Structural Steel Design / module 3
Structural forms, load paths and modelling
The building and the model are different objects. Knowing exactly which parts of the building the model does not contain is most of the skill.
What you will understand
- Follow a load from the cladding to the foundations of a steel building
- Distinguish a physical member from the analytical element that represents it
- Say what each element type captures and, more importantly, what it misses
- Classify a joint by its stiffness relative to the beam it connects
- Explain why a connection detailed as simple may still attract moment
- Account for eccentricity that the model has quietly removed
Lessons
From building to model
Every modelling decision throws something away. The skill is knowing what, and where it gets picked up again.
Joints are never pinned or rigid
Those are the two ends of a scale. Everything real is in between, and a connection built stiffer than it was assumed will attract moment whether or not you asked it to.
Module summary
- A rendered 3D model looks like the building and contains almost none of it
- A beam element knows nothing about plates — local buckling is handled entirely outside the analysis
- A joint is never truly pinned or rigid; those are the ends of a continuous scale
- Joint stiffness is judged against EI/L of the beam, not in absolute terms
- At ρ = kL/EI = 2, a 'simple' connection already carries half the fixed-end moment
- Modelling members on their centrelines removes eccentricity the real detail has
- Everything the model omits must be handled somewhere else — usually in a member check
- The question to ask of any model is not 'is it right' but 'what is it not telling me'
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 member carries 320 kN with the load line 52 mm from the modelled axis. What moment results, in kNm?
Practice
A beam has I = 40 000 cm⁴ and spans 10.0 m. What is EI/L, in kNm/rad? Take E = 210 000 N/mm².
Practice
A joint has ρ = 6 on a beam whose fixed-end moment is 200 kNm. What end moment develops, in kNm?
Practice
For that same beam, if the simply supported midspan moment is 300 kNm, what is the midspan moment with the ρ = 6 joints, in kNm?
Practice
A joint of 30 000 kNm/rad sits on a beam with EI/L = 6000 kNm/rad in a braced frame. What is the ratio, and does it classify as rigid?
Practice
A beam is modelled centreline to centreline between 450 mm deep columns. The clear span is 9.1 m. By what percentage does the modelled span exceed the clear span?