Bending Performance of Grouted Micro Steel Tube Piles

Literature Overview

Yin Changjun, Yi Danping, and Gong Jiaxing from Xiangtan University published this study in the Chinese Journal of Applied Mechanics in 2023, supported by the National Natural Science Foundation. The research investigates the flexural behavior of grouted micro steel tube piles through orthogonal experimental design, proposing a new ultimate flexural capacity expression validated by finite element analysis.

Experimental Design and Methodology

The experimental program employed an orthogonal test scheme with 12 specimens varying three factors:

• Steel tube diameter
• Steel tube wall thickness
• Mortar strength grade

Pure bending tests were conducted to isolate flexural behavior without axial load interference. The bending response was divided into three distinct stages:

1. Elastic stage
2. Elastic-plastic stage
3. Strain hardening stage

Key Findings and Parametric Sensitivity

The sensitivity ranking of parameters on ultimate load was determined as:

A critical finding is that the steel tube contributes approximately 78% of the total flexural capacity, while the internal mortar contributes approximately 22%. This distribution has significant implications for material selection and cost optimization in pile design.

Proposed Ultimate Flexural Capacity Expression

The authors derived a new expression for ultimate flexural capacity that showed better agreement with experimental results compared to existing code formulas. The model explicitly accounts for the composite action between the steel tube and the grouted mortar, recognizing that current codes do not adequately capture the synergistic behavior of these materials under bending.

Finite Element Validation

ANSYS finite element models were developed to validate the proposed analytical expression. The simulation results confirmed that the internal mortar significantly enhances the bending performance of the micro steel tube pile, verifying the correctness of the proposed expression. The finite element analysis also provided insight into stress distribution patterns and deformation mechanisms that are difficult to observe experimentally.

Engineering Practice Integration

For steel tube fabrication and welding quality control, the findings emphasize that tube diameter is the most critical geometric parameter. Manufacturing tolerances on diameter should be tightly controlled. The wall thickness, while less influential, still requires adequate control to ensure the steel tube contributes its full 78% capacity share.

The grouting quality is also critical. Poor grouting with voids or incomplete filling would reduce the mortar contribution below the expected 22%, and more importantly, could lead to localized buckling of the thin steel tube wall under bending. Quality control procedures should include grout density testing and possibly radiographic inspection for critical applications.

The three-stage bending behavior (elastic, elastic-plastic, strain hardening) is consistent with the expected response of composite steel-concrete members. The strain hardening stage indicates that the steel tube material continues to carry increasing load even after yielding, which is a favorable characteristic for ductile design.

Key Reflections and Study Insights

The finding that existing code formulas show significant deviation from experimental results is an important warning for engineers relying on standard design provisions for micro steel tube piles. The proposed expression provides a more accurate tool, but its adoption requires validation against a broader database. From a welding perspective, if the micro steel tube piles involve field-welded splices, the weld quality would directly affect the continuity of the 78% steel tube contribution. Any weld defects would create stress concentrations that could initiate premature failure in the critical elastic-plastic transition stage.