Reinforced Concrete Design / module 11
Foundations and pile caps
Where structural design meets the ground — and where the two most consequential mistakes are both about which limit state you are working in.
What you will understand
- Separate the geotechnical sizing from the structural design, and use the right loads for each
- Size a pad footing from an allowable bearing pressure
- Check bearing pressure under combined axial load and moment, including uplift
- Design a pad for bending, beam shear and punching shear
- Explain why a pile cap is designed by strut and tie, not by bending
- Recognise when minimum reinforcement governs, which in footings is often
Lessons
Pad footings
Two calculations on the same base, at two different limit states, with two different sets of loads.
Eccentric loads and pile caps
When the base lifts off at one edge, and when the member is too deep for bending theory to apply at all.
Module summary
- SIZE the base on characteristic loads against an allowable pressure — a serviceability check
- DESIGN the concrete on factored loads at the ultimate limit state
- The base's own weight counts when sizing, and cancels when designing
- Outside the middle third the base lifts and the pressure is triangular, not trapezoidal
- Bending is taken at the column face; beam shear at d from it; punching on a perimeter
- In a thick compact pad the punching perimeter encloses most of the base, so punching rarely governs
- Minimum reinforcement very often governs a footing, not strength
- A pile cap is a deep member: the reinforcement is a TIE, and it is never curtailed
Teaching basis and review status
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 notesCheck your understanding
Check yourself
Why does the base's own weight not appear in the structural design of a pad footing?
Practice
A column carries 1600 kN characteristic. The allowable pressure is 175 kPa and the base is 700 mm thick. What square side is required, in m?
Practice
A 2.5 m square base carries a factored load of 1500 kN from a 350 mm square column. What is the ultimate bearing pressure, in kPa?
Practice
For that base, what is the design moment at the column face, in kNm?
Check yourself
A bearing-pressure calculation returns a minimum pressure of −40 kPa. What does this mean?
Practice
Two piles at 2000 mm centres carry a factored column load of 2800 kN through a cap with d = 750 mm. What is the tie force, in kN?