This article aims to explain the basics of how FEA works with a worked example.
Scenario
Analysis is required for a wood pole, without a guy, between two termination poles and will use ruling span load source. There is a line deviation angle which will apply a force to the pole, even under everyday conditions.
For instructions on running FEA, see How to run FEA.
Model Segmentation
The first step the FEA engine will do is segment the structure model. In this example the pole has been split up into 5 segments, which will be referred to as S0 to S4.
For instructions on how to view the FEA model as per below, see Viewing FEA model.
Iterating Until Equilibrium Reached
The FEA engine will then iterate until it reaches an equilibrium. Note, even though the point loads in this example on the pole are constant due to ruling span load source, there are non-linear behaviours that can only be solved iteratively. For example, as the pole deflects an additional bending moment is generated on the pole due to vertical loads contributing to the cantilever load (known as p-delta effect). This increase in load will cause the pole to deflect more and the FEA engine must iterate until equilibrium is reached.
The video below shows how the FEA engine iteratively solves the final position of the pole.
The starting position of the pole is the vertical position
The next iteration adds the point loads (depicted as short orange lines near the pole's tip)
Subsequent iterations show the pole deflecting. Initially there are large changes to the poles deflection and as the simulation continues the deflections reduce until equilibrium. At equilibrium the force applied from the conductors as point loads is perfectly balanced by the opposing force created by the pole's deflection.
FEA Results
There are Reports available in the FEA panel which provide a range of details, including the offsets from the ground at which each segment starts and ends with S0 always being the ground position.
To illustrate, the diagram below has the FEA model with S0 and S1 labelled based on the data from the FEA results. Note the naming is the pole name, followed by the frame number, followed by the segment number and if it is the start (s) or end (e) of the segment.
The FEA engine calculates the geometry of the beam at the start and end of each segment using the dimensions given in the library and linear interpolation. Using this geometry the maximum structural limits are calculated such as the normal stress capacity, shear stress capacity, bending moment capacity, and displacement limits. The FEA engine also calculates all the forces and moments at the start and end of each segment and assesses them against the calculated structural limits.
There is a general naming pattern for FEA results in these reports:
Columns containing Ultimate are the maximum allowable value without applying the Strength Reduction Factors (SRF)
SRF are defined in Strength States within the Environment tab, see Strength States
Ultimate values are provided for completeness and are not used in structural assessments
Columns containing Limit are the maximum allowable value after applying the SRF values
For example, where wood poles have an SRF of 0.5, the Limit value will be half the Ultimate value.
Columns containing just the structural term such as Moment are the calculated value, with load factors already applied.
The column titled just Moment is the vector sum of the calculated Moment X, Moment Y and Moment Z.
Columns containing the word Utilization are the structural value being calculated divided by the Limit.
