Pushover Analysis of Composite Beam Square Steel Tube Concrete Column Frame Seismic Performance

Literature Overview

This study by Liu Jingbo, Guo Bing, and Liu Yangbing from the Department of Civil Engineering at Tsinghua University, published in Journal of Earthquake Engineering and Engineering Vibration in 2008 (Volume 28, Issue 5, pages 87-93), presents a pushover analysis of a 15-story composite beam-square steel tube concrete column frame structure. The research was supported by the National Natural Science Foundation of China (Key Program Grant 50438020). The study addresses the seismic performance evaluation of composite frame structures that combine composite beams with square steel tube concrete columns, a structural system gaining increasing attention in modern high-rise construction.

Technical Background and Structural System Description

The composite beam-square steel tube concrete column frame system combines the advantages of composite construction (steel-concrete interaction for increased stiffness and strength) with the high load-bearing capacity and ductility of steel tube concrete columns. This hybrid system is particularly suitable for tall buildings where both gravity load resistance and seismic performance are critical design requirements.

The composite beams in this system typically consist of a steel beam with a concrete slab connected by shear connectors. The square steel tube concrete columns provide high axial load capacity, good energy dissipation, and enhanced fire resistance compared to conventional reinforced concrete columns.

Structural Model Parameters

The study focuses on a 15-story frame structure with the following general characteristics:

Pushover Analysis Methodology

Section Constitutive Model for Composite Beams

A key contribution of this study is the development of a section constitutive model that accounts for the asymmetric behavior of composite beam sections under positive and negative bending. In composite beams, the positive bending (sagging) and negative bending (hogging) moments produce different stiffness, strength, and load-bearing capacity due to the multi-material cross-section comprising steel, concrete, and shear connectors.

The constitutive model captures the nonlinear material behavior of both steel and concrete, the slip behavior at the steel-concrete interface, and the progressive degradation of stiffness under cyclic loading. This is essential for accurate pushover analysis, which relies on the correct representation of section nonlinearity.

Elastic-Plastic Analysis Model

The pushover analysis was conducted using an elastic-plastic analysis framework that incorporates the developed section constitutive model. The analysis procedure involves:

1. Applying a lateral load pattern representative of the first-mode shape or uniform load
2. Incrementally increasing the lateral load while allowing plastic hinges to form at critical sections
3. Recording the base shear and roof displacement at each load increment
4. Identifying the yield point, ultimate point, and collapse point on the pushover curve

The analysis was performed for both frequent earthquake (small-to-moderate seismic intensity) and rare earthquake (major seismic intensity) scenarios, following the Chinese seismic design code requirements.

Comparative Analysis Results

Dynamic Characteristics Comparison

The study compares the dynamic characteristics of three structural systems:

The composite beam-square CFST column frame system exhibits a shorter fundamental period compared to the other two systems, indicating higher lateral stiffness. This is attributed to the increased flexural stiffness of the composite beams, which reduces the overall structural flexibility and seismic displacement demands.

Seismic Performance Evaluation

The pushover analysis results demonstrate that the composite beam-square CFST column frame system offers superior seismic performance compared to both the steel beam-square CFST column and steel beam-RC column systems. The key advantages include:

• Higher initial stiffness due to the composite action in the beams
• Greater energy dissipation capacity from the combination of composite beam and CFST column plastic hinges
• More uniform distribution of plastic deformation along the structural height
• Better post-yield ductility due to the steel tube confinement of the concrete columns

Performance Under Different Seismic Scenarios

Under frequent earthquake conditions, all three structural systems remain in the elastic range with minimal damage. Under rare earthquake conditions, the composite beam-square CFST column system shows controlled plastic deformation with plastic hinges forming at the beam-column joints and column bases, as intended by the capacity design approach. The other two systems exhibit more concentrated damage and larger displacements.

Engineering Practice Implications

Design Considerations for Composite Frame Structures

The pushover analysis results provide several important design insights:

1. The composite beam contribution to structural stiffness should be properly accounted for in the seismic design, as it significantly affects the fundamental period and lateral load distribution
2. The asymmetric section behavior of composite beams under positive and negative bending must be considered in the pushover analysis to avoid non-conservative predictions
3. The plastic hinge locations should be verified to ensure they form at the intended locations (beam ends and column bases) rather than at unintended locations

Modeling Recommendations

For practical pushover analysis of composite frame structures, the following modeling recommendations are derived from this study:

• Use a section constitutive model that captures the positive-negative asymmetry of composite beams
• Include the shear connector behavior in the composite beam model, particularly the slip degradation under cyclic loading
• Model the CFST columns with a constitutive model that accounts for the concrete confinement effect, such as the Mander model or a similar approach
• Apply a realistic lateral load pattern that represents the actual seismic demand, considering higher-mode effects for tall structures

Quality Control for Construction

The seismic performance of composite frame structures depends heavily on construction quality. Key quality control points include:

• Shear connector weld quality and spacing verification
• Concrete placement quality within the CFST columns, ensuring full consolidation
• Steel tube fabrication quality, including wall thickness uniformity and weld integrity
• Connection detail quality at beam-column joints, which are critical for seismic energy dissipation

Summary and Conclusions

This pushover analysis study demonstrates that the composite beam-square steel tube concrete column frame system offers superior seismic performance compared to alternative frame systems. The key advantages stem from the increased lateral stiffness of composite beams and the enhanced ductility and energy dissipation of CFST columns. The development of an asymmetric section constitutive model for composite beams is a significant methodological contribution that improves the accuracy of pushover analysis for this structural system. The study provides a valuable reference for the seismic performance evaluation of composite frame structures and for the application of pushover analysis methodology in this context. Future research should extend to nonlinear time-history analysis for more detailed seismic response predictions and to experimental validation of the pushover analysis results through shake table testing or full-scale structural testing.