Research Progress on Square Steel Tube Concrete Structures

Literature Overview

The paper by Qu Zhanmei and Zhou Lishan, published in China Building Materials Technology (2005, Vol. 14, No. 4, pp. 7–12), provides a comprehensive review of research on square steel tube concrete (SSTC) members in China over several decades. The authors, representing Hengshui Municipal Engineering Office and Tsinghua University respectively, survey the state of knowledge on compressive bearing capacity, seismic performance, and fire resistance of SSTC components. This review is significant because square steel tubes, while less common than circular ones in tube concrete applications, offer distinct advantages in terms of space efficiency, architectural flexibility, and ease of connection to other structural elements.

Compressive Bearing Capacity of Square Steel Tube Concrete

The compressive behavior of SSTC members is governed by the interaction between the confined concrete core and the steel tube. Unlike circular tubes, where the confining pressure is uniformly distributed, square tubes exhibit non-uniform confining stress with higher confinement at the corners and lower confinement at the mid-span of each face. This geometric effect reduces the overall confinement efficiency compared to circular sections of equivalent cross-sectional area.

Key findings from the reviewed literature include:

• The ultimate load of SSTC columns exceeds the sum of the individual capacities of the steel tube and concrete core, with an enhancement ratio typically ranging from 1.2 to 1.6 depending on the confinement ratio.
• The confinement ratio, defined as the ratio of the steel tube cross-sectional area to the concrete cross-sectional area, is the primary geometric parameter influencing strength enhancement.
• The aspect ratio (height-to-width ratio) significantly affects the failure mode, with slender members exhibiting buckling-dominated failure and stocky members showing concrete crushing failure.

From a materials engineering perspective, the concrete mix design for SSTC is critical. High-strength concrete with compressive strength above 50 MPa is commonly used, but the workability of such concrete must be sufficient to fill the interior of the square tube without voids. Pumping concrete with appropriate admixtures (superplasticizers, viscosity-modifying agents) is essential. The steel tube material should conform to GB/T 6728 or GB/T 3094 for square hollow sections, with yield strength typically in the range of 235–355 MPa.

Seismic Performance

The seismic performance of SSTC members is a critical design consideration, especially for structures in seismic zones. The reviewed studies indicate that SSTC columns exhibit good ductility and energy dissipation capacity, attributed to the confining effect of the steel tube on the concrete core. Under cyclic loading, the steel tube restrains the concrete from spalling, maintaining axial load-carrying capacity even after significant lateral deformation.

However, square tubes present unique challenges under seismic loading. The corner regions, where the confining pressure is highest, may experience local buckling under compressive stress. The mid-face regions, with lower confinement, may develop outward bulging that can lead to premature local buckling of the steel tube. To mitigate these effects, internal concrete reinforcement (such as spirals or cross-ties) is sometimes used, although this complicates construction.

The hysteresis loops of SSTC members under cyclic loading typically show a stable, full shape with good energy dissipation. The equivalent viscous damping coefficient is generally in the range of 0.08–0.12, which is adequate for seismic design purposes. The ductility factor, defined as the ratio of ultimate displacement to yield displacement, typically ranges from 3 to 6 for well-designed SSTC columns.

Fire Resistance

Fire resistance is another important aspect addressed in the review. Steel tube concrete members inherently possess better fire resistance than unprotected steel members because the concrete core provides thermal insulation to the steel tube. However, under fire conditions, the steel tube may lose strength at temperatures above 500°C, and the concrete core may spall due to thermal gradients and steam pressure buildup.

The review summarizes several strategies for improving fire resistance of SSTC members:

• Increasing the concrete cover thickness (the distance from the steel tube inner surface to the fire-exposed surface, though in SSTC the concrete is inside the tube, so the fire exposure is on the tube exterior).
• Applying fire-resistant coatings or boards to the exterior of the steel tube.
• Using steel tubes with thicker walls to provide greater thermal mass.
• Incorporating fire-resistant concrete mixes with lower thermal conductivity.

Welding and Fabrication Considerations

The fabrication of SSTC members involves significant welding activity. The steel tubes are cut to length, and the concrete is pumped into the tube through access openings that are subsequently welded shut. The quality of these closure welds is critical, as they must be leak-tight to prevent concrete leakage during pouring and must be structurally sound to maintain the integrity of the composite section.

Welding procedures for SSTC fabrication should conform to relevant standards such as GB/T 985 (welding groove preparation) and NB/T 47014 (welding procedure qualification). The weld type is typically a fillet weld or a full-penetration groove weld depending on the wall thickness and structural requirements. Preheating may be required for high-carbon or high-strength steels to prevent cold cracking, with preheat temperatures of 100–200°C for steels with carbon equivalent (CE) above 0.4%.

Study Insights and Engineering Practice

This review paper serves as an excellent reference for engineers entering the field of steel tube concrete design. The key takeaway is that square tubes, while slightly less efficient in confinement than circular tubes, offer practical advantages in terms of architectural integration, connection simplicity, and ease of formwork. The choice between circular and square tubes should be made based on a holistic assessment of structural performance, constructability, and aesthetic requirements. For engineers working on high-rise buildings or bridges in seismic zones, SSTC members represent a viable and efficient structural solution that warrants further research into optimization of confinement geometry and fire protection strategies.