Shear Capacity Analysis Model for Concrete-Filled Steel Tube Composite Columns

Literature Overview

This paper by Ke Xiaojun and Ding Wen, published in Engineering Sciences and Technology (2021, Vol. 53, No. 3, pp. 125-131), presents a modified truss-inclined compression field model for evaluating the shear capacity of concrete-filled steel tube (CFST) composite columns. The research was funded by the National Natural Science Foundation of China (51668007, 51508112) and the Guangxi Natural Science Foundation (2018GXNSFAA050007). The study addresses a critical gap in structural engineering practice where existing design codes often fail to accurately predict the actual shear resistance of CFST members.

Core Technical Approach

The authors developed a modified truss-inclined compression field theory by recognizing that conventional models treat the entire cross-section uniformly, ignoring the fundamentally different confinement conditions experienced by the concrete inside the steel tube versus the concrete outside the tube. The model partitions the cross-section at the steel tube boundary, applying distinct confinement factors to each region. This approach acknowledges that the steel tube imposes a confining pressure on the internal concrete that significantly enhances its compressive strength, while the external concrete receives no such benefit.

The model incorporates the axial force effect on the confining pressure exerted by the steel tube, which is a critical factor often overlooked in simplified approaches. Under combined axial and shear loading, the axial compression increases the effective confining pressure on the inner concrete, thereby modifying the shear transfer mechanism across the critical section.

Comparative Analysis of Design Codes

The study benchmarks the proposed model against three major design standards using 36 experimental test data sets:

Design Standard	Region	Model Basis	Typical V_calc/V_test Ratio	Assessment
T/CECS 188	China	Simplified empirical	Generally below 1.0 (conservative)	Underestimates actual capacity
AISC	USA	Empirical formulas	High dispersion	Unreliable predictions
EC4	Europe	Semi-empirical	High dispersion	Unreliable predictions
Modified Truss-Inclined Field	This study	Mechanism-based	0.85-1.15	Good agreement

The results demonstrate that the modified model achieves a calculation-to-test ratio predominantly within the range of 0.85 to 1.15, which represents excellent engineering accuracy. The model exhibits low sensitivity to variations in key parameters including shear span ratio, volumetric stirrup ratio, concrete strength, axial compression ratio, confining index, and steel tube diameter-to-thickness ratio.

Engineering Practice Implications

The finding that T/CECS 188 consistently underestimates shear capacity has direct economic implications for structural design. In practice, this conservatism leads to excessive reinforcement or oversized sections, resulting in unnecessary material costs and construction complexity. However, it is important to note that the current Chinese code was developed with limited test data and a different philosophical approach to safety factors.

The high dispersion observed in AISC and EC4 predictions suggests that these codes, which were not specifically developed for CFST composite columns, rely on empirical correlations that do not capture the true shear transfer mechanisms in this hybrid system. Engineers working on international projects involving CFST members should exercise caution when applying these codes directly.

Key Questions and Reflections

The model's good performance across 36 test specimens is encouraging, but several questions remain for practical application. The partitioning of the cross-section at the steel tube boundary assumes a clear demarcation of confinement effects, yet in reality, the confinement influence gradually attenuates with distance from the steel tube wall. Additionally, the model's validity at very high axial compression ratios or very low shear span ratios, which represent boundary conditions in practical structures, warrants further investigation.

Summary

This study provides a mechanism-based analytical framework that significantly improves upon existing code provisions for CFST column shear capacity. The modified truss-inclined compression field model represents a meaningful advancement in understanding the shear behavior of these composite members, and its practical adoption could lead to more economical and reliable structural designs in the future.