Spread Footings

Part of Structural Engineering

Designing and building the concrete or masonry pads that distribute column and wall loads over enough soil area to prevent settlement and failure.

Why This Matters

A masonry wall applies a concentrated line load to the ground. A column applies an even more concentrated point load. The soil directly beneath these loads would be overloaded and yield if the load were not spread out. A spread footing does exactly that — it projects beyond the base of the wall or column like a flat flange, spreading the load over a much larger area of soil.

The spread footing is the most common foundation type for ordinary buildings and the most practical to construct without heavy equipment. It requires only excavation, formwork, and concrete (or carefully laid masonry). Getting it right requires three calculations: total load, soil capacity (from the Soil Bearing Capacity article), and required footing dimensions.

A spread footing that is too small concentrates stress and causes settlement. One that is too thick wastes concrete. One that is too shallow is susceptible to frost heave. These three failure modes are all avoidable with systematic design.

Footing Types

Strip footing (continuous wall footing): A continuous strip of concrete running under the full length of a masonry or framed wall. Width projects beyond the wall on each side to spread the load. Most common footing for load-bearing walls.

Pad footing (isolated column footing): A square or rectangular concrete pad under a single column or post. Width and length are calculated to stay within the soil bearing capacity.

Combined footing: A single footing under two adjacent columns when the columns are too close together for individual footings or when one column is too close to a property line to spread its load symmetrically.

Raft (mat) foundation: A continuous concrete slab under the entire building footprint, used when soil is very weak or column loads are very close together. Functions as a very large spread footing.

Step-by-Step Strip Footing Design

Step 1: Determine the load per foot of wall. For each foot of wall length:

• Dead load = (wall thickness × wall height × 1 ft) × material density
• Floor/roof load = tributary area (per foot of wall length) × load per sq ft

Example: 18-inch thick stone wall, 14 feet high, supporting a roof with 40 lb/sq ft dead + 30 lb/sq ft live load, 10-foot tributary width:

• Wall weight per foot: (1.5 × 14 × 1) × 140 lb/cu ft = 2,940 lb/ft
• Roof load per foot: 10 ft × 70 lb/sq ft = 700 lb/ft
• Total load: 3,640 lb/ft

Step 2: Determine allowable soil bearing capacity. From field tests and soil classification (see Soil Bearing Capacity article). Assume compact sand: 3,000 lb/sq ft.

Step 3: Calculate required footing width. Width = Total load per foot / Allowable bearing capacity Width = 3,640 / 3,000 = 1.21 ft → use 1.5 ft minimum (round up significantly to provide margin)

Step 4: Set footing thickness. For an unreinforced concrete footing, the minimum thickness is the greater of:

• 8 inches minimum
• Projection beyond wall (overhang on each side) × 1.5

For a 1.5-ft wide footing under an 18-inch wall: overhang = (18 - 18)/2 = 0 inches. Use 8-inch minimum. For a 2.5-ft wide footing: overhang = (30 - 18)/2 = 6 inches. Minimum thickness = 6 × 1.5 = 9 inches. Use 10 inches.

Step 5: Check bending in the footing. The footing projects beyond the wall and is loaded from below by soil pressure. This creates bending in the footing slab — it wants to curl upward at the edges. For unreinforced concrete, keep the bending stress below the tensile capacity of concrete (approximately 200–300 PSI for plain concrete).

Bending stress check: σ = 1.5 × q × (B/2 - b/2)² / t² Where q = soil pressure under footing, B = footing width, b = wall width, t = footing thickness

If bending stress exceeds the allowable, either increase footing thickness or add iron reinforcing rods along the bottom of the footing in the direction of projection.

Step-by-Step Pad Footing Design

Step 1: Determine total load on column. Sum all floors above, walls, roof, and any other tributary loads.

Example: corner column in a two-story stone building, each floor 15 × 15 ft tributary area:

• Floor 1 dead load: 225 × 60 lb/sq ft = 13,500 lb
• Floor 1 live load: 225 × 40 lb/sq ft = 9,000 lb
• Floor 2 dead load: 225 × 60 = 13,500 lb
• Floor 2 live load: 225 × 40 = 9,000 lb
• Roof dead + snow: 225 × 60 = 13,500 lb
• Column weight: assume 3,000 lb
• Total: 61,500 lb

Step 2: Required footing area. Soil bearing capacity: 3,000 lb/sq ft (compact sand) Area = 61,500 / 3,000 = 20.5 sq ft → use 4.5 ft × 4.5 ft = 20.25 sq ft → round to 5 ft × 5 ft

Step 3: Footing thickness. Column base 12 inches square on a 5-foot square footing: Projection = (60 - 12)/2 = 24 inches Minimum thickness = 24 × 1.5 = 36 inches for unreinforced concrete (very thick)

At this size, adding iron reinforcing bars at the bottom of the footing and reducing thickness to 18–24 inches is more economical. Iron bars in the tension zone (bottom of footing) resist the bending caused by upward soil pressure.

Construction Procedure

Excavation:

1. Mark the footing outline on the ground, extending 6 inches beyond the design edge for working room
2. Excavate to the design depth — must reach firm soil and must be below the frost line
3. Remove all loose disturbed material from the bottom — tamp the bottom to firm it
4. Add 4 inches of compacted gravel to the bottom for drainage and a clean working surface

Setting level: Use a water level, spirit level, or batter boards to establish the top-of-footing elevation around the perimeter. The footing top must be level — walls built on an unlevel footing will be out of level from the start.

Formwork: For strip footings, set boards on edge at the design width apart and prop them from outside with stakes. Check that forms are the correct width along their full length. For pad footings, set a box of planks to the exact dimensions.

Concrete placement:

1. Mix concrete — a 1:2:4 (cement:sand:aggregate) mix by volume is standard
2. Add just enough water to make workable concrete, not soupy
3. Pour in layers and rod (prod repeatedly) to eliminate air pockets
4. Strike the top surface level with a straightedge
5. Do not place concrete in freezing weather — the concrete will not cure properly

Curing: Cover with wet burlap or wet straw. Keep wet for at least 7 days. Do not load or build on the footing for at least 5–7 days for normal loads, or 14 days for heavy masonry.

Inspection Checklist

Before accepting a footing installation:

• Bottom is in firm, undisturbed soil (not loose fill or topsoil)
• Depth is at or below the frost line for this location
• Width is at least the calculated minimum
• Footing is level within 1/4 inch over its full length/width
• Concrete is fully placed without cold joints or honeycombing
• No cracking visible after curing (minor shrinkage hairline cracks are normal; structural cracks are not)

Document the footing dimensions, depth, and soil condition at the bottom of excavation. This information will be needed if future modifications or repairs are required.