Seismic Performance of Composite Steel Tube Concrete Columns with Different Configurations

Literature Overview

This paper by Zhao Lidong and colleagues from Beijing University of Technology, published in the Journal of Harbin Institute of Technology in 2019, presents full-scale low-cycle reversed loading tests on six specimens representing three cross-sectional configurations and two shear spans of composite steel tube concrete (CSTC) columns. The study was funded by the National Natural Science Foundation of China (Grant 51878015) and conducted under the Key Laboratory of Urban and Engineering Safety and Disaster Reduction of the Ministry of Education. The investigation addresses a critical gap in structural engineering: how different internal steel tube arrangements and compartmentalized welded steel plate configurations influence the seismic behavior of composite columns under cyclic lateral loading.

Core Technical Findings

Specimen Configuration and Test Parameters

The experimental program involved six full-scale specimens subjected to identical axial loads with varying shear span ratios (2.2 and 3.0) and three distinct cross-sectional steel configurations: conventional circular steel tube, outer-circular-inner-circular composite steel tube, and composite steel tube with compartmentalized welded steel plates. The primary response parameters evaluated include failure modes, hysteresis characteristics, load-bearing capacity, stiffness degradation, ductility, and energy dissipation.

Key Seismic Performance Results

The most significant finding is that all specimens except the shear span ratio 2.2 specimen without internal compartmentalization exhibited full and stable hysteresis curves, indicating reliable seismic performance. This observation has direct implications for seismic design of composite columns in high-seismicity zones. The study demonstrates that increasing the section steel ratio or raising the shear span ratio produces fuller hysteresis curves at equivalent displacement angles, meaning the member absorbs more energy before reaching a given deformation level.

The introduction of an inner circular steel tube or compartmentalized welded steel plates substantially improves load-bearing capacity. More importantly, these configurations significantly enhance ductility for specimens with smaller shear span ratios, which is particularly relevant for short columns in building frames where shear demand is high. Lowering the shear span ratio markedly increases initial stiffness but accelerates stiffness degradation and amplifies cumulative damage effects. At ultimate displacement angles, specimens with internal compartmentalization and smaller shear spans exhibited higher cumulative energy dissipation.

Welding and Fabrication Considerations

From a fabrication and quality control perspective, the compartmentalized welded steel plate configuration deserves particular attention. The welded steel plates that create internal compartments introduce additional weld joints within the column cross-section. These internal welds must be designed and executed to meet stringent quality requirements because they serve as critical stress-transfer interfaces between the outer steel tube and the internal concrete core.

Based on my experience with pipe fitting welding processes, the following quality control points are essential for such configurations:

• Weld joint design: Internal compartmentalization welds should follow fillet weld or full-penetration butt weld configurations per GB/T 985.1 or ISO 2553, with weld sizes determined by the thickness of the steel plates and the expected stress transfer requirements.
• Welding process selection: For internal steel plates, GTAW (tungsten inert gas welding) or FCAW (flux-cored arc welding) processes are typically preferred due to accessibility constraints within the confined column interior. PAW (plasma arc welding) may also be considered for thin steel plates where high penetration rates are needed.
• Non-destructive testing: All internal welds must undergo UT (ultrasonic testing) or MT (magnetic particle testing) per GB/T 11345 or ASTM E1650, with acceptance criteria referencing GB/T 3323 or ASME V Section 9.
• Residual stress management: The welding sequence for compartmentalization plates must be carefully planned to minimize distortion and residual stress concentration, which could compromise the composite action between steel and concrete during seismic loading.

The welding sequence should follow a symmetric pattern to minimize angular distortion. For multi-compartment configurations, welding should proceed from the center outward, with intermediate stress-relief operations (such as low-temperature hammering or controlled cooling) between passes. The heat input per pass should be limited to prevent excessive HAZ softening, particularly in high-strength steel tubes where the HAZ may experience reduced yield strength if heat input exceeds 25 kJ/mm.

Standards and Code Relevance

The practical load-bearing capacity calculation method employed in this study yielded results that are conservative relative to experimental values. This conservatism is consistent with general structural engineering practice but may lead to over-design in seismic applications where material efficiency is important. The findings support the continued development of more refined analytical models that can capture the composite interaction between steel tubes and concrete cores under cyclic loading.

Relevant standards for this type of composite column design include GB 50011 (Seismic Design Code for Buildings), GB 51248 (Technical Specification for Concrete-Filled Steel Tubular Structures), and GB/T 14976 (Seamless Steel Tubes for Structural Use). The steel tube materials typically conform to GB/T 8162 or ASTM A500, with concrete grades ranging from C40 to C80 depending on the structural requirements.

Engineering Practice Implications

For structural engineers designing composite columns in seismic zones, the following design recommendations emerge from this research:

1. Internal steel tube configuration: Adding an inner circular steel tube provides a significant capacity boost and improves ductility, particularly for short columns. This is analogous to the concept of "nested tubes" in pipe design, where concentric configurations provide progressive collapse resistance.
2. Compartmentalized steel plates: The use of welded steel plates to create internal compartments offers a practical alternative to full inner tubes, reducing material cost while maintaining seismic performance. However, the weld quality must be rigorously controlled.
3. Shear span ratio optimization: A shear span ratio of 3.0 generally provides better ductility and energy dissipation than 2.2, but designers must balance this against architectural and structural layout constraints. For shear-critical members, the internal compartmentalization becomes even more critical.
4. Cumulative damage consideration: The accelerated stiffness degradation observed in low shear span ratio specimens without internal compartments suggests that such configurations may not be suitable for structures subject to frequent seismic events (e.g., aftershock sequences). The cumulative damage effect must be accounted for in performance-based seismic design.

Study Insights and Reflections

This research contributes meaningfully to the understanding of how internal steel configurations influence seismic behavior. From a materials science perspective, the key mechanism is the enhanced confinement effect provided by the internal steel elements, which delays concrete crushing and maintains composite action at large deformations. The compartmentalized steel plates function similarly to transverse reinforcement in reinforced concrete columns, but with the advantage of being integral to the steel tube system.

The conservative nature of the existing capacity calculation method highlights the need for further refinement of analytical models. Future research should incorporate more sophisticated constitutive models that capture the progressive degradation of the steel-concrete interface bond under cyclic loading, as well as the effect of welding-induced residual stresses on the overall member behavior. For engineering practice, the findings strongly support the use of internal steel configurations in seismic design, provided that welding quality is maintained to the highest standards.