Torsional Resonance Modes in East-West Aligned Solar Trackers

Torsional resonance begins with simple physics: any rotating structure has a natural tendency to twist at specific frequencies determined by its stiffness and inertia. For solar trackers, the torque tube acts like a long, thin beam fixed at discrete piers—its torsional behavior is highly sensitive to how rigidly those piers resist rotation in the soil. Unlike lateral or vertical vibrations, torsional modes involve minimal visible deflection, making them difficult to detect without instrumentation.

As wind flows across the array, pressure fluctuations generate alternating aerodynamic moments along the torque tube length. When these moments contain energy near a torsional natural frequency, energy transfers efficiently into the structure—like pushing a swing at just the right moment. The resulting angular acceleration stresses welds, bearings, and foundation connections far beyond static design loads. Critically, damping in this system is extremely low: soil hysteresis contributes most dissipation, yet typical field-measured ζ values fall below 1.5%, meaning even modest wind energy can cause large-amplitude torsion.

Advanced analysis requires coupling structural dynamics with site-specific wind spectra and soil–structure interaction. Modern practice uses substructuring: the torque tube is modeled as a Timoshenko beam with variable torsional rigidity; piers are represented as rotational springs calibrated to CPT or SPT-derived G₀ profiles; and wind forcing incorporates phase lag between span segments using coherence functions from wind tunnel data (e.g., NREL WT-301). Industry validation shows that ignoring pier–soil rotational compliance overestimates fₜ by 22–38%, leading to false confidence in resonance avoidance.