Model development

Creating the computational model

Question: How does

Software create a computational model from an object-based bridge model?

Answer: For response, please see the general description and additional topics presented on the Bridge components page.

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: For response, please see the Applying point, line, and area loads to bridge objects tutorial. 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 are their elevations defined?

Answer: For response, please see the Substructure and bearing elevations page.

Bearing links

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

Answer: For response, please see the Link creation and function topic of the Bridge bearings FAQ page.

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.

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 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, and add all elements and [apply the modifiers] 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.

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.

Analysis

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.

Design

What is the procedure for the design of bridge object? Is any documentation available?

See Bridge Design topic.

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.

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.

Reviewing Results

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

How does the program obtain bridge forces and stresses diagrams?

See Bridge object force and stress diagrams page.

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] test problem.

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.

How can I obtain girder reactions for individual girders?

You can obtain girder reactions from forces in the link elements representing the bearings. The [reactions] could be also estimated from girder end shears that can be displayed via the "Bridge Object Response" plots.

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

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).

See Also

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