Steel Roof Truss Design -- Types, Load Paths & Worked Example
Steel roof trusses are one of the most efficient structural systems for spanning large distances with minimal material weight. From industrial warehouses and aircraft hangars to sports stadiums and commercial buildings, the triangulated framework of a steel roof truss converts applied loads primarily into axial tension and compression in its members, avoiding the bending stresses that make solid beams heavier and more expensive.
Common steel roof truss configurations
Four truss types account for the vast majority of steel roof truss design in building construction:
- Pratt truss: Diagonal web members slope downward toward the centre. Diagonals are in tension under gravity loading; verticals are in compression. The most common configuration for roof trusses in the 20-40 m span range.
- Howe truss: Diagonal web members slope upward toward the centre. Under gravity loading, diagonals are in compression and verticals are in tension.
- Warren truss: Equilateral triangulation with no vertical members. All diagonals alternate between tension and compression. Very efficient for distributed loading.
- Fink truss: Sub-divided panel configuration with secondary triangulation. Used primarily for shorter spans in residential and light-commercial roofs.
For most steel roof truss design applications, the Pratt configuration offers the best balance of fabrication cost and structural efficiency.
Load paths in a steel roof truss
The load path sequence is: roof cladding âÃÂàpurlins âÃÂàtop chord âÃÂàweb members âÃÂàbottom chord âÃÂàconnections âÃÂàbearings. Each element in this chain must be designed for the forces that pass through it.
Worked example: 30 m span Pratt roof truss
A 30 m span parallel-chord Pratt roof truss supports an industrial warehouse roof. The truss depth is 2.5 m with 8 panels at 3.75 m each. Steel grade ASTM A992 (Fy = 50 ksi). Purlin point loads of 18.2 kips per panel point govern the design.
Critical member forces
Peak chord forces from truss analysis: top chord max compression = -425 kN, bottom chord max tension = +380 kN, max diagonal tension = +145 kN.
Design check (HSS 203x203x8.0 chords)
Top chord in compression: KL = 3750 mm, Fe = 845 MPa, Fcr = 293 MPa, phi_c * Pn = 1634 kN. Pu = 425 kN — OK at 26% utilisation.
Bottom chord in tension: phi_t * Pn = 1925 kN. Pu = 380 kN — OK at 20% utilisation.
Connection design: M20 Grade 8.8 bolts in double shear provide 88.8 kN per bolt. Two bolts sufficient per gusset plate connection.
Key considerations
Chord forces reverse under wind uplift, so both gravity and wind load combinations must be checked. Connection detailing with gusset plate thickness and bolt layout often governs the design more than member section sizes.
Educational reference only. All steel roof truss designs must be independently verified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.
Try It Yourself
Ready to try this yourself? Use our free Beam Capacity Calculator to verify chord and web member capacity, and the Load Combinations Calculator to generate code-compliant load cases for your truss analysis.