Reinforced Concrete Design / module 2
Design basis and limit states
Where the safety factors come from, what a characteristic value actually is, and why choosing the wrong load combination is a more common failure than getting a formula wrong.
What you will understand
- Explain why design is a statement about probability, not certainty
- Distinguish a characteristic value from a mean and from a design value
- Say what each partial factor is compensating for
- Build the ultimate and serviceability combinations for a real floor
- Identify which variable action should lead, and why you must try each
- Recognise when an action is favourable, and factor it correctly
Lessons
Design under uncertainty
Why safety factors exist at all, and what each one is actually paying for.
Actions, and getting the classification right
Permanent or variable, favourable or unfavourable — the classification decides the factor, and it is where most combination errors begin.
Combinations of actions
Building the ULS and SLS combinations for a real floor, and why the largest variable action is not always the leading one.
Module summary
- Nothing is certain: loads vary, materials vary, and the model is approximate
- A characteristic value is a defined fractile, not an average — 95th for loads, 5th for strengths
- γF covers load uncertainty and modelling error; γM covers material variability and workmanship
- The ULS asks whether it survives; the SLS asks whether it is fit to use
- ULS loads are factored, SLS loads are not — a factored serviceability check is meaningless
- ψ factors exist because two variable actions rarely peak together
- A favourable permanent action is factored DOWN; a favourable variable action is dropped
- Every variable action must be tried as the leading one — the biggest is not always the worst
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
What does the '30' in C30/37 concrete actually specify?
Practice
A 180 mm slab (25 kN/m³) with 1.5 kN/m² of finishes carries a 3.0 kN/m² imposed load. Give the ULS design load in kN/m².
Practice
For that same slab, give the quasi-permanent load in kN/m², taking ψ₂ = 0.3.
Check yourself
A cantilever balcony is held down by the self-weight of the floor slab behind it. How should that back-span self-weight be factored when checking the cantilever's stability?
Practice
A frame carries gk = 25 kN/m, imposed 8 kN/m (ψ₀ = 0.7) and wind 10 kN/m (ψ₀ = 0.5). Give the governing ULS load in kN/m.
Check yourself
Why are serviceability checks carried out with unfactored loads?