Wind Tunnel Study of Shape Coefficients for Transmission Steel Tubular Towers

Literature Overview

This paper, published in the journal "Power System Technology" (2010, Vol. 34, No. 9), presents wind tunnel test results for a 1000 kV UHV transmission steel tubular tower (SZT2 type) located on the Huainan–Shanghai transmission line. The study was conducted by researchers from Tongji University's School of Civil Engineering and Fujian Electric Power Survey and Design Institute. The authors employed a high-frequency force balance system to measure wind loads on scaled models of the tower body and cross-arms, obtaining average wind loads and shape coefficients across various wind direction angles.

Core Findings and Technical Parameters

The investigation focused on determining the shape coefficient (Cd) for structural members of the steel tubular tower under varying wind attack angles. The key experimental parameters and results are summarized below:

Interpretation of Technical Points

Wind Direction Angle Effects

The identification of 15° and 75° as the most unfavorable wind direction angles is a critical finding for structural design. In conventional transmission tower design, wind loads are often assumed to be applied perpendicular to the main plane of the structure (90° incidence). However, this study demonstrates that oblique wind incidence can produce higher aerodynamic forces due to complex interference effects between structural members. The 15° angle is particularly significant because it creates a near-tangential flow condition that can induce asymmetric pressure distributions on tubular members, resulting in higher drag forces than the perpendicular case.

Shape Coefficient Determination

The recommended shape coefficients of 0.80 for the tower body and 0.85 for cross-arms represent conservative values that account for the actual aerodynamic behavior of the full-scale tower configuration. These values differ from idealized cylinder values (Cd ≈ 1.0–1.2 for smooth circular cylinders at subcritical Reynolds numbers) because:

• The actual tower geometry includes lattice-like bracing members, gusset plates, and connection details that modify the local flow field.
• Surface roughness of weathered steel members promotes earlier boundary layer transition, potentially reducing Cd in certain configurations.
• The interference between closely spaced tubular members creates sheltering effects that reduce the effective drag area.

High-Frequency Force Balance Method

The use of a high-frequency force balance (HFB) is noteworthy because it captures both the mean (steady) and fluctuating (dynamic) components of wind loads. This is essential for UHV transmission towers where dynamic effects—particularly vortex-induced vibrations and galloping—can govern the design of slender members. The HFB method provides superior data quality compared to strain gauge-based measurement systems, as it directly measures aerodynamic forces without requiring separate conversion through structural response analysis.

Standards Context and Engineering Practice

The shape coefficient is a fundamental input for wind load calculations per GB 50009 (Load Code for the Design of Building Structures) and DL/T 5154 (Design Code for Overhead Transmission Lines). The standard provides a default Cd of 0.7–1.0 for tubular members depending on Reynolds number and surface roughness. This study validates and refines these values for a specific tower type, providing project-specific data that can improve design accuracy.

In engineering practice, the following considerations apply:

• The shape coefficient should be applied to the projected area perpendicular to the wind direction, not the surface area.
• For oblique wind conditions, the effective projected area changes with wind direction angle, and the interaction between the shape coefficient and projected area must be considered simultaneously.
• The 15° unfavorable angle suggests that wind load calculations for transmission towers should not be limited to 90° incidence only.

Reflections and Implications

This study highlights an important gap in conventional design practice: the assumption that the most severe wind load occurs at 90° incidence may be conservative for some angles but non-conservative for others. For UHV towers with complex geometries, wind tunnel testing remains the most reliable method for determining accurate aerodynamic coefficients. The recommended Cd values of 0.80 and 0.85 are practical design parameters that balance safety with economy. Future work should investigate the dynamic shape coefficients (including flutter and galloping contributions) and their implications for fatigue design of tubular tower members.