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This page is devoted to *frequently asked questions* (FAQ) related to the [bridge modeler|kb:bridge modeler].

Please note that this page applies to both [CSiBridge|csibridge:home] and the *bridge modeler*, available with [SAP2000|sap2000:home] releases through version 14.{info}

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h1. Model development


h2. How is a computational model developed from an object-based bridge model?

*Answer:* Please see the general description and additional topics presented on the [Bridge components|kb:Bridge components] page for response.


h2. How are loads from railings and utilities applied to bridge objects, and will they remain after update?

*Answer:* Please see the [Applying point, line, and area loads to bridge objects|tutorials:Applying point, line, and area loads to bridge objects] tutorial for response. Further, when enhancing a model created in the [bridge modeler|kb:Bridge modeler] by linking it to [SAP2000|sap2000:home], please follow the guidelines outlined on the [Mixed modeling|kb:Mixed models] page.

Some suggestions for indirect definition of superimposed loading include the following:

* Increase the unit weight of the bridge-deck slab material to account for superimposed loading.

* Apply weight modifiers to the shell section used to model the concrete slab.


h2. When a bridge is simply supported under dead load and continuous under live load, how is loading applied?

*Answer:* This can be done using [staged-construction|kb:Staged construction] analysis. Please see the [Staged construction for precast girder bridges|tp:Staged construction analysis of 2-span precast girder bridge] test problem for an illustrated overview of this procedure.


h2. To what do _bearing elevation_ and _bent elevation_ refer?

*Answer:* Please see the [Substructure and bearing elevations|kb:Substructure and bearing elevations] page for response.


h2. What do the links, created at each bearing location, represent?

*Answer:* Please see the [Link creation and function|kb:Bridge bearings FAQ#Link creation and function] topic of the Bridge bearings FAQ page for response.


h2. How can I model a reinforced-concrete box-girder bridge object with a rigid, bearing-less connection between the deck and bents/abutments?

*Answer:* This can be accomplished through the following process:

# Define a fixed bridge bearing through the Bridge > Bearings > Add New Bridge Bearing option and set a [fixed release|kb:Constraint] type for all DOF.
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# Define an abutment with integral-girder support conditions through the Bridge > Abutments > Add New Bridge Abutment option.
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# Define a bridge bent with integral-girder support conditions using the Bridge > Bents > Add New Bridge Bent option.
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# Use the Bridge > Bridge Objects > Modify/Show Bridge Object option to assign the previously-defined fixed bearing to the abutments and bents. This establishes a rigid connection to the superstructure. Use the Bridge Object Data form to complete the following steps:
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## Under Modify/Show Assignments, select Abutments > Modify/Show, then assign the previously-defined fixed bearing to the abutments.
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## Also under Modify/Show Assignments, select Bents > Modify/Show, then assign the previously-defined fixed bearing to the bents.
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# Review the entities created by the bridge modeler at each superstructure to substructure connection to ensure that they correctly represent modeling intention.

Please also see the [Bridge girder FAQ] page for additional information on integral bents and abutments.


h2. How can I change section properties for all individual elements of a bridge object?

*Answer:* A complex bridge design, such as a curved deck with variable cross section, may generate hundreds of individual elements. It would be impractical to modify each of these individually.

There are several ways to efficiently edit superstructure-element section properties. One procedure is as follows:

* Use the Bridge Object Data > Staged Construction Groups option to define a group which will contain all superstructure frame elements.

* Use the Define > Name Property Sets > Frame Modifiers Define option to define the frame property modifier to be used.

* Define a [staged-construction|staged construction] [load case|kb:Load case], add all elements to be modified, [apply the modifiers|Staged construction operations] in the first stage, then apply other loads in the subsequent stages.

This approach is most flexible, since structures with full and reduced stiffness coexist in a single model. Please note that users without staged-construction [licensing|kb:Licensing] are permitted to run a single-stage analysis. This is sufficient for evaluation of multiple structural configurations in that users may run subsequent analyses by using stiffness at the end of this single-stage staged-construction load case.

An alternate approach is to use [Interactive Database Editing] (Edit > Interactive Database Editing) to assign a modifier to multiple elements.


h2. For solid models, how are solid elements connected to diaphragm shell elements?

*Answer:* [Solid|kb:Solid] and [shell|kb:Shell] elements are connected at their common [joints|kb:Joint]. While all DOF are active at common joints, only shell elements contribute to rotational stiffness. This is reasonable because diaphragm in-plane stiffness is the dominant contribution to this performance measure.


h1. Analysis


h2. How do I perform seismic analysis using uniform-load, single-mode, and time-history methods?

*Answer:* According to _AASHTO ASD 2002 (17th Edition), Division IA - Seismic Design, Section 4 (Analysis Requirements)_, based on the complexity and regularity of the bridge, designers must implement one of the following analysis methods:

* Uniform-load method

* Single-mode spectral method

* Multi-mode spectral method

* Time-history method

{new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software enables analysis using any of the methods listed. The uniform-load and single-mode spectral methods are essentially equivalent static methods, for which response may be determined under equivalent static loading.

Users may also define [load cases|kb:Load case] of [response-spectrum|kb:Response-spectrum analysis] and [time-history analysis|kb:Time-history analysis] type, which would enable evaluation using the multi-mode and time-history methods.


h1. Design


h2. What is the bridge-object design procedure, and is documentation available?

*Answer:* Please see the [Bridge design|kb:Bridge design] topic for response.


h2. How is flexural mild reinforcement added into a bridge model?

*Answer:* In [CSiBridge|csibridge:home], users may add mild reinforcement through the Bridge Tab > Bridge Objects > Girder Rebar option. However, this reinforcement is used only for design checks, and does not contribute stiffness to the model. To add stiffness, users may model mild reinforcement as [tendon references|kb:Tendon references] with zero force. This, however, would only be truly accurate for the stress check, since code distinguishes between the two types of reinforcement, and would treat mild reinforcement as tendon elements for the flexural check.


h2. How is the tension limit calculated for the AASHTO LRFD 2007 principal check?

*Answer:* The tension limit is calculated from the specified concrete compressive strength, f '~c~ (defined on the Material Property Data menu), and a tension limit factor, _Ten Lim_ (defined on the Superstructure Design Request menu). _Ten Lim_ defaults to 0.19 for ksi units and 0.5 for MPa units.

The tension limit formulation is as follows:

!Tension limit.PNG|align=center,border=0!

h2. Why are bridge girder forces used for design slightly different from girder forces calculated by analysis?

Analysis moments are always calculated from the analysis, while design moments are calculated from the analysis only live load distribution method on the design request is set to "Use Directly Girder Forces from Analysis". For the other types of live load distribution method, the total bridge section moment is divided by the number of girders and applied to each girder for design. Please note that the previous two sentences apply to non-live moments. This is the reason why you may observe a slight difference in the analysis and design moments.

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Related Incidents:

* {incident:no=43325|comment=Bridge analysis and design forces}
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h1. Reviewing Results


h2. Why is the bridge-girder moment diagram not smooth?

*Answer:* In the Bridge Object Response Display menu, jumps in the frame moment diagram at node locations are caused by a portion of the total moment acting on the [composite section|composite behavior] being carried by bridge deck elements. The total moment across the entire deck should be continuous.

{related-email:date=2008-07-11|from=tc|to=-|subject=-|comment=-}


h2. How are bridge force and stress diagrams obtained?

*Answer:* Please see the [Bridge object force and stress diagrams|kb:Bridge object force and stress diagrams] page.


h2. What causes nonzero moment at abutment locations?

*Answer:* When bearing links are located at the bottom of the girder, and have nonzero stiffness in the longitudinal direction, this results in longitudinal force acting on a lever arm about the neutral axis of the section. This is kinematically correct behavior. Releasing the abutment bearing links in the longitudinal direction (U3) should produce zero moment at the abutments. For stability, ensure that the model has a longitudinal restraint, such as at one abutment or bent.

To avoid removing longitudinal restraint, set the bearing elevation to the neutral axis of the superstructure section. This de-couples longitudinal and bending behavior, though it may not be physically realistic.

Please see the [Effect of insertion point on beam reactions|tp:Effect of insertion point on beam reactions] test problem for additional information.

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*Related Incidents:*

* {incident:no=25410|comment=Large longitudinal reactions for simply supported (pin-roller) bridge model with only vertical loads applied. Explanation: this is due to constraint provided by the deck. See 6/15/2010 email reply.}
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* {incident:no=39862|comment=Negative moments at abutment of simply supported by highly skewed bridge}
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** {email:date=11/7/2011|from=ok|to=rs|subject=Negative moments at abutment of simply supported by highly skewed bridge|comment=|id=8093768}
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h2. Is there an explanation for why my spine bridge model exhibits uplift and unexpected bearing response?

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[Body constraint DRAFT] to be finished and referenced in following paragraph
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*Answer:* At a given substructure unit, a spine bridge model uses a single body constraint to connect a superstructure frame-element joint to all other joints which represent the top of bearing links. While this modeling approach is adequate to obtain global structural response, it does not fully capture the distribution of superstructure load to individual bearings. For bearing and pier-cap design, users should update the Linked Bridge Model as an Area Model to compute more meaningful bearing forces.


h2. How are girder reactions obtained for individual girders?

*Answer:* Users may obtain girder reactions from [link element|kb:Link] forces. Link elements represent the [bridge bearings|kb:Bridge bearings]. [Reactions|kb:Base reactions] may also be estimated from girder end shear, displayed through the Bridge Object Response plots.


h2. How may I obtain transverse bridge forces?

*Answer:* To obtain detailed results in the transverse direction, the linked bridge object should be updated as an area or solid model. Then users may review the forces acting on individual [shell|kb:Shell] or [solid|kb:Solid] elements using the graphical user interface through Display > Show Forces / Stresses, or in a tabular format through Display > Show Tables > Analysis Results. Users may also define [section cuts|kb:Section cut] to obtain the forces over a given design length, rather than for individual shell or solid elements.


h2. Why are bridge girder moments larger than those of individual girder frame elements?

*Answer:* When displaying girder forces, the Display > Show Bridge Forces/Stresses menu displays moments resisted by the composite section of the girder and its tributary deck. The frame forces displayed through Display > Show Forces/Stresses > Frames/Cables are only moments resisted by the girder itself, without the deck contribution.


h1. See Also

Additional bridge modeler FAQ pages for specialized topics include the following:

* [Bridge bent FAQ|kb:Bridge bent FAQ]
* [Bridge bearings FAQ|kb:Bridge bearings FAQ]
* [Bridge girder FAQ|kb:Bridge girder FAQ]
* [Bridge deck section FAQ|kb:Bridge deck section FAQ]
* [Bridge load rating FAQ|kb:Bridge load rating FAQ]
* [Layout line FAQ|kb:Layout line FAQ]