Structural Analysis¶
The fifth tab. It builds the finite element model of the pile on an elastic foundation and returns the internal forces that will be used to design the section.
What the model represents¶
The pile becomes a 3D frame discretised metre by metre, with a soil spring at each node. The springs come from the soil reaction: \(K_h\) in the horizontal directions and, optionally, \(K_v\) in the vertical.
The analysis runs per axis — one model for X and another for Y —, and the two results are combined in the design as biaxial bending with axial force.
Options¶
| Option | Effect |
|---|---|
| Consider \(K_v\) | Enables the vertical springs. Matters above all for raked piles |
| Axis | Chooses which model to view, X or Y |
| Modulus of elasticity | \(E_c\) of the pile; if omitted, the program adopts one by pile type |
| Poisson's ratio | Default 0.20 |
| Self-weight | Distributed load along the shaft |
When to enable \(K_v\)
In a vertical pile under horizontal load, the vertical springs change little. In a raked pile they are essential: without \(K_v\), nothing prevents the pile from sliding along its own axis, and the displacements come out unrealistic.
Results¶
The tab shows, along the depth:
| Diagram | Used for |
|---|---|
| Horizontal displacement | Serviceability check — it is the usual criterion for laterally loaded piles |
| Bending moment | Designs the longitudinal reinforcement |
| Shear force | Designs the stirrups |
| Axial force | Shows how much of the load has already been transferred by friction |
The charts are interactive and can be included in the report.
Where the maximum moment is
In a pile under horizontal load, the maximum moment is rarely at the head: it usually appears a few metres below the surface, where the soil stiffness is still low but the pile has already gained lever arm. That peak is what designs the reinforcement, and that is why the longitudinal reinforcement cannot be stopped just below the cap.
Precautions¶
The model is linear elastic
The concrete does not crack and the soil does not yield. For service displacements this is acceptable; close to failure, it is not. The flexural stiffness uses the gross concrete section — NBR 6118 allows a reduction for cracking, which would increase displacements.
See limits of validity.
- The discretisation is 1 m. In short or very stiff piles, the moment peak may fall between nodes and be underestimated.
- Shallow nodes have no spring. Above 0.10 m depth the stiffness is deliberately set to zero: the surface soil is subject to erosion, excavation and seasonal variation.
- Springs do not interact. In a cap with small spacing, the effective stiffness per pile is lower than calculated.
Processing¶
The analysis opens a progress window. The time grows with the number of piles and with the depth — a cap with eight piles on a 20 m borehole runs sixteen models, two per pile.