# P-Δ vs. P-𝛿
A P-delta analysis considers the effect of deflection on the loads carried by the structure: the difference between the two options being that the P-
The features introduced to the design layer in GSA 10.1 make it much more straightforward to consider both P-
# What are P-Δ and P-𝛿 effects?
To highlight the difference, consider a simple vertical column of height h subject to a transverse force F at the top and a vertical load P there will be a linear bending moment distribution from 0 at the top to
In a linear analysis there is no interaction between the axial and transverse load. However, the effect of this transverse load is to create a deflection at the top of the column. If this deflection is
This is the P-
Note: If the column is in tension the P-
effect reduces the moment.
While this takes into account the global effect of the interaction of load and deflection, there is a local effect which this misses. The way the element deflects means that the moment at mid height can be more than that predicted by the P-
At mid height there is an additional moment of
While this is not important for the response of the structure as a whole it can be significant to the design of individual members, especially if this effect coincides with the position of maximum bending in the member.
# When are P-Δ and P-𝛿 important to consider?
Note: When the analysis is no longer linear, combination cases are not valid. Take care to define the design loads for the analysis cases rather than base load cases (e.g., dead, live, etc.).
This includes the AISC direct-analysis method, removing the need for a side sway buckling analysis to determine K.
See Static P delta analysis options for more details on how GSA calculates these forces.
# How to adjust your GSA model to account for P-Δ and P-𝛿 effects
To do this:
- Select the column members.
- Set the element size to approximately half (or an appropriate fraction of) the storey height.
- When using the Create elements from members tool, GSA will generate multiple elements per column allowing the analysis to pick up both the P-
and P- effects.
effects are global. P- effects are local.