Bearing Capacity Calculation of Rectangular Steel Tube Concrete Compression-Bending Members Under Repeated Loads

Overview of the Study

The paper by Yang Youfu and Han Linhai, published in Earthquake Engineering and Engineering Dynamics (Vol. 23, No. 5, 2003, pp. 125–129), presents a comprehensive comparison of bearing capacity calculation methods for rectangular steel tube concrete (SRC) compression-bending members under repeated (cyclic) loading. The research was supported by the Fujian Province Science and Technology Plan Major Funding Project (Grant No. 2002H007). The study is particularly significant for seismic engineering applications, where compression-bending members are subjected to repeated loading cycles during earthquake events.

The authors compare seven different calculation methods from international and national standards against experimental results obtained from cyclic loading tests on rectangular SRC members. The standards evaluated include British Standard BS5400 (1979), American Concrete Institute ACI 318-99 (1999), American Institute of Steel Construction AISC LRFD (1999), Japanese AIJ (1997), European Code EC4 (1994), Chinese GJB4142-2000 (2001), and a Fujian Province local standard (draft for review).

Experimental Background and Test Configuration

The experimental program involved cyclic loading tests on rectangular steel tube concrete compression-bending members. The specimens were designed to simulate typical structural members found in seismic-resistant buildings, where combined axial compression and bending moments are subjected to repeated reversal of loading direction.

Test Specimen Parameters

The cyclic loading was applied in a displacement-controlled manner, with the loading amplitude progressively increased to simulate the increasing intensity of earthquake ground motion. The loading protocol followed the typical equal-amplitude double-cycle procedure, with each amplitude level repeated twice before progressing to the next level.

Comparison of Calculation Methods

The core contribution of this paper is the systematic comparison of bearing capacity predictions from seven different standards against the experimental results. The comparison reveals significant differences in the conservatism of each method, which has important implications for structural design and seismic assessment.

Bearing Capacity Comparison

The results clearly demonstrate that all seven methods predict bearing capacities that are lower than the experimental values, meaning all methods are conservative. However, the degree of conservatism varies significantly. The numerical finite element method provides the closest predictions to the experimental results, with an average deviation of only 5%. The Chinese standard GJB4142-2000 provides the second-best predictions, with an average deviation of 11%.

The American ACI 318-99, European EC4, and Fujian local standard all provide predictions that are approximately 20% lower than the experimental values. The British BS5400, American AISC LRFD, and Japanese AIJ standards are the most conservative, with predictions that are over 30% lower than the experimental values.

Influence of Axial Load Ratio on Bearing Capacity

The comparison reveals that the numerical method captures the nonlinear interaction between axial load and bending moment under cyclic loading most accurately. The standards that are more conservative tend to underestimate the composite action between the steel tube and the concrete core, particularly under reversed cyclic loading where the confinement effect of the steel tube on the concrete is fully mobilized.

Engineering Practice Implications

The findings of this study have important implications for the design and assessment of rectangular steel tube concrete members in seismic regions. Several key points emerge:

1. Standard Selection: For projects in seismic regions where rectangular SRC members are used, the Chinese standard GJB4142-2000 provides the most reasonable predictions among the evaluated standards. However, even this standard is somewhat conservative, suggesting that the actual bearing capacity may be higher than what is calculated using current design codes.
2. Design Optimization: The conservatism of most international standards means that designs based on these standards may be over-conservative, leading to unnecessary material usage and increased costs. Engineers should consider using numerical methods for detailed design analysis, particularly for critical structural members where efficiency is important.
3. Seismic Assessment: For existing structures undergoing seismic assessment, the bearing capacity predictions from different standards will lead to different conclusions regarding structural adequacy. The more conservative standards (BS5400, AISC LRFD, AIJ) may classify structures as inadequate when they are actually adequate, leading to unnecessary and costly retrofitting measures.
4. Composite Action: The study highlights the importance of properly accounting for the composite action between the steel tube and the concrete core. The steel tube provides confinement to the concrete, enhancing its compressive strength and ductility, while the concrete provides lateral support to the steel tube, delaying local buckling. This interaction is most effectively captured by numerical methods.

Design Recommendations

Study Insights and Recommendations

The study by Yang and Han provides a valuable benchmark for evaluating the accuracy of different design standards for rectangular steel tube concrete members under cyclic loading. The systematic comparison against experimental data reveals that current design codes are generally conservative, with the degree of conservatism varying significantly between standards.

From a practical standpoint, the study suggests that engineers should not rely solely on code-based calculations for the design of seismic-resistant SRC structures. Instead, a combination of code-based preliminary design and detailed numerical analysis should be employed to achieve both safety and efficiency. The numerical finite element method, when properly calibrated against experimental data, provides the most reliable predictions of bearing capacity.

The study also highlights the need for continued development of design standards that more accurately capture the behavior of steel tube concrete members under cyclic loading. The current standards were largely developed for monotonic loading conditions and may not adequately represent the behavior under repeated loading reversal. Future standards should incorporate the effects of cyclic loading, including the degradation of stiffness and strength under repeated cycles, the Bauschinger effect, and the progressive damage accumulation.

In conclusion, the study by Yang and Han demonstrates that the bearing capacity of rectangular steel tube concrete compression-bending members under repeated loads is consistently overestimated by current design standards, with deviations ranging from 5% (numerical method) to over 30% (several international standards). Engineers should use this information to make informed decisions about design methodology, safety factors, and the level of analysis required for their specific projects. The findings also underscore the importance of experimental validation in the development of design standards for composite structural systems.