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

Literature Overview

This 2005 paper by Cao Meigen, Zhou Fulin, Xu Zhonggen, and Liu Zhiyong (Guangzhou University Engineering Seismic Research Center and Guangdong Electric Power Guangzhou Power Supply Company) investigates the dynamic characteristics of steel pipe composite large-span transmission tower structures. Funded by the National Science and Technology Major Basic Research Project (2004CCA03300), the study uses the ANSYS finite element analysis software to establish a member-beam hybrid analysis model for the SZK series of steel pipe composite transmission towers. The subspace iteration method is employed to obtain the first five modal characteristics of the structure.

Core Technical Content

Model Development and Modal Analysis

The member-beam hybrid analysis model was established using ANSYS, combining beam elements for the tower members and appropriate element types for the conductor and ground wire effects. The subspace iteration method was used to compute the first five natural frequencies and corresponding mode shapes of the structure.

The first-order natural period calculation formula was obtained by comparing the finite element results with empirical period formulas derived from measured data. The empirical formula was corrected to provide a more accurate first-order period calculation for this type of structure without considering conductor and ground wire effects.

Conductor and Ground Wire Effects

The study further considers the influence of conductors and ground wires on the dynamic characteristics of the transmission tower. Different vertical spans were analyzed to understand how the conductor tension and mass distribution affect the structural dynamics. A period magnification factor accounting for conductor and ground wire effects was introduced, and the first-order period calculation formula was corrected accordingly.

Period Formula Correction

The corrected first-order period calculation formula provides a more accurate estimate of the fundamental period for steel pipe composite large-span transmission towers. The formula accounts for the specific geometric and material characteristics of the SZK series towers and can be used for preliminary dynamic assessment and seismic/wind design.

Engineering Practice Implications

For engineers involved in the design, manufacturing, and quality control of large-span transmission towers, the findings have several practical implications:

• Dynamic design verification: The corrected period formula provides a rapid estimation tool for the fundamental period of steel pipe composite transmission towers. This is essential for seismic and wind design, as the natural period determines the dynamic amplification of inertial loads. Engineers can use the formula for preliminary design checks and to identify structures that require detailed dynamic analysis.
• Conductor tension effects: The period magnification factor introduced to account for conductor and ground wire effects highlights the importance of considering the coupled dynamics of the tower-conductor system. In practice, conductor tensions vary with temperature, ice loading, and wind conditions, which means the dynamic characteristics of the tower are not constant but vary with operating conditions.
• Steel pipe member quality: The dynamic characteristics of the tower are directly influenced by the stiffness and mass distribution of the steel pipe members. Any deviation in wall thickness, material grade, or geometric dimensions from the design specifications will alter the natural frequencies and mode shapes, potentially affecting the seismic and wind response of the structure.
• Welding quality at tower joints: Large-span transmission towers typically use bolted or welded connections between steel pipe members. The integrity of these connections directly affects the structural stiffness and, consequently, the dynamic characteristics. Poorly executed welds or improperly torqued bolts can introduce flexibility that shifts the natural frequencies, potentially into resonance with wind or seismic excitation frequencies.

Study Insights and Reflections

The introduction of a period magnification factor to account for conductor and ground wire effects is a practical engineering approach that simplifies the complex coupled dynamics problem into a correction factor applied to the structural-only period. This approach is consistent with the engineering practice of using equivalent static analysis for structures where dynamic effects can be captured through period-based corrections.

The use of the subspace iteration method for modal analysis is appropriate for large-scale structural models, as it efficiently computes the lower-order modes that dominate the structural response. For transmission tower structures, the first few modes typically account for the majority of the dynamic response energy, making lower-order modal analysis sufficient for most design purposes.

The study's focus on the SZK series of towers suggests that the findings are specific to this design series, but the methodology and correction approach can be adapted to other tower configurations. The key insight is that the conductor and ground wire effects, while often neglected in preliminary design, can significantly influence the dynamic characteristics of large-span transmission towers and should be considered in the design process.

From a quality control perspective, the dynamic characteristics of a completed tower can be verified through ambient vibration testing or impact testing. Comparing the measured natural frequencies with the predicted values from the corrected period formula provides a practical method for verifying that the tower was constructed in accordance with the design specifications. Significant deviations between predicted and measured frequencies may indicate construction defects, material non-conformance, or connection quality issues that require investigation.