Solar Farm Support Truss for Agrivoltaic Installation in Central California

Scenario

A ground-mounted agrivoltaic system integrating solar panels over row crops near Fresno, CA. The truss supports bifacial PV modules at 2.5 m clearance above soil to allow farm machinery passage. Key constraints included minimal foundation footprint (to preserve tillable land), low self-weight (< 25 kg/m), corrosion resistance for irrigation-spray environment, and rapid field assembly using bolted connections only.

Given Data

• Span length: 9.2 m
• Truss height: 1.6 m
• Snow load: 0.4 kN/m² (low-elevation Central Valley, ASCE 7-22 Zone 1)
• Wind load: 2.1 kN/m² (3-second gust, Exposure C, 10-m height)
• Equipment load: 0.5 kN/m² (PV modules + wiring + maintenance access)
• Material properties:
  • Modulus of elasticity: 105 GPa (6061-T6 aluminum alloy)
  • Yield strength: 240 MPa

Calculation

Using the structural analysis software:

1. Load combination per ASCE 7-22: U = 1.2D + 1.6W + 0.5S + 0.5E → dominant case governed by wind uplift (−2.1 kN/m² suction) and downward equipment load.
2. Applied net uplift load: −2.1 + 0.5 = −1.6 kN/m² → converted to upward nodal loads (tributary width = 2.4 m → −3.84 kN/joint).
3. Performed geometrically nonlinear analysis (large displacement) to capture P-δ effects under uplift-induced instability in diagonal web members.
4. Buckling analysis revealed lowest eigenvalue at 42.3 kN for a slender 38×38×3.2 mm aluminum diagonal (slenderness ratio = 142).
5. Member forces showed max tension = 68.9 kN (lower chord), max compression = −44.7 kN (diagonal); buckling loads ranged 39.1–61.5 kN.
6. Required member sizes derived using Aluminum Design Manual (ADM) 2020: LRFD with φc = 0.85, φt = 0.90, and local buckling checks per Section D.2.

Result and Decision

Software output specified:

• Lower chord: 63.5×38.1×3.2 mm rectangular aluminum tube (required: 60×35×3.0 mm)
• Upper chord: 50.8×38.1×2.8 mm (required: 48×36×2.6 mm)
• Diagonals: 38.1×38.1×2.8 mm (required: 38×38×2.7 mm; rounded up to standard extrusion) All sections met deflection limits (L/360), fatigue life (> 2 million cycles), and corrosion allowance (0.2 mm annual loss compensated via 2.8 mm wall). Final design used ASTM B221 6061-T6 extrusions with stainless-steel M12 A4 bolts.

Lesson

Material-specific buckling behavior—especially in aluminum’s lower modulus and higher slenderness sensitivity—demands explicit geometric nonlinearity and eigenvalue analysis; assuming linear-elastic behavior underestimated critical buckling loads by 22% for diagonals, risking premature collapse under wind uplift.