When using OpenBrIM, a thorough understanding of the coordinate systems is essential for various purposes, such as accurately modeling bridge elements, interpreting results correctly, and performing other tasks. It is crucial to be familiar with the coordinate systems employed in the software and understand how to apply them effectively. This knowledge is vital for ensuring the proper modeling of the bridge and the accurate interpretation of the results.
OpenBrIM uses three coordinate systems:
1.Global Coordinate System: The axis directions can be viewed using the cube symbol located in the top-right corner of the application.
2.Alignment Coordinate System: In this system, the PGL direction corresponds to the positive X-axis, regardless of the shape of the PGL. For instance, in the case of a curved PGL, the X-axis follows the curve. The Y-axis represents the transverse direction, where the positive Y-axis points to the right when looking upstream along the PGL. The Z-axis has a value of 0 at the PGL, with positive Z values pointing upwards and negative Z values pointing downwards (gravity).
3.Element’s Local Coordinate System: The element’s local axis can align with the alignment coordinate system, the global coordinate system, or be a modified version of either.
An element's coordinate system can be viewed in the FEA tab of the workflow, under Geometry > Nodes (when the lock icon is clicked), in the 'Coordinate System' column.The displayed coordinate system can either be the Alignment Coordinate System or the object's coordinate system.
Coordinate systems utilized for FEA results can be summarized as follows:
Node Displacements (Local):
Node displacements are calculated and presented with respect to the node's associated coordinate system, which is the same as the alignment's coordinate system, unless a rotation is defined by the Bearing Rotation parameter under the Superstructure-Bearing objects. The definition of this parameter will directly affect the node's coordinate system by rotating the axes according to the definition of the bearing rotation parameter.
Node Reactions (Local):
Node reactions are calculated and presented to the user with respect to the node’s coordinate system, which is the same as the alignment's coordinate system unless the bearing rotation parameter is defined with a value other than zero.Node Reactions (Global):
Node reactions in this section are calculated and presented to the user with respect to the global coordinate system.Element End Forces (External - Local):
The forces at the ends of each FE line generated are calculated and presented to the user with respect to the element's local coordinate system in this section.Element End Forces (External - Global):
The forces at the ends of each FE line generated are calculated and presented to the user with respect to the global coordinate system in this section.Composite Element Forces (Sectional):
For composite elements (e.g., girders) and several other elements (e.g., piles), results can be accessed in this section of the FEA tab. The results will be displayed based on the following convention: a negative sign indicates compression (for both moment and force), and positive results indicate tension. This sign convention is generated during the calculation stage and then presented to the user in the UI.FELine Forces (Internal):
FELine internal forces can be viewed under this section. The results are displayed according to the sign convention, where negative values represent compression and positive values represent tension.FELine Stresses:
Stresses calculated for FELines can be viewed under this section.FESpring Forces (Global):
The forces on the nodes of the springs can be viewed in this section. The calculations are done with respect to the global coordinate system.FESurface Forces (Internal):
Composite Element Stresses:
Why do the node coordinates show a negative y direction even though the positive transverse offset is going in the right direction looking ahead station in most bridge workflows?
In a typical 3D coordinate system, there are three mutually perpendicular axes: X, Y, and Z. The positive direction of the x-axis points to the right, the Z-axis is vertical and the positive direction of the Z-axis points upwards, and the Y-axis is horizontal and pointing away from you. However, in the context of bridge engineering, positive transverse offset values are typically used in the right direction when looking ahead at a station. To ensure consistency with how bridge engineers think, many library components in OpenBrIM Platform convert +Y values to -Y and -Y values to +Y in their input parameters.
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