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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. Creating the computational model

*Question:* How does {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software create a computational model 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. Superimposed loading on bridge objects

*Question:* How are superimposed loads, such as those from railings and utilities, applied to bridge objects? Also, will applied loading remain after model updates?

*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. Loading for different bridge conditions

*Question:* How is loading applied when the bridge condition is simply supported under dead load and continuous under live load?

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


h2. Bearing and bent elevations

*Question:* In the Bridge Object Bent Assignments menu, to what do _bearing_ and _bent elevations_ refer, and where, geometrically, are their elevations defined?

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


h2. Bearing links

*Question:* What do the several 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. Modeling rigid connection between deck and supports

*Question:* How can I use the [bridge modeler|kb:Bridge modeler] to model a [reinforced-concrete|kb:Concrete] box-girder bridge where the deck is rigidly (without [bearings|kb:Bridge bearings]) connected to its [bents|kb:Bridge bent] and 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. Modifying multiple elements

*Question:* How can I efficiently reduce section properties for all superstructure frame elements created by the bridge modeler? 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.

*Answer:* 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.

Because structural systems with full and reduced stiffness coexist in a single model, the approach previously described is the most flexible. Users without staged-construction [licensing|kb:Licensing] are permitted to run a single stage. 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. Solid and shell element connection

*Question:* For solid models, how does the [bridge modeler|kb:Bridge modeler] connect [solid|kb:Solid] elements to diaphragm [shell|kb:Shell] elements?

*Answer:* Solid and 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. Analysis methods

*Question:* 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. Bridge-object design

*Question:* What is the bridge-object design procedure, and is documentation available?

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


h2. Modeling additional mild reinforcement for flexure

*Question:* How do I add flexural mild reinforcement 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. Tension limit for principal check

*Question:* 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!


h1. Reviewing Results


h2. WhenDiscontinuities Iin display moment diagram for a bridge girder ( plot

*Question:* Why is the bridge-girder moment diagram, displayed on the "Bridge Object Response Display" form)menu, the diagram is not smooth.? 
Why?
*Answer:* Jumps in the frame moment diagram at thenode nodeslocations are caused by the fact that a portion of the total moment acting on the [composite section|composite behavior] isbeing carried by the 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 does the programForce and stress diagrams

*Question:* How does {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software obtain bridge forcesforce and stressesstress diagrams?

*Answer:* Please Seesee the [Bridge object force and stress diagrams] page.


h2. Why am I getting|kb:Bridge object force and stress diagrams] page.


h2. Effect of insertion point on beam reactions

*Question:* What causes nonzero moment at theabutment abutmentslocations?

*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 ana lever arm about the neutral axis of the section. This is kinematically correct behavior. Releasing the abutment bearing links in the longitudinal direction (U3) direction should giveproduce you zero momentsmoment at the abutments. JustFor makestability, sureensure youthat have the model has a longitudinal restraint somewhere, such as at one abutment or bent.

IfTo you do not want to remove the avoid removing longitudinal restraint, then you can set the bearing elevation to the neutral axis of the superstructure section. This de-couples the longitudinal and bending behavior, though butit may not be physically realistic.

SeePlease see alsothe [ImpactEffect of insertion point on horizontalbeam reactions of simply supported (pin-pin) beam|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. Why the bearing reactions for bridge spineSpine bridge model lookbearing incorrect?reactions

*Extended Question:* The bearing reactions forFor my spine bridge model, dothe notproportion seem to be correct. The loadsof load taken by each bearing areseems notincorrect. inAlso, theunexpected anticipateduplift proportionoccurs. to each other and I am even getting unexpected uplift forces for some bearings.Is there an explanation for this? 

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[Body constraint DRAFT] to be finished and referenced in following paragraph
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*Answer:* The At a given substructure unit, a spine bridge model uses a single [Bodybody constraint DRAFT]to to connect a joint of the superstructure frame-element elementjoint withto all theother joints which representingrepresent the top of bearing links at a given substructure unit. While this modeling approach is adequate to obtain global structural response of the structure, it does not fully capture the distribution of the superstructure load to the individual bearings. You would need toFor bearing and pier-cap design, users should update the Linked Bridge Model as an Area Model to getcompute more meaningful bearing forces.
for
the
designh2. ofIndividual bearingsgirder andreactions
the pier caps.


h2. How can I obtain 
*Question:* How are girder reactions obtained for individual girders?

*Answer:* YouUsers canmay obtain girder reactions from forces in the [link elementselement|kb:Link] representing forces. Link elements represent the [bridge bearings|kb:Bridge bearings]. The [reactions|reactionReactions|kb:Base reactions] couldmay also be also estimated from girder end shears that can beshear, displayed viathrough the "Bridge Object Response" plots.


h2. Transverse bridge forces

*Question:* How canmay I obtain transverse bridge forces?
in transverse direction?

*Answer:* To obtain detailed results in the transverse direction, the linked bridge object should be updated as an area or solid model. Then, youusers canmay review the forces acting on individual [shell|kb:Shell] or solid [solid|kb:Solid] elements inusing the graphical user interface viathrough "Display > Show Forces / Stresses" menu command, or in a tabular format viathrough "Display > Show Tables > ANALYSIS RESULTS" menu commandAnalysis Results. YouUsers couldmay also define [section cuts|kb:Section cut] to obtain the forces over a somegiven design length, rather than for individual shell or solid elements.


h2. 

*Question:* Why are the bridge girder moments displayed viathrough "Display > Show Bridge Forces/Stresses" larger than the girderthose moments displayed for individual girder frame elements viathrough "Display > Show Forces/Stresses > Frames/Cables"?

The "Display > *Answer:* When displaying girder forces, the Display > Show Bridge Forces/Stresses (show forces for girders)" form menu displays moments resisted by the composite section of the girder and its tributary deck,. while theThe frame forces displayed viathrough "Display > Show Forces/Stresses > Frames/Cables" are only moments resisted by the girder itself, (in other words, without the deck contribution is not included in the frame forces).


h1. See Also

SpecializedAdditional bridge modeler FAQsFAQ devotedpages tofor individualspecialized topics of the bridge modeler, such as bridge deck section, bridge bearings, etc.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]