Question

There are differences between the axial capacity obtained with the SACS tool, "Plot Pile Capacity," and that calculated with the Static Analysis with Pile/Soil Interaction module, using the same SOIL card. What is the explanation?  


Answer

The key difference between the capacities value relates to how they're calculated:

Pile Capacity Plot

- Treat the piles individually (isolated), ignoring any pile head springs or superstructure restraints.
- SACS imposes large axial displacements until every depth reaches peak shear, and ignores lateral deflection entirely.
- It assumes full mobilization of skin friction and end bearing over the entire depth without any lateral deflection, P-delta effects or structural coupling.

Static with PSI analysis

- accounts for group effects, structure interaction and the piles are under combined axial and lateral loads which might reduce the axial capacity.
- Other factors such as displacement, P-Delta effects and load case differences also contribute to the reduced axial capacity.
- The pile head is connected to the superstructure. Any moment restraints, pile head springs limit the axial displacement at the top, thus preventing full mobilization of skin friction at all depths.
- The pile can deflect laterally under eccentric or lateral loads so the axial load that actually mobilizes along the shaft is less than if you pushed axially in isolation.
- Any lateral load case (wave or wind) will be combined with axial loads. Because the soil springs are nonlinear, the resulting axial reaction is lower than if axially loaded in isolation.

Because of these interactions, the maximum axial capacity that PSI reports under the specific load case (including superstructure stiffness, lateral loads, p-delta,etc) is always lower than the theoretical, fully mobilized value from the Pile Capacity Plot.

The pile capacity plot is intended to find the absolute ultimate axial limit of a single pile under pure tension/compression with no coupling with lateral springs or superstructure.

Static PSI solves a coupled, nonlinear foundation structure equilibrium. It includes P-delta, soil nonlinearity, pile head springs, and any specified lateral loads. Because of those interactions, the axial capacity that PSI reports under realistic load combinations is always lower than Pile Capacity axial peak.

So, if you need an unconstrained, envelope-type estimation of how strong a single pile could be under axial load, use the capacity in the PILE CAPACITY PLOT.

If you require the working axial capacity under ACTUAL combined load cases (including lateral, rotational, and support stiffness), use the capacity in STATIC ANALYSIS with PSI.