Module 02
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 this module covers
- 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
Why safety factors exist at all, and what each one is actually paying for.
Start lesson →Permanent or variable, favourable or unfavourable — the classification decides the factor, and it is where most combination errors begin.
Start lesson →Building the ULS and SLS combinations for a real floor, and why the largest variable action is not always the leading one.
Start lesson →
Module checkpoint
Check what you have taken in
6 questions
Question 1
What does the '30' in C30/37 concrete actually specify?
Question 2
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.
Question 3
For that same slab, give the quasi-permanent load in kN/m², taking ψ₂ = 0.3.
Question 4
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?
Question 5
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.
Question 6
Why are serviceability checks carried out with unfactored loads?