FEA Results
FEA calculates the deflection and rotation of the poles and cross arms. To view the calculated deflection, click on the Structure Summary and select the pole of interest.
Scroll down in the report to the pole and cross arm results (cross arms results shown below).
Displacement
The displacement is the amount an object has moved from its at rest position when a force is applied to it. It is important to note that the displacement is shown in the X, Y and Z directions of the specified coordinate system (not the LTV of the pole). It is also important to note that the deflection of a cross arm includes any deflection of the pole as well. Hence two values might need to be subtracted to get the actual cross arm deflection (see validation example below).
Validation of Displacement
Consider the scenario of a termination cross arm where the conductors have 5kN pulling at a distance of 1m from the kingbolt. The width of the cross arm is 100mm and the depth 150mm and the modulus of elasticity 1GPa. Note that the conductors have been aligned with the x axis to make it easier to interpret the results.
The deflection of a beam can be approximately calculated by hand using the formula:
In this scenario the Second Moment of Area in the direction of the force would be 1250cm^4. Hence the deflection would be:
To run the FEA the Load Source was set to Ruling Span so the tension does not decrease as the pole and cross arm deflect. It is important to note that the FEA results include the pole displacement when showing the cross arm displacement. The cross arm offset of 0m is one end of the cross arm (where the conductor attaches), 1m is the kingbolt and 2m the other end of the cross arm. The displacement in the x-axis (direction of force) would be 195.28mm - 67.12mm = 128.16mm or 0.128m. This is very close to the hand calculation above of 0.133m. The difference is likely due to the fact that as the cross arm deflects the force perpendicular to the cross arm decreases and hence the displacement decreases (which is all taken into account in the FEA engine).
Rotation
The rotation angle is the angle at the end of the beam as shown below:
It is important to note that Neara does not graphically display the curve of a cross arm which could give the impression that the rotation angle is the same along the beam. However internally the curve of the beam is calculated and this is the rotation angle that will be shown in the results.
Rotation Validation
The rotation angle at the end of a beam can be calculated with the formula:
Hence using the above scenario for deflection the angle of rotation would be:
The results from the FEA simulation are very close to this at 11.1 degrees. Again the FEA calculates a slightly lower angle due to the fact that as the cross arm deflects the force perpendicular to the cross arm decreases and hence the displacement decreases.
