Structural Steel Design / module 15
Single-storey buildings and portal frames
The first whole building. A portal frame is designed by a work equation, made economic by a haunch, and kept standing by stays that are on the drawing for one reason most people never learn.
What you will understand
- Derive the three collapse mechanisms of a portal frame
- Say which mechanism governs, and what decides it
- Explain what a haunch buys and why it is the length it is
- Say which flange is in compression along a rafter, and where
- Explain why every portal has stays, and what they are worth
- Say why a portal's stability is not a storey-drift calculation
Lessons
Collapse mechanisms, and the haunch
A portal is one of the few structures designed directly by plastic collapse. Three mechanisms, three work equations, and the largest answer wins.
Rafter stability, and why every portal has stays
Near the eaves the purlins restrain the wrong flange. That single fact explains a detail on every portal frame drawing, and it is the one thing about them worth never forgetting.
Module summary
- Beam mechanism Mp = wL²/16, sway Mp = Hh/2, combined Mp = wL²/16 + Hh/4 — all derived
- The combined mechanism governs whenever wL² > 4Hh, which is the usual case
- The 30 m frame needed 592 kNm, and a 610 × 229 × 113 column gives λ = 2.18
- A 10% haunch took the rafter's design moment from 592 to 420 kNm — a 29% saving
- The moment changes sign 69% of the way to the apex, 10.4 m along the rafter
- Over all of that the INSIDE flange is in compression and the purlins are on the wrong side
- Unstayed, χLT = 0.242 and Mb,Rd = 200 kNm against a demand of 592
- Two stays took it to 656 kNm — the same restraint argument as Module 8
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 portal spans 24 m with w = 12 kN/m. What does the beam mechanism require, in kNm?
Practice
That frame is 7 m to eaves with H = 60 kN and pinned bases. What does the combined mechanism require, in kNm?
Practice
For a frame with H = 60 kN at h = 7 m, above what value of wL² does the combined mechanism govern over the sway one? Give wL² in kNm.
Practice
A rafter hogs 400 kNm at the eaves and sags 150 kNm at the apex, over a 12 m length. How much of it has the inside flange in compression, in m?
Practice
Three stays are placed within an 8.73 m unrestrained length. What is the segment length, in m?
Practice
A 24 m rafter rises 2.0 m and carries 12 kN/m. What thrust does the arch analogy give, in kN?
Check yourself
A portal frame's collapse load factor is λ = 1.6. What does that tell you?
Check yourself
Why does this course refuse to give a portal frame an αcr from a sway calculation?