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:
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2.Alignment Coordinate System: In OpenBrIM, any node or point assigned to an alignment will have local X-Y-Z values in the alignment coordinate system. In this system, moving upstation and downstation along the PGL direction corresponds to the positive X-axis, regardless of the shape of the PGLwill change the local X value of points or nodes. For instance, in the case of a curved PGLalignment, the X-axis follows the horizontal curve, so local X values displayed at nodes or points can be interpreted as station values. The Y -axis represents values, in this case, represent the transverse direction , where and are perpendicular to the PGL, with the positive Y-axis points pointing to the right when looking upstream upstation along the PGL and negative values pointing to the left. The Z -axis has a value of is 0 at the PGL, with positive Z values pointing upwards and negative Z values pointing downwards (gravity)from the PGL.
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3.Finite 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 eitherelement's local axes can be viewed directly by activating 'Display Local Axis.' The line representing the element's direction corresponds to the X-axis, the second line represents the Y-axis, and the third line represents the Z-axis, as shown in the figure below.
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An element's node’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 . Typically, for most bridge workflows, nodes follow the alignment coordinate system. However, for bearing nodes, if the bearing rotation is defined, this parameter will directly affect the node's coordinate system by rotating the axes according to the definition of the bearing rotation parameterdefinition.
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Node Reactions (Local):
Node reactions are
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presented to the user
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based on the node's local coordinate system. In most cases, this coordinate system aligns with the alignment coordinate system. However, if the bearing rotation parameter is defined with a value other than zero, the node's coordinate system will rotate accordingly, and the reactions will follow the updated orientation.
For example:
Force X: Represents the longitudinal alignment direction.
Positive values: Indicate forces resisting movement in the downstation direction.
Negative values: Indicate forces resisting movement in the upstation direction.
Force Y: Represents the transverse direction, perpendicular to the alignment.
Positive values: Indicate forces resisting movement to the left when looking upstation.
Negative values: Indicate forces resisting movement to the right when looking upstation.
Force Z: Represents the vertical direction.
Positive values: Indicate forces resisting downward movement, often corresponding to compressive reactions.
Negative values: Indicate forces resisting upward movement, often corresponding to uplift or tension.
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 FELine and FESurface are presented to the user in this section with respect to the element's local coordinate system in this section.
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If the user intends to use these forces or Element End Forces (External - Global) for design purposes, they should exercise caution, as the sign convention for element end forces is opposite with respect to the element's local coordinate system. For example, a positive axial force at one end of the element will appear as a negative axial force at the opposite end. This reversal is inherent to how forces are represented and must be accounted for during the design process to avoid errors. Alternatively, using FELine internal forces may provide a simpler approach. |
Element End Forces (External - Global):
The forces at the ends of each FE line FELine and FESurface 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
FEComposite forces represent the combined effects of internal forces across multiple finite elements that make up a composite structure or a group of connected elements.
These forces are calculated by summing or integrating the internal forces (e.g.,
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axial forces, shear forces, and bending moments) from all contributing finite elements along the path of the composite element. The resulting forces provide a simplified and consolidated representation of the overall structural behavior, making it easier to interpret and use for design and analysis purposes.
Summation of Forces:
The forces from all finite elements within the composite structure are aggregated to produce a single resultant value for each force component (e.g.
, axial, shear, and moment).
Simplified Interpretation:
Instead of analyzing individual element forces, FEComposite forces provide a macro-level understanding of the force distribution within the entire structure or substructure.
Applications in Bridge Engineering:
Useful for analyzing composite sections such as girders, deck slabs, or combined bridge components where the contribution of multiple elements needs to be evaluated as a whole.
The path of a typical FEComposite force for bridge elements is illustrated in the figure below.
For girders, the composite force follows the girder's physical path along its centerline.
Fx (Axial Force): Represents the force along the girder's longitudinal axis. For example, a positive Fx indicates tensile forces, while a negative Fx indicates compressive forces.
Fy (Shear Force in the Y-Direction): Represents the transverse shear force acting perpendicular to the girder's length.
Fz (Shear Force in the Z-Direction): Represents the vertical shear force acting in the vertical plane of the girder.
Mx (Torsional Moment): Represents twisting along the girder's axis.
M_y (Bending Moment about the Y-Axis): Represents major-axis bending, typically caused by vertical loads.
Mz (Bending Moment about the Z-Axis): Represents minor-axis bending, typically caused by lateral forces.
FELine Forces (Internal):
FELine internal forces can be viewed under are displayed in this section . The results are displayed according to the sign convention, where negative values represent compression and positive values represent tensionalong the local axes of the FELine element. These forces include axial, shear, and bending components:Force X: Represents the axial force within the element. Negative values indicate compression, while positive values indicate tension.
Force Y: Represents the shear force in the local Y-axis direction, perpendicular to the element's axis.
Force Z: Represents the shear force in the local Z-axis direction, also perpendicular to the element's axis.
Moment X: Represents the torsional moment about the local X-axis.
Moment Y: Represents the bending moment about the local Y-axis, where positive and negative values indicate different bending directions.
Moment Z: Represents the bending moment about the local Z-axis, following the same convention as Moment Y.
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 spring forces at each node are displayed in this section. The calculations are done with respect to the global coordinate systemresults for each node are reported based on the node's coordinate system, which, in most cases, aligns with the alignment coordinate system.In a typical bridge workflow, if the bearing rotation angle is 0:
X-axis: Represents the longitudinal direction of the alignment.
Positive values: Indicate forces acting in the upstation direction.
Negative values: Indicate forces acting in the downstation direction.
Y-axis: Represents the transverse direction, perpendicular to the alignment.
Positive values: Indicate forces acting to the left when looking upstation.
Negative values: Indicate forces acting to the right when looking upstation.
Z-axis: Represents the vertical direction.
Positive values: Indicate upward forces.
Negative values: Indicate downward forces, often corresponding to compressive reactions.
FESurface Forces (Internal):
Composite Element Stresses:
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