Pile Cap Design Calculator — ACI 318-08 / 318-19, ETABS Import

Design basis

Design code

Design for

One service and one ultimate load per column, as in the original tool.

Include column moments (off: concentric, axial only)

Loads from ETABS

Sort the file's combinations once into Strength / Service, assign a joint (or pier) to each column, and bring in the governing rows or every combination. Pile grouping by required pile count is in the same dialog.

Columns

Col	Pu kip	Mux k·ft	Muy k·ft

Column A is the origin; Δx, Δy place the others. P compression +; +Mx loads the +Y piles, +My the +X piles.

Column A section: Rectangle

Piles

Number of pilesSpacing Sin

Pile typeDiameter / width DinAllowable (service)kipAllowable upliftkipRotation θdeg

S ≥ 2.5D recommended (Bowles). Rotation turns the piles and the cap about Column A; the columns stay put.

Cap self-weight on the piles (service)Cap self-weight in the strength design (CRSI: × 1.6)

Soil over the capftγ soilpcf

Cap

Auto edge distance, max(D, 12 in) from pile centre

Thickness tinEffective depth din

Pile cap centre, shifted from Column A

ΔxinΔyin

Concrete & reinforcement

f'cpsifypsiBottom coverinTop coverin

Bottom X fromBottom Y fromTop

Outer bottom layerMinimum steel

Piles 3 in further from the column than drawn (CRSI allowance, column shear)

CRSI deep-cap checks at the column face (Limit States 4–6; one rectangular column)

CRSI one-way shear at corner piles (Limit State P4; CRSI: conservative for thick caps)

Show the d each check needs (slower)

Everything you have entered is in the address bar, so the link reproduces this exact calculation.

Adequate

Governing: Pile capacity (service) · DCR 0.70

4 piles · cap 84 × 84 × 34 in · d = 30.00 in · bottom X #8 @ 7.7 in, Y #8 @ 7.7 in

⚠ Every pile lies within d of a column face, so the one-way shear sections carry no pile load. Such a deep cap works as struts and ties — the sectional checks here do not capture that; check it by strut-and-tie (ACI 318-08 Appendix A), or turn on CRSI's deep-cap checks at the column face.

Plan

Coordinates

Item	Δx, in	Δy, in	cx × cy, in
Cap centre
	origin		30 × 30

Reinforcement

Bottom, X bars	11-#8 @ 7.70 in	d = 30.50 in · the largest any combination needs at any face · standard 90° hooks at both ends
Bottom, Y bars	11-#8 @ 7.70 in	d = 29.50 in · standard 90° hooks at both ends
Top	minimum mat	single column: temperature and shrinkage steel each way

Sections

Design checks

Check	Demand	Capacity	DCR
Piles (service)
Pile capacity (service)Geotechnical allowable · P1; rigid-cap reactions + 5.21 kip/pile cap self-weight	70.21 kip	100.0 kip	0.70	OK
Punching (two-way) shear
▸Punching shear at pilesACI 318-08 11.11.2.1; CRSI 5.3 · P4: b0 = 73.7 in, run out to the cap edges (CRSI P3); vc = 4√f'c	97.50 kip	419.5 kip	0.23	OK
▸Column A — Punching shear at pedestalACI 318-08 11.11.2.1 · Eq. 11-33; b0 = 240 in; Vu = Pu − piles inside the perimeter	110.0 kip	1366.1 kip	0.08	OK
One-way (beam) shear
▸Column A — Beam shear (X-dir)ACI 318-08 11.2.1 / 15.5.2 · bw = 84 in at d from the pedestal face, x = 45 in; Eq. 11-3: 2√f'c — no pile beyond the section	0.00 kip	239.1 kip	0.00	OK
▸Column A — Beam shear (Y-dir)ACI 318-08 11.2.1 / 15.5.2 · bw = 84 in at d from the pedestal face, y = 45 in; Eq. 11-3: 2√f'c — no pile beyond the section	0.00 kip	239.1 kip	0.00	OK
Flexure
▸Column A — Flexure (X-dir)ACI 318-08 10.3 / 15.4.2 · +X face; b = 84 in, d = 30.5 in; 11-#8 @ 7.70 in; φ = 0.900	139.7 kip·ft	1157.0 kip·ft	0.12	OK
▸Column A — Flexure (Y-dir)ACI 318-08 10.3 / 15.4.2 · +Y face; b = 84 in, d = 29.5 in; 11-#8 @ 7.70 in; φ = 0.900	141.6 kip·ft	1117.9 kip·ft	0.13	OK
Bar development
▸Column A — Bar development (X-dir)ACI 318-08 12.5 (standard hook) · Straight ld = 47.43 in does not fit in 24 in from the +X face to the cap edge less cover — hook the #8 bars: ldh = 13.28 in	13.28 in	24.00 in	0.55	OK
▸Column A — Bar development (Y-dir)ACI 318-08 12.5 (standard hook) · Straight ld = 47.43 in does not fit in 24 in from the +Y face to the cap edge less cover — hook the #8 bars: ldh = 13.28 in	13.28 in	24.00 in	0.55	OK
Design strips
Design strip X1 — flexureACI 318-08 10.3 / 15.4.2 · at the column faces, b = 84 in	139.7 kip·ft	1157.0 kip·ft	0.12	OK
Design strip X1 — one-way shearACI 318-08 11.2.1 · At d from column faces, b = 84 in	0.00 kip	239.1 kip	0.00	OK
Design strip Y1 — flexureACI 318-08 10.3 / 15.4.2 · at the column faces, b = 84 in	141.6 kip·ft	1117.9 kip·ft	0.13	OK
Design strip Y1 — one-way shearACI 318-08 11.2.1 · At d from column faces, b = 84 in	0.00 kip	239.1 kip	0.00	OK

Click a check for its calculation, pile by pile.

Pile reactions

Pile	x, in	y, in	Service max, kip	Ultimate max / min, kip	Punching DCR
P1	-24.0	-24.0	70.2	82.5 / 82.5	0.20
P2	24.0	-24.0	70.2	88.8 / 88.8	0.21
P3	-24.0	24.0	70.2	91.3 / 91.3	0.22
P4	24.0	24.0	70.2	97.5 / 97.5	0.23

Service reactions include the cap's own weight, 20.8 kip (5.21 kip per pile).

Design strips — shear, moment and reinforcement

Run strip checks

X-strips on planY-strips on plan

Suggested through each column line; they follow the columns until you edit one. Width 0 = the full cap. Click a band on the plan, or a strip below, to edit it.

Strip	Piles	Vu / φVc, kip	Mu / φMn, kip·ft	Bars	DCR
X-1 at y = 0 in, w = full	2	0.0 / 239.1	139.7 / 1157.0	11-#8 @ 7.70 in	0.12
Y-1 at x = 0 in, w = full	2	0.0 / 239.1	141.6 / 1117.9	11-#8 @ 7.70 in	0.13

X-1 · shear at d from column faces

Y-1 · shear at d from column faces

A strip narrower than the cap takes only the piles and columns inside its band; unless that set balances, its diagrams need not close to zero — read it as a tributary-width approximation, beside the main checks.

Report

Report / project titleThis design's mark

Give the design a mark, add it, change the inputs for the next cap, add that — then print or export the whole package. Kept in this browser.

Design checks

Check	Demand	Capacity	DCR	Status
Pile capacity (service)	70.210 kip	100.000 kip	0.702	✓ OK
Punching shear at piles	97.500 kip	419.490 kip	0.232	✓ OK
Column A — Beam shear (X-dir)	0.000 kip	239.070 kip	0.000	✓ OK
Column A — Beam shear (Y-dir)	0.000 kip	239.070 kip	0.000	✓ OK
Column A — Punching shear at pedestal	110.000 kip	1366.100 kip	0.081	✓ OK
Column A — Flexure (X-dir)	139.690 kip·ft	1157.010 kip·ft	0.121	✓ OK
Column A — Flexure (Y-dir)	141.560 kip·ft	1117.900 kip·ft	0.127	✓ OK
Column A — Bar development (X-dir)	13.280 in	24.000 in	0.553	✓ OK
Column A — Bar development (Y-dir)	13.280 in	24.000 in	0.553	✓ OK
Design strip X1 — flexure	139.693 kip·ft	1157.007 kip·ft	0.121	✓ OK
Design strip X1 — one-way shear	0.000 kip	239.068 kip	0.000	✓ OK
Design strip Y1 — flexure	141.568 kip·ft	1117.902 kip·ft	0.127	✓ OK
Design strip Y1 — one-way shear	0.000 kip	239.068 kip	0.000	✓ OK

Calculation

1Pile reactions (rigid cap, about the pile group's centroid)
$R_i = \frac{\sum P}{n} + \frac{M_x\,(y_i - \bar y)}{\sum (y-\bar y)^2} + \frac{M_y\,(x_i - \bar x)}{\sum (x-\bar x)^2}$
$R_{u,max} = 97.5,\quad R_{u,min} = 82.5$
$R_{u,max}$ 97.5 kip
2Cap self-weight on the piles (service)
$W = A_{cap}\,(t\,\gamma_c + h_s\,\gamma_s),\quad W/n$
$W = 20.8,\quad W/n = 5.21$
$W/n$ 5.21 kip
3Effective depth (average for shear; per layer for flexure)
$d = t - c - 0.75\,d_{b,outer} - 0.25\,d_{b,inner}$
$d = 30.00,\quad d_x = 30.50,\quad d_y = 29.50$
$d$ 30.00 in

Warnings

Every pile lies within d of a column face, so the one-way shear sections carry no pile load. Such a deep cap works as struts and ties — the sectional checks here do not capture that; check it by strut-and-tie (ACI 318-08 Appendix A), or turn on CRSI's deep-cap checks at the column face.

Notes

Straight bottom bars do not fit past the face in both directions — hook them (standard 90° hooks at the cap edge).
All 13 design checks pass. Governing DCR = 0.70 (Pile capacity (service)).

What this calculator does not check

Rigid-cap pile reactions (R = P/n ± M·c/Σc² about the pile group's centroid); pile axial stiffness and group effects (block failure, negative skin friction) are not considered. Pile capacities are taken as entered.
Sectional design: punching at d/2 from every pile and pedestal, one-way shear at d from each pedestal face, flexure at the faces. Pile punching follows the CRSI Design Guide for Pile Caps (2015) 5.3: vc = 4√f'c, a corner pile also on its perimeter run out to the two cap edges, and two piles together where their perimeters overlap or combine shorter. A pile straddling a shear section counts by ACI 318-08 15.5.4: straight-line on its centre over ±dp/2, on each side of a punching perimeter it crosses (at a perimeter corner the two shares multiply); CRSI counts it by its centre moved 3 in away from the column, which is more severe where piles sit near a section; a tick-box takes the piles 3 in further out in each column's shear checks before ACI's share. Unbalanced-moment transfer at the pedestal and deep-cap (strut-and-tie) action are not included; CRSI's deep-cap checks at the column face, and its one-way shear at corner piles (P4), are options.
On a multi-column cap each column's sections take the full free body beyond them — the other columns' loads and moments included; the top steel between columns uses the same beam idealisation along the column line. Neither replaces a plate (FEM) analysis of a large core cap.
The service pile reactions carry the cap's own weight (and any soil over it) shared equally among the piles. The strength design leaves it out unless the cap-weight tick-box is on; CRSI includes it (×1.6), which in its Examples 1–3 reads up to 2.8 % more one-way shear. With it on, each column-face section takes off the weight beyond it; pile punching keeps the whole reaction, as CRSI; the strips and the top steel between columns leave it out.
Every strength combination is designed with the same bottom mat, chosen for the largest steel per inch any combination needs at any face; imported combinations are used as given.
Verified against Design Examples 1, 2 and 3 (16, 5 and 6 piles) of the CRSI Design Guide for Pile Caps (2015) and independent hand calculations — see golden.spec.ts and README.md.

Results are for preliminary sizing and educational use. They are not a design and must be reviewed, verified and sealed by a licensed or registered engineer before being used in construction.

pile-cap-design v2.5.1 · ACI 318-08 · input f5b9b43faa4f5fdb