Reinforced Concrete Design / Foundations and pile caps
Pad footings
Two calculations on the same base, at two different limit states, with two different sets of loads.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Corrections checked in this pass
- Searched internal punching perimeters; checked the loaded face; reconciled bar spacing and the two orthogonal effective depths.
Checks needed for project use
- A concrete-design reviewer must check the stated first-generation clauses, National Annex choices, detailing rules and factor sets. Numerical agreement alone does not establish code compliance.
- Confirm the ground model, GEO resistance, settlement and anchorage. The structural model is limited to centred uniform pressure and complete internal perimeters.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
A foundation is the one element where a structural engineer's work is governed by someone else's number — the allowable bearing pressure. Two mistakes follow from not understanding what that number is. Confirm whether the ground pressure supplied is an allowable service value or a design resistance, then use the matching action combination. Complete the required geotechnical and structural ultimate checks as well as settlement checks.
By the end of this lesson you should be able to
- State which limit state governs sizing and which governs the concrete design
- Size a square pad from an allowable pressure, allowing for its own weight
- Calculate the design moment and the two shear checks
- Explain why the base's self-weight cancels out of the structural design
What you should already know
- Combinations of actions, and the difference between ULS and SLS (Module 2)
- Flexural design (Module 4)
- Shear and punching shear (Module 5)
Match the action combination to the ground criterion
This example supplies an allowable service pressure, so its plan-area comparison uses the stated characteristic load including the pad weight. A ground report may instead supply design resistance values requiring ultimate combinations. Establish that basis before sizing the foundation.
Then verify settlement and the relevant geotechnical ultimate limit states as well as the structural bending, shear and anchorage checks. There is no universal percentage by which a factored-load comparison oversizes a footing.
Worked example
A square pad footing, from ground to reinforcement
Given
- Internal column 400 mm × 400 mm
- Characteristic axial load 1400 kN; design (factored) axial load 1900 kN
- Allowable bearing pressure 200 kPa = 0.20 N/mm²
- C30/37 concrete, grade 500 reinforcement, 50 mm cover
- Trial base thickness 650 mm
Find
The plan size, the reinforcement, and whether the shear checks pass.
Assumptions
- The stated 200 kPa is an allowable service pressure for the specified characteristic combination; separate geotechnical ultimate and settlement verification is still required
- Centred column load, uniform upward pressure, no openings or moment transfer; concrete unit weight 25 kN/m³
- 15H16 each direction in two touching orthogonal layers; dy = 592 mm, dz = 576 mm, punching mean d = 584 mm
- Use fcd = 17 MPa and γc = 1.5 for the stated first-generation teaching checks
Practice
A column carries a characteristic load of 900 kN. The allowable bearing pressure is 150 kPa and the base is 600 mm thick (concrete at 25 kN/m³). What square base side is required, in m?
Practice
A 3.0 m square pad carries a factored column load of 2100 kN from a 450 mm square column. What is the design moment at the column face, in kNm?
Check yourself
For the allowable service-pressure comparison specified in this example, which load should be used?
Summary
- Sizing: characteristic loads plus base self-weight, against the allowable pressure
- Design: factored loads, and the self-weight cancels exactly
- Bending is taken at the column face; beam shear at d from it
- Punching is checked on a perimeter at 2d, with the enclosed pressure deducted
- For a thick compact pad the punching perimeter nearly reaches the edge, so punching rarely governs
- Minimum reinforcement usually governs a footing, because d is large and K is tiny