Full-Scale Testing of Q690 Steel Tube Concrete Transmission Line Poles

Literature Overview

This paper by Zhang Bin, Guo Yonghua, and Sun Qing, published in the Journal of Henan Institute of Urban Construction (2021, Vol. 30, No. 4), presents the results of full-scale load testing on a Q690 steel tube concrete (STC) transmission line pole. The Z1 pole, with a total height of 30.6 meters, was subjected to simulated wind loading at a 90-degree angle. The study combines strain measurements, fracture surface analysis, and engineering evaluation to assess the feasibility of using high-strength Q690 steel in STC transmission line pole applications.

Test Configuration and Loading Conditions

The full-scale pole test represents a significant engineering investment and provides data that cannot be obtained from scaled-down models. The 30.6-meter height and the use of actual Q690 steel tubes make this one of the most comprehensive field-representative tests of its kind.

Test Parameters

Core Findings and Technical Analysis

Strain Distribution Analysis

The strain measurements revealed important stress concentration patterns:

• Ribs and flange junctions: High stress concentrations were observed at the intersection of stiffening ribs and flange plates. This is consistent with the well-known stress concentration effects at geometric discontinuities, where the abrupt change in cross-section creates localized stress amplification.
• Flange root: Relatively lower stresses were measured at the root of the flange plates, suggesting that the load path through the pole structure distributes stresses away from this critical location more effectively than anticipated.
• Bolt behavior: The bolt strain data provides insight into the load-sharing mechanism between the bolted connections and the welded structure, which is critical for understanding the overall structural behavior.

Fracture Surface Analysis

The SEM examination of the outer steel tube fracture surfaces revealed a critical failure mechanism:

• Layer tearing (lamellar tearing): The fracture analysis identified that deformation in the thickness direction of the steel plate, when constrained by the concrete core, could lead to layer tearing. This is a well-documented failure mode in thick steel sections, particularly those with poor through-thickness ductility.
• Thickness-direction strain: The concrete confinement effect restricts lateral expansion of the steel tube, forcing the material to deform primarily in the thickness direction. When the through-thickness ductility is insufficient, microvoids initiate and coalesce into cracks parallel to the plate surface.

Engineering Practice Implications

Q690 Steel Selection Considerations

The use of Q690 steel in transmission line poles offers significant advantages in terms of weight reduction and cost savings compared to conventional Q345 or Q420 grades. However, the test results highlight several critical considerations:

1. Through-thickness quality: Q690 steel plates must be procured with adequate through-thickness ductility (Z-direction properties). Standards such as GB/T 5313 or ASTM A770 should specify Z-grade requirements for plates used in this application.
2. Welding procedures: The higher strength of Q690 steel necessitates careful control of preheat temperatures, interpass temperatures, and post-weld heat treatment to prevent cold cracking and maintain toughness in the heat-affected zone (HAZ).
3. Stress concentration mitigation: The rib-flange junction requires detailed design attention, including generous fillet radii, optimized rib geometry, and potentially additional stiffening elements.

Layer Tearing Prevention Measures

Key Questions and Reflections

The test results raise several important questions for future research and engineering practice. First, the layer tearing mechanism identified in the outer steel tube suggests that the concrete-steel interface interaction under bending loads is more complex than currently modeled in design codes. The confinement effect that benefits axial compression performance may become detrimental under bending, particularly when the steel tube is forced to deform in the thickness direction.

Second, the stress concentration at rib-flange junctions indicates that current design practices for STC transmission line poles may underestimate local stresses in these regions. Finite element analysis with detailed modeling of these junctions, followed by comparison with strain gauge measurements, would provide valuable validation data for design methodology improvements.

Third, the recommendation to pilot-apply Q690 STC poles in transmission line projects is well-founded, but the transition from pilot to widespread application requires additional work on long-term performance, including fatigue behavior under cyclic wind loading, corrosion resistance of the high-strength steel in outdoor environments, and maintenance requirements.

Study Insights and Implications

This full-scale test provides irreplaceable real-world data for the advancement of high-strength steel tube concrete technology in the power transmission sector. The identification of layer tearing as a potential failure mechanism is particularly valuable, as it highlights a failure mode that might be overlooked in conventional design approaches focused on bending capacity and stability. For engineers involved in transmission line pole design, the key lessons are: (1) always consider through-thickness properties when specifying high-strength steel plates, (2) pay special attention to geometric discontinuities in the pole structure, and (3) validate design assumptions through full-scale or large-scale testing whenever possible. The cost savings potential of Q690 STC poles, combined with the demonstrated structural adequacy, makes this a promising technology for future transmission line infrastructure projects.