Seismic Performance of Prefabricated Steel Tube Concrete Composite Frames with End-Plate Connections

Literature Overview

This paper by Wang Donghua, Wang Jingfeng, Li Beibei, Pan Xuebei, and Liu Bingkang, published in the China Civil Engineering Journal (Vol. 50, No. 8, 2017, pp. 20–28), investigates the seismic performance of prefabricated steel tube concrete (CFST) composite frames. The research is funded by the National Natural Science Foundation of China (Grants 51478158 and 51178156) and the Ministry of Education's New Century Excellent Talent Support Program (NCET-12-0838). The study addresses an increasingly important topic in modern construction: the seismic behavior of prefabricated structural systems that combine steel tube concrete columns with steel-concrete composite beams.

Structural System Description

The proposed prefabricated composite frame system consists of circular or square steel tube concrete columns connected to steel beams through one-sided high-strength bolted end-plate connections. The floor system employs steel-concrete composite slabs formed by reinforced steel truss decks. Two connection types were investigated: flush end-plate connections and extended (outstanding) end-plate connections. The prefabrication approach aims to reduce on-site construction time while maintaining seismic performance comparable to conventional welded or bolted steel frames.

Experimental Program

Two two-story single-span composite frames were tested under horizontal low-cycle reversed loading. The frames differed in column cross-sectional form (circular versus square steel tubes) and end-plate connection type (flush versus extended). The steel tube concrete columns had the same steel ratio in both configurations to ensure a fair comparison.

Key Experimental Findings

Hysteresis and Energy Dissipation Performance

The test results demonstrated that the prefabricated CFST composite frames with one-sided bolted end-plate connections exhibited satisfactory hysteresis performance and energy dissipation capacity. The key seismic performance indicators obtained from the tests are summarized below:

Performance Indicator	Range	Significance
Ductility coefficient (μ)	2.13 – 4.28	Indicates acceptable to good ductility for seismic design
Energy dissipation coefficient (E)	0.652 – 0.90	Reflects effective energy absorption through inelastic deformation
Stiffness degradation	Gradual	No abrupt stiffness loss observed
Hysteresis loop shape	Full and stable	Good seismic behavior

Comparison Between Circular and Square Column Configurations

The comparison between circular and square steel tube concrete columns revealed interesting differences:

Load-bearing capacity: Under the same steel ratio condition, circular CFST frames had lower load-bearing capacity than square CFST frames.
Ductility: Circular CFST frames exhibited superior ductility compared to square CFST frames.
Energy dissipation: Circular CFST frames demonstrated better energy dissipation performance than square CFST frames.

This trade-off between strength and ductility is consistent with the well-known behavior of circular CFST members, where the uniform confinement provided by the circular steel tube promotes more uniform concrete crushing and better post-yield deformation capacity.

Failure Modes and Damage Patterns

The detailed observation of the loading process revealed the following damage evolution patterns:

Initial cracking: Floor slab cracks initiated at the beam-column joint region and propagated as lateral displacement increased.
Bolt slip and yielding: One-sided bolts in the end-plate connections underwent slip and yielding before significant frame deformation.
Steel tube yielding: Local buckling and yielding of the steel tube occurred near the column ends.
Concrete crushing: Confined concrete experienced progressive crushing, with the CFRP-like confinement from the steel tube maintaining residual load capacity.

Technical Analysis and Engineering Implications

Connection Design Considerations

The one-sided bolted end-plate connection represents a practical solution for prefabricated construction, where on-site welding is difficult or impossible. The connection must accommodate both gravity loads and seismic lateral forces. Key design considerations include:

Bolt sizing and layout: The number, diameter, and arrangement of high-strength bolts must be designed to ensure adequate moment resistance and slip resistance.
End-plate thickness: The end-plate must be thick enough to prevent yielding or excessive deformation, while remaining thin enough to allow sufficient rotation for ductile behavior.
Connection ductility: The connection should be designed as a ductile element that can undergo inelastic deformation without brittle fracture.

Prefabrication Advantages and Challenges

Aspect	Advantage	Challenge
Construction speed	Rapid on-site assembly	Requires precise factory fabrication
Quality control	Factory-controlled welding and machining	Transport damage and handling
Seismic performance	Good hysteresis and energy dissipation	Connection behavior under cyclic loading
Cost	Reduced on-site labor	Higher transportation costs
Sustainability	Reduced construction waste	Material efficiency

Integration with Seismic Design Codes

The findings of this study have direct implications for seismic design provisions in codes such as GB 50011 (Seismic Design Code for Building Structures) and GB 51249 (Technical Specification for Steel Tube Concrete Structures). The demonstrated ductility coefficients of 2.13–4.28 suggest that these prefabricated frames can meet seismic design requirements for moderate seismic zones when properly detailed. The energy dissipation coefficients of 0.652–0.90 indicate effective energy absorption through inelastic deformation of the connections and column ends.

Key Questions and Reflections

Several important questions arise from this study:

Connection fatigue under repeated seismic loading: The one-sided bolted connections undergo significant cyclic deformation. Long-term fatigue performance and the effect of bolt pretension loss on connection behavior require further investigation.
Effect of composite slab participation: The steel-concrete composite slab acts as a diaphragm that distributes seismic forces to the frames. The interaction between the slab and the frame connections should be considered in detailed design.
Pushover analysis and nonlinear static analysis: While low-cycle reversed loading tests provide valuable information, pushover analysis and dynamic time-history analysis should be conducted to evaluate the overall seismic performance of multi-story prefabricated CFST composite buildings.
Standardization and detailing guidelines: The development of standardized connection details and detailing guidelines is essential for widespread adoption of prefabricated CFST composite frames in seismic regions.

Study Insights and Practical Value

This research makes a significant contribution to the understanding of seismic behavior in prefabricated steel tube concrete composite frames. The experimental results provide empirical evidence that one-sided bolted end-plate connections can achieve satisfactory seismic performance, addressing a critical gap in the design of prefabricated composite structures. The trade-off between strength and ductility observed between circular and square column configurations offers valuable guidance for structural engineers selecting the appropriate column shape for different seismic design objectives. The findings support the feasibility of prefabricated CFST composite frames as a viable alternative to conventional welded steel frames, particularly in applications where rapid construction and high-quality factory fabrication are prioritized. Future work should focus on developing comprehensive design provisions, conducting dynamic analysis, and addressing the long-term durability of the bolted connections under seismic and environmental loading.