Steel Bridges: FEA Not Completing

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Although the error indicates that a specific node (CFX55_11.MIN) is causing instability, the issue may not be related to that specific node. Therefore, additional checks should be performed.

Definitions Related to Properties Section

One of the most common mistakes when defining materials and sections occurs with the ‘RigidMaterial' and 'RigidSection' definitions. These cannot be imported from the OpenBrIM database and must be manually defined by entering values. 'RigidMaterial' and 'RigidSection' are used for defining 'Rigid Beams’, which are unit elements in finite element analysis typically used to connect two separate bridge elements. Therefore, correctly defining their properties is essential.

Correct Definition for Rigid Material

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Correct Definition for Rigid Section

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Incorrect Definition for Rigid Material

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Incorrect Definition for Rigid Section

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properties edited.mp4

Bearing Definitions

There are nine parameters when defining bearings:

1. Insertion Points: A previously defined insertion point must be used to define the location of the bearings.
2. Tx [Free/Fixed/Stiffness]: Translation along the X-axis (Longitudinal). When steel girders are used, there may be several bearings associated with a specific girder. To ensure the stability of the girder, at least one bearing must have the Tx value set to 'Fixed' or to a specific Stiffness value.
3. Ty [Free/Fixed/Stiffness]: Translation along the Y-axis (Transverse). When steel girders are used, there may be several bearings associated with a specific girder. To ensure the stability of the girder, at least one bearing must have the Ty value set to 'Fixed' or to a specific Stiffness value.
4. Tz [Free/Fixed/Stiffness]: The translation along the Z-axis should always be set to 'Fixed' or assigned a specific stiffness value to ensure the bridge remains stable and does not move in the direction of gravity.
5. Rx [Free/Fixed/Stiffness]: Rotation along the X-axis (Rx) is typically set to 'Free'; however, when a diaphragm or cross frame is present and these elements are constructed after the girders, Rx should not be set to 'Free' and should be defined as 'Fixed' or assigned a specific Stiffness value. If these elements are constructed together with the girders, Rx should be set to 'Free'.
6. Ry [Free/Fixed/Stiffness]: Rotation along the Y-axis. This value is typically set to 'Free'; however, in some cases, such as having girder embedment into an abutment, Ry can be defined as 'Fixed' or with a specific Stiffness value.
7. Rz [Free/Fixed/Stiffness]: Rotation along the Z-axis. This value is typically set to 'Free'; however, in some cases, such as having girder embedment into an abutment, Rz can be defined as 'Fixed' or with a specific Stiffness value.
8. Bearing Rotation: Can be specified if needed.
9. Transfer Forces to Substructure [YES/NO]: This value can be set to 'No' under two conditions: if there is no substructure, or if there is an object that has 'FEM' in not generated mode such as abutments. If piers are present, which will be considered in FEA, this parameter should be set to 'YES'.

For instance, as it can clearly be seen from the screenshot below, steel girder G4 is modeled with four insertion points and followingly four bearings.

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More on the Transfer Forces to Substructure Parameter:

When the 'Transfer Forces to Substructure' parameter is set to 'NO', one-node springs are generated for FEM and analysis. When set to 'YES', two-node springs are generated. Therefore, with such arrangements, as shown in the example case, the visual representation should be as follows:

Transfer forces to substructure edited.mp4

Pile Definitions

Correct Definitions for Piles (FEA Definitions)

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Incorrect Definitions for Piles (FEA Definitions)

pile wrong.mp4

By using the spreadsheet, you can define the location of the nodes, assign stiffness values [Free/Fixed/Stiffness] (Tx, Ty, Tz, Rx, Ry, Rz) to those nodes, and obtain FEM results accordingly.

Therefore, it is recommended to:

Set Tx, Ty, Tz, Rx, Ry, and Rz to ‘Fixed’ or define them with high stiffness values at the depth of fixity.

Visual Difference: When Tx, Ty, and/or Tz are set to ‘Fixed,’ a one-node spring (illustrated as a rectangular prism) will be visible, indicating that at least one of the parameters is defined as ‘Fixed.’ When stiffness values are defined, or the option ‘Free’ is set for the related nodes, a one-node spring (illustrated as a line) should be visible.

Constructing the Model

In addition to the definitions related to the 3D model, construction-related definitions of bridge elements must be provided to obtain a complete FEA. The importance of constructing the elements lies not only in the definition itself but also in the sequence.

To ensure accuracy:

1.Construction stages must be defined with several inputs, with two of the most commonly mistaken definitions being Stage Name and Prior Stage.

2.Assigning related elements as either a complete construction case or a partial construction case must be done in the specified stage.

Correct Definition of Construction Stages

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Incorrect Definition of Construction Stages

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One common mistake during this stage occurs when constructing bearings.

• Bearings must be constructed at the stage in which the related girder is erected. If an earlier stage is chosen, you will likely encounter a failed FEA; if a later stage is chosen, the related deformation calculations of the model will be inaccurate.

Correct Definition - Bearing Construction

bearing construction correct.mp4

Incorrect Definition - Bearing Construction

bearing construction wrong prior stage.mp4

bearing construction-following stage wrong.mp4