Mechanism and wind resistance performance assessment of internal cable reinforcement for transmission towers under extreme winds

Transmission towers are highly sensitive to wind loading. Improving the wind resistance of transmission towers is important for ensuring grid reliability. Prestressed cables provide easy installation and minimal spatial intrusion. An internal cable reinforcement method was proposed to improve the tower. Four internal cables were installed at the main member of the tower to reduce horizontal wind loads.

The applicability of the internal cable method was confirmed using theoretical analysis. A performance evaluation framework for transmission towers with internal cables was introduced. The framework determines the cable reinforcement scheme through theoretical analysis. Truss idealization, pinned-joint modeling, and small-deformation assumptions were adopted to investigate axial forces in main and diagonal members after cable reinforcement. The optimal reinforcement scheme was determined based on the comparative reduction in member axial forces arising from reinforcement. Nonlinear stability analysis was implemented on finite element models of transmission towers with and without internal cables. The member stability cycle criterion was performed to ascertain failure modes and critical wind speeds. Tower failure is regarded as either the buckling failure of a single member or the yielding of a main element below the cable installation location.

A 220 kV transmission tower was used as the case study. Using the framework, the reinforcement effect under extreme wind was explored. The results revealed that the axial-to-force ratio in the lower leg member improved considerably after internal cable installation, while the axial-to-force ratio in diagonal members reduced substantially under extreme wind loading. Local yielding in the lower leg member below the cable anchorage precipitated structural collapse. The failure mode changed from diagonal buckling to leg failure after reinforcing. The critical wind speed rose from 35.05 m/s to 40.36 m/s. The critical wind speed increased by 15.0%, which led to a 32.6% enhancement in horizontal load-bearing capacity. Then, the influence of cable prestress levels and cross-sectional areas were investigated. Under varying cable parameters, the critical wind speed of the tower increased by 13.0%-16.0%. Therefore, cable cross-sectional area and prestress exert only a minor effect on wind resistance performance. As cable prestress and cross-sectional area increased, the marginal gain in critical wind speed progressively diminished. Raising the cable cross-sectional area decreased its ultimate stress, with further area increases above 100.00 mm² producing only marginal changes.

The framework can be used to design cable-reinforcement schemes for transmission towers and evaluate the wind resistance performance of the reinforced tower-cable system. Cable reinforcement reduces axial forces in diagonal members along the tower. It reduces axial forces in lower leg members but raises them in upper leg members. For towers prone to leg-member buckling, internal cables with small inclination angles intensify compressive stresses in loaded legs. Therefore, external cables with larger angles are recommended. In towers where diagonal buckling governs failure, internal cable reinforcement substantially increases critical load capacity. The failure mode shifts to local yielding of lower leg members, and the capacity gain relies on the wind speed at which these members yield. Raising cable prestress or cross-sectional area decreases the critical wind speed for transmission towers with internal cables. The optimal wind resistance enhancement is realized with a cable prestress of 5.0 kN and a cross-sectional area of 100.00 mm². The research results of this paper can provide a reference for the optimization of wind resistance design of transmission towers.