Page Comparison

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

{info}Please note that this page applies largely to [CSiBridge|csibridge:home], though some information may pertain to the *bridge modeler* available with [SAP2000|sap2000:home] releases through version 14.{info}

{on-this-page}

h1. Developing Model

h2. How does program create analysis model from structural model for bridge objects? Could you please provide complete documentation with illustrative sketches?

_See_ _[bridge modeler parameters and analysis model|Bridge components#FAQ]_.

h2. How can I apply superimposed loads (such as railing and utilities) to a bridge modeler object? Will this loading remain to be assigned correctly when the discretization changes?

See [tutorials:Applying point, lineal and area loads to bridge objects] tutorial.

In older versions of the program, this type of loading had to be applied manually, which could get modified when the linked bridge model is updated and would need to be manually re-applied. See note on [mixed models|mixed models] for additional information. The following manipulations could be used in the older versions to indirectly define the loading within the bridge modeler definition:

* Increase the unit weight of the bridge deck slab material to account for the superimposed dead loads. This would be the most straightforward solution.
* Apply weight modifiers to the shell section that is used to model the concrete slab.

h2. How can I model bridge simply supported for dead load and continuous for live load?

This can be done by using construction stage analysis. Please refer to [precast girder bridge test problem|tp:Staged construction for precast girder bridge] for an illustration of how this can be done.

h2. What do the bearing and bent elevations specified at the Bridge Object Bent Assignments form represent?

*Extended question:* Bridge > Bridge Objects > Modify/Show Bridge Object > Modify/Show Assignments for Bents > Elevation (Global Z)": at which point is the elevation defined? C.g. of pier cap?

_See_ _[Substructure and bearing elevations|kb:Substructure and bearing elevations]_ _page._

h2. There are several links created at each bearing location. What do these links represent?

_See_ _[Bridge bearings FAQ|kb:Bridge bearings FAQ]_ _page._

h2. How can I model a concrete box bridge using bridge modeler for which its deck is rigidly (without bearings) connected to its bents and abutments?

This can be accomplished using the following steps (in V12.0.2):

# Define fixed bridge bearing via "Bridge > Bearings > Add New Bridge Bearing" and set [fixed release type|Fixed DOF in links] for all [DOFs|DOF].
# Define abutment with integral girder support condition (via "Bridge > Abutments > Add New Bridge Abutment").
# Define bridge bent with integral girder support condition (via "Bridge > Bents > Add New Bridge Bent").
# Use "Bridge > Bridge Objects > Modify/Show Bridge Object" menu command to assign the previously defined fixed bearing to the abutments and bents with rigid connection to the superstructure. The menu command will open Bridge Object Data form that can be used the complete the following actions:
## Under "Modify/Show Assignments" heading, choose Abutments and click the "Modify/Show" button. Then assign the previously defined fixed bearing to the abutments.
## Under "Modify/Show Assignments" heading, choose Bents and click the "Modify/Show" button. Then assign the previously defined fixed bearing to the bents.
# Review the entities created by the bridge modeler at each superstructure to substructure connection to make sure that they correctly represent the modeling intention.

See also [Girder support condition] page for additional information on integral bent and abutments.

h2. How can I efficiently reduce section properties for all superstructure frame elements created by the bridge modeler?

*Extended Question:* For my particular model, the [Bridge Modeler|bridge modeler] generates hundreds frame sections since the superstructure is highly curved and the cross-section varies. If I want to reduce the moment of inertia of the superstructure (cracked concrete, Icr = 0.5 Ig, Jcr = 0.05 J), do I need to go to Section "Property > Frame Properties" and set modifiers one by one? This is lots of work and very inefficient, I was wondering if there’s any easier way to model the cracked concrete for the superstructure.

*Answer:* There are several ways you can efficiently assign reduced sectional properties for the superstructure members. For example, you could follow the procedure described below:

* Define a group that will contain all superstructure frame elements. Such group can be defined directly on the "Bridge Object Data" form via the "Staged Construction Groups" assignment.
* Define frame property modifier to be used via "Define > Name Property Sets > Frame Modifiers" menu command.
* 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. 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 [kb:Bridge object forces and stresses 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.

{hidden-content}
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}
{hidden-content}



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. 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. See Also

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