Shear Performance of Square and Rectangular Steel Tube Concrete Members

Literature Overview

This comprehensive review article by Li Zhiqiang and Chen Yiyi from Tongji University, published in Journal of Architecture and Civil Engineering (2013, Vol. 30, No. 3, pp. 62-70), provides a systematic survey of the research status on shear behavior of square and rectangular steel tube concrete (SRC) members. Funded by the National Natural Science Foundation of China (NSFC) and the Ministry of Railways, the paper synthesizes experimental and analytical findings from multiple countries, identifies gaps in current knowledge, and proposes directions for future research. The work is particularly relevant to structural engineers designing composite bridge piers, building columns, and energy-absorbing structural elements.

Fundamental Shear Behavior of SRC Members

The shear behavior of square and rectangular steel tube concrete members is governed by three primary parameters:

• Shear span-to-depth ratio (a/d): Determines the failure mode—shear failure (a/d < 1.0), shear-bending transition (1.0 < a/d < 2.0), and bending failure (a/d > 2.0). For square and rectangular tubes, the confinement effect is anisotropic, with corner regions experiencing different confinement pressures than flat panel regions.
• Axial compression ratio (n): The ratio of axial load to the compressive strength of the concrete cross-section. Higher axial compression generally improves shear capacity up to a critical value, beyond which brittle failure becomes more likely.
• Confinement effect: The steel tube confines the concrete core, increasing its compressive strength and ductility. However, the confinement effectiveness varies across the cross-section—corner regions provide the highest confinement, while the middle of each flat panel provides the lowest.

Key Research Findings and Comparisons

The authors compared various shear capacity formulas from different countries and found significant discrepancies in predicted values, attributable to differences in:

1. Assumptions about the contribution of the steel tube to shear resistance (some formulas include a direct steel tube shear term, others do not).
2. Treatment of the concrete-steel interface friction (some models assume full bond, others assume partial slip).
3. Confinement model used (uniform confinement vs. non-uniform confinement accounting for corner effects).
4. Failure mode classification boundaries (different researchers use different a/d thresholds for defining failure modes).

Engineering Practice Implications

For structural engineers designing SRC members in practice, the following guidelines emerge from the literature:

1. Design shear span ratio: Maintain a/d ≥ 1.5 for ductile shear-bending behavior, particularly in seismic zones where energy dissipation through controlled plastic deformation is desirable.
2. Axial load management: Limit the axial compression ratio to n ≤ 0.3 for optimal shear ductility, especially in bridge pier applications where cyclic loading is expected.
3. Steel tube geometry: Prefer square tubes over rectangular tubes with high aspect ratios when shear capacity is critical, as the more uniform confinement distribution provides more predictable shear behavior.
4. Corner detailing: Ensure adequate corner radius (typically r ≥ t, where t is the steel tube wall thickness) to avoid stress concentrations that can initiate shear cracks.
5. Concrete placement: Use self-compacting concrete or vibrated concrete with fine aggregate to ensure complete filling of the steel tube, as voids significantly reduce confinement effectiveness and shear capacity.

Critical Analysis and Gaps

The review identifies several important gaps in the current research landscape:

• Unified failure mode classification: There is no internationally agreed-upon criterion for distinguishing between shear failure, shear-bending failure, and bending failure in SRC members. This leads to inconsistencies in design codes and safety assessment.
• Cyclic and fatigue behavior: Most available data are from monotonic loading tests. The effect of cyclic shear loading on the shear capacity degradation of SRC members is poorly understood, which is a critical gap for seismic design.
• Size effect: The influence of member size on shear capacity has not been adequately investigated. Scale effects in concrete structures are well documented, but the interaction between size effects and steel tube confinement in SRC members requires further study.
• Long-term behavior: The effect of sustained loading, creep, and shrinkage on the shear capacity of SRC members is largely unexplored.

Study Insights and Recommendations

This review is a valuable resource for structural engineers working with composite SRC members, particularly those involved in bridge and building design in seismic regions. The identification of the lack of unified failure criteria is particularly important, as it directly affects the reliability of design calculations and safety assessments. The recommendation to develop a unified shear capacity model that accounts for the anisotropic confinement effect of square and rectangular tubes would significantly improve the predictability and safety of SRC designs.

From a materials perspective, the confinement effect of the steel tube on the concrete core represents a synergistic interaction that cannot be fully captured by simple superposition models. The concrete, in turn, provides lateral support to the steel tube panels, preventing local buckling and enhancing the overall shear capacity. This mutual support mechanism is the fundamental reason why SRC members outperform both plain concrete and empty steel tubes in shear performance.

Future research should focus on developing finite element models validated against multi-source experimental data, incorporating realistic material constitutive laws for both the confined concrete and the steel tube. Additionally, large-scale cyclic loading tests on square and rectangular SRC members with varying shear span ratios and axial loads would provide the data necessary to develop reliable seismic design provisions.