Direct stiffness with basic settings is a simple linear analysis. Direct stiffness with second-order bending enabled is equivalent to what some standards refer to as p-delta analysis.
P-delta analysis accounts for second-order eccentric loads on the structure, particularly additional moments caused by vertical forces as the pole deflects. P-delta attempts to approximate these effects by increasing the loads on the pole to reflect the increase from these eccentric forces but maintaining the linear relationship between load and deformation.
Co-rotational analysis is what we consider to be geometric non-linear analysis. This method accounts for, and tracks, the movement of each load on the pole, as well as continual changes in the geometry of the structure as it reacts to each load and deforms. This means that co-rotational analysis can account for both geometric nonlinearities (the relationship between load and deformation) and load nonlinearities (such as p-delta effects or changes in wire tensions).
The biggest impact this will have on analysis results is due to the ability to update wire tensions as the structure deforms and deflects. In reality, as a pole bends towards a particular span, the sag in the wires will increase and the wire tension will decrease. This effect is accounted for when “Update Tensions Based on Displacement” is enabled.
When this “Update Tensions” option is enabled, all tension imbalances tend to quickly balance and come to equilibrium with each other after the pole begins to deflect. This means the pole is forced to support far less of the original load and has a much lower maximum stress.
When the analysis option “Update Tensions Based on Displacement” is disabled for anchor and guy analysis, this means the tension imbalances will not support each other or come to equilibrium themselves. Rather, the force of the imbalance must be completely supported by the pole and guy wires. When components are extremely overloaded during co-rotational analysis, the model does not converge to a stable geometry and is reported back to the user as unstable or infinitely loaded.
It should be noted that, with co-rotational analysis and updating tensions based on displacement, “bad” designs can pass weather load. The simplest case of this is an unguyed, deadend structure. Obviously, such a structure in the real world will slowly creep towards failure as the pole begins to lean over the course of weeks, months, or years. However, the stress loading on the pole would likely not fail the pole if “Update Tensions Based on Displacement” was enabled. Once the pole started to deflect, the wire tension would decrease, and the model would reach a stable geometry at a large deflection. Care should be taken to always adhere to your engineering standards for design guidelines.
View the Displacement-Based Loading article for more information on the option to update tensions based on displacement.