Design a steel roof truss

When a roof spans further than a portal frame can economically reach — or when you want a deep, light structure that services can pass through — a roof truss is the classic answer. This tutorial designs pitched Pratt trusses on columns with the Steel truss roof template, and explains why trusses are so efficient.

#How a truss carries load

A truss turns bending into axial force. Under gravity the top chord is squeezed and the bottom chord is pulled — together they form a couple that resists the bending moment, with a lever arm equal to the truss depth. The web members (verticals and diagonals) carry the shear between them. Because every member works mostly in tension or compression, the steel is used much more efficiently than in a solid beam.

In a Pratt truss the diagonals slope down towards mid-span. Under gravity this puts the long diagonals in tension and the short verticals in compression — the best arrangement, since compression members must be stocky to avoid buckling.

Wind uplift reverses all of this: the bottom chord goes into compression, and because it is not restrained by purlins it can buckle sideways over long lengths. That is why roof trusses always need a check for load reversal.

#Step 1 — Pick the template

1. In the gallery choose Industrial › Steel truss roof.
2. The default is a 24 m span, six bays of trusses at 6 m, 6 m columns and a 3 m rise, with pinned bases.

#Step 2 — Set the size

Parameter	Section	Default	Range
Span	Span	24 m	12–40 m
Bays	Plan	6	2–12
Truss spacing	Plan	6 m	4–10 m
Column height	Height	6 m	3–12 m
Truss rise	Roof	3 m	1.5–6 m
Truss panels per half	More options	4	2–8

The rise is the depth of the truss at the ridge; at the eaves it comes to a point on the column. A purlin sits at every top-chord panel point, so more panels means more purlins and shorter top-chord buckling lengths. The top chord is one continuous member from eave to ridge on each slope and the bottom chord runs end to end; the webs frame into them.

#Step 3 — Materials and sections

1. Pick the Design code and steel Grade.
2. Sections for Columns, Chords, Web members and Purlins. Web members are truss members (pinned ends, axial force only); the chords are continuous and also carry some bending from the purlin loads between nodes.
3. Keep Size members for me on.

#Step 4 — Supports and loads

1. Bases are pinned. Across the building the columns and trusses form a frame; along it, the stability comes from the rigid joints between purlins and column tops only.
2. Set the roof Finishes and services load. The roof live load comes from the code.
3. Wind loads the side and gable walls. As for any light roof, uplift on the roof sheeting is not generated: add it as line loads on the purlins if your site needs it (Full interface, Add a line load).

#Step 5 — Analyse

Press Run analysis (F5). Check:

1. Axial: top chord in compression, bottom chord in tension under gravity, largest at mid-span. A hand check: chord force ≈ M / d = (w L² / 8) / rise. For the default 24 m span and 3 m rise, with w = 10 kN/m on each truss (about 1.7 kN/m² over the 6 m spacing): 10 × 24² / 8 / 3 = 240 kN.
2. Pick a wind case and see whether any web or the bottom chord changes sign.
3. Displacement: the ridge deflection under service load — trusses are stiff, so it is usually small.
4. The horizontal movement at the column tops along the building — with no bracing it relies on the column and purlin bending.

#Step 6 — Design

1. Press Run design (F6). Chords and webs are checked for tension, compression with flexural buckling, and combined bending where present. Buckling lengths come from the nodes where other members frame in — purlins restrain the top chord out of plane, the webs in plane.
2. Connections are designed, including gusset plates where the webs meet the chords. Show Steel connections and click a node.
3. Show Utilization: the end diagonals and the mid-span chords are usually the hardest-working members.

#Typical problems and fixes

• Top chord fails in buckling. Increase Truss panels per half (shorter lengths between purlins), or use a section stiff about both axes.
• Very large chord forces. The truss is too shallow. Increasing the rise lowers chord forces in direct proportion.
• Bottom chord fails under uplift. Add bottom-chord restraint (ties to the purlins or a longitudinal tie in the real building) and model it, or increase the chord section.
• Webs oversized. All webs share one group, so the end diagonals set the size for all of them. Accept it for simple fabrication, or change a member's Group in its properties.

#Rules of thumb

• Truss depth at mid-span ≈ span / 8 to span / 12 for pitched trusses, span / 10 to / 15 for parallel-chord ones.
• Angle between diagonals and chords: 35–55°.
• Steel weight of a truss roof: 15–25 kg per m² for 20–30 m spans.

For two-way long spans see the space frame roof.