Seismic Performance of Steel Tube Concrete Frame Structures

Literature Overview

This paper, published in the Journal of Harbin Institute of Technology (2009, Vol. 41, No. 10), authored by Du Guofeng, Xu Lihua, Xu Chengxiang, and Fan Hong from Yangtze University and Wuhan University, presents the results of shaking table tests and finite element analysis on an eight-storey steel tube concrete column and H-beam frame structure model. The study was supported by the Hubei Provincial Department of Education Research Fund (Grant Q20091210) and addresses the seismic performance of steel tube concrete (STC) frame structures, which are increasingly used in seismic zones due to their favourable combination of strength, ductility, and energy dissipation capacity.

Core Technical Findings

Shaking Table Test Programme

The experimental programme involved an eight-storey reduced-scale model of an STC frame structure with steel tube concrete columns and H-section steel beams. The model was subjected to three seismic wave excitations:

El-Centro wave (a classic benchmark earthquake record)
Tianjin wave (N-S component, representing a regional earthquake)
Wuhan artificial seismic wave (representing local seismic conditions)

The test parameters included varying peak ground acceleration (PGA) levels to simulate increasing earthquake intensities. The measured responses included maximum seismic forces, inter-storey shear forces, strain distributions, and displacement responses at multiple levels of the structure.

Key Results

The study found that the experimental results and numerical simulation results were in good agreement, validating the finite element modelling approach. The principal findings are summarised below:

Response Parameter	Observed Behaviour	Interpretation
Structural displacement	Dominated by low-order modes at lower PGA	Flexural response governs at moderate intensities
Higher PGA response	Higher-order modes become more significant	Shear response increases with intensity
Deformation pattern	Bending-shear type	Combined flexural and shear deformation
ANSYS 8.1 simulation	Good agreement with test results	Validated modelling approach

The study concluded that STC frame structures exhibit good seismic performance, with the displacement response being primarily governed by low-order vibration modes at lower intensities and higher-order modes becoming more significant at higher intensities. The structure deforms in a bending-shear pattern, which is characteristic of multi-storey frame structures.

Interpretation of Technical Points

Seismic Behaviour of STC Frame Structures

The seismic performance of STC frame structures is governed by several key mechanisms:

Column ductility: The steel tube provides lateral confinement to the concrete core, enhancing ductility and preventing brittle failure. The confinement effect is particularly important at plastic hinge formation, where the steel tube continues to confine the concrete even after significant deformation.
Beam-column joint performance: The connection between steel tube concrete columns and H-beams is a critical detail that must be designed for adequate strength and ductility. The joint must accommodate the relative rotation between the column and beam without failure.
Energy dissipation: The STC frame structure dissipates seismic energy through plastic deformation of the steel tube, cracking and crushing of the concrete core, and yielding of the steel beams. The energy dissipation capacity is a key indicator of seismic performance.
P-delta effects: The axial load on the columns creates additional moments under lateral displacement, which can amplify the seismic response. The P-delta effect is more significant for slender columns and higher structures.

Vibration Mode Analysis

The study's observation that low-order vibration modes dominate the response at lower PGA levels and higher-order modes become more significant at higher PGA levels is consistent with the non-linear behaviour of structures under increasing earthquake intensity. At low intensities, the structure responds primarily in its first mode of vibration, which is typically a global flexural mode. As the intensity increases and the structure enters the non-linear range, higher-order modes are excited due to stiffness degradation and geometric non-linearity.

This mode evolution has important implications for seismic design. The design must account for the potential contribution of higher-order modes to the seismic response, particularly for structures that are expected to experience significant non-linear deformation during strong earthquakes. The inclusion of higher-mode effects in the seismic design analysis can be achieved through response spectrum analysis or time history analysis, rather than relying solely on equivalent static force procedures.

Numerical Modelling Validation

The good agreement between the shaking table test results and the ANSYS 8.1 finite element simulation results validates the numerical modelling approach for STC frame structures. The key modelling considerations include:

Proper representation of the steel tube-concrete interface interaction
Appropriate material models for both steel and concrete, including non-linear behaviour
Accurate modelling of the beam-column joint connections
Correct boundary conditions and mass distribution
Inclusion of geometric non-linearity for large displacement analysis

The validation of the numerical model through physical testing is essential for building confidence in the simulation predictions, particularly for structures that will be subjected to extreme loading events such as strong earthquakes.

Process and Standards Analysis

Relevance to Seismic Design Standards

The seismic performance of STC frame structures is addressed in several design standards, including:

GB 50011-2010 (Code for Seismic Design of Buildings)
GB 51248-2016 (Technical Code for Concrete-Filled Steel Tubular Structures)
GB/T 22515-2018 (Steel tube concrete structures)
ASCE 41-17 (Seismic Evaluation and Retrofit of Existing Buildings)
ACI 318-19 (Building Code Requirements for Structural Concrete)

The study findings support the use of STC frame structures in seismic zones, provided that the design accounts for the non-linear behaviour observed under strong earthquake loading. The key design considerations include:

Adequate ductility of columns through steel tube confinement
Strong beam-weak column design philosophy to ensure plastic hinge formation in beams rather than columns
Proper detailing of beam-column joints to accommodate large rotations
Seismic design analysis that includes non-linear effects for high-intensity earthquakes

Material and Fabrication Considerations

For the fabrication of STC frame structures, the following considerations are important:

Steel tube material grade: Q235, Q345, or higher grades are commonly used, with Q345 being preferred for seismic applications due to its superior ductility
Concrete grade: C30 to C60 is typically used, with higher grades providing greater strength but potentially reduced ductility
Steel tube welding: Longitudinal and circumferential welds must be qualified for seismic applications, with attention to HAZ toughness and ductility
Concrete placement: Proper compaction and curing are essential to ensure uniform concrete properties and avoid voids or weak zones

Integration with Engineering Practice

Application in Seismic Zone Design

The study findings directly inform the design of STC frame structures in seismic zones. The good seismic performance demonstrated by the eight-storey model structure provides confidence in the use of STC frames for mid-rise and high-rise buildings in seismic regions. The key design recommendations based on the study findings include:

Use response spectrum analysis or time history analysis for seismic design of STC frame structures, rather than relying solely on equivalent static force procedures
Design for adequate ductility through proper detailing of columns and joints, with steel tube confinement being the primary ductility mechanism
Include non-linear analysis for performance-based design, particularly for structures in high seismic hazard zones
Validate numerical models through physical testing or comparison with validated models from similar structures
Consider the effect of higher-order vibration modes on the seismic response, particularly for structures expected to experience significant non-linear deformation

Case Study Insight: Mid-Rise Building in Seismic Zone

For a typical eight-storey STC frame building in a seismic zone with a design PGA of 0.2g, the study findings suggest that the structure will respond primarily in its first mode of vibration under moderate earthquake loading. Under strong earthquake loading (PGA > 0.4g), higher-order modes will become significant, and the structure will exhibit combined bending-shear deformation. The design should account for this mode evolution by using non-linear analysis methods that capture the stiffness degradation and energy dissipation behaviour of the STC frame.

Key Questions and Reflections

The study raises several important questions about the seismic design of STC frame structures. First, the shaking table test was conducted on a reduced-scale model, and the scaling effects on the seismic response must be considered when extrapolating the results to full-scale structures. Second, the study did not investigate the effect of soil-structure interaction on the seismic response, which can be significant for structures on soft soils. Third, the study did not address the seismic performance of STC frame structures after damage, which is important for progressive collapse prevention and post-earthquake usability.

From a personal perspective, I find the validation of the numerical model through physical testing to be the most valuable aspect of this study. The good agreement between the shaking table test results and the ANSYS simulation results provides confidence in the use of numerical analysis for seismic design of STC frame structures. However, it is important to recognise that numerical models are only as good as the material property data and modelling assumptions used, and continuous validation against physical testing is essential.

Study Insights and Implications

This study provides valuable experimental and numerical evidence for the seismic performance of STC frame structures. The good agreement between the shaking table test results and the finite element simulation results validates the numerical modelling approach, and the observed seismic behaviour supports the use of STC frames in seismic zones. The key insight is that the seismic response of STC frame structures is governed by a transition from low-order to higher-order vibration modes as the earthquake intensity increases, and the design must account for this non-linear behaviour. For engineering practice, this means that non-linear seismic analysis methods should be used for the design of STC frame structures in seismic zones, with proper validation of numerical models against physical testing data. The study also highlights the importance of proper detailing and material selection for achieving the desired seismic performance of STC frame structures.