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Combined Footing

Single footing supporting two or more columns — used when columns are close (<2 m) or property line restricts.

Also calledrectangular combinedtrapezoidal combined
Related on InfraLens
CODES
IS 456
Definition

A combined footing is a single foundation supporting two or more columns. Used when (a) two columns are too close together for separate isolated footings (typical for boundary-line columns in narrow plots), (b) one column is on a property line where extending an isolated footing is impossible, requiring the footing to extend toward an interior column, or (c) one column carries much higher load than adjacent columns, where uniform soil pressure across both is achieved via combined footing. Per IS 456:2000 Cl. 34 + IS 1904:1986 + IS 2950:1981 (rafts).

Geometric configurations: (1) Rectangular combined footing — symmetric supporting two equal-loaded columns; centroid of footing aligns with centroid of column loads (no eccentric soil pressure). (2) Trapezoidal combined footing — for two columns with unequal loads or property-line constraint; the wider end is at the more-heavily-loaded column. (3) Strap footing (or cantilever footing) — two separate isolated footings connected by a strap beam, often used when one column is at the property line and an isolated footing under it would project beyond the property. (4) Mat or raft — when footing area exceeds about 50% of building plan area.

Design per IS 456 Cl. 34: (a) plan dimensions sized so soil pressure ≤ SBC under combined load; (b) flexural design as a beam supported on two columns, with continuous beam analysis (the soil pressure provides a UDL between columns; the columns provide point loads); (c) shear check at d/2 from column face for punching shear, at d from column face for one-way shear; (d) reinforcement for both top and bottom flexure (footing acts as both simply supported and continuous beam depending on loading); (e) separate transverse reinforcement perpendicular to the long axis. The most-overlooked design point is the 'distribution moment' — when combined footing load is unbalanced, the cross-section across the footing also experiences moment that must be reinforced.

Where used
  • Narrow property-line plots — boundary column with adjacent interior column
  • Adjacent columns too close for isolated footings (overlap)
  • Unequal-load columns where uniform soil pressure across two columns is desired
  • Strap footings — boundary column connected to interior column via strap beam
  • Industrial structures where columns are clustered (silos, pump houses)
Acceptance / threshold
Per IS 456 Cl. 34 + IS 1904: soil pressure under service load ≤ SBC at all points; structural design as continuous beam with punching shear check at d/2 from column face; reinforcement top and bottom; minimum cover ≥ 75 mm against soil.
Site example
Site reality: a Pune residential project had two adjacent columns (300 × 600 with 600 kN load each) at 1.8 m spacing. Standard isolated footings would each be 1.8 × 1.8 m — overlapping centre-to-centre. The structural engineer designed a 2.4 × 4.5 × 0.5 m combined footing instead, eliminating the overlap. Cost differential: ₹85,000 less than two separate isolated footings (less concrete, less rebar, single placement). Combined footings often more economical than overlapping isolated for adjacent columns.
Frequently asked
When is combined footing used?
Combined footings are used when (a) two columns are too close for separate isolated footings (the would overlap), (b) one column is on a property line preventing extension toward the outside, requiring the footing to extend toward an interior column, (c) one column has much higher load than adjacent ones — uniform soil pressure achieved by combining. For typical Indian residential construction, combined footings are common for adjacent boundary-line and interior columns.
How is combined footing designed?
Plan dimensions sized so soil pressure ≤ SBC at all points under combined service load. Structural analysis as continuous beam supported at column locations with soil reaction as UDL. Flexural design for moment between columns and at column locations (top reinforcement). Punching shear at d/2 from column face. One-way shear at d from column face. Transverse reinforcement perpendicular to the long axis.
What is the difference between combined footing and strap footing?
Combined footing is a single rectangular or trapezoidal slab spanning beneath multiple columns. Strap footing is two separate isolated footings connected by a strap beam — used when an isolated footing under one column would project beyond a property line. The strap beam transfers eccentric load from the boundary column to a deeper isolated footing under the interior column. Both follow IS 456 + IS 1904; combined is structurally simpler, strap is geometrically more flexible.
Related foundation terms
Isolated Footing
Footing supporting a single column. Most common for low-rise buildings on soils with adequate bearing capacity.
Raft Foundation
Single thick slab covering the entire building footprint — for soft soil, heavy loads, or when isolated footings overlap.
Pile Cap
Reinforced concrete cap connecting tops of piles to the column. Designed as a deep beam or strut-and-tie.
Well Foundation
Hollow shaft sunk through soft soil to firm strata, filled with concrete — used for major bridge piers in rivers.
Strip Footing
Continuous footing under a load-bearing wall — width = wall thickness + 150 mm projection on each side.
Stepped Footing
Footing with stepped/tapered profile — saves concrete on large footings while maintaining bearing pressure distribution.
Bored Cast-in-situ Pile
Pile constructed by drilling a hole and filling with concrete + reinforcement. Diameter 300-2400 mm, depth up to 60 m.