Bridge modeler FAQ
This page is devoted to frequently asked questions (FAQ) related to the Bridge Modeler.
Please note that this page applies to both CSiBridge and the bridge modeler, available with SAP2000 releases through version 14.
On this page:
- 1 Model development
- 1.1 How is a computational model developed from an object-based bridge model?
- 1.2 How are loads from railings and utilities applied to bridge objects, and will they remain after update?
- 1.3 When a bridge is simply supported under dead load and continuous under live load, how is loading applied?
- 1.4 To what do bearing elevation and bent elevation refer?
- 1.5 What do the links, created at each bearing location, represent?
- 1.6 How can I model a reinforced-concrete box-girder bridge object with a rigid, bearing-less connection between the deck and bents/abutments?
- 1.7 How can I change section properties for all individual elements of a bridge object?
- 1.8 For solid models, how are solid elements connected to diaphragm shell elements?
- 2 Analysis
- 3 Design
- 3.1 What is the bridge-object design procedure, and is documentation available?
- 3.2 How is flexural mild reinforcement added into a bridge model?
- 3.3 How is the tension limit calculated for the AASHTO LRFD 2007 principal check?
- 3.4 Why are design bridge-girder moments slightly different from those calculated during analysis?
- 4 Reviewing Results
- 4.1 Why is the bridge-girder moment diagram not smooth?
- 4.2 How are bridge force and stress diagrams obtained?
- 4.3 What causes nonzero moment at abutment locations?
- 4.4 Why are my bridge objects returning zero force during moving-load analysis?
- 4.5 Is there an explanation for why my spine bridge model exhibits uplift and unexpected bearing response?
- 4.6 How are girder reactions obtained for individual girders?
- 4.7 How may I obtain transverse bridge forces?
- 4.8 Why are bridge girder moments larger than those of individual girder frame elements?
- 4.9 What is the difference between beam and girder forces displayed on the "Bridge Object Response Display" form?
- 5 See also
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:
Define a fixed bridge bearing through the Bridge > Bearings > Add New Bridge Bearing option and set a fixed release type for all DOF.
Define an abutment with integral-girder support conditions through the Bridge > Abutments > Add New Bridge Abutment option.
Define a bridge bent with integral-girder support conditions using the Bridge > Bents > Add New Bridge Bent option.
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:
Under Modify/Show Assignments, select Abutments > Modify/Show, then assign the previously-defined fixed bearing to the abutments.
Also under Modify/Show Assignments, select Bents > Modify/Show, 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 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:
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.