Module 04
Flexural behaviour and section analysis
The central calculation of reinforced concrete, built from three principles and nothing else — and an honest account of which parts a code decides.
What this module covers
- Follow a section from zero load through cracking and yield to failure
- Derive the moment of resistance from strain compatibility and equilibrium
- Distinguish what is mechanics from what the stress block assumes
- Design tension reinforcement for a rectangular section, and check the bars fit
- Explain why compression reinforcement is added for ductility, not strength
- Analyse a flanged section, and know when it stops behaving as one
Lessons
Four distinct states, and the one the design calculation actually describes.
Start lesson →Three principles, no formula quoted — and a clear account of which step the code decides.
Start lesson →Running the derivation backwards — and the step that turns a number into a buildable beam.
Start lesson →What to do when the section is working too hard, and why a T-beam is usually easier than it looks.
Start lesson →
Module checkpoint
Check what you have taken in
5 questions
Question 1
A section has As = 1200 mm² with fyd = 434.8 N/mm², b = 250 mm and η fcd = 17.0 N/mm². What is the depth of the rectangular stress block, in mm?
Question 2
In the flexural derivation, which step is pure mechanics rather than a code choice?
Question 3
A beam has h = 600 mm, cover 35 mm, 10 mm links and 20 mm bars in one layer. What is the effective depth, in mm?
Question 4
Why does design limit the neutral-axis depth ratio x/d?
Question 5
For a T-beam with a 900 mm wide flange, a tension force of 500 kN and η fcd = 17.0 N/mm², what is the stress-block depth, in mm?