Design a space frame roof

A space frame is a truss that works in two directions at once. Two layers of chords — a top grid and a bottom grid offset by half a module — are joined by diagonal web members into a stiff, light plate of steel. It covers large column-free areas such as exhibition halls, airport terminals, canopies and sports arenas. The Space frame roof template builds a square-on-square offset grid on perimeter columns.

#How a space frame carries load

Like a flat slab, a space frame spans both ways: load at any point spreads across the whole grid to all the supports. Like a truss, each member carries mostly axial force. Under gravity the top layer is compressed and the bottom layer is pulled, most strongly at the middle of the roof; the webs carry the shear, which is largest near the supports.

The two-way action makes the grid very stiff for its weight and forgiving — if one member is overloaded, the load finds another path. The price is many members and many joints, so node design and fabrication drive the cost.

#Step 1 — Pick the template

1. In the gallery choose Industrial › Space frame roof.
2. The default is 10 × 8 modules of 3 m (a 30 × 24 m roof), 2 m deep, on 6 m columns placed every 2 modules round the edge, with pinned bases.

#Step 2 — Set the size

Parameter	Section	Default	Range
Modules in X	Plan	10	4–20
Modules in Y	Plan	8	4–16
Module size	Plan	3 m	2–5 m
Grid depth	Roof	2 m	1–4 m
Column height	Height	6 m	3–12 m
Column every … modules	More options	2	1–4

Columns stand under the top-layer edge nodes, at every n-th module along each side and always at the corners. Fewer columns means longer edge spans and larger forces in the edge chords.

#Step 3 — Materials and sections

1. Choose the Design code and steel Grade.
2. Three groups: Columns, Chords (both layers) and Web members (truss members, axial force only). Circular hollow sections are the usual choice for chords and webs — they buckle equally in every direction.
3. Keep Size members for me on.

#Step 4 — Supports and loads

1. Bases are pinned. The column tops are rigidly joined to the grid, which with pinned bases still gives sway stiffness. Switch on Pad footings to size foundations.
2. The roof sheeting on the top layer gets the roof live load and the Finishes and services load, passed to the top chords.
3. With no cladding walls, Wind acts as horizontal drag on the members. Roof uplift is not generated; add it as line loads if needed.
4. For earthquake, the heavy roof on slender columns usually sways as a single-storey structure.

#Step 5 — Analyse

Press Run analysis (F5). Check:

1. Displacement (service): the centre of the roof deflects most, in a smooth dish. Rule of thumb: span / 250 to / 360 of the shorter span.
2. Axial: compression in the top chords and tension in the bottom chords, peaking at mid-roof; the webs next to the columns carry the most shear.
3. Look at the corners: corner regions of two-way grids can show uplift tendencies and force reversals — the Reactions table shows if any column goes into tension.

#Step 6 — Design

1. Press Run design (F6). Chords are checked for axial force with buckling (and any bending), webs for tension and compression, columns for combined axial and bending.
2. Show Utilization: the top chords at mid-roof and the webs at the supports usually govern.
3. The Calculation report lists the governing member of each group in full.

#Typical problems and fixes

• Top chords fail in buckling. Reduce the Module size (shorter chords) or increase the Grid depth (lower chord forces).
• Large edge deflections between columns. Place columns closer (Column every … modules = 1) or increase the chord section.
• Too many heavy members. One section per group means the worst chord sets all chords. Real grids are graded, with lighter members away from the peak forces — use the Utilization view and the member forces table to see where lighter sections would do, and treat the auto-sized result as the upper bound.

#Rules of thumb

• Grid depth ≈ span / 15 to span / 25 (span between supports).
• Module size ≈ 1 to 1.5 × the depth; diagonals at about 45°.
• Steel weight: about 15–30 kg per m² for 30–60 m spans.