Precast Bridges: FEA Not Completing
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 is made when defining materials and sections.
The most common mistake made when defining ‘RigidMaterial’ and ‘RigidSection’ is that these definitions cannot be imported from the OpenBrIM database and must be defined manually by entering numbers. The ‘RigidMaterial’ and ‘RigidSection’ are used as connectors to bridge elements and must have the proper definitions to not become obstacles when running FEA.
Correct Definition for Rigid Material
Correct Definition for Rigid Section
Incorrect Definition for Rigid Material
Incorrect Definition for Rigid Section
Bearing Definitions
There are nine parameters when defining bearings:
Insertion Points: A previously defined insertion point must be used to define the location of the bearings.
Tx [Free/Fixed/Stiffness]: Translation along the X-axis (Longitudinal). When precast girders are used, there must be two 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.
Ty [Free/Fixed/Stiffness]: Translation along the Y-axis (Transverse). When precast girders are used, there must be two 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.
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.
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'.
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.
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.
Bearing Rotation: Can be specified if needed.
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'.
Girder Definitions
As clearly shown in the screenshot below, unlike steel girders, precast girders can only be defined by two insertion points and, consequently, two bearings. It is important to define two separate bearings for each girder; in other words, two adjacent girders cannot share a common insertion point.
Correct Definition
Incorrect Definition
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:
Pile Definitions
Correct Definitions for Piles (FEA Definitions)
Incorrect Definitions for Piles (FEA Definitions)
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, and Tz, as well as Rx, Ry, and Rz, to ‘Fixed’ or define them with high stiffness values at the depth of fixity.
Visual Difference: When Tx, Ty, and Tz are set to ‘Fixed,’ a one-node spring (illustrated as a rectangular prism) will be visible. When a stiffness value is defined, a two-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:
Construction stages must be defined with several inputs, with two of the most commonly mistaken definitions being Stage Name and Prior Stage.
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
Incorrect Definition of Construction Stages
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
Incorrect Definition - Bearing Construction
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