Stiffener Arrangement and Ultimate Bearing Capacity of K-Joints in Steel Tubular Towers

Literature Overview

The 2011 paper by Hu Xiaoguang, Yang Jingbo, Li Maohua, Zhu Binrong, and Gao Yuan, published in Science & Technology Review, investigates the K-joint design for UHV (Ultra-High Voltage) transmission line steel tubular towers. The study addresses a critical engineering challenge: the K-joints in wineglass-shaped UHV towers have brace-to-chord angles θ less than 30°, which falls outside the applicability range of existing design codes. The authors propose novel stiffener arrangements and validate their effectiveness through ANSYS finite element analysis.

Core Technical Content

The Design Challenge

In UHV transmission towers, the wineglass configuration creates K-joints where the brace tubes intersect the chord tubes at acute angles. The existing Chinese code (GB 50017) and international standards (CIDECT, Eurocode 3) for K-joint capacity calculations are valid only for θ ≥ 30°. For UHV towers with θ < 30°, the following issues arise:

Issue	Description	Consequence
Stress concentration	Acute angles create severe stress gradients at the brace-to-chord intersection	Premature fatigue failure
Local buckling	Reduced overlap ratio between braces at acute angles	Loss of load-carrying capacity
Chord wall yielding	High local compressive stress at the intersection	Local deformation and capacity reduction
Code inapplicability	θ < 30° outside validated range	No reliable design method available

Three Novel Stiffener Arrangements

The authors designed and analyzed three stiffener configurations:

External ring stiffeners — circular plates welded to the exterior of the chord tube at the K-joint intersection
Internal saddle stiffeners — plate stiffeners placed inside the chord tube at the brace intersection
External sector stiffeners — arc-shaped plates welded externally at the critical stress concentration zones

Finite Element Analysis Results

The ANSYS nonlinear analysis revealed the following:

Without stiffeners, the maximum stress at θ < 30° K-joints can exceed the material yield strength by 40-60%
Stiffener arrangements significantly redistribute stress, reducing maximum stress by 30-50%
The external sector stiffener provides the most uniform stress distribution improvement
All three stiffener types eliminate stress concentrations at the brace-to-chord intersection

Interpretation of Technical Points

Welding Implications for Stiffener Attachment

The stiffener arrangements require specific welding considerations:

Weld Type	Location	Quality Requirement	Inspection Method
Fillet weld	External ring stiffener to chord	Full penetration equivalent	MT + UT per GB/T 11345
Groove weld	Internal saddle stiffener	Full penetration	RT per GB/T 3323
Fillet weld	External sector stiffener	Complete fusion	MT + visual inspection
Seam weld	Stiffener to brace tube	Continuous weld	MT per GB/T 26956

The welding of stiffeners to steel tubes presents unique challenges:

Heat input control — Excessive heat input can cause local distortion of the chord tube wall, particularly for thin-walled tubes common in transmission towers
Residual stress management — Stiffener welds introduce additional residual stresses that interact with the existing tube forming and seam weld stresses
Fit-up tolerance — The gap between stiffener plate and curved tube surface must be controlled within ±1 mm for reliable weld quality
Pre-heat and interpass temperature — For higher-strength steels (Q345, Q390), pre-heat of 60-80°C is recommended to prevent cold cracking

Material Selection for UHV Tower Components

UHV transmission towers require steel materials with specific properties:

Chord tubes: Typically Q235B or Q345B, ERW or SAW welded, with minimum Charpy V-notch impact energy of 27 J at -20°C
Brace tubes: Same material grade as chords, with dimensional tolerances per GB/T 6728
Stiffener plates: Rolled steel plates per GB/T 709, with thickness matched to the chord tube wall thickness
Welding consumables: E43 series for Q235, E50 series for Q345, with guaranteed impact toughness

Connection with Engineering Practice

The practical implications of this research extend to several areas of steel tower engineering:

Fabrication Considerations

Stiffener welding should be performed before the tube-to-tube connection welds to minimize distortion accumulation
Backing bars may be required for internal stiffener groove welds to achieve full penetration
Post-weld heat treatment (PWHT) may be necessary for stiffener welds to relieve residual stresses, particularly for towers in high-stress regions

Quality Control Procedures

Based on FMEA analysis of potential failure modes:

Failure Mode	Likelihood	Severity	Detection	Risk Priority	Countermeasure
Incomplete stiffener weld fusion	Medium	High	UT/MT	High	Increase weld current, verify fit-up
Stress concentration at stiffener edge	Medium	High	FEA verification	High	Add fillet radii, smooth transitions
Chord tube local buckling	Low	Critical	Visual + dimensional	Medium	Verify stiffener stiffness, limit wall thickness
Fatigue crack initiation at weld toe	Medium	High	MT periodic inspection	High	Grind weld toes, apply anti-fatigue details

Design Code Recommendations

The authors recommend continuing to use existing code provisions for the basic K-joint capacity calculation while supplementing with stiffener design to address the θ < 30° limitation. This pragmatic approach acknowledges the gap in design methodology while providing a practical engineering solution.

Key Questions and Reflections

A significant question remains about the long-term fatigue performance of stiffened K-joints under the cyclic loading conditions typical of transmission towers (wind-induced vibration, galloping, and ice loading). The static ultimate capacity improvement demonstrated by stiffeners may not translate directly to fatigue life improvement, as fatigue cracks typically initiate at weld toes regardless of the overall structural capacity.

Another reflection concerns the constructability of internal stiffeners in large-diameter chord tubes. For UHV towers with chord tube diameters exceeding 500 mm, accessing the interior for welding and inspection becomes challenging. External stiffener arrangements may be preferred for constructability, even if internal stiffeners offer marginally better stress distribution.

Study Insights and Implications

This research provides a practical solution to a well-recognized gap in steel tubular tower design methodology. The stiffener approach is economically viable because it uses simple plate elements with standard welding procedures, avoiding the need for complex geometric modifications to the tube connections. For steel tube manufacturers and fabricators, the key implication is that UHV tower projects will increasingly require pre-fabricated stiffener assemblies that must be welded to tubes in the fabrication shop with controlled quality. The finite element validation approach demonstrated in this paper should become standard practice for novel joint designs, providing engineering justification where code provisions are insufficient.