Dynamic Characteristics Study of Steel Pipe Composite Large-Span Transmission Tower Structures

Literature Overview

This paper by Cao Meigen, Zhou Fulin, Xu Zhonggen, and Liu Zhiyong, published in the Journal of Guangzhou University (Natural Science Edition) in 2005, investigates the dynamic characteristics of steel pipe composite large-span transmission tower structures of the SZK series. Supported by the National Science and Technology Major Basic Research Project (No. 2004CCA03300), the research employs the finite element analysis software ANSYS to establish a rod-beam hybrid analysis model of the transmission tower. The authors use the subspace iteration method to obtain the first five modal characteristics of the structure and compare them with measured periods using approximate formulas.

Numerical Modeling and Modal Analysis

The rod-beam hybrid analysis model represents a practical approach to modeling transmission towers where members can be classified as either rod-type (axial force dominant) or beam-type (bending moment dominant). This classification allows for computational efficiency while maintaining adequate accuracy for dynamic analysis. The subspace iteration method is a well-established technique for extracting eigenvalues and eigenvectors from large-scale finite element models, making it suitable for the modal analysis of complex transmission tower structures.

Period Formula Development and Conductor Influence

The authors first obtained a more precise first-order period calculation formula that does not consider conductor influence by correcting the approximate formula based on measured data. Subsequently, they considered the influence of conductors and studied the dynamic characteristics of transmission towers under different vertical spans. The introduction of a period amplification factor that accounts for conductor influence represents a practical engineering solution that bridges the gap between simplified models and actual structural behavior.

The conductor influence on dynamic characteristics is significant because conductors add mass to the structure without contributing stiffness, thereby reducing natural frequencies and increasing natural periods. This effect is particularly important for seismic and wind design because longer natural periods may shift the structure into different response regions of the design spectrum. The vertical span of conductors directly affects the amount of conductor mass participating in the structural dynamics, making it a critical parameter in the period amplification factor.

Engineering Implications for Seismic and Wind Design

The dynamic characteristics of transmission towers directly influence their seismic and wind response. The natural periods determine which portions of the design spectrum are most critical for structural response, while the mode shapes indicate the deformation patterns that govern member forces and displacements. Understanding these characteristics is essential for developing appropriate seismic isolation strategies and wind-resistant design measures.

For large-span transmission towers, the first-order period is typically the most critical for seismic design because it governs the primary response of the structure. The modified period formula developed in this study provides engineers with a more accurate tool for estimating the fundamental period, which directly affects the design base shear and overturning moment. The consideration of conductor influence is particularly important for towers carrying high-voltage conductors with significant sag and span, where the dynamic mass contribution can be substantial.

Study Insights and Practical Recommendations

The research methodology employed in this study demonstrates the value of combining numerical simulation with experimental validation for developing practical design tools. The progressive refinement of the period formula, first without conductor influence and then with it, illustrates a systematic approach to improving analytical accuracy. Engineers working on transmission tower design should pay particular attention to the dynamic characteristics when evaluating seismic and wind vulnerability, as these loads are governed by the structural modal properties. The period amplification factor concept provides a simple yet effective means of incorporating conductor effects into simplified design calculations, which is valuable for preliminary design assessments and code-based design procedures.