Seismic Performance of CFST Unequal-Span Frames Simulated via OpenSees

Literature Overview

The paper by Liu Xiaoqiang, Xu Chengxiang, Li Lei, and Yang Zhao (2021), published in World Information on Earthquake Engineering, investigates the seismic behaviour of a single-bay, single-storey unequal-span plane frame composed of square CFST columns and H-shaped steel beams. The authors used the OpenSees finite element platform to replicate low-cycle reversed loading tests and examined how several key design parameters influence the hysteretic response, load-carrying capacity, and ductility of the system. The work was supported by the National Natural Science Foundation of China (Grant No. 51678457).

This topic sits at the intersection of structural seismic engineering, steel-concrete composite systems, and numerical modelling, and it has direct relevance to engineers who design or assess composite frames in seismic zones.

Core Findings and Technical Points

The authors demonstrated that a plastic hinge fibre-section element, when properly calibrated, reproduces the experimental load-displacement curves of bolted-welded H-beam connections with acceptable accuracy. The following conclusions were drawn:

• Concrete strength has a relatively minor effect on the overall seismic performance of the frame, which is consistent with the observation that the concrete core in CFST columns primarily provides confinement and post-yield ductility rather than initial stiffness.
• Steel grade plays a more significant role. Increasing steel strength improves both the load capacity and ductility. Among the grades examined, Q345 was identified as offering the best balance between cost and performance.
• Column width-to-thickness ratio (b/t): a smaller ratio yields higher frame capacity, reflecting the well-known local buckling behaviour of hollow steel sections under combined axial and bending loads.
• Axial compression ratio: higher values degrade seismic performance substantially. The authors strongly advise against high axial compression ratios in practical design.
• Span-to-depth ratio and beam-column line stiffness ratio: these govern the seismic mechanism. The authors recommend that designers select beam heights carefully to balance bending capacity against the stiffness distribution between beams and columns, thereby ensuring a beam-hinge rather than column-hinge failure mode.

Key Design Parameters Summary

Engineering Practice Reflections

From a steel pipe and fitting fabrication perspective, several observations emerge. First, the emphasis on the column width-to-thickness ratio directly relates to the fabrication quality of square CFST columns. In practice, this means that the flatness, weld quality, and dimensional tolerance of the square steel tubes must be tightly controlled. A deviation in the wall thickness, even if within the general tolerance band, can shift the local buckling threshold and alter the expected plastic hinge formation.

Second, the finding that Q345 provides the best cost-benefit ratio aligns with the current market availability and weldability of this grade. However, engineers should note that Q345 steel requires careful control of heat input during welding to avoid coarse-grained HAZ microstructures that could compromise the connection ductility. Pre-heating at ambient temperatures below 5 °C and limiting interpass temperatures to below 250 °C are typical measures.

Third, the warning against high axial compression ratios has implications for the selection of column cross-sections. In tall buildings or heavy industrial structures where axial loads are large, designers may be tempted to use larger sections, but the paper's findings suggest that this can inadvertently reduce the ductility margin. A more balanced approach—perhaps using a slightly larger section with a lower axial load path—is preferable.

Questions and Further Investigation

One question that arises from reading this paper is whether the OpenSees fibre-section model adequately captures the interaction between the concrete core and the steel tube at the plastic hinge region. The confinement effect of the steel tube on the concrete, particularly under large inelastic strains, is a complex phenomenon that may not be fully represented by a simple confined concrete model. Future work could explore the use of more sophisticated constitutive models for the concrete core, such as those based on the Mander or Kent-Park formulations, and compare the results with experimental data.

Another point worth noting is that the study focuses on a single-bay, single-storey frame. The seismic response of multi-storey CFST frames may differ significantly due to P-Δ effects, story drift concentration, and the cumulative influence of multiple plastic hinges. Extending the numerical methodology to full-scale multi-storey frames would provide more directly applicable design guidance.

Summary

This paper provides a useful numerical framework for evaluating the seismic performance of unequal-span CFST frames. The findings on steel grade selection, axial compression ratio control, and the importance of the column b/t ratio are directly actionable in engineering practice. The OpenSees-based approach offers a cost-effective alternative to full-scale cyclic tests, provided the calibration is validated against experimental data. Engineers involved in the design of composite structures in seismic regions should pay close attention to the interplay between member sizing, material selection, and connection detailing to ensure a ductile, beam-hinge failure mechanism.