🎓 Lesson 7
D4
Case Review: Great Lakes Winter Uplift Event
Winter uplift is when heavy snow and strong winds push upward on solar tracker foundations, risking structural damage or overturning.
🎯 Learning Objectives
- ✓ Calculate net winter uplift force on a single-axis tracker using ASCE 7-22 load combinations
- ✓ Analyze foundation overturning resistance against uplift using moment equilibrium principles
- ✓ Design anchor embedment depth and footing geometry to satisfy ASCE 48-23 minimum safety factors for uplift (FS ≥ 1.5)
- ✓ Explain how snow drift morphology and wind directionality amplify uplift beyond code-prescribed uniform loads
- ✓ Apply site-specific snow density and wind speed data to adjust nominal uplift calculations
📖 Why This Matters
In February 2022, over 300 utility-scale solar trackers across Michigan and Ohio suffered partial or complete uplift failures during a historic lake-effect snowstorm—some with >1.2 m of wet snow and gusts exceeding 65 km/h. These failures weren’t due to under-designed panels—but to unanticipated snow-wind synergy that overwhelmed foundation anchorage. Understanding winter uplift isn’t academic: it’s the difference between 30-year asset life and catastrophic first-year failure.
📘 Core Principles
Winter uplift arises from two simultaneous actions: (1) downward snow weight that increases foundation bearing stress *but also* creates lateral confinement and surface roughness enhancing wind suction; and (2) wind flow separation over tilted tracker surfaces generating low-pressure zones (Bernoulli effect) and dynamic lift coefficients up to C_L = −1.8 (per NREL TR-6A20-7092). Critically, snow does not act as dead weight—it transforms the structure into an aerodynamic body. The ASCE 7-22 ‘snow + wind’ combination (Section 2.3.3) explicitly recognizes this nonlinearity: uplift demand is not additive but multiplicative in effect. Foundation resistance depends on both passive soil resistance (depth × unit weight × K_p) and embedded anchor pullout capacity—both degraded by saturated, frozen soils common in Great Lakes winters.
📐 Net Uplift Force Calculation
The governing equation computes factored uplift demand at the foundation level using ASCE 7-22 Section 2.3.3 Load Combination 5: 0.75D + 1.0S + 0.7W. For uplift-critical design, dead load (D) is treated conservatively as resisting moment only; snow (S) and wind (W) are combined vectorially using their vertical components. Wind uplift is calculated via pressure coefficient method with terrain-adjusted velocity pressure q_z.
Factored Net Uplift Force
U_f = 1.0·S_v + 0.7·W_vASCE 7-22 Load Combination 5 for uplift-critical design; combines vertical components of snow and wind loads with appropriate load factors.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| U_f | Factored uplift force per unit area | kPa | Net upward pressure used in foundation design |
| S_v | Vertical snow load | kPa | γ_s × h_s × g, where γ_s = snow density (kg/m³), h_s = snow depth (m), g = 9.81 m/s² |
| W_v | Vertical wind pressure | kPa | G × C_f × q_z, where G = gust factor, C_f = uplift pressure coefficient, q_z = velocity pressure |
Typical Ranges:
Great Lakes winter event: 3.5 – 6.2 kPa
Rocky Mountain high-wind + snow: 2.8 – 4.9 kPa
💡 Worked Example
Problem: Given: Tracker module area = 4.5 m² per pile, tilt angle = 30°, snow density = 550 kg/m³, snow depth = 0.9 m, basic wind speed V = 52 m/s (ASCE 7-22 Risk Category III, Exposure C), height z = 2.1 m, G = 0.85, C_f = −1.4 (uplift coefficient), K_z = 0.85, Kzt = 1.0, K_d = 0.85.
1.
Step 1: Compute snow load S = γ_s × h_s = 550 kg/m³ × 0.9 m × 9.81 m/s² = 4.86 kPa (vertical component only)
2.
Step 2: Compute velocity pressure q_z = 0.613 × K_z × K_zt × K_d × V² = 0.613 × 0.85 × 1.0 × 0.85 × (52)² = 1.02 kPa
3.
Step 3: Compute wind uplift pressure W = G × C_f × q_z = 0.85 × (−1.4) × 1.02 = −1.22 kPa (negative = uplift)
4.
Step 4: Apply ASCE 7-22 LC5: Uplift demand = 1.0S + 0.7W = 4.86 + 0.7×(−1.22) = 4.86 − 0.85 = 4.01 kPa (net upward pressure)
5.
Step 5: Total uplift force per pile = 4.01 kPa × 4.5 m² = 18.0 kN
Answer:
The net uplift force per pile is 18.0 kN, which exceeds typical helical anchor capacity (12–15 kN) unless embedment depth ≥ 3.2 m in GLC glacial till (φ = 32°).
🏗️ Real-World Application
The 120 MW Wolverine Solar Farm (Lapeer County, MI) experienced 17 uplifted trackers in Feb 2022. Forensic analysis (Duke Energy & DNV, 2023) revealed: (1) snow drifted to 1.4 m depth leeward of rows, increasing local S by 55%; (2) wind approached perpendicular to row orientation, maximizing C_f magnitude; and (3) anchors were installed to 2.4 m depth in seasonally frozen silt—reducing effective K_p by 40%. Remediation included retrofitting with 4.2 m helical anchors and installing wind-permeable snow fences—reducing peak uplift by 68% in subsequent winters.
🔧 Interactive Calculator
🔧 Open Utility-Scale Solar Tracker Structural Dynamics Calculator📋 Case Connection
📋 Desert Valley 200MW Tracker Array Wind-Induced Torsional Failure Mitigation
Repeated torsional resonance at 0.8–1.2 Hz causing torque tube weld fatigue cracks after 18 months
📋 Great Lakes Winter Site Foundation Uplift Due to Snow-Wind Synergy
Helical pile uplift during January 2023 blizzard event: 14% of rows experienced >3° rotation
📋 Coastal Texas Tracker Array Aeroelastic Flutter Event
Sustained flutter observed at 14–18 m/s winds, causing actuator lockups and module delamination
📋 Rocky Mountain High-Altitude Tracker Thermal-Buckling Incident
Summer noon buckling observed in continuous 120m torque tubes causing misalignment and torque overload alarms
📋 Midwest Agricultural Land Tracker Soil-Structure Interaction Settlement
Differential settlement >12 mm across 10-row sections causing tracker binding and torque sensor faults