Numerical Analysis of Grouted Micro Steel Pipe Pile Bending Capacity

Literature Overview

The paper by Tang Xianyuan and colleagues, published in Concrete in 2019, presents a numerical analysis of the bending capacity of grouted micro steel pipe piles, based on experimental data from nine specimens of different diameters. The authors use finite element analysis (FEA) to simulate the effects of steel ratio, steel yield strength, and core material strength on the bending capacity of the composite member. The study provides valuable insights into the design optimization of grouted micro steel pipe piles, which are increasingly used in foundation engineering for their high stiffness, rapid installation, and cost-effectiveness.

Test Methodology and Specimen Details

The experimental program involved the bending testing of nine grouted micro steel pipe pile specimens with different diameters and material properties. The test specimens were fabricated as follows:

The specimens were tested under four-point bending loading, with the span-to-depth ratio set at 4.0 to ensure a pure bending region at the midspan. The load was applied through a steel loading plate to distribute the reaction force over a sufficient area, and the deflection was measured using displacement transducers at the midspan and at the supports.

The steel pipes used for the test specimens were fabricated from ERW steel tubes conforming to GB/T 3091, with the weld seam located at the top of the pipe to minimize its effect on the bending behavior. The concrete core was cast in place within the steel tube after the tube was installed in a vertical orientation, and the specimens were allowed to cure for 28 days before testing.

Finite Element Model and Parameter Study

The FEA model was developed using the ABAQUS finite element software, with the steel tube modeled using shell elements (S4R) and the concrete core modeled using solid elements (C3D8R). The interface between the steel tube and the concrete core was modeled using a cohesive zone model that accounts for the bond strength and the potential for debonding.

The material models used in the FEA are as follows:

• Steel: Elastic-plastic material model with bilinear hardening, using the yield strength and elastic modulus measured from coupon tests.
• Concrete: Concrete Damaged Plasticity model, with the tensile and compressive strengths, elastic modulus, and Poisson's ratio defined according to GB 50010.
• Interface: Cohesive zone model with a linear elastic behavior up to the bond strength, followed by a softening behavior that simulates the progressive debonding.

The FEA results were compared with the experimental results, and the comparison showed good agreement, with the errors ranging from 6.4 percent to 13.6 percent. The FEA model was then used to perform a parametric study to investigate the effects of the following parameters on the bending capacity:

Key Findings

The parametric study yielded the following key findings:

1. The steel ratio is the most influential parameter on the bending capacity, with a doubling of the steel ratio resulting in an approximately 80 percent increase in the ultimate moment capacity. This is because the steel tube contributes significantly to the bending resistance through both the direct contribution of the steel cross-section and the confinement effect on the concrete core.
2. The steel yield strength has a significant effect on the bending capacity, with an increase from Q235 to Q460 resulting in a 45 to 55 percent increase in the ultimate moment capacity. This is because the higher yield strength allows the steel tube to sustain higher stresses before yielding, which delays the onset of plastic deformation and increases the ultimate capacity.
3. The core concrete strength has a negligible effect on the bending capacity, with an increase from C20 to C50 resulting in less than a 5 percent increase in the ultimate moment capacity. This is because the concrete core primarily contributes to the shear resistance and the confinement effect, rather than the bending resistance, and the bending capacity is dominated by the steel tube.
4. The pipe wall thickness and diameter have moderate effects on the bending capacity, with thicker and larger pipes providing higher capacities due to the increased steel cross-sectional area and the enhanced confinement effect.

Engineering Implications and Cost Optimization

The findings of this study have significant implications for the cost optimization of grouted micro steel pipe pile foundations. Since the core concrete strength has a negligible effect on the bending capacity, engineers can use lower-strength concrete for the core material without compromising the structural performance. This can result in significant cost savings, as the cost of concrete is directly proportional to its compressive strength.

For example, using C20 concrete instead of C40 concrete for the core material can reduce the concrete cost by approximately 30 percent, while the reduction in bending capacity is less than 5 percent. This cost saving can be offset by a slight increase in the steel ratio or the steel yield strength, which can be achieved at a lower cost than the increase in concrete strength.

From a steel pipe fabrication and welding perspective, the study highlights the importance of the steel ratio and the steel yield strength in determining the structural performance of the composite member. The steel ratio is directly related to the wall thickness and the outer diameter of the steel pipe, which are critical fabrication parameters that must be controlled with high precision. The steel yield strength is determined by the material grade of the steel pipe, which must be verified through chemical analysis and mechanical testing per GB/T 228.1 and GB/T 229.

Reflections on Welding Quality and Material Selection

The study underscores the critical role of steel pipe quality in the performance of grouted micro steel pipe piles. The ERW steel tubes used for the test specimens must be manufactured to meet the dimensional tolerances and mechanical property requirements specified in GB/T 3091, and any deviation from these requirements can lead to significant variations in the bending capacity.

The weld seam in an ERW steel tube is a potential weak point that can affect the bending performance of the composite member. The HAZ of the weld seam may exhibit different mechanical properties compared to the base metal, including a higher yield strength but lower ductility. This microstructural heterogeneity can lead to localized stress concentrations and premature fatigue crack initiation at the weld seam under cyclic loading. Therefore, the location of the weld seam relative to the critical sections of the pile should be considered in the design, and the weld seam should be inspected using non-destructive testing methods such as ultrasonic testing per GB/T 11345.

The selection of the steel grade for the pipe is another critical factor that affects the bending capacity and the cost of the foundation. Higher-strength steel grades, such as Q345 or Q460, provide higher bending capacities but may be more expensive and less readily available than lower-strength grades. Engineers must balance the structural requirements with the economic constraints to select the optimal steel grade for each application.

The numerical analysis presented in this paper provides a reliable and efficient tool for the design optimization of grouted micro steel pipe piles, and its accuracy depends on the fidelity of the material properties and the fabrication quality of the steel pipe components. Steel pipe manufacturers must ensure that the dimensional accuracy, material properties, and weld quality of the steel tubes meet the specifications required for the composite member to perform as predicted.