Mechanical Performance Analysis of Hollow Sandwich Steel Pipe Concrete Wind Power Towers Under Wind Load

Literature Overview

This study addresses the structural behavior of hollow sandwich steel pipe concrete (HSSPC) wind power towers subjected to wind loading. Wind turbine towers are among the tallest civil structures in energy infrastructure, and their design must withstand extreme wind speeds, fatigue cycles, and seismic events over a 25-year design life. The sandwich configuration—typically consisting of an inner steel pipe, an outer steel pipe, and a concrete layer sandwiched between them—offers a hybrid structural system that combines the tensile strength of steel with the compressive capacity and damping characteristics of concrete.

Core Technical Viewpoints

The sandwich structure leverages the composite action between steel and concrete through interfacial bonding, often enhanced by mechanical interlocks such as ribs, studs, or corrugations on the inner pipe surface. Under wind load, the tower experiences primarily bending moments, with the outer fibers of the cross-section carrying the highest stresses. The sandwich geometry effectively increases the section modulus and moment of inertia while keeping the overall diameter within transportation and erection limits.

A critical aspect of this design is the load transfer mechanism between the steel shells and the concrete core. The interfacial shear stress governs the degree of composite action, and insufficient bonding can lead to partial composite behavior, reducing the effective stiffness and strength of the tower. The study examines how wind-induced lateral loads create a complex stress state involving axial compression, hoop tension, and shear flow at the steel-concrete interface.

Key Technical Parameters and Design Considerations

Structural Analysis Under Wind Load

Wind loading on wind power towers follows a power-law vertical profile, where wind speed increases with height. The tower acts as a cantilever beam with a variable cross-section, and the first natural frequency typically ranges from 0.15 Hz to 0.35 Hz for heights between 80 m and 160 m. The design must ensure that this frequency does not coincide with the vortex shedding frequency (Strouhal number) or the rotor passing frequency (1P, 3P excitations), as resonance would amplify dynamic responses beyond acceptable limits.

The sandwich configuration provides several advantages over monolithic steel tubular towers. First, the concrete layer increases the mass per unit length, which shifts the natural frequency downward and improves the tower's capacity to resist dynamic wind loads. Second, concrete provides inherent damping (typically 2–5% critical damping versus 0.5–1% for steel), reducing fatigue damage accumulation. Third, the composite section offers superior buckling resistance compared to a single steel shell of equivalent weight.

However, the sandwich design introduces challenges that must be carefully managed. The concrete placement process within a vertical or inclined pipe is technically demanding and requires controlled vibration and compaction to avoid voids or honeycombing. The steel-concrete interface must be protected against corrosion, as the presence of steel in contact with concrete can lead to localized galvanic corrosion if the concrete cover is insufficient or the pH environment is compromised. Additionally, thermal expansion mismatch between steel and concrete creates differential stresses, particularly in regions of rapid temperature change such as near the nacelle interface.

Engineering Practice and Quality Control

From a quality control perspective, the following checkpoints are essential:

1. Pre-welding inspection: Verify steel pipe dimensions, straightness, and surface condition. Confirm that the inner pipe surface preparation (studs or ribs) meets the specified geometry and spacing.
2. Welding quality: The longitudinal and circumferential welds of both inner and outer pipes must meet full-penetration requirements. Ultrasonic testing (UT) per ASME V or GB/T 11345 is mandatory, with acceptance criteria typically at Level B or better.
3. Concrete placement: Monitor slump flow, air content, and temperature of the concrete mix. Use low-noise vibrators and ensure continuous placement to avoid cold joints. Post-placement, conduct rebound testing and ultrasonic pulse velocity testing to verify concrete integrity.
4. Interfacial bond verification: Pull-off tests on representative samples can quantify the bond strength between steel and concrete. Values below 1.5 MPa may indicate insufficient composite action.
5. Post-erection inspection: Visual inspection, magnetic particle testing (MT) of welds, and dimensional survey to confirm plumbness within ±1/1000 of tower height.

Study Insights and Implications

The hollow sandwich steel pipe concrete concept represents a promising evolution in wind turbine tower design, particularly for offshore and high-wind-speed onshore applications where structural efficiency and fatigue resistance are paramount. The key insight from this literature is that the composite action between steel and concrete, when properly designed and executed, can deliver structural performance that exceeds what either material alone could achieve. However, the realization of this potential depends critically on construction quality—particularly the interfacial bond and concrete integrity—which must be rigorously controlled through inspection protocols and process monitoring. For engineers involved in wind energy infrastructure, understanding the load transfer mechanisms and failure modes of sandwich structures is essential for safe and economical design.