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Model development

Frequently asked questions associated with model development are discussed as follows:

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 page for response.

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 tutorial for response. Further, when enhancing a model created in the bridge modeler by linking it to SAP2000, please follow the guidelines outlined on the Mixed modeling 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.

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 analysis. Please see the Staged construction for precast girder bridges test problem for an illustrated overview of this procedure.

To what do bearing elevation and bent elevation refer?

Answer: Please see the Substructure and bearing elevations page for response.

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

Answer: Please see the Link creation and function topic of the Bridge bearings FAQ page for response.

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:

1. Define a fixed bridge bearing through the Bridge > Bearings > Add New Bridge Bearing option and set a fixed release type for all DOF.
2. Define an abutment with integral-girder support conditions through the Bridge > Abutments > Add New Bridge Abutment option.
3. Define a bridge bent with integral-girder support conditions using the Bridge > Bents > Add New Bridge Bent option.
4. 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:
   
   
   1. Under Modify/Show Assignments, select Abutments > Modify/Show, then assign the previously-defined fixed bearing to the abutments.
   2. Also under Modify/Show Assignments, select Bents > Modify/Show, then assign the previously-defined fixed bearing to the bents.
5. 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.

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 load case, add all elements to be modified, apply the modifiers 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 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.

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

Answer: Solid and shell elements are connected at their common joints. 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.

Analysis

Frequently asked questions associated with analysis are discussed as follows:

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

CSI 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 of response-spectrum and time-history analysis type, which would enable evaluation using the multi-mode and time-history methods.

Design

Frequently asked questions associated with design are discussed as follows:

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

Answer: Please see the Bridge design topic for response.

How is flexural mild reinforcement added into a bridge model?

Answer: In CSiBridge, 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 tendons 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.

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:

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Why are design bridge-girder moments slightly different from those calculated during analysis?

Answer: Analysis moments are calculated through the analysis process, while design moments are retrieved from analysis results only when the live-load distribution method is specified in the design request by selecting Use Direct Girder Forces from Analysis. Otherwise the total bridge-section moment is divided by the number of girders, then applied to each girder for non-live design forces.

Reviewing Results

Frequently asked questions associated with reviewing results are discussed as follows:

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 being carried by bridge deck elements. The total moment across the entire deck should be continuous.

How are bridge force and stress diagrams obtained?

Answer: Please see the Bridge object force and stress diagrams article.

What causes nonzero moment at abutment locations?

Answer: Please see the Nonzero moment at abutment locations article.

Why are my bridge objects returning zero force during moving-load analysis?

Answer: The request for saving section forces should be made such that bridge objects return the proper response during moving-load analysis.

Is there an explanation for why my spine bridge model exhibits uplift and unexpected bearing response?

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.

How are girder reactions obtained for individual girders?

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

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 or 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 to obtain the forces over a given design length, rather than for individual shell or solid elements.

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.

What is the difference between beam and girder forces displayed on the "Bridge Object Response Display" form?

Please see the Beam vs girder forces displayed on the Bridge Object Response Display form page.

See also

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

• Bridge bent FAQ
• Bridge bearings FAQ
• Bridge girder FAQ
• Bridge deck section FAQ
• Bridge load rating FAQ
• Layout line FAQ