Pad footings, also called isolated or spread footings, are the most common foundation type for columns carrying moderate loads. Each footing is a square or rectangular reinforced concrete slab that spreads the column load over enough soil area to keep the bearing pressure inside the allowable value. The design sequence is fixed: size the base from service loads, apply ultimate load factors, check shear in two directions, then calculate bending reinforcement. When columns sit close enough together that individual pads overlap, a combined footing design becomes the practical alternative, and comparing the two layouts before fixing the geometry saves rework later. The worked example in this article follows Eurocode 2, EN 1992-1-1, using a 900 kN dead load, a 400 kN live load, and an allowable bearing pressure of 175 kN/m2.
Initial Sizing from Service Loads
Design starts before any reinforcement is calculated. The plan dimensions of the pad come from two inputs: the allowable bearing pressure from the geotechnical report and the service loads transmitted by the column. Every later check uses the geometry fixed in this step, so the sizing decision deserves careful attention.
For columns that carry horizontal loads, wind, or moments, the pressure distribution is not uniform, and the footing is then sized so the maximum edge pressure stays below the allowable value. The example in this article assumes a concentric load, which keeps the pressure uniform across the base and simplifies every later check.
| Parameter | Value |
|---|---|
| Concrete grade fck | 30 N/mm2 |
| Reinforcement yield strength fyk | 500 N/mm2 |
| Column size | 400 mm square |
| Allowable bearing pressure | 175 kN/m2 |
| Assumed footing thickness | 500 mm |
| Assumed bar diameter | 16 mm |
The Standard Design Sequence
A pad footing is designed in six ordered checks, and skipping any one of them leaves a weak link in the foundation.
- Calculate the size of the footing from the allowable bearing pressure and the service load.
- Calculate the bearing pressure under ultimate loads.
- Check the vertical line shear at the face of the column.
- Check for punching shear around the column perimeter.
- Calculate the reinforcement required for bending.
- Check shear again at the critical section.
The same six checks appear in national codes with local adjustments to factors and critical sections. The base dimensions chosen in step one also feed into the wider building layout, because column positions, cladding lines, and service zones are settled during the architectural design and building envelope design process. A change in that layout shifts column loads and reopens the footing calculation.
Sizing the Base Area
The service load is the unfactored combination of permanent and variable actions plus the self-weight of the footing. For the design example, the permanent load Gk is 900 kN, the variable load Qk is 400 kN, and the footing weight is estimated at 150 kN.
Worked Example: Required Footing Area
Design service load = 1.0Gk + 1.0Qk = 900 + 150 + 400 = 1450 kN. Required footing area = 1450 / 175 = 8.3 m2. A 2.9 m square footing provides an area of 8.41 m2, rounded up to give a small margin.
Rounding up is standard practice. The chosen plan area must never be smaller than the calculated value, and the service bearing pressure is recalculated with the actual dimensions: 1450 / 8.41 = 172 kN/m2, which sits below the 175 kN/m2 allowable pressure.
Ultimate Loads and Bearing Pressure
Service loads size the footing, but the structural checks use factored loads. Eurocode 2 applies partial factors of 1.35 to permanent actions and 1.5 to variable actions in the fundamental combination.
| Load component | Service value (kN) | Partial factor | Ultimate value (kN) |
|---|---|---|---|
| Permanent load Gk | 900 | 1.35 | 1215 |
| Variable load Qk | 400 | 1.5 | 600 |
| Total ultimate NEd | 1300 | – | 1815 |
Ultimate Bearing Pressure
The total ultimate axial load is NEd = 1.35 x 900 + 1.5 x 400 = 1815 kN. The ultimate bearing pressure is this load divided by the actual base area: 1815 / (2.9 x 2.9) = 216 kN/m2. This pressure drives the shear and bending checks, while the service pressure of 172 kN/m2 is only used to confirm the base area.
The 216 kN/m2 ultimate pressure is about 1.26 times the 172 kN/m2 service pressure, close to the average load factor applied to the total load.
Comparing Code Approaches
The partial factors are not universal. ACI 318 uses its own load combinations, and older BS 8110 designs applied 1.4 and 1.6. When a project crosses code boundaries, the column footing design method used on the structural drawings must match the code that governs the whole frame, and this example keeps the Eurocode factors throughout.
Shear Checks: Vertical Line Shear and Punching Shear
A pad footing can fail in shear before it fails in bending, so the slab depth is usually governed by shear rather than by the moment. Two shear mechanisms are checked: vertical line shear acting across the full width of the footing and punching shear acting around the column perimeter.
Effective Depth of the Slab
Both shear checks need the effective depth. Assume a footing thickness of 500 mm, 16 mm diameter bars at the base, and 40 mm cover to the reinforcement. The effective depth is d = 500 – 40 – 16/2 = 452 mm, measured from the compression face to the centroid of the tension reinforcement.
Vertical Line Shear at the Column Face
Vertical line shear, also called one-way shear, is checked on a section at the face of the column where the shear force is highest. The design shear stress from the ultimate pressure acting on the cantilever portion of the slab is compared with the concrete shear resistance. Where the section is inadequate, the footing thickness is increased; shear reinforcement is rarely economical in a pad footing.
Punching Shear at the Column Perimeter
Punching shear, or two-way shear, is checked around the column. The basic control perimeter is set at a distance of 2.0d from the column face, and the applied shear stress is compared with the punching shear resistance of the concrete. A failed check is corrected with a thicker slab, a larger column head, or shear reinforcement around the column.
For most pads, the governing check is punching shear at the column, because the concentrated load creates high stresses close to the column face. Designers set the thickness first and verify, then increase it if either check fails. A thicker slab is usually cheaper than adding shear links and reduces deflection.
The choice between pad, strip, and raft foundations depends on soil conditions and load intensity. The pad foundation design principles, types, and selection criteria guide that decision, because a pad is only the right answer when the soil at footing level can carry the concentrated load without excessive settlement.
Bending Reinforcement for the Base Slab
With the depth verified in shear, the base is reinforced for the bending moment from the upward soil pressure. The footing bends like an inverted cantilever: the column pushes down at the centre while the soil pushes up across the full base, so tension develops at the bottom of the slab.
Critical Section and Design Moment
The critical section for bending is taken at the face of the column. The design moment comes from the ultimate bearing pressure acting on the cantilever span between the column face and the footing edge. For a 2.9 m square footing with a 400 mm column, the cantilever span is (2900 – 400) / 2 = 1250 mm.
For a uniform pressure q over the cantilever span L, the design moment per metre width is M = q x L x L / 2. Using the ultimate pressure of 216 kN/m2 and a span of 1.25 m gives M = 216 x 1.25 x 1.25 / 2 = 169 kNm/m, the value used to select the bottom bars.
Minimum Reinforcement and Bar Layout
The calculated steel area is compared with the code minimum, which controls cracking and handles shrinkage and temperature effects. Bars are laid in a uniform grid in both directions because the moments are similar in each.
Spacing and Anchorage Rules
Keep the maximum bar spacing at or below 450 mm, and check the national annex for the footing clause, since some codes tighten this to three times the slab thickness. Bars need enough anchorage length beyond the face of the column to develop their full capacity, and the edge cover protects the steel from soil moisture and aggressive ground.
The same geometry is handled differently in other codes. The isolated footing design guidelines based on ACI 318-14 show how critical sections and development lengths are defined in the American approach, which is useful when drawings must be produced for a US project.
Detailing Rules and Practical Construction Notes
Detailing decisions determine whether the calculated reinforcement actually performs. Cover, spacing, bar diameter, and lapping are set out on the drawings, and the site team should be able to build the footing without ambiguity.
Practical Detailing Rules
- Provide at least 40 mm nominal cover where the concrete is cast against a blinding layer; without blinding, codes demand a larger cover for concrete cast directly against the ground.
- Lay the bars in a uniform grid in both directions and keep the maximum spacing at or below 450 mm.
- Leave enough clearance around the column starter bars so the vertical reinforcement fits inside the footing grid.
- Lap the starter bars to the column reinforcement with the lap length specified by the code.
Blinding concrete, typically 50 to 75 mm thick, gives a clean working surface, protects the reinforcement from mud, and keeps the cover uniform. The excavation sides must be stable and the base level before the blinding is placed.
Related Foundation Types
Pad footings are one member of a family of shallow foundations. When loads run along a wall rather than at a point, the analysis and design of RC wall footing based on ACI 318-19 applies, and the strip footing that results behaves more like an inverted beam than a two-way slab.
A footing is rarely designed in isolation from the structure above it. The base plate and anchor bolts transfer column forces into the concrete, and when the column is a steel section, the connection is worked out with structural steel design principles before the footing reinforcement is finalised. Checking the interface between the base plate and the concrete pad closes the design loop and matches the assumptions made at the start of the calculation.