Ductility Assessment of Phased-Construction Steel Tube Concrete Composite Columns

Literature Overview

The paper by Lu Huasen, Zhang Yao, Chen Qing, and Wu Bing, published in Cement (2025, Issue 7, pp. 149-151), investigates the ductility of steel tube concrete composite columns constructed in phases. The study is supported by multiple research grants from the Guangxi Higher Education Commission for Young and Middle-aged Teachers (2024KY1168, 2023KY1169, 2023KY1175, 2025KY1527). The research addresses a practically significant problem in construction engineering: the effect of staged construction on the structural performance of composite columns where the outer reinforced concrete jacket is applied after the inner CFST core has already been subjected to service loads.

Constraint Effect Mechanism in Composite Columns

The fundamental concept of a steel tube concrete composite column (also known as a CFST composite column or partially encased column) relies on the lateral confinement provided by the outer reinforced concrete jacket to the inner CFST core. This confinement enhances the ductility and energy dissipation capacity of the column under cyclic loading. However, when the column is constructed in phases—first the CFST core is erected and loaded, then the outer RC jacket is applied—the concrete in the outer jacket does not experience the initial axial loads that the CFST core has already sustained.

This creates a critical asymmetry in the strain state between the inner and outer components. The outer jacket's concrete and stirrups begin their structural life at a higher axial load level than they would in a monolithic construction scenario. Consequently, the lateral strain in the outer jacket's stirrups develops less than it would in a conventionally constructed column, leading to insufficient passive confining pressure. This reduced confinement directly translates to lower ductility in the outer jacket region.

Influence of Initial Load on Lateral Strain Development

The study's stress-strain analysis model, built upon the hoop strain formula for confined concrete, reveals that the lateral strain development in the outer jacket is directly proportional to the axial load carried by that jacket. Since the outer jacket does not participate in carrying the initial loads applied during the first construction phase, its concrete remains at a lower stress state compared to the CFST core. When the column is subsequently subjected to cyclic loading (such as seismic forces), the outer jacket's stirrups require additional axial load increment before they can develop the lateral strains necessary for effective confinement.

This delayed confinement development has important implications for the column's hysteretic behavior. During the initial loading cycles, the outer jacket contributes less to energy dissipation than expected, and the column's overall ductility is governed primarily by the CFST core. As the loading progresses and the outer jacket accumulates strain, its confinement effectiveness gradually improves, but by this point, significant damage may have already occurred in other parts of the column.

Ductility Reduction and Design Implications

The study quantifies the ductility reduction caused by phased construction as ranging from 0 to 15 percent within the parameter range examined. While this range may appear modest, it represents a meaningful degradation in seismic performance, particularly for columns designed to achieve high ductility demands in earthquake-prone regions. The 0 percent lower bound corresponds to scenarios where the initial load is small relative to the column's total capacity, while the 15 percent upper bound corresponds to cases where a substantial portion of the column's axial capacity is consumed during the first construction phase.

From a design perspective, this finding suggests that phased-constructed composite columns require additional confinement reinforcement to compensate for the ductility loss. Engineers should consider increasing the stirrup density or using higher-strength stirrups in the outer jacket region. Alternatively, the design axial load ratio should be limited to ensure that the outer jacket retains sufficient load capacity for ductile behavior. The stress-strain model proposed in this study provides a clear physical basis for these design adjustments and can be incorporated into structural analysis software for more accurate performance prediction.

Study Insights and Reflections

The phased construction issue is increasingly relevant in modern construction practices where sequential construction of building elements is common for schedule optimization. This study provides the first systematic quantitative assessment of how this construction sequence affects the seismic ductility of CFST composite columns. The proposed stress-strain model with its clear physical interpretation offers a practical tool for engineers to evaluate the impact of different construction sequences on structural performance. Future research should extend this analysis to include the effects of construction sequence on the steel tube-concrete interface bond, as the differential strain history between the CFST core and the outer jacket may also affect the interfacial shear transfer capacity, which is critical for composite action under seismic loading.