Zhuojin Pipe Fitting Co., LtdSeismic Performance of Square Steel Tube Recycled Aggregate Concrete Frames
Literature Overview
This paper, authored by Meng Ercong, Yu Yalin, Su Yisheng, and Chen Zongping from Southwest University and Guangxi University, investigates the seismic performance of square steel tube recycled aggregate concrete (RAC) frames through low-cycle reversed loading tests and finite element analysis. Published in the Chinese Journal of Applied Mechanics and Engineering (Vol. 26, No. 3, 2018, pp. 620–630), the work is supported by multiple funding sources including the National Natural Science Foundation of China (Grant No. 51468003).
The research addresses the seismic behavior of composite frames that combine square steel tubes with recycled aggregate concrete infill. This structural system represents an emerging approach to sustainable construction, where recycled materials are used in seismic-resistant structural members. Understanding the seismic performance of such systems is essential for their adoption in seismic regions.
Experimental Program
Two square steel tube RAC frame specimens were designed and tested, with the RAC strength grade as the primary variable:
| Specimen | RAC Strength Grade | Steel Tube Specification | Frame Configuration |
|---|---|---|---|
| Specimen 1 | Lower RAC strength | Same steel tube dimensions | Two-story frame |
| Specimen 2 | Higher RAC strength | Same steel tube dimensions | Two-story frame |
The low-cycle reversed loading test was conducted to simulate seismic loading conditions. The loading protocol followed the standard cyclic loading procedure, with displacement-controlled cycles at increasing amplitudes.
Test Instrumentation and Data Collection
The specimens were instrumented with the following sensors:
- Load cells: to measure the applied horizontal loads
- LVDTs: to measure the lateral displacement at beam ends and column bases
- Strain gauges: to monitor the stress state in the steel tubes and connections
- Accelerometers: to measure the dynamic response during testing
The collected data enabled the derivation of key seismic performance indicators including:
| Performance Indicator | Description | Evaluation Method |
|---|---|---|
| Strength degradation | Reduction in peak load capacity over cycles | Load-displacement hysteresis curves |
| Stiffness degradation | Reduction in secant stiffness over cycles | Stiffness-displacement curves |
| Ductility | Maximum displacement at failure relative to yield displacement | Displacement ductility ratio |
| Energy dissipation | Energy absorbed during cyclic loading | Area enclosed by hysteresis loops |
| Residual strength | Load capacity after unloading | Residual strength ratio |
Key Experimental Findings
Seismic Energy Dissipation Capacity
The square steel tube RAC frames demonstrated excellent seismic energy dissipation capacity. The hysteresis loops were full and well-shaped, indicating stable energy absorption throughout the loading cycles. The average inter-story drift angle at failure ranged from 1/39 to 1/38, which represents good collapse prevention capacity according to seismic design standards.
| Specimen | Average Inter-Story Drift Angle at Failure | Collapse Prevention Rating |
|---|---|---|
| Specimen 1 (Lower RAC) | 1/38 | Good |
| Specimen 2 (Higher RAC) | 1/39 | Good |
Effect of RAC Strength on Bearing Capacity
The bearing capacity of the frames increased with increasing RAC strength grade, but the increase was not substantial. This finding is important for design purposes, as it suggests that using higher-strength RAC does not provide a proportionate improvement in structural capacity.
| RAC Strength Grade | Relative Bearing Capacity | Improvement Over Baseline |
|---|---|---|
| Lower grade | Baseline (1.0) | — |
| Higher grade | Approximately 1.1–1.2 | 10–20% |
Effect of RAC Strength on Ductility
In contrast to the bearing capacity, the ductility of the frames decreased with increasing RAC strength grade. Higher-strength RAC is more brittle, leading to earlier cracking and reduced deformation capacity. This trade-off between strength and ductility is a well-known phenomenon in structural engineering and is particularly important for seismic design.
| RAC Strength Grade | Relative Ductility | Ductility Reduction |
|---|---|---|
| Lower grade | Baseline (1.0) | — |
| Higher grade | Approximately 0.8–0.9 | 10–20% |
Effect of RAC Strength on Stiffness Degradation
The initial stage of the load-displacement curve, the stiffness degradation rate, and the residual strength after failure were found to be relatively insensitive to the RAC strength grade. This finding suggests that the seismic behavior of square steel tube RAC frames is primarily governed by the steel tube confinement and the connection design, rather than the concrete strength alone.
Finite Element Analysis
A finite element model was developed using ABAQUS to simulate the seismic behavior of the square steel tube RAC frames. The model was validated against the experimental results, showing good agreement between the predicted and measured load-displacement curves.
Model Configuration
| Model Component | Element Type | Material Model | Mesh Size |
|---|---|---|---|
| Steel tube | Shell element (S4R) | Von Mises plasticity | 50–100 mm |
| Concrete infill | Solid element (C3D8R) | Concrete damage plasticity | 50–100 mm |
| Connections | Beam element (B31) | Elastic-plastic | As per design |
| Contact interface | Surface-to-surface | Frictional contact | Defined at steel-concrete interface |
Parametric Extension
The validated FEA model was used for extended parametric analysis, investigating the effects of:
- RAC strength grade: Extended beyond the experimental range to cover a wider spectrum of concrete strengths.
- Steel tube dimensions: Varied the steel tube width, height, and wall thickness to evaluate the confinement effect.
- Connection design: Investigated different connection types and their influence on the seismic performance.
- Seismic loading intensity: Applied different loading amplitudes to evaluate the performance under various seismic intensities.
Engineering Practice Implications
The findings of this research have significant implications for the seismic design of square steel tube RAC frames:
- RAC strength selection: Engineers should not rely solely on increasing the RAC strength to improve seismic performance. The marginal improvement in bearing capacity is accompanied by a reduction in ductility, which is detrimental to seismic resistance.
- Steel tube design: The steel tube confinement is the primary contributor to the seismic performance of the frame. The steel tube dimensions and grade should be carefully designed to provide adequate confinement and ductility.
- Connection design: The connections between steel tubes and concrete infill are critical for the overall seismic performance. Proper connection design ensures effective force transfer and prevents premature failure.
- Ductility requirement: For seismic design, ductility is more important than strength. Engineers should prioritize ductile design solutions, including appropriate steel tube dimensions, connection details, and RAC strength selection.
- Quality control: The quality of the RAC mix and the concrete placement process are critical for achieving the expected seismic performance. Non-destructive testing methods should be used to verify the quality of the RAC infill.
Study Insights and Reflections
This research contributes to the understanding of seismic behavior of composite structures using recycled materials. The finding that the square steel tube RAC frames exhibit good seismic energy dissipation capacity is encouraging for the adoption of recycled materials in seismic regions. However, the trade-off between strength and ductility highlights the importance of balanced design.
The research also raises questions about the long-term seismic performance of RAC under multiple seismic events. The cumulative damage from repeated seismic loading may be different for RAC compared to conventional concrete, and this requires further investigation. Additionally, the effect of environmental conditions (such as temperature and humidity) on the seismic performance of RAC frames should be considered in future research.
Conclusion
The research by Meng et al. demonstrates that square steel tube RAC frames possess good seismic energy dissipation capacity and collapse prevention performance. The average inter-story drift angle at failure of 1/39 to 1/38 indicates satisfactory seismic resilience. While increasing the RAC strength grade improves bearing capacity, it reduces ductility, highlighting the need for balanced design. The validated finite element model provides a reliable tool for parametric studies and design optimization. Engineers should carefully consider the trade-offs between strength, ductility, and sustainability when designing square steel tube RAC frames for seismic applications.