Structures
Design a steel cantilever canopy
Design a steel cantilever canopy in 3D: a single row of fixed columns with propped arms, edge beams and purlins, cantilever moments, uplift and tip deflection.
Canopies over entrances, platforms, petrol stations and loading bays are often held from one side only: a single row of columns with arms reaching out over the open space. That makes them a perfect lesson in cantilevers — and in how a single prop can transform one. This tutorial uses the Cantilever canopy template.
How a cantilever canopy carries load
The roof sheeting loads the purlins and the edge beam at the tip, which span between arms. Each cantilever arm carries its share back to its column. A cantilever has nothing at the far end to help it, so the moment at the root is large — for a uniform load w over a reach L it is wL²/2, four times the moment of a simply supported beam of the same span — and the tip deflection is wL⁴/8EI, about 10 times larger.
A prop — a diagonal strut from the column up to the arm — cuts the arm's free length and turns most of the bending into axial force in the prop and the column. Because there is only one row of columns, the columns must have fixed bases: they resist the whole overturning moment by bending.
Step 1 — Pick the template
- In the gallery choose Elements › Cantilever canopy.
- The default is five 6 m bays, 4 m columns and 5 m arms with a 3° fall to the tip and one prop under each arm. The bases are fixed by the template.
Step 2 — Set the size
| Parameter | Section | Default | Range |
|---|---|---|---|
| Bays | Plan | 5 | 1–15 |
| Column spacing | Plan | 6 m | 3–10 m |
| Column height | Height | 4 m | 2.5–8 m |
| Cantilever length | Canopy | 5 m | 2–10 m |
| Roof slope | More options | 3° | 0–15° |
| Props (0 none, 1 under each arm) | More options | 1 | 0–1 |
The roof falls from the column towards the tip. Purlins are spaced at about 1.8 m or less along each arm (at least two divisions). The prop runs from mid-height of the column to a point about 60 % of the way along the arm.
Step 3 — Materials and sections
- Choose the Design code and steel Grade.
- Four groups: Columns, Cantilever arms, Edge beams & purlins and Props (truss members).
- Keep Size members for me on.
Step 4 — Supports and loads
- The bases are fixed — a single row of columns needs them. Switch on Pad footings: expect large footings, since they resist the overturning moment.
- Set the roof Finishes and services load. The roof live load comes from the code.
- With no walls, Wind acts as horizontal drag on the members.
On an open canopy, wind blowing underneath causes strong uplift — often larger than the downward load. Uplift is not generated by the wind cases. Add it as upward line loads on the purlins (Full interface, select a purlin and use Add a line load), with the pressure coefficient for canopies from your code. Under uplift the prop goes into tension and the arm's bottom flange into compression.
Step 5 — Analyse
Press Run analysis (F5). Check:
- Displacement under the service combination: the tip of the arms moves most. Rule of thumb for cantilevers: tip deflection ≤ reach / 180 (twice the span/360 rule, since the cantilever's "span" is twice its reach).
- Moment: with props, the arm moment peaks over the prop and at the root; without, the root moment approaches wL²/2. Compare by setting Props to 0.
- Axial: the prop is in compression under gravity and the column carries both axial force and the moment.
- Reactions: the base moment of each column — the number the footing and base plate have to resist.
Step 6 — Design
- Press Run design (F6). Arms, columns, purlins and props are checked and sized; base plates with anchors, end plates and gussets at the props are designed.
- Show Steel connections and click a column base — the base plate is the most heavily loaded connection on a canopy.
- Produce the Calculation report and Shop drawings.
Typical problems and fixes
- Tip deflection governs. Increase the arm depth, add props, or shorten the reach. Deflection grows with the fourth power of the reach.
- Columns bend sideways along the row. Along the canopy, the columns are linked only by purlins and must cantilever from their bases. Check Minor moment under the wind along the row, and add bracing between two columns if needed.
- The arm passes but the deflected tip "bounces". Light, long cantilevers are lively. Keep the first frequency above about 3–5 Hz.
Rules of thumb
- Unpropped cantilever arm depth ≈ reach / 8 to reach / 12; with a prop, about half that.
- Back-spans (an arm running back past the column) halve the column moment — consider them where space allows.
- Footings for single-row canopies are governed by overturning: check their stability under uplift as well as bearing.