Combined / Strap Footing Generator

About combined and strap footings

A combined footing supports two (or more) columns on a single concrete base. IS 456:2000 Clause 34 is the design code; IS 1904:1986 and IS 6403:1981 govern the soil-bearing analysis. The footing is proportioned so that its plan centroid coincides with the resultant of the column loads — this is what makes the soil pressure uniform and the design economical. Where that is geometrically impossible, the footing is shaped as a trapezoid or split into a strap arrangement.

Structurally a combined footing behaves like an inverted beam — the upward soil reaction is the load, and the two columns act as the supports. Between the columns the beam hogs (tension on top), so top longitudinal steel between the columns is mandatory. Under each column the cantilever projection sags (tension at bottom), requiring bottom steel. Missing the top steel is the single most common and most damaging design error.

Rectangular vs trapezoidal vs strap — when each

• Rectangular combined footing: equal column loads, or unequal loads where the footing can project beyond both columns enough to bring the centroid onto the resultant. Constant width B; simplest formwork and reinforcement. Length L = 2 × (distance of resultant from the free edge).
• Trapezoidal combined footing: the heavier column is close to a property line or free edge so the footing length is restricted, and a rectangular pad cannot move its centroid far enough toward the heavy column. Two different end widths (B1 at C1, B2 at C2) shift the area centroid to the resultant within the available length. Choose B1 > B2 when C1 is the heavier column near the edge.
• Strap (cantilever) footing: column C1 sits exactly on a property line and the footing cannot project past it. Two independent pads — one under each column — are joined by a stiff strap beam that carries no soil bearing itself but transfers the eccentricity of the exterior column to the interior one, keeping both pad pressures uniform. Economical when columns are far apart and a single solid pad would waste concrete.

Design steps (what the generator does)

1. Locate the resultant: R = P1 + P2; its distance from C1 is x̄ = P2 · S / R. Including any edge offset, the resultant is x̄ + offset from the free edge.
2. Size for uniform pressure: required area A = R / SBC. For a rectangular footing the length is set to L = 2 × (resultant distance from the free edge) so the geometric centroid lands on the resultant; width B = A / L. For trapezoidal, B1 and B2 are solved so the trapezoid centroid x̄ = L/3 · (B1 + 2B2)/(B1 + B2) equals the resultant location.
3. Soil pressure: if the centroid coincides with the resultant the gross pressure is uniform (p = R / A) and must be ≤ SBC. If not (length overridden), the pressure is trapezoidal: p = (R/A)(1 ± 6e/L) — check that p_max ≤ SBC.
4. Bending moment diagram: treat the footing as a beam on the net upward soil pressure with the two columns as supports — a cantilever at each end (sagging) plus a simply-supported span between the columns that hogs. Maximum hogging moment is near the heavier column where shear crosses zero.
5. Reinforcement: bottom steel for the cantilever sagging moment (full length under columns); top steel for the hogging moment between columns, curtailed a development length beyond the point of contraflexure; transverse steel designed as a column-band over a width ≈ column width + 2d.
6. Shear: one-way (beam) shear checked at d from each column face; two-way punching shear checked on a perimeter at d/2 around each column — usually the heavier column governs punching. The strap beam (strap variant) is designed for the full transferred shear and moment.

Common mistakes

1. No top steel between the columns — the footing hogs between supports; omitting top reinforcement causes wide transverse cracks on the top face and is the most frequent combined-footing failure. IS 456 Cl. 34 requires it.
2. Footing centroid not placed at the resultant — designers often centre the footing on the column spacing midpoint instead of the load resultant. The pressure then becomes eccentric/trapezoidal and one end overstresses the soil.
3. Ignoring eccentric (trapezoidal) pressure — using the average pressure for design when the centroid is off the resultant. Always check p_max against SBC, not the mean.
4. Strap beam under-designed for shear — the strap carries the full cantilever shear from the exterior column to the interior pad with little or no soil relief; a beam sized only for moment fails in shear. Provide closed stirrups for the full transferred shear.
5. Missing the punching-shear check at the heavier column — punching is checked at the lighter column or not at all; the heavier column on a thin footing usually governs (IS 456 Cl. 31.6).
6. Inadequate bottom cover — 75 mm is required at the earth-facing bottom (IS 456 Cl. 26.4.2 / Table 16). Using 50 mm to save concrete leads to corrosion within a few years.
7. Differential settlement between the two columns — combined footings are used precisely because the columns interact; unequal soil stiffness or staged loading tilts the rigid base. Verify settlement (IS 8009) and the soil model before sizing.

Related references

• IS 456:2000 — RCC code (Clause 34 — combined footings is the primary)
• IS 1904:1986 — Foundation design general requirements
• IS 6403:1981 — Bearing capacity of shallow foundations
• IS 1786:2008 — TMT rebar specification (the steel)
• IS 13920:2016 — Ductile detailing (seismic Zone III–V)
• IS 8009 Parts 1 + 2 — Settlement calculation (companion to bearing capacity)
• InfraLens Isolated Footing Generator — single-column footing drawings
• InfraLens Pile Cap Generator — when soil is too weak for spread footings
• InfraLens CAD Library — more parametric DXF generators
• Safe Bearing Capacity (Handbook) — quick-reference values for Indian soils