Ultimate Bearing Capacity Analysis of Steel Pipe Lattice Columns Under Eccentric Compression

Literature Overview

The paper by Ou Zhijing and Chen Baochun from Fuzhou University, published in the Journal of Fuzhou University (Natural Science Edition) (Vol. 34, No. 4, 2006, pp. 578-582), presents experimental and analytical studies on the ultimate bearing capacity of steel pipe lattice columns under eccentric compression. Two steel pipe lattice column specimens were tested, and the test process, failure modes, and load-displacement behavior are documented. The authors compare the experimental results with predictions from the Chinese Code for Design of Steel Structures (GB 50017-2003) and identify limitations in the current design methodology.

Core Technical Points

Steel pipe lattice columns are composed of multiple steel pipe chords connected by diagonal and horizontal struts, forming a truss-like structure. The eccentric compression test subjects the column to a combined axial load and bending moment, which creates a non-uniform stress distribution across the cross-section. The failure mode is typically governed by the local buckling of the most stressed chord pipe or the yielding of the chord pipe under combined compression and bending.

The experimental results show that the GB 50017-2003 design formula provides reasonably accurate predictions of the ultimate bearing capacity. However, the authors identify a specific issue: when the slenderness ratio (λ) is small, the equivalent slenderness ratio calculation that accounts for shear deformation effects (using an additive approach) produces results that are unreasonably large, which is conceptually inconsistent with the physical behavior of the column.

Design Formula Comparison

Method	Description	Accuracy for Low λ	Accuracy for High λ
GB 50017-2003 (additive)	λ_eq² = λ_b² + λ_s²	Overestimates λ_eq	Reasonable
Proposed (magnification factor)	λ_eq = λ_b × K_s	Improved	Reasonable
Direct finite element	Full nonlinear analysis	Accurate	Accurate

The authors propose using a magnification factor method instead of the additive method for calculating the equivalent slenderness ratio when λ is small. The magnification factor K_s accounts for the shear deformation effect by multiplying the flexural slenderness ratio, which provides a more physically consistent result.

Failure Modes and Load-Displacement Behavior

The test specimens exhibited the following failure characteristics:

Local buckling of the chord pipe at the maximum moment location, typically near the mid-span or at the eccentric load application point.
Progressive yielding of the chord pipe, with the compressive fiber reaching the yield stress before the tensile fiber.
Diagonal strut yielding, which reduces the shear capacity of the lattice and accelerates the failure process.
Overall column buckling, which occurs at a higher load level and is governed by the equivalent slenderness ratio of the entire lattice column.

The load-displacement curves show an initial linear elastic stage, followed by a nonlinear stage where the chord pipe begins to yield, and finally a post-peak stage where the load capacity decreases as the chord pipe buckles locally.

Relevance to Steel Pipe Fabrication and Welding

Steel pipe lattice columns present unique challenges for steel pipe fabrication and welding. The lattice structure requires numerous pipe-to-pipe connections, each of which must be designed and fabricated to ensure adequate load transfer. The welding quality at these connections is critical for the overall structural performance.

Weld Connection Types and Quality Requirements

Connection Type	Welding Process	Inspection Method	Acceptance Criteria
Chord-to-diagonal (K-joint)	SAW or SMAW	RT + MT	No cracks, no lack of fusion
Chord-to-chord (butt)	GTAW or SMAW	RT 100%	Class II per GB/T 3323
Chord-to-end plate	SMAW or GMAW	MT + PT	No surface cracks
Gusset plate to chord	SMAW or GMAW	MT	No surface defects

The K-joint connections between the chord pipes and diagonal struts are the most critical welds in the lattice structure. These joints experience complex multiaxial stress states under eccentric loading, and the weld geometry must be designed to minimize stress concentrations. The weld throat thickness should be calculated based on the maximum stress at the joint, and the weld leg length should be sufficient to transfer the full load without yielding.

Steel Pipe Material and Dimensional Requirements

For steel pipe lattice columns, the following material and dimensional requirements are critical:

The chord pipes should be made of hot-finished seamless steel pipes (per GB/T 8163) to ensure uniform material properties and minimal weld defects.
The diagonal and horizontal struts may be made of ERW or HFW welded pipes (per GB/T 3091), provided that the weld quality is verified by RT.
The wall thickness tolerance should be ±10% of the nominal thickness, and the ovality should be limited to 1% of the diameter.
The steel grade should be selected based on the stress level and the required ductility. Q345 is commonly used for chord pipes, while Q235 may be acceptable for diagonal struts in non-critical applications.

Engineering Practice and Design Recommendations

The paper provides valuable insights for the design and fabrication of steel pipe lattice columns:

For low slenderness ratio columns (λ < 40), the magnification factor method should be used instead of the additive method for calculating the equivalent slenderness ratio.
The chord pipe diameter and wall thickness should be selected to ensure that local buckling does not occur before the overall column reaches its design capacity.
The weld connections should be designed for the maximum stress at the joint, with adequate weld throat thickness and proper weld geometry.
The fabrication tolerances for pipe straightness and dimensional accuracy should be controlled to minimize initial imperfections.
The load-displacement behavior should be verified through nonlinear finite element analysis, particularly for complex lattice geometries.

The experimental results confirm that the GB 50017-2003 design formula is generally applicable, but the proposed modification for low slenderness ratio columns improves the accuracy and physical consistency of the predictions. Engineers should be aware of this limitation when designing steel pipe lattice columns with low slenderness ratios.

Summary

This paper presents a valuable experimental and analytical study of steel pipe lattice columns under eccentric compression. The authors identify a specific limitation in the current design code (GB 50017-2003) for low slenderness ratio columns and propose a practical modification using a magnification factor method. The experimental results validate the overall design approach while highlighting the need for improved treatment of shear deformation effects. For steel pipe engineers, the key implications are: careful selection of pipe materials and dimensions for chord and diagonal members, rigorous weld quality control at all lattice joints, and awareness of the design formula limitations for low slenderness ratio columns. The proposed modification to the equivalent slenderness ratio calculation provides a more physically consistent design approach that should be considered in future code revisions.