Structural Steel Design / module 2
Design basis, actions and combinations
The same combination rules as any other material — and a set of cases that only bite steel, because steel structures are light.
What you will understand
- Apply ULS and SLS combinations, and say which one a check belongs to
- Identify when a permanent action is FAVOURABLE, and use the right factor
- Carry out an uplift check on a light roof
- Explain why snow drift is worse for a purlin than uniform snow
- Calculate the force in a restrained member under temperature change
- Explain why crane dynamic factors belong to the action, not to the safety margin
Lessons
Combinations, and the case where load helps you
Most of this is shared with every other material. The part that is not is knowing when a load is on your side.
The actions that catch steel out
Drifted snow, restrained temperature and crane dynamics — three cases where the obvious calculation is not the governing one.
Module summary
- The combination rules are material-independent — EN 1990 does not know what the structure is made of
- What IS steel-specific is which case governs, and lightness is why
- Permanent load resisting uplift is FAVOURABLE and takes γG = 1.0, never 1.35
- Using 1.35 by habit can turn a real uplift into an apparent hold-down
- Drifted snow can be twice the uniform load over a short length — a purlin's worst case
- Thermal force does not depend on member length; only the free movement does
- Crane dynamic factors are a real dynamic effect applied before any partial factor
- The governing combination is rarely the one with the biggest single number in it
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 roof weighs 0.30 kN/m² and the characteristic wind suction is 1.00 kN/m². What is the net design action, in kN/m², taking downward as positive?
Practice
A fully restrained member of 3000 mm² sees a 50 K temperature change. What force develops, in kN? Take E = 210 000 N/mm², α = 12 × 10⁻⁶/K.
Practice
By how much does a free 30 m steel member move under a 45 K change, in mm? Take α = 12 × 10⁻⁶/K.
Practice
A crane's static wheel load is 40 kN self-weight plus 120 kN hoisted. With dynamic factors of 1.1 and 1.3, what is the amplification ratio of dynamic to static total?
Practice
A transverse crane force of 18 kN acts 400 mm above the beam's shear centre. What torque does it apply, in kNm?
Practice
A member's full-restraint thermal force is 400 kN. Its axial stiffness is 2000 N/mm and the restraint's is 2000 N/mm. What force actually develops, in kN?