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 this module covers
- 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
A portal is one of the few structures designed directly by plastic collapse. Three mechanisms, three work equations, and the largest answer wins.
Start lesson →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.
Start lesson →
Module checkpoint
Check what you have taken in
8 questions
Question 1
A portal spans 24 m with w = 12 kN/m. What does the beam mechanism require, in kNm?
Question 2
That frame is 7 m to eaves with H = 60 kN and pinned bases. What does the combined mechanism require, in kNm?
Question 3
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.
Question 4
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?
Question 5
Three stays are placed within an 8.73 m unrestrained length. What is the segment length, in m?
Question 6
A 24 m rafter rises 2.0 m and carries 12 kN/m. What thrust does the arch analogy give, in kN?
Question 7
A portal frame's collapse load factor is λ = 1.6. What does that tell you?
Question 8
Why does this course refuse to give a portal frame an αcr from a sway calculation?