Seismic Performance of Thin-Walled Rectangular Concrete-Filled Steel Tube Piers with End Stirrups

Literature Overview

This paper by Sun Hao, Lv Fei, Ding Faxing, and Li Biao (2024), published in the Journal of Railways and Road Engineering, investigates the seismic performance of thin-walled rectangular concrete-filled steel tube (CFST) piers with end stirrups. The research was funded by the National Natural Science Foundation of China (Grants 51978664 and 52008400) and the Hunan Provincial Natural Science Foundation (Grant 2022JJ40609). The study is particularly relevant to railway bridge engineering, where rectangular CFST piers are increasingly used due to their superior moment of inertia and stability compared to circular CFST piers. The authors conducted quasi-static tests on two square and two rectangular CFST piers, and developed a detailed three-dimensional finite element model using ABAQUS that incorporates concrete cracking and steel ductile damage.

Core Technical Findings

The study demonstrates that the end stirrup confinement significantly improves the seismic performance of rectangular CFST piers. The key performance improvements are quantified below.

The most striking finding is the 49.3% improvement in yield load and the 42.8% improvement in peak horizontal load when end stirrups are added to the rectangular CFST pier. These improvements are substantial and demonstrate the effectiveness of the end stirrup confinement in enhancing the seismic resistance of rectangular CFST piers. The study also shows that strong axis loading produces higher seismic performance indicators than weak axis loading, which is expected because the rectangular cross-section has a larger moment of inertia about the strong axis.

Finite Element Model and Damage Analysis

The finite element model developed in this study is a notable contribution because it incorporates both concrete cracking and steel ductile damage, which are critical for accurately predicting the seismic performance degradation of CFST piers under cyclic loading. The model captures the progressive damage accumulation in the concrete and the plastic deformation in the steel, leading to a realistic representation of the load-displacement hysteresis curves. The model also captures the "pinching" effect of the hysteresis curves, which is a well-known phenomenon in reinforced concrete and composite columns under cyclic loading.

The agreement between the finite element results and the experimental results is described as good, which validates the modeling approach. The crack height position predicted by the finite element model is also found to be in basic agreement with the experimental observations, which is important for damage assessment and repair planning.

From a welding perspective, the end stirrups are typically welded to the rectangular steel tube at the ends of the pier. The quality of these welds is critical because they must transfer the cyclic shear forces between the stirrups and the tube wall. The welds should be designed and executed in accordance with the requirements of GB/T 985 for weld preparation and GB/T 3323 for radiographic testing. For cyclic loading applications, the weld details should be designed to avoid stress concentrations that could initiate fatigue cracks. The use of full-penetration groove welds with adequate weld size and proper weld preparation is recommended.

Engineering Practice Implications

The results of this study have direct implications for the design of railway bridge piers and other structures subjected to seismic loading. The following practical recommendations emerge:

1. End stirrups should be incorporated into the design of rectangular CFST piers to improve the seismic performance. The stirrups should be designed to provide adequate confinement over the plastic hinge region, which is typically located at the base and top of the pier.
2. The loading direction should be considered in the design. Strong axis loading produces higher seismic performance, so the rectangular pier should be oriented to maximize the seismic performance in the critical loading direction.
3. The finite element model developed in this study can be used for the detailed design of rectangular CFST piers, including the prediction of damage patterns and the assessment of residual deformations.
4. The residual deformation after cyclic loading is an important consideration for post-earthquake repair and serviceability. Engineers should evaluate the residual deformation of the pier after a design-level earthquake and plan for repair accordingly.

Key Questions and Reflections

A significant question that arises from this study is how the seismic performance of the rectangular CFST pier would be affected by the wall thickness of the thin-walled steel tube. The study focuses on thin-walled tubes, but the wall thickness is a critical parameter that affects both the local buckling behavior and the composite action with the concrete. A thinner wall provides less confinement but is more susceptible to local buckling, while a thicker wall provides greater confinement but increases the weight and cost. Engineers should perform a parametric study to determine the optimal wall thickness for a given application.

Another reflection concerns the interaction between the end stirrups and the longitudinal weld seams of the rectangular tube. In ERW or HFW welded rectangular tubes, the longitudinal weld seam runs along the length of the tube, and the end stirrups are welded to the tube wall near the ends. The proximity of the stirrup welds to the longitudinal weld seam could create a complex stress interaction that affects the fatigue performance of the connection. Engineers should consider this interaction in the design of the weld details and may need to modify the weld sequence or the weld preparation to avoid adverse effects.

Study Insights and Implications

This study provides valuable experimental and numerical evidence for the use of end stirrups to improve the seismic performance of rectangular CFST piers. The substantial improvements in yield load, peak horizontal load, and energy dissipation capacity demonstrate the effectiveness of the end stirrup confinement concept. The detailed finite element model that incorporates concrete cracking and steel ductile damage offers a powerful tool for the design and assessment of rectangular CFST piers under cyclic loading. For railway bridge engineers and structural engineers working on seismic-resistant design, this study offers practical guidance and validated design tools that can be directly applied to the design of rectangular CFST piers with end stirrup confinement. The study also highlights the importance of welding quality control and the consideration of loading direction in achieving the full seismic performance potential of rectangular CFST piers.