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The substructure seismic-design process in the [bridge modeler|kb:Bridge modeler] is documented in the [_Bridge Seismic Design_|doc:Bridge seismic analysis and design] manual. This manual, based on AASHTO specifications for LRFD bridge seismic design, describes the automated seismic-design procedure for bridges. Displacement-demand formulation is based on Article 4.8 of the AASHTO Guide Specifications for LRFD bridge seismic design.

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h1. Applicability

Design procedures implemented in the [bridge modeler|kb:Bridge modeler] are applicable to reinforced-concrete columns.

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*Related incident:*
* Incident 23273 - Discussion that the automatic seismic bridge design is not available for steel columns
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h1. Seismic design category

The seismic design category, determined through AASHTO Article 3.5, is based on the design spectral acceleration of a one-second period.

h1. Hinges

Caltrans [hinges|kb:Hinge] are assigned according to the bridge-seismic-design request. Because Caltrans hinges are specific to certain types of cross section, users should ensure that column sections are rectangular, circular, or from the Section Designer.

h1. Pushover analysis

During [pushover analysis|kb:Pushover], when a structure is pushed beyond its displacement capacity, numerical problems arise due to instability. Under such conditions, analysis terminates prior to reaching target displacement. In spite of termination, the calculated pushover capacity displacements are still correct since they are automatically identified from the pushover curve just before strength loss.

h2. Pushover analysis for integral substructure

For bridge models with integral substructure, users should reconsider [pushover analysis|kb:Pushover] for individual [bents|kb:Bridge bent] since the integral connection between bents and superstructure will cause frame action. Users may choose to investigate pushover response for the entire bridge (including deck) along the longitudinal axis. This may be done outside the bridge modeler.

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*Related Incident:*
* Incident 23572 - (outgoing email 4/23/2010)
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h2. Pushover target displacement ratio

Pushover target displacement ratio generates target displacement as follows:
* Demand displacement is derived from [response-spectrum analysis|kb:Response-spectrum analysis]

* Target displacement for pushover analysis is derived as the product of demand displacement and target displacement ratio

During implementation, a pushover target displacement ratio equal to 1 pushes the structure to demand displacement. Setting this parameter equal to 2 pushes the structure to twice the demand displacement. Since pushover-curve drop indicates bent failure, users should review pushover curves to determine whether or not the structure achieves failure, and increase the target displacement ratio as necessary.

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*Related Incident:*
* Incident 32307 - Explanation of pushover displacement ratio
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h1. See Also

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*Related Incident:*
* Incident 37517 - Number of modes for seismic design request
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** {email:from=ok|to=kl|date=8/23/2011|subject=Number of modes for seismic design request|comment=|id=7636559}
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* [_Bridge Seismic Design_|doc:Bridge seismic analysis and design] manual
* [Bridge seismic design request|tutorials:Model from Bridge Seismic Design Request manual] tutorial - further discussion on the _Bridge Seismic Design_ manual model
* [Caltrans hinge|kb:Caltrans hinge]