Pile cap setup¶
The first tab. It defines the pile cap geometry, the position of the piles, the columns and the actions.
Piles¶
| Field | Note |
|---|---|
| Number of piles | From 1 to 30 per cap |
| Position | X and Y coordinates of each pile |
| Diameter | Defines the area of the node over the pile |
| Axis spacing | Governs the lever arm of the struts |
The outline of the pile cap is the convex hull of the piles plus the edge distance — not a circumscribed rectangle. That polygon is what enters the self-weight calculation and the finite element mesh.
Spacing is the most sensitive variable
Moving the piles apart flattens the strut and increases the tie reinforcement, as well as enlarging the cap in plan. Moving them closer reduces the tie, but there is a construction minimum — piles that are too close interfere with each other during construction and in capacity.
Columns¶
PCO accepts more than one column per pile cap, each in its own tab, with its own section, position and actions.
Available sections¶
| Section | |
|---|---|
| Rectangular | |
| Circular | |
| I/H section | |
| U section | |
| Hollow rectangular | |
| Hollow circular |
Non-rectangular sections become an equivalent dimension
Blévot's formulas require a column dimension \(a_p\) in the direction considered. For sections that are not rectangles, the program calculates the equivalent dimension preserving the contact area that defines the compression node under the column.
It is the correct approximation for the strut model, which sees the column as the region through which the load enters the cap — not as its exact shape.
Actions¶
Each column receives the five force components:
| Component | Meaning |
|---|---|
| \(N\) | Axial force |
| \(M_x\), \(M_y\) | Moments |
| \(F_x\), \(F_y\) | Shears |
Actions are classified as permanent and variable, and the program builds the normal ultimate combinations of NBR 8681 from there.
Classify permanent and variable correctly
It is not a formality. The code requires \(\gamma_g = 1.0\) on the permanent action when it relieves the effect of a variable one, and \(1.4\) when it aggravates it — and PCO tests both assumptions precisely because "favourable" changes from check to check.
An action entered as permanent when it is variable — or the reverse — invalidates the whole combination. See Load combinations.
The self-weight of the pile cap is calculated by the program from the real volume, with \(\gamma_{concreto} = 25\) kN/m³ (NBR 6120), and enters as a permanent action.
Finite element simulation¶
With the geometry and actions entered, the tab solves the pile cap as a solid.
| Option | When to use it |
|---|---|
| Hexahedral mesh | Regular geometry — converges better with fewer elements |
| Tetrahedral mesh | Irregular arrangements, cut-outs, many columns |
The result gives the stress field, the pile reactions and the stresses at the nodes — under the column and over the piles.
FEM is optional for design
Blévot and MBT are analytical. The finite element model serves to check the assumptions and to verify the nodes against the real stress field, which is more rigorous than the analytical check.
On a critical pile cap, run it. See Finite elements.
3D view¶
The pile cap is drawn with the piles and columns in position. Use it before calculating: a swapped coordinate is the most common input error, and it is invisible in the table.