Eccentric Compression Bearing Capacity of Multi-Chamber Polygonal Steel Tube Concrete Columns

Literature Overview

This paper by Xu Lihua et al. (2019), published in Engineering Mechanics, investigates the eccentric compression behavior of multi-chamber polygonal steel tube concrete (CFT) columns, using the giant columns of the Tianjin Gaoyin 117 Building as the engineering prototype. The authors designed and tested 11 scaled specimens at a 1/20 scale and conducted extensive finite element analysis (FEA) using ABAQUS to examine the influence of steel tube wall thickness, slenderness ratio, eccentricity ratio, and concrete strength on the ultimate bearing capacity. As a welding and metallurgy specialist, I find the multi-chamber polygonal geometry particularly interesting from a fabrication and welding perspective, as it involves complex intersection welds and multi-wall tube configurations that present unique challenges in pipe fitting manufacturing.

Specimen Design and Experimental Setup

The test specimens were designed based on a 1/20 scale reduction of the actual giant columns in the Tianjin Gaoyin 117 Building. The multi-chamber polygonal geometry consists of multiple interconnected steel tubes forming a polygonal cross-section with internal chambers. This configuration is essentially a complex welded assembly where multiple steel tubes are joined at various angles, creating a lattice-like internal structure within the polygonal envelope.

Welding Considerations in Multi-Chamber Polygonal Construction

From a pipe fitting fabrication standpoint, the multi-chamber polygonal column represents a significant welding challenge. The geometry requires:

• Multi-angle intersection welds: Steel tubes are joined at non-standard angles, requiring careful weld design and procedure qualification.
• Internal chamber connections: The internal steel tube connections within the polygonal envelope create confined welding spaces that may require specialized access techniques.
• Thermal management: The sequential welding of multiple tubes in close proximity can lead to cumulative distortion and residual stress accumulation, which directly impacts the structural performance under eccentric loading.

The fabrication of such columns typically involves SMAW (Shielded Metal Arc Welding) or GTAW (Gas Tungsten Arc Welding) for the primary structural welds, with careful control of interpass temperature and welding sequence to minimize residual stress. The weld metal selection must be compatible with the base steel grade, and the HAZ microstructure must be controlled to ensure adequate toughness, particularly in the confined internal chambers where cooling rates may be accelerated.

Experimental Results and Failure Modes

The eccentric compression tests revealed that the multi-chamber polygonal CFT columns primarily failed through bending-type instability. This failure mode is consistent with the behavior of slender columns under eccentric loading, where the combination of axial compression and bending moment leads to lateral-torsional buckling or flexural buckling.

The load-lateral deflection curves exhibited a characteristic three-stage behavior: an initial elastic stage, a post-buckling stage with increasing deflection, and a final failure stage. The load-strain curves demonstrated that the concrete strength and steel tube wall thickness have a positive effect on the ultimate bearing capacity, while the slenderness ratio and eccentricity ratio have a negative effect.

Influence of Internal Reinforcement Cage

A significant finding is that the internal reinforcement cage within the chambers can significantly improve the ductility and post-peak bearing capacity of the columns. This is consistent with the well-known confinement effect of reinforcement on concrete under compression, but in this case, the reinforcement cage also contributes to the overall structural integrity of the multi-chamber configuration. The reinforcement cage effectively bridges the internal chambers, enhancing the load transfer between adjacent steel tubes and providing additional resistance to local buckling of the steel tube walls.

Finite Element Analysis and Capacity Formulas

The ABAQUS-based parametric study systematically examined the influence of key design parameters on the ultimate bearing capacity. The results were used to develop bearing capacity calculation formulas applicable to hexagonal six-chamber and pentagonal four-chamber configurations. These formulas are valuable for practical engineering design, as they provide a direct means of predicting the capacity of non-circular, multi-chamber CFT columns without the need for full-scale testing.

The FEA results confirmed the experimental trends and provided additional insights into the internal stress distribution within the columns. The stress concentration at the internal tube intersections is a critical factor that influences the overall structural performance, and the FEA model is capable of capturing these localized effects that are difficult to measure experimentally.

Engineering Practice Implications

For engineers involved in the design of multi-chamber polygonal CFT columns, several practical recommendations emerge from this study:

• The 38.6% capacity reduction when the slenderness ratio increases from 24 to 70 underscores the importance of limiting slenderness in eccentrically loaded columns. In practice, this means that the height-to-width ratio of such columns must be carefully controlled, particularly in tall building applications where the column height is dictated by architectural requirements.
• The 54.1% capacity reduction from an eccentricity ratio of 0.2 to 1.0 highlights the sensitivity of these columns to the position of the applied load. Designers must ensure that the eccentricity ratio remains within acceptable limits, and that the column cross-section is oriented to minimize the effect of the applied moment.
• The internal reinforcement cage is not merely a construction requirement but a structural necessity that significantly enhances ductility and post-peak capacity. The design of the reinforcement cage should be optimized to provide adequate confinement without interfering with the concrete placement or the welding of the internal tube connections.

Study Insights and Outlook

This study makes a valuable contribution to the understanding of multi-chamber polygonal CFT columns, which are increasingly being used in super-tall building applications where the conventional circular or square CFT columns are insufficient to meet the structural demands. The development of bearing capacity formulas for specific polygonal configurations provides a practical tool for engineers, although the applicability of these formulas to other polygonal geometries should be verified through additional testing. From a welding and fabrication perspective, the multi-chamber polygonal configuration presents unique challenges that require careful attention to weld design, welding sequence, and residual stress management. Future research should focus on the long-term behavior of these columns under cyclic loading, as well as the effects of fire and corrosion on the multi-chamber configuration, which are critical considerations for the design life of super-tall buildings.