Model development

Creating the computational model

Question: How does

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 page for response.

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 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.

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

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 page for response.

Bearing links

Question: What do the several links created at each bearing location represent?

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

Modeling rigid connection between deck and supports

Question: How can I use the bridge modeler to model a [reinforced-concrete] box-girder bridge where the deck is rigidly (without bearings) connected to its bents and 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.

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

Solid and shell element connection

Question: For solid models, how does the bridge modeler connect solid elements 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

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

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

Bridge-object design

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

Answer: Please see the Bridge design topic for response.

Modeling additional mild reinforcement for flexure

Question: How do I add flexural mild reinforcement 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 [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.

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:

Reviewing Results

Discontinuities in moment diagram plot

Question: Why is the bridge-girder moment diagram, displayed on the Bridge Object Response Display menu, not smooth?

Answer: 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.

Force and stress diagrams

Question: How does

Software obtain bridge force and stress diagrams?

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

Effect of insertion point on beam reactions

Question: 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 test problem for additional information.

Spine bridge model bearing reactions

Question: For my spine bridge model, the proportion of load taken by each bearing seems incorrect. Also, unexpected uplift occurs. Is there an explanation for this?

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.

Individual girder reactions

Question: 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.

Transverse bridge forces

Question: 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.

Differences between global and local bridge-girder moment displays

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

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.

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