Shear Performance of Rectangular Steel Tube Concrete Members Without End Plates

Literature Overview

This paper by Shi Yanli, Zhou Xuhong, Xian Wei, and Wang Wenda (2018, Engineering Mechanics, Vol. 35, No. 12, pp. 25-33) presents an experimental and numerical investigation into the fundamental shear behavior of rectangular steel tube concrete (SRC) members without end plates. The study was funded by the National Natural Science Foundation of China (Grants 51268035 and 51768038) and the Gansu Provincial Higher Education Collaborative Innovation Team Program (2018C-08). Eight full-scale specimens were tested under basic shear conditions, examining the influence of cross-sectional dimensions and shear span ratio (0.3 to 0.8) on shear capacity and the slip behavior between the steel tube and the core concrete. The authors further validated their experimental findings using ABAQUS finite element analysis and investigated the effects of friction-bonding between the steel tube and concrete, the presence or absence of end plates, and the loading principal axis direction.

Core Technical Findings

The experimental program revealed several critical insights into the shear mechanics of end-plate-free rectangular SRC members. At a shear span ratio of 0.3, specimens exhibited pure shear failure characterized by diagonal cracking of the concrete core and localized yielding of the steel tube walls. As the shear span ratio increased beyond 0.3, the failure mode progressively transitioned from shear-dominated to bending-dominated failure. This transition is consistent with classical beam theory but is particularly significant for rectangular cross-sections where the shear stress distribution is non-uniform compared to circular sections.

The friction coefficient of 0.25 identified in this study is consistent with values reported in prior literature on steel-concrete composite interfaces, typically ranging from 0.2 to 0.35 depending on surface roughness, concrete strength, and steel grade. This parameter is critical for accurate finite element modeling of SRC members, as it governs the load transfer mechanism between the steel tube and the concrete core.

Finite Element Modeling and Validation

The authors employed ABAQUS to develop a three-dimensional finite element model that reproduced the experimental results with satisfactory accuracy. The modeling approach incorporated elastic-plastic material constitutive models for both the steel tube and the concrete core, with a Coulomb friction interface defined between them. The concrete was modeled using the Concrete Damaged Plasticity (CDP) model, which captures both cracking and crushing behavior under multiaxial stress states. The steel was modeled using a bilinear isotropic hardening model.

The numerical simulations confirmed that the ABAQUS model with a friction coefficient of 0.25 reproduced the experimental load-displacement curves with good agreement, particularly in the elastic and early plastic ranges. In the post-peak region, minor discrepancies were observed, which is typical for finite element analyses of concrete-steel composite members where the progressive degradation of the interface bond is difficult to capture precisely.

Engineering Practice Implications

From a practical design perspective, this study provides several actionable insights. First, the significant reduction in shear capacity with increasing shear span ratio underscores the importance of shear span ratio control in the design of SRC members, particularly in beam-column joints and short members subjected to seismic loading. Second, the finding that end plates do not significantly influence the yield load but do affect post-yield hardening suggests that end plate design should focus on ensuring adequate confinement in the plastic hinge region rather than on enhancing initial strength. Third, the sensitivity of shear capacity to the loading axis direction highlights the anisotropic behavior of rectangular SRC members, which must be accounted for in design when the loading direction is not aligned with the principal axes of the cross-section.

The study also has implications for the design of SRC columns and beams in seismic regions, where shear resistance is a governing design criterion. The transition from shear failure to bending failure with increasing shear span ratio aligns with the ductile design philosophy of ensuring flexural yielding before shear failure, a principle enshrined in most modern seismic design codes.

Key Questions and Reflections

One question that arises from this study is how the results would differ for circular steel tube concrete members, where the uniform confinement provided by the circular steel tube creates a more predictable stress state in the concrete core. Another consideration is the long-term behavior of the steel-concrete interface under cyclic loading, which is not addressed in this monotonic shear study but is critical for seismic design. The friction coefficient of 0.25, while validated for monotonic loading, may be reduced under reversed cyclic loading due to the degradation of the interface bond.

Additionally, the study does not address the effect of concrete strength grade on the shear behavior, which is an important parameter in modern high-strength concrete applications. The interaction between concrete strength, steel tube thickness ratio (confinement ratio), and shear span ratio on shear capacity warrants further investigation.

Summary and Conclusions

This paper makes a solid contribution to the understanding of shear behavior in rectangular SRC members without end plates. The experimental database of eight specimens, combined with validated finite element modeling, provides reliable design guidance for engineers working on composite steel-concrete structures. The identified friction coefficient of 0.25 and the quantification of the shear span ratio effect are particularly valuable for practical design applications. The study reinforces the importance of considering the loading direction and the progressive transition of failure modes in the design of SRC members, and it sets a foundation for further research into cyclic shear behavior and high-strength concrete applications.