Eccentric Compression Performance of Steel Tube Concrete Members with Internal Lattice-Type Steel Bones

Literature Overview

Chen Qingsheng, Li Bofan, Kong Long, Rong Lu, Hou Xinyu, and Wang Xiantie from State Grid Gansu Economic and Technological Research Institute and Xi'an University of Architecture and Technology present experimental and numerical investigations into the eccentric compression behavior of steel tube concrete (STC) members reinforced with internal lattice-type steel bones. Published in Building Structure (Vol. 52, No. 13, 2022, pp. 77-85) and supported by the National Natural Science Foundation of China (Grant No. 51678474), this study examines four groups of eight specimens with varying internal reinforcement configurations.

Experimental Program and Specimen Configuration

The experimental program included four groups of specimens, each with two replicates, resulting in eight total specimens. The groups varied in their internal reinforcement configuration:

All specimens were subjected to eccentric compression loading, with the eccentricity ratio varied to study the effect of load eccentricity on structural behavior.

Experimental Results

The failure mode for all specimens was consistent: overall bending failure with local outward bulging of the steel tube on the compression side. The presence of internal reinforcement significantly influenced the failure characteristics:

• Longitudinal stiffeners reduced local bulging of the steel tube, providing additional lateral confinement.
• The lattice-type steel bone contributed substantially to load capacity by directly bearing compressive forces and distributing stresses more uniformly.

The combined reinforcement configuration yielded the highest improvement, suggesting a synergistic effect between the two reinforcement types. The longitudinal stiffeners provide local confinement and reduce tube wall buckling, while the lattice steel bone provides global load-bearing capacity and improves stress distribution.

Numerical Simulation and Parametric Analysis

Finite element analysis was conducted to investigate the stress development and load-bearing mechanisms within the members. The parametric study examined the following variables:

• Concrete compressive strength
• Steel tube yield strength
• Steel tube diameter-to-thickness ratio
• Steel bone yield strength
• Steel bone dimensions
• Load eccentricity ratio

Key findings from the parametric study:

1. Increasing concrete strength improved load capacity but had diminishing returns at higher grades.
2. Increasing steel tube strength had a more pronounced effect on load capacity than increasing concrete strength.
3. A lower diameter-to-thickness ratio improved local buckling resistance but increased material usage.
4. Higher steel bone strength and larger steel bone dimensions both contributed positively to load capacity.
5. Increasing eccentricity reduced load capacity and shifted the neutral axis, increasing the demand on the tension-side steel tube.

Proposed Load Capacity Formula

The authors propose a simplified calculation formula for the eccentric compression load capacity of STC members with internal lattice-type steel bones. The formula incorporates contributions from the concrete core, steel tube, longitudinal stiffeners, and lattice steel bone, with appropriate reduction factors for eccentric loading. The calculated results showed good agreement with experimental data.

Welding and Fabrication Considerations

The fabrication of STC members with internal lattice-type steel bones involves several critical welding operations:

1. Internal lattice-to-tube welds: The lattice steel bone must be welded to the inner surface of the steel tube, typically through pre-cut access holes or using internal welding techniques. These welds should be full-penetration groove welds with thorough root preparation.
2. Stiffener-to-tube welds: Longitudinal stiffeners welded to the inner tube wall should use fillet welds of adequate size, with weld metal matched to the base steel.
3. Diaphragm welds: Transverse diaphragms, if present, should be welded to the lattice steel bone and tube wall to ensure composite action.
4. Access hole repair: Any access holes used for internal welding must be sealed with plug welds or cap plates welded to the tube exterior.
5. NDT requirements: All internal welds should be inspected using ultrasonic testing (UT) or magnetic particle testing (MT) where accessible, with acceptance criteria per GB/T 11345 or ISO 17635.

Study Reflection

This research demonstrates the significant potential of internal lattice-type steel bones to enhance the structural performance of steel tube concrete members. The 14.33% improvement in ultimate load capacity for the combined reinforcement configuration is substantial and could translate into meaningful material savings or increased span capabilities in practical applications. However, the fabrication complexity associated with internal lattice reinforcement is non-trivial. Internal welding operations are inherently more difficult to inspect and control than external welds, and the quality of these welds is critical to achieving the predicted structural performance. In my experience, the success of composite reinforcement systems depends as much on fabrication quality as on material selection. Engineers should develop detailed welding procedure specifications (WPS) for internal welds, conduct welder qualification tests under conditions representative of actual fabrication, and implement rigorous NDT protocols to ensure weld integrity. The proposed load capacity formula provides a useful design tool, but its accuracy is contingent upon the assumption of full composite action between the internal reinforcement and the steel tube, which in turn depends on weld quality.