Strap (Cantilever) Footing Design

Nilson & Darwin rigid-body statics for an eccentric exterior footing tied by a rigid strap beam to a concentric interior footing — bearing, one-way and punching shear and flexural design for both footings, and flexure/shear design for the strap beam itself, by ACI 318-08.

ACI 318-08ASCE 7

Column spacing & property line

Column 1→2 spacingLcftProperty line to column 1edge1ft

Founding depthDfft

e = 2.50 ft, k = 1.185

Footing 1 (exterior, eccentric)

c1 (span dir.)inc2 (transverse)in

Position

Length L1ftWidth B1ft

PDkipPLkipPEkip

Footing 2 (interior, concentric)

c1 (span dir.)inc2 (transverse)in

Length L2ftWidth B2ft

PDkipPLkipPEkip

Strap beam

Width bwinDepth hin

Strap bar

Soil & materials

Allowable bearingqaksfSoil unit weightpcf

Concrete unit weightpcfFooting thicknessTin

Coverin

f'cksifyksi

Footing bar (each way)

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

9 checks · see warnings

Plan view

Net strap force

63.0 kip

Reaction-split factor

1.185

Governing DCR

2.09

Design checks

Check	Ref	Demand	Capacity	DCR	Governs	Status
Bearing, footing 1 (gross, service)	ASCE 7 §2.4	7.67 ksf	4.00 ksf	1.92	Governing combo: D+L	FAIL
Bearing, footing 2 (gross, service)	ASCE 7 §2.4	6.61 ksf	4.00 ksf	1.65	Governing combo: D+L	FAIL
One-way shear, footing 1, L-dir	ACI §11.2.1 / Eq. 11-3	85.15 kip	96.13 kip	0.89	qu = 9.59 ksf	PASS
One-way shear, footing 1, B-dir	ACI §11.2.1 / Eq. 11-3	65.76 kip	112.15 kip	0.59	qu = 9.59 ksf	PASS
Punching shear, footing 1	ACI §11.11.2.1	351.12 kip	214.95 kip	1.63	Eq. 11-33, position: edge	FAIL
One-way shear, footing 2, L-dir	ACI §11.2.1 / Eq. 11-3	123.47 kip	128.17 kip	0.96	qu = 8.14 ksf	PASS
One-way shear, footing 2, B-dir	ACI §11.2.1 / Eq. 11-3	123.47 kip	128.17 kip	0.96	qu = 8.14 ksf	PASS
Punching shear, footing 2	ACI §11.11.2.1	454.72 kip	365.81 kip	1.24	Eq. 11-33, position: interior	FAIL
Strap beam, one-way shear (concrete)	ACI §11.2.1 / Eq. 11-3	55.64 kip	26.62 kip	2.09	#6 bars fit at this width	FAIL

Footing 1 steel (each way)

8-#6 @ 9.7 in o.c.

Footing 2 steel (each way)

12-#6 @ 8.6 in o.c.

Strap beam bottom steel (sagging throughout — no top steel required)

3-#6

⚠ Governing DCR = 2.09 exceeds 1.0, or the strap beam's bars don't fit — see the design checks table.

Design checks

Check	Demand	Capacity	DCR	Status
Bearing, footing 1 (gross, service)	7.671 ksf	4.000 ksf	1.918	✗ FAIL
Bearing, footing 2 (gross, service)	6.612 ksf	4.000 ksf	1.653	✗ FAIL
One-way shear, footing 1, L-dir	85.150 kip	96.125 kip	0.886	✓ OK
One-way shear, footing 1, B-dir	65.761 kip	112.146 kip	0.586	✓ OK
Punching shear, footing 1	351.125 kip	214.947 kip	1.634	✗ FAIL
One-way shear, footing 2, L-dir	123.475 kip	128.167 kip	0.963	✓ OK
One-way shear, footing 2, B-dir	123.475 kip	128.167 kip	0.963	✓ OK
Punching shear, footing 2	454.717 kip	365.810 kip	1.243	✗ FAIL
Strap beam, one-way shear (concrete)	55.643 kip	26.619 kip	2.090	✗ FAIL

Calculation

1Footing 1 eccentricity and the rigid-body reaction-split factor
$e = \frac{L_1}{2} - \text{edge}_1, \quad k = \frac{L_c}{L_c - e}$
$e = 7.00/2 - 1.00 = 2.50, \quad k = \frac{16.00}{16.00-2.50} = 1.185$
$k$ 1.185
2Strap beam design moment and shear at column 2's face
$M_u = T_u L_m - \frac{w_u L_m^2}{2}, \quad V_u = T_u - w_u L_m$
$M_u = 63.0 \times 15.25 - \frac{0.480 \times 15.25^2}{2} = 904.4$
$M_u$ 904.4 kip·ft
Governing combo for R1/Tu: 1.2D+1.6L

Warnings

Governing DCR = 2.09 exceeds 1.0, or the strap beam's bars don't fit — see the design checks table.

What this calculator does not check

Footing 1's own bending and shear are designed as a plain concentric footing under R1 — its eccentricity is resolved entirely by the whole-system rigid-body statics (the k factor), not re-applied inside its own cantilever geometry.
Neither footing is auto-sized or iterated — both are direct user inputs, gated only by a lever-arm validity check (Lc - e > 0.5 ft); if a check fails, resize the footings by hand and recompute.
The strap beam's shear check is concrete capacity (φVc) only — no stirrup design. Only its bottom (sagging) steel is designed; a net downward tip demand (the strap going into reverse) is clamped to zero rather than flagged, matching the source, so nominal top hanger bars are practice only, not a computed requirement.
No eccentric punching-shear moment transfer (ACI §11.11.7) for either footing, and no anchorage/development-length check anywhere in this tab — the source defines helpers for both elsewhere but never calls them here.
The strap beam carries only its own self-weight — no user-entered surcharge or soil overburden on top of it, and only one design section (at column 2's face) is checked, not an envelope along its length.
No published worked example has been reproduced against this port; verification is internal cross-checks with the original tool's own engine, disclosed as such.

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.

strap-footing-design v1.0.0 · ACI 318-08 · input de0fc17062f947e5