Learn · guide

How to use the pile cap calculator

A column on a group of piles: what each input means, how to read the pile reactions and the strut-and-tie model, and why the anchorage so often governs.

All guides Inputs Result Calc sheet Walk-through Mistakes

1. What the calculator does

The pile cap calculator designs a reinforced concrete cap that carries a column onto a group of piles. It shares the column load and moments between the piles, checks each pile reaction against the pile capacity, and designs the cap: the tie bars between the piles, shear, punching at the column and at the piles, and the anchorage of the tie bars past the piles.

The code routes are SANS 10100, BS 8110, Eurocode 2 (EN 1992-1-1) and SANS 51992-1-1 (SA Eurocode, 2023 draft annex). The page badge says which method: the truss method to BS 8110 cl. 3.11.4 on SANS and BS, strut-and-tie per EN 1992-1-1 cl. 6.5 and 9.8.1 on Eurocode 2.

Pile cap calculator header with the PASS badge — The header: PASS, governed by the tie bars (the bobbed-end anchorage), with the method named in the badge.

2. The inputs, group by group

Input	What it means	Typical value
Column load	Factored axial N and moments M_x, M_y. The SLS load under More inputs lets the pile reactions be checked at SLS.	N 2500 kN ULS
Piles	Layout (preset 2 to 6 piles, or a general grid or coordinates), number, diameter, spacing and the pile capacity R_all.	4 square, ø450, spacing 1350, 800 kN
Cap	Column size, cap depth and the overhang past the piles.	400 × 400, h 900, overhang 150
Materials	Concrete f_cu and rebar f_y.	30 MPa, 450 MPa
Durability	Cover and exposure.	75 mm, in contact with soil
Tie bars	Bar size and layers; the engine designs the bars per band over the piles. Bars and starters lets you set them.	Y20, 2 layers

Pile cap inputs: column load, piles, cap, materials, durability and tie bars — The inputs. The column load is ULS (factored); the pile capacity R_all is the allowable value per pile.

3. How to read the result

The plan draws the cap with each pile and its reaction, the column and the tie bars per band. The section shows the cap depth and the piles below. The strut-and-tie model shows the load path: struts from the column down to the piles and the tie between the pile heads, with its force.

Pile cap plan with the pile reactions — Plan: four ø450 piles at 1350 mm, each carrying 654 kN, with 2 × 5-Y20 per band.

Pile cap section and elevation — Section: cap depth 900 mm on the piles.

Strut-and-tie model with the tie force — The truss: N = 2614 kN (column plus cap self-weight) and the tie force T = 551.5 kN.

The badge names the governing check; here the anchorage of the tie bars past the piles (bobbed ends). Pile caps are often governed by the anchorage, the punching at a corner pile or the shear, more than by the tie area. Anything the engine could not run shows as NOT CHECKED.

Look at the pile reaction check: without an SLS load, the page compares the ULS reaction with R_all and says so with “(ULS)”, which is on the safe side. Give the SLS axial load to check the reaction at working load.

Design checks and bending schedule card for the pile cap — Design checks: SANS 10100 · 18 pass, and the bending schedule card with “+ Add to a project”.

4. Calc sheet, drawing and schedule

The result (pass / fail and utilisation) stays free for everyone. Every check with its clause, the calc sheet and the bending schedule are open to signed-in users. Sign in free: 3 designs a month (one calculator, open for 24 hours). Unlimited with a calculator pack.

The bending schedule card reads: “Sign in (free): 3 schedules a month. CSV with a paid plan.” The full reinforcement detail drawing is part of the paid plan.

Reinforcement detail panel marked paid plan — The reinforcement detail drawing, sections and bar editing are part of the paid plan.

5. A 2-minute walk-through

Open the calculator: a 400 × 400 column with N 2500 kN on four ø450 piles at 1350 mm, cap 900 mm deep.
Read the pile reactions (654 kN each) and the tie force (551.5 kN).
Add M_x 300 kNm and see the reactions split between the two rows of piles.
Reduce the depth to 750 mm: the tie force goes up and the shear checks get harder.
Try three piles in a triangle for a lighter column.

6. Common mistakes

Allowable pile capacity against a ULS reaction without knowing it. Give the SLS load, or accept the conservative check.
Short overhang. The tie bars need room to anchor past the pile; a small overhang makes the anchorage govern.
Pile spacing too close. Spacing below about three diameters cuts the pile group capacity; check it with the pile capacity calculator.
Forgetting the cap self-weight. The engine includes it by default (More inputs); leave it on.

8. Learn more

Try it nowThe calculator opens with the walk-through inputs above. The result is free; sign in for the checks, calc sheet and schedule.
Open the pile cap calculator

A guide to using the calculator, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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