Pile cap

A rigid pile cap on 2 to 9 piles in a standard pattern, or on any layout by coordinates, designed to ACI 318-19 (13.4.6, Chapters 22 and 25) with ASCE 7-22 loads. Pile reactions follow the rigid-cap group formula.

Inputs of the pile cap #

• Piles: the pattern (2 in line, 3 triangle, 4 square, 5 square plus centre, 6 two rows of 3, 7 hexagon plus centre, 8 staggered 3-2-3, 9 grid) with spacing s, or entered coordinates from the column centre; the edge distance (pile centre to cap edge); the pile diameter and shape; the allowable compression, tension (0 = none allowed) and lateral load per pile, from the pile design.
• Cap: thickness h, the distance from the cap bottom to the bottom bars (pile embedment plus the clearance above the pile tops), soil cover and unit weights. The cap is the rectangle around the piles plus the edge distance.
• Column: concrete, or steel on a base plate, as for the spread footing; loads per load type at the top of the cap (P, Mx, My, Vx, Vy).
• Reinforcement: bars each way (size and number), bottom layer, straight or hooked ends, side cover.

Auto-design (editor toolbar) varies the thickness (2-in. steps) and the bottom bars each way (size and count); the pile layout is kept. Objectives: lowest cost index (concrete, formwork, reinforcement); least concrete, then cost. Every other input is kept; see Auto-design.

Checks performed #

The drawings (screen, report, DXF and PDF sheet) show the cap plan with the piles (dashed), the column or steel column on its base plate, the bottom bars both ways, the pile rows and edge distances, and section A along x through the column with the piles below the cap, the clearance to the bottom bars, the bars (hooked ends when chosen) and the soil cover. Pile embedment and reinforcement, column dowels and anchor rods are not part of the calculation and are not drawn.

Limitations #

• No strut-and-tie design is performed; for deep caps (most caps on closely spaced piles) the sectional shear checks may be unconservative (R13.4.6.5) and the advisory asks for a strut-and-tie check.
• The cap is rectangular (triangular caps with clipped corners are drawn and weighed as their bounding rectangle).
• Batter piles and the pile design itself are not covered; lateral load is shared equally.
• Two piles in a line cannot resist a moment across the line: that case is not feasible (brace the cap with grade beams).
• The minimum steel and spacing of 8.6.1.1 and 8.7.2.2 are applied to caps by analogy with two-way footings.

Verification examples #

Published worked example (backend/tests/test_verify_pile_cap.py): the free StructurePoint example that reproduces Fanella, Reinforced Concrete Structures, 2nd Ed., Example 10.13 (ACI 318-14): an 8.5 ft × 7.75 ft × 33 in cap on eight 12 in piles under a 22 in column, D = 425, L = 250 kips. The example neglects the cap's own weight; the program includes it, which nearly cancels in the moments and in punching.

Hand calculation of the default 4-pile cap.