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This page contains various frequently asked questions related to the bridge modeler that are organized by topic. *On this page:* {toc} h1. Developing Model h2. How does SAP2000 internally distributes moving loads from lanes to frames? {open-question} h2. Why cannot I run the step-by-step moving load analysis when the vehicle defined using uniform load? Vehicles defined uniform load can be used only in influence-based moving load analysis. To use the vehicle in the step-by-step moving load analysis, you would need to replace the uniform load by a series of equivalent point loads. h2. How does SAP2000 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? As of SAP2000 V14.0.0, this type of loading would need to be applied manually. This loading may however 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. Alternatively, you could use one of the following manipulations 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 does the bearing and bent elevations specified at the Bridge Object Bent Assigments 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 [Bridge bearings FAQ|kb:Bridge bearings FAQ] 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 SAP2000 builds a model using the bridge modeler, the model can be a solid object model with diaphragms modeled as area elements. How can SAP2000 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 SAP2000 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 SAP2000 enables to perform analysis using any of the procedures listed above. Procedures 1 and 2 are essentially equivalent static methods and SAP2000 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? The program uses [section cuts|Section cut] defined by groups to obtain bridge forces and stresses diagrams. These section cuts are automatically formed as part of the bridge model using the parametric definition and can be reviewed via "Display > Show Tables > MODEL DEFINITION > Bridge Data > Bridge Object Data > Table: Bridge Section Cuts 01 - General and Table: Bridge Section Cuts 02 Groups". Any manually added objects are not added to the section cuts. The force/moment diagrams will be correct for the final structure at all sections that have not been modified. Manually added members, such as diaphragms, may carry load and take it away from objects that are part of a section cut, thus reducing the section cut force. They can only affect section cuts to which they are connected. _See [Bridge forces and stresses diagrams] for additional details._ 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. {related-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.} h2. Why the bearing reactions for SAP2000/Bridge spine model look incorrect? *Extended Question:* The bearing reactions for my spine SAP2000/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] 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? The design is for prestressed concrete box girder sections only and as of SAP2000 V12 there are no provisions for mild reinforcement. 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 SAP2000 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.: {children}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. |
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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:
- 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.
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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.
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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.
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Is there an explanation for why my spine bridge model exhibits uplift and unexpected bearing response?
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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: