Oblong, or Rectangle), dimensions, Cover Thickness, Longitudinal Bar Size, Transverse Bar Size, and reinforcement layout. (see Moment-Curvature Report)
Define material properties in the Concrete Material, Rebar Material, and Circular Hoop-Spiral Confinement Details tabs. Include concrete strength, elastic modulus, rebar yield/fracture stress, steel grade, transverse reinforcement type, spacing, and strain limits.
In the Axial Forces tab, define axial force values covering the expected load range. Include both tension (positive) and compression (negative) values. Example: For a column with –750 kip dead load, enter values such as 250, 0, –50, –100, –200, –400, –600, –750, –1000 kips with intervals of ~50 kips near maximum load.
In the Output tab, click the three-dot menu (⋮) next to Report Output and select "Compute". The system will calculate moment-curvature curves and populate the output field.
The Moment-Curvature Report must be computed before it can be assigned to a Hinge Property. If the report is not computed, the hinge will not have valid stiffness data.
Step 2: Create Hinge Property
Hinge Properties define the nonlinear behavior at potential plastic hinge locations in columns during pushover analysis.
Navigate to Properties > Pushover and create a new Hinge Property object. In the Nonlinear tab, set Hinge Stiffness Option to "From Report" and select the computed Moment-Curvature Report from Step 1. The hinge stiffness values (Ry and Rz) will be automatically extracted. In the Linear tab, set Tx, Ty, Tz, and Rx to "Fixed" for typical pier columns. (see Hinge Property)
Step 3: Assign Hinge Properties to Column
Now that the Hinge Property is defined, assign it to the pier column to enable nonlinear pushover analysis.
Navigate to your Column object under Substructure > Pier. In the Pushover section, set Generate Top Hinge to "YES", select the Hinge Property from Step 2 in Top Hinge Property, and specify Top Hinge Length. Repeat for Generate Bottom Hinge, Bottom Hinge Property, and Bottom Hinge Length as needed. For detailed parameter descriptions, see Pier Column
Step 4: Define Pushover Analysis Case
Create a Pushover Analysis case to simulate the incremental lateral loading of the structure until failure.
Navigate to Construction > Construction Stage Generate a new construction stage and assign a Prior Stage that identifies structure stage that Pushover Analysis will be conducted. Also, set navigate to the Nonlinear and assign nonlinear as 'Yes'.
Navigate to Loading > Loads > Pushover Load > Pushover Case and create a new Pushover Analysis Case. Specify the Pushover Direction (e.g., Ty for transverse seismic) and the Stopping Criteria. For detailed parameter descriptions, see Pushover Case.
Navigate to Loading > Loads > Pushover Load and create Pushover Load objects. Specify the Point where the load will be applied, the Pushover Case (select the case created above), and initial force values (Fx, Fy, Fz) that will be incrementally increased during analysis. Select the Element type where the load is applied. For detailed parameter descriptions, see Pushover Load.
Ensure that the pushover direction is consistent with the expected direction of seismic forces.
Step 5: Define Response Spectrum Analysis Case
Create a Response Spectrum Analysis (RSA) case to determine the elastic seismic demand on the structure.
Navigate to Construction > Construction Stage Generate a new construction stage and assign a Prior Stage that identifies structure stage that Eigenvalue Analysis will be conducted. Loads other than self weight has to be specified in Eigenvalue and RSA Case which is discussed in next steps.
Navigate to Construction > Changes > Pier Section Property Override Assign Cracked Moment of Inertia modifier for columns. Cracked Moment of Inertia modify factor is displayed in Moment-Curvature Report.
Navigate to Loading > Loads > Dynamic Loads > Response Spectrum Curve and create a new Response Spectrum Curve. Specify the data points for Response Spectrum Function (see Response Spectrum Curve).
Navigate to Loading > Loads > Dynamic Loads > Eigenvalue and RSA Case and create a new Eigenvalue and RSA Case. Specify the Response Spectrum Curve. Then specify the parameters for Eigenvalue and RSA Case Direction of seismic excitation, Damping Ratio, Number of Modes, Modal Combination Method, and Directional Combination Method. For detailed parameter descriptions, see Eigenvalue and RSA Case.
Step 6: Run Analysis
Execute both the Pushover Analysis and Response Spectrum Analysis to obtain results.
Ensure the FEM button is active to compile the model, then wait for the Analysis button to appear. Click Analysis, ensure stages for the both Pushover Analysis Case and RSA Case are active, and click Run Analysis. Monitor the progress until completion.
Step 7: Generate Pushover Graph Report
Create a Static Pushover Graph to visualize the capacity curve and compare with RSA demand.
Navigate to Reports > Static Pushover Graph and create a new Static Pushover Graph. In the General tab, select the Pushover Case and RSA Case from Step 4 and 5. In the Advanced tab, set Result Coord. System to "Alignment" or "Global" (see Static Pushover Graph).
Click the Design button to calculate the pushover graph. Once complete, click Report to view the capacity curve showing base shear vs. displacement with the RSA demand point overlaid.
The pushover capacity curve shows the nonlinear force-displacement relationship of the structure. The RSA demand point should fall below the capacity curve, indicating adequate seismic capacity.
Understanding the Results
The pushover graph displays base shear vs. control node displacement. The curve shows elastic behavior initially, then yielding as hinges form, and finally a plateau or drop-off at ultimate capacity. The RSA displacement demand point should fall within the acceptable range of the capacity curve.
Additional Tips
Common Issues and Solutions
Hinge not forming: Verify that Generate Top/Bottom Hinge is set to YES and the correct Hinge Property is assigned.
Report Output empty: Ensure the Moment-Curvature Report was computed using the "Compute" cell action before assignment.
RSA demand not showing: Check that both Pushover Case and RSA Case are properly defined and analyzed.
Coordinate mismatch: Confirm that Result Coord. System setting matches your expected output (Alignment vs. Global).
Related Documentation
For more detailed information on individual components, refer to:
Moment-Curvature Report - Detailed parameter descriptions
Hinge Property - Hinge definition and assignment
Pier Column - Column object documentation including Pushover section
Pushover Case - Pushover analysis case configuration
Response Spectrum Curve - Response spectrum function definition
Eigenvalue and RSA Case - Response spectrum analysis case setup
Static Pushover Graph - Report generation and interpretation
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