The procedure used by SAP2000 for the bridge substructure seismic design is documented in Automated seismic design of bridges based on AASHTO guide specifications for LRFD seismic bridge design manual. The structure displacement demand is calculated based on Article 4.8 of the AASHTO Guide Specifications for LRFD Seismic Bridge Design.

Applicability

The design implemented in SAP2000 is applicable for reinforced concrete columns.

Seismic Design Category

Seismic design category is determined in accordance with AASHTO Article 3.5, based on the value of one second period design spectral acceleration.

Hinges

The program creates Caltrans hinges for bridge seismic design request, but Caltrans hinges are only available for Section Designer section, regular rectangular sections or circular section. Therefore, the column section should be one of the sections for which Caltrans hinges can be generated.

Pushover Analysis

Sometimes, the pushover analysis may not get completed, because the structure is being pushed beyond its capacity and it becomes unstable, thus causing numerical problems and the analysis being terminated prior to reaching the full target displacement. However, the calculated pushover capacity displacements are still correct, because they are automatically identified from the pushover curve just before the strength loss.

Pushover Analysis for Integral Substructure

If your bridge model contains integral substructure, you should evaluate whether the pushover analysis of individual bents is appropriate for your situation. Since the integral connection between the bents and the superstructure will results in frame action, you may consider also investigating pushover analysis for the entire bridge (including the deck) in longitudinal direction. This type of pushover analysis would need to be setup manually, outside of the bridge modeler.

Pushover target displacement ratio

The pushover target displacement ratio is used as follows: The program first determines the demand displacement from the response spectrum analysis. This demand displacement is then multiplied by the specified target displacement ratio to obtain target displacement for the pushover analysis.

For example, setting this parameter to 1 would result in the structure being pushed exactly to the demand displacement, while setting this parameter to 2 would result in the structure being pushed to twice the demand displacement. The bent failure is determined from the pushover curve drop, so it may be a good idea to review the pushover curves to determine whether the structure is being pushed to full failure and increase the value of this parameter if necessary.

See Also

• Bridge Seismic Design manual
• [Bridge seismic design request tutorial] - additional discussion for the model developed in the "Bridge Seismic Design" manual.
• [Caltrans hinge]