Reinforced Concrete Design / module 9
Reinforced-concrete slabs
One-way and two-way action, flat slabs and the punching failure that gives no warning, yield lines, and stairs — where the load is not where you think it is.
What you will understand
- Derive how a two-way slab shares its load, rather than looking it up
- Decide when a one-way idealisation is justified
- Design a two-way slab, and recognise when minimum steel governs
- Check punching shear at a flat-slab column and say why it is the critical failure
- Explain yield-line analysis as an upper-bound method, and why that matters
- Load a stair correctly, allowing for the slope
Lessons
One-way and two-way action
Why a slab chooses a direction, and why the choice is decided by the fourth power of the span ratio.
Flat slabs, punching shear and yield lines
The failure that takes buildings down, and the analysis method that finds more capacity than there is if you let it.
Module summary
- Load share follows the FOURTH power of the span ratio — a 2:1 slab sends 94% the short way
- That single result explains the whole one-way/two-way distinction
- Minimum steel and maximum spacing govern most lightly loaded slabs, not strength
- Punching shear is the flat slab's characteristic failure: brittle, local, and progressive
- Yield-line analysis is an UPPER bound — an assumed mechanism gives a load at or above the true one
- So yield line finds MORE capacity than an elastic method, and must be used with judgement
- A stair waist is measured perpendicular to the soffit but loaded on plan: the slope increases the load
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
Practice
A panel is 3.5 m × 5.0 m. What fraction of the load goes the short way? Give the decimal.
Check yourself
Why is yield-line analysis described as an upper-bound method?
Practice
A 5.0 m square simply supported slab is reinforced with m = 20 kNm/m. What collapse load does yield-line analysis give, in kN/m²?
Practice
A 200 mm slab has 25 mm cover and H12 bars in the outer layer. What is the effective depth to the outer layer, in mm?
Check yourself
A punching check fails, and the stress at the COLUMN FACE also exceeds vRd,max. What is the correct response?
Practice
A stair has a 175 mm waist, 250 mm going and 175 mm rise. What is the slope factor applied to the waist self-weight?