Finite Element Analysis of Stress Redistribution in Round-End Steel Tube Concrete Short Columns Under Variable Loading Conditions

Literature Overview

This study by Li Bing, Xie Jin, Yang Yongsheng, and Tong Zhou (2021), published in the Journal of Shenyang Jianzhu University (Natural Science Edition), investigates the mechanical behavior of round-end steel tube concrete (CFT) short columns under split loading conditions. The research is supported by the National Natural Science Foundation of China (Grant No. 52027811), the Liaoning Provincial Key R&D Program (2019JH8/10100099), and the Shenyang Science and Technology Plan (20-206-4-13). The work employs ABAQUS finite element analysis to explore how different loading configurations and loading pad geometries affect the load-bearing capacity and stress distribution within these special-shaped columns.

Core Technical Content

The round-end steel tube concrete column is a composite cross-section that can be conceptually decomposed into two circular concrete columns combined with a rectangular concrete column. This geometric complexity introduces non-uniform stress distribution, particularly under axial compression, which is the central concern of this study.

Loading Configurations Analyzed

The researchers examined two distinct loading scenarios:

Single-pad loading: Individual pads (square, rectangular, or elliptical) placed at the geometric center of the entire cross-section
Dual-pad loading: Two pads (both square or both circular) placed separately at the centers of the two semicircular ends

Key Findings

Loading Configuration	Pad Shape	Relative Load Capacity	Stress Uniformity
Single pad at section center	Square	Baseline	Moderate non-uniformity
Single pad at section center	Rectangular	Higher than square	Improved
Single pad at section center	Elliptical	Highest among single-pad	Best conformity to tube shape
Dual pads at semicircle centers	Both square	Lower than optimal	Poor at inner rectangular zone
Dual pads at semicircle centers	Both circular	Highest overall	Best stress distribution
With I-section steel reinforcement	Various	Significantly enhanced	Markedly improved

Stress Redistribution Mechanism

The fundamental issue with round-end CFT columns is the inherent geometric discontinuity at the transition between the semicircular ends and the rectangular midsection. Under axial compression, the load transfer path is not straightforward, leading to stress concentrations at the internal corners where the semicircles meet the rectangular zone. The finite element results demonstrate that when the loading pads are positioned at the centers of the semicircular ends (dual-pad configuration), the load is introduced more directly into the primary load-bearing paths, resulting in higher overall capacity.

The shape conformity principle emerges clearly: the closer the loading pad geometry matches the external steel tube profile at the loading point, the more effectively the load is transferred into the composite section. This is consistent with the fundamental principle that loading should be applied as uniformly as possible across the cross-section to minimize bending moments and secondary stresses.

Engineering Practice Implications

Design Recommendations

Loading pad selection: For round-end CFT columns, dual circular pads placed at the semicircular centers should be the preferred loading configuration, as they provide the highest load capacity and most uniform stress distribution.
Shape matching principle: Loading pads should be designed to conform to the local geometry of the steel tube at the bearing surface. Elliptical pads for single-pad loading at the section center provide better performance than square pads due to their superior geometric compatibility.
Internal steel reinforcement: Incorporating I-section steel (wide-flange sections) within the concrete core significantly enhances load-bearing capacity by providing additional load paths and improving stress uniformity across the cross-section.

Fabrication and Welding Considerations

From a steel pipe manufacturing perspective, the round-end geometry requires careful attention during fabrication:

The transition welds between the semicircular tube ends and the rectangular midsection are critical quality zones
Full-penetration butt welds with complete backing are essential at these geometric transitions
Post-weld heat treatment may be required to relieve residual stresses at the internal corner regions
Non-destructive testing (UT or RT) of transition welds is mandatory given the high stress concentrations

Connection with Composite Column Design Standards

The findings align with provisions in GB 50936-2014 (Code for Design of Concrete-Filled Steel Tubular Structures) and the general philosophy of composite column design, which emphasizes that the interaction between steel and concrete components must be optimized through proper detailing. The study reinforces the importance of load introduction geometry in composite column design, a factor that is sometimes overlooked in practice.

Key Reflections and Technical Insights

The most significant insight from this study is the demonstration that load introduction geometry is not merely a construction convenience issue but a fundamental design parameter that directly affects structural capacity. In engineering practice, loading pads are often designed based on available equipment or standard dimensions rather than optimal geometric matching with the column section.

The concept of stress migration (应力迁移) is particularly relevant to quality control in composite column fabrication. During construction, the steel tube is first erected and then filled with concrete. If the initial loading conditions during construction differ from the design loading conditions, residual stresses may develop in the steel tube that affect the ultimate performance. This connects directly to the topic of initial stress in steel tubes (as discussed in Topic 5 of this batch), suggesting that construction sequencing should be carefully planned to minimize unfavorable initial stress states.

The finite element modeling approach adopted here—decomposing the complex cross-section into simpler geometric elements—is a practical methodology that can be extended to other special-shaped composite columns. However, engineers should be aware that the accuracy of such models depends heavily on the proper representation of the steel-concrete interface behavior, including bond-slip and local buckling of the steel tube.

This research provides valuable guidance for the design of special-shaped CFT columns used in architectural structures where aesthetic requirements dictate non-standard geometries. The recommendation to use internal steel reinforcement for capacity enhancement is practically feasible and should be considered in preliminary design stages rather than as a retrofit solution.