Industrial Warehouse Truss Design in Northern Ontario

Scenario

A 3-bay pre-engineered metal building for a logistics distribution center near Timmins, Ontario. The site experiences heavy seasonal snowfall (design snow load per NBCC 2020) and moderate wind exposure (Class B terrain). Constraints included a tight construction schedule, limited crane access requiring lightweight trusses ≤ 12 m span, and strict fire-rating requirements limiting steel thickness options.

Given Data

• Span length: 12.5 m
• Truss height: 2.8 m
• Snow load: 3.2 kN/m² (local NBCC Zone 4, unsheltered roof)
• Wind load: 1.3 kN/m² (positive pressure, windward side)
• Equipment load: 0.7 kN/m² (HVAC units and conduit supports)
• Material properties:
  • Modulus of elasticity: 200 GPa
  • Yield strength: 350 MPa (ASTM A992 Grade 50 steel)

Calculation

Using the structural analysis software:

1. Applied combined dead + live (snow + equipment) + wind load combinations per CSA S16-19: U = 1.25D + 1.5S + 0.5W + 0.5E → effective uniform load = 1.25(0.4) + 1.5(3.2) + 0.5(1.3) + 0.5(0.7) = 6.2 kN/m² (roof projection area).
2. Converted to nodal loads on truss joints using tributary width (3.0 m bay spacing): 6.2 kN/m² × 3.0 m = 18.6 kN/m line load → discretized into 12 joint loads.
3. Ran linear static analysis with second-order (P-Δ) effects enabled due to high slenderness ratio (L/r > 100 for top chord).
4. Performed elastic buckling analysis (Eigenvalue) on compression members — identified critical mode at 127.4 kN (top chord mid-span panel).
5. Member forces output showed max tension = 214.6 kN (bottom chord), max compression = −189.3 kN (top chord); buckling loads ranged from 112.1–158.7 kN across compression elements.
6. Required member sizes computed using CSA S16-19 Clause 13.3 (axial resistance) and Clause 13.4 (buckling resistance), applying φ = 0.9 and γₘ = 1.1.

Result and Decision

The software recommended:

• Bottom chord: 127×76×9.5 mm RHS (required: 125×75×8.0 mm; selected larger for constructability & future retrofit capacity)
• Top chord: 152×89×11.1 mm RHS (required: 148×89×10.2 mm; selected to exceed buckling threshold by 18%)
• Web members: mixed 89×89×6.4 mm and 76×76×6.4 mm RHS All members satisfied serviceability (deflection < L/240) and ultimate limit states (utilization ratios ≤ 0.87). Final design approved for fabrication with welded connections and hot-dip galvanizing.

Lesson

Local snow load values must be verified against current regional codes—not assumed from generic defaults—because underestimating by just 0.5 kN/m² increased top chord compression demand by 14%, triggering a full reselection of section sizes and connection detailing.