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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. 

*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 modifythe frame properties property modifier to be used.

* Define [staged construction load case|staged construction], and add all elements and [apply the modifiers|Staged construction operations] in the first stage. Then apply other loads in the subsequent stages.

Using the above approach is the most flexible, since the structures with full and reduced stiffnesses coexist in a single model. Please note that as of V14.0.0, we allow to run staged construction analysis with a single stage, even for users that do not have the staged construction license. This allows to evaluate multiple configurations of the structure which is exactly your case. You could then run subsequent analyses by using stiffness at the end of this single-stage staged construction load case.

You could also use [Interactive database editing] ("Edit > Interactive Database Editing") to assign modifier to multiple members from a single location.


h2. How does the program connect solid elements with diaphragm shell elements for solid models?

*Extended Question:* When the program builds a model using the bridge modeler, the model can be a solid object model with diaphragms modeled as area elements. How can the program connect these two elements which have different degrees of freedom?

*Answer:* The solid and shell elements get connected at their common joints. All DOFs are activated for such common joints, but only the shell elements contribute rotational stiffness. This is a reasonable modeling, because the main contribution of the diaphragms is due to their in-plane stiffness.


h1. Analysis


h2. Can the program perform a seismic analysis using single mode, uniform load or time history method?

I am assuming that you are referring to AASHTO ASD 2002 (17th Edition), Division IA - Seismic Design, Section 4 (Analysis Requirements). Based on the complexity and regularity of the bridge, this section requires to use one of the following analysis methods:

* Procedure 1: Uniform load method
* Procedure 2: Single-Mode Spectral Method
* Procedure 3: Multimode Spectral Method
* Procedure 4: Time History Method

The program enables to perform analysis using any of the procedures listed above. Procedures 1 and 2 are essentially equivalent static methods and the program can be used to determine the response under the equivalent static loading. You can also define load cases of "Response Spectrum" and "Time History" types which would enable you to evaluate the response using procedures 3 or 4, respectively.


h1. Design


h2. What is the procedure for the design of bridge object? Is any documentation available?

_See_ _[Bridge Design topic|Bridge design]__._


h2. How do I add flexural mild reinforcement into the bridge modeler model?

In CSiBridge, mild refinforcement can be added via "Bridge Tab > Bridge Objects > Girder Rebar". However, this reinforcement is used only for design checks and does not add any stiffness to the model. If you need to add the stiffness of the reinforcement to the model,  you could possibly model mild steel reinforcement as a [Tendon references] with zero force, but the design would not really be correct. For example, the mild reinforcement would be treated as tendon for flexural check, but the code distinguishes between the two. Only the stress check would provide adequate results.


h2. How does the program define the tension limit under the AASHTO LRFD 2007 principal check request in bridge design? Which formula does the program use?

The tension limit is calculated from the specified concrete compressive strength, f'_c (this is defined on the "Material Property Data" form),  and  a "Ten Lim" factor (this is defined on the "Superstructure Design Request" form) as a sqrt\{f'_c\} multiplied by the "Ten Lim" factor. Please note that the "Ten Lim" factor depends on the units used. The default  value for ksi units is 0.19, while the default value for MPa units is 0.5.


h1. Reviewing Results


h2. When I display moment diagram for a bridge girder (on the "Bridge Object Response Display" form), the diagram is not smooth. Why?

Jumps in the frame moment diagram at the nodes are caused by the fact that a portion of the total moment acting on the [composite section|composite behavior] is carried by the bridge deck elements. The total moment across the entire deck should continuous.

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


h2. How does the program obtain bridge forces and stresses diagrams?

See [Bridge object force and stress diagrams] page.


h2. Why am I getting nonzero moment at the abutments?

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 an arm about the neutral axis of the section. This is kinematically correct behavior. Releasing the abutment bearing links in the longitudinal (U3) direction should give you zero moments at the abutments. Just make sure you have longitudinal restraint somewhere, such as at one abutment or bent.

If you do not want to remove the 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, but may not be physically realistic.

See also [Impact of insertion point on horizontal reactions of simply supported (pin-pin) beam|tp:Impact of insertion point on horizontal reactions of simply supported (pin-pin) beam] test problem.

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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.}
* {incident:no=39862|comment=Negative moments at abutment of simply supported by highly skewed bridge}
** {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 spine model look incorrect?

*Extended Question:* The bearing reactions for my spine bridge model do not seem to be correct. The loads taken by each bearing are not in the anticipated proportion to each other and I am even getting unexpected uplift forces for some bearings.

*Answer:* The spine bridge model uses a single [Body constraint DRAFT] to connect a joint of the superstructure frame element with all the joints representing the top of bearing links at a given substructure unit. While this modeling approach is adequate to obtain global response of the structure, it does not fully capture the distribution of the superstructure load to the individual bearings. You would need to update the Linked Bridge Model as Area Model to get more meaningful bearing forces for the design of bearings and the pier caps.


h2. How can I obtain girder reactions for individual girders?

You can obtain girder reactions from forces in the [link elements|Link] representing the [bearings|Bridge bearings]. The [reactions|reaction] could be also estimated from girder end shears that can be displayed via the "Bridge Object Response" plots.


h2. How can I obtain bridge forces in transverse direction?

To obtain detailed results in transverse direction, the linked bridge object should be updated as area or solid model. Then, you can review the forces acting on individual shell or solid elements in the graphical user interface via "Display > Show Forces / Stresses" menu command or in a tabular format via "Display > Show Tables > ANALYSIS RESULTS" menu command. You could also define [section cuts|Section cut] to obtain the forces over some design length rather than for individual shell or solid elements.


h2. Why are the bridge girder moments displayed via "Display > Show Bridge Forces/Stresses" larger than the girder moments displayed for individual girder frame elements via "Display > Show Forces/Stresses > Frames/Cables"?

The "Display > Show Bridge Forces/Stresses (show forces for girders)" form displays moments resisted by the composite section of the girder and its tributary deck, while the frame forces displayed via "Display > Show Forces/Stresses > Frames/Cables" are only moments resisted by the girder itself (in other words, the deck contribution is not included in the frame forces).


h1. See Also

Specialized bridge modeler FAQs devoted to individual topics of the bridge modeler, such as bridge deck section, bridge bearings, etc.