Shear Characteristics of Steel-Concrete Interface in Steel-Concrete Composite Piles with Built-In Longitudinal Ribs

Literature Overview

This study focuses on the interfacial shear transfer mechanism between steel and concrete in composite piles incorporating built-in longitudinal ribs. Steel-concrete composite piles are widely used in deep foundation engineering for their combined advantages of steel's tensile strength and concrete's compressive capacity. The interface between the steel tube and concrete core is critical to the composite behavior, governing load transfer, slip resistance, and overall structural integrity. The introduction of longitudinal ribs on the inner surface of the steel tube is intended to enhance mechanical interlock and improve shear transfer capacity, which is particularly important for piles subjected to lateral loads and seismic actions.

Core Technical Points

The interfacial shear behavior of steel-concrete composite members is governed by three mechanisms: chemical adhesion, friction, and mechanical interlock. In conventional smooth-walled steel tubes, the first two mechanisms dominate, and the shear capacity is relatively limited. The incorporation of longitudinal ribs introduces a powerful mechanical interlock mechanism that significantly enhances the shear transfer capacity.

The study examines the following key parameters:

The experimental results demonstrate that the shear capacity of ribbed interfaces is 2–4 times that of smooth interfaces. The load-slip curves exhibit a distinct peak followed by a gradual decline, indicating a transition from elastic interlock to plastic deformation and eventual concrete crushing around the ribs. The peak shear stress can reach 15–25 MPa for ribbed interfaces, compared to 5–8 MPa for smooth interfaces.

Interpretation of Technical Points

The shear transfer mechanism in ribbed steel-concrete interfaces operates through a sequence of events as slip develops:

1. Elastic stage: The ribs and surrounding concrete deform elastically, and shear stress increases linearly with slip. The mechanical interlock provides the primary resistance mechanism.
2. Plastic interlock stage: As slip increases, the ribs begin to yield, and plastic deformation occurs at the rib-concrete interface. The shear stress reaches its peak value, which corresponds to the combined resistance of rib yielding and concrete bearing.
3. Post-peak stage: After the peak, concrete crushing around the ribs reduces the interlock capacity, but friction and residual interlock continue to provide shear resistance. The shear stress declines gradually, maintaining a residual capacity of approximately 60–80% of the peak value.

The rib geometry plays a decisive role in determining the shear capacity. Taller ribs provide greater interlock depth but may lead to stress concentration and premature concrete failure. The optimal rib height is typically 5–8 mm for concrete grades C30–C50. The rib spacing must be sufficient to allow concrete to develop its full bearing capacity without interference from adjacent ribs, but close enough to ensure uniform load distribution along the interface.

The study also investigates the effect of concrete confinement on the interfacial shear behavior. The radial confinement provided by the steel tube enhances the bearing capacity of the concrete around the ribs, delaying concrete crushing and maintaining higher residual shear capacity. This confinement effect is particularly significant for high-strength concrete (C50 and above), where the unconfined concrete is more susceptible to brittle failure.

From a FMEA (Failure Mode and Effects Analysis) perspective, the primary failure modes at the ribbed interface include:

• Rib root fracture due to stress concentration
• Concrete crushing around rib tips
• Rib pull-out from the steel tube
• Interface debonding due to insufficient chemical adhesion

Each failure mode has different implications for the structural performance and requires specific design countermeasures.

Process and Standards Analysis

The manufacturing of ribbed steel tubes for composite piles involves several specialized processes:

1. Rib formation: Longitudinal ribs can be formed by cold-rolling, hot-rolling, or welding processes. Cold-rolling is the most common method, where a roller with a rib profile is pressed against the inner surface of the tube during the forming process. This method produces ribs with good dimensional accuracy and surface finish.
2. Welded ribs: For retrofit applications or special requirements, ribs can be welded to the inner surface of the steel tube. This method requires careful control of the welding process to avoid distortion and residual stresses that could compromise the tube's structural integrity.
3. Surface preparation: The inner surface of the steel tube should be cleaned and, in some cases, roughened to enhance chemical adhesion between the steel and concrete. Shot blasting or acid etching are common surface preparation methods.

The relevant standards for steel-concrete composite piles include:

• GB 50010 (Chinese code for concrete structures)
• GB 50007 (Chinese code for foundation design)
• SY/T 6543 (Steel pipe for oil and gas industry)
• EN 1993-1-1 (Eurocode 3 for steel structures)
• AISC 360 (American steel construction specification)

These standards provide guidelines for the design of composite members but do not specifically address the behavior of ribbed interfaces. The study fills this gap by providing empirical data and analytical models for the design of ribbed interfaces.

Integration with Engineering Practice

The findings of this study have direct applications in the design of composite piles for deep foundation engineering. In seismic regions, the enhanced shear transfer capacity of ribbed interfaces improves the lateral load resistance of composite piles, reducing the risk of interface failure during earthquake loading. The study provides design equations that can be used to calculate the shear capacity of ribbed interfaces:

τ_u = α f_c (h_r / s_r) + β f_y (w_r h_r) / (s_r t)

where τ_u is the ultimate shear stress, f_c is the concrete compressive strength, f_y is the steel yield strength, h_r is the rib height, s_r is the rib spacing, w_r is the rib width, and t is the tube wall thickness. The coefficients α and β are empirical factors determined from experimental data.

For practical implementation, the following recommendations are made:

• Rib height should be selected based on the concrete grade, with taller ribs for lower-strength concrete and shorter ribs for high-strength concrete.
• Rib spacing should be optimized to balance interlock capacity and concrete bearing capacity, typically in the range of 100–150 mm.
• The steel tube wall thickness should be adequate to resist the bearing pressure from the ribs without excessive deformation.
• Surface preparation is essential to ensure reliable chemical adhesion and prevent interface debonding.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation. First, the long-term behavior of ribbed interfaces under cyclic loading and environmental exposure is not fully addressed. Cyclic slip can lead to progressive degradation of the interlock mechanism, and the cumulative damage may reduce the shear capacity over time.

Second, the study focuses on monotonic shear loading, but practical piles are subjected to complex loading conditions including axial compression, lateral bending, and torsion. The interaction between these loading components and their effect on the interfacial shear behavior requires further investigation.

Third, the study does not extensively address the effect of construction quality on the interfacial performance. Variations in rib dimensions, surface condition, and concrete placement quality can significantly affect the actual shear capacity. Quality control measures during construction are essential to ensure the design performance is achieved.

Study Insights and Implications

The research provides valuable insights into the interfacial shear behavior of ribbed steel-concrete composite piles. The key finding is that longitudinal ribs significantly enhance the shear transfer capacity through mechanical interlock, making them a practical and effective solution for improving the performance of composite piles. The design equations and empirical data provided in the study can be directly applied to the design of ribbed interfaces in composite pile applications.

The study also highlights the importance of rib geometry optimization. The rib height, spacing, and width must be carefully selected to balance interlock capacity, concrete bearing capacity, and manufacturing feasibility. The optimal rib configuration depends on the concrete grade, steel tube properties, and loading conditions, and should be determined through detailed analysis and testing.

Reference Value and Outlook

This research contributes to the advancement of composite pile design by providing empirical data and analytical models for ribbed interfaces. Future work should extend the investigation to include cyclic loading, combined loading, and long-term durability of ribbed interfaces. The integration of numerical modeling with experimental validation should continue to refine the design equations and provide more accurate predictions of structural behavior under complex loading conditions.

In conclusion, this study demonstrates that longitudinal ribs are an effective means of enhancing the interfacial shear capacity of steel-concrete composite piles, and provides practical guidance for their design and implementation in deep foundation engineering.