Application of Prestressed Steel Strand in Wind Power Tower Cylinders

The prestressing system is one of the core technologies in wind power hybrid towers, playing a vital role in enhancing the load-bearing capacity, wind resistance, and structural stability of tower cylinders. By symmetrically arranging steel strands on the cylindrical walls and applying prestress to form a pre-compressed stress field, it effectively suppresses cracks, improves stiffness and bending resistance, serving as a critical measure for ensuring safe operation of the tower body.

Prestressed steel strands are among the core load-bearing components in wind power hybrid towers (referred to as "wind power hybrid towers"). Through pre-applied stress optimization of concrete structures, they adapt to the special working environment of wind power towers—high loads, large displacements, and long-term exposure to alternating loads—thereby significantly enhancing tower safety, economy, and durability.

The following elaborates on application background, core functions, technical highlights, and advantages:

I. Application Background:

Demand and Challenges for Wind Power Hybrid Towers Wind turbine towers must support the nacelle (including generators) and blades (with total weight reaching hundreds of tons), typically exceeding 100 meters (even over 200 meters) in height. They must withstand complex loads:
> Static loads: Self-weight, nacelle weight, blade weight;
>Dynamic loads: Wind-induced vibrations (impact and alternating stresses under strong winds), periodic loads from blade rotation;
>Environmental loads: Earthquakes, temperature changes, corrosion (in coastal/industrial areas). Traditional pure steel structure towers face challenges beyond 100 meters: High steel consumption (costly), insufficient rigidity (vibration-prone), difficult anti-corrosion maintenance; conventional reinforced concrete towers struggle with poor crack resistance (easily cracked under tension leading to rebar corrosion) and excessive self-weight (increasing foundation load).
Therefore, prestressed concrete hybrid towers (consisting of lower concrete sections with upper steel structures or fully prestressed concrete) have become the preferred choice, with prestressed steel strands serving as their core technical support

II. Core Functions of Prestressed Steel Strands 

By pre-embedding high-strength steel strands in concrete tower segments and applying prestress to the concrete during tensioning, these strands counteract tensile stresses caused by external loads, achieving the following functions:
>Enhancing Crack Resistance: Wind pressure and vibration loads cause hoop and vertical tensile stresses in concrete cylinder walls. The prestress from steel strands offsets part of these stresses, maintaining long-term compression of concrete to prevent cracking (which leads to rebar corrosion and reduced stiffness).
> Strengthening Stiffness and Stability: Prestress ensures more uniform stress distribution in concrete, reducing deformation (bending, torsion), decreasing tower vibration amplitude under strong winds, and preventing resonance with blades and nacelles (which exacerbates structural fatigue).
>Optimizing Cross-Section and Reducing Self-Weight: Prestress enhances concrete's "effective load-bearing capacity," allowing smaller cross-sectional dimensions (especially at variable-diameter upper sections), reducing concrete usage and lowering foundation costs (accounting for over 30% of total infrastructure expenses).
>Improving Fatigue Resistance: Wind turbines endure alternating loads (with load cycles changing with each blade rotation). Using low-relaxation, high-fatigue-strength materials like 1860MPa-grade steel strands, prestressed steel strands can withstand prolonged alternating stresses and prevent fractures.

III. Technical Specifications

1. Material Selection Steel Strand Type:
Primarily 1860MPa grade high-strength low-relaxation steel strands with tensile strength ≥1860MPa and relaxation rate ≤2.5% (1000 hours) to ensure minimal long-term prestress loss. Corrosion Protection: Protected by "non-bonded coatings (grease+PE sheath)" or "bonded channel grouting (cement slurry+corrosion additives)" to withstand corrosive environments such as humid and coastal areas.
2. Layout Method Based on tower load characteristics,
prestressed steel strands are arranged in circumferential and vertical configurations:
Circumferential Prestress: Uniformly distributed along the tower circumference (dozens per circle) to resist wind-induced circumferential tensile stress (similar to a "tightening spell"), particularly requiring denser spacing at large-diameter sections (maximum wind load). Vertical Prestress: Distributed vertically along the tower height to resist vertical bending stress from self-weight and nacelle loads, with optimized distribution at diameter transition sections to avoid stress concentration.
3. Construction Techniques Pre-embedded Channels: Pre-install corrugated pipes (or plastic corrugated pipes) before concrete pouring as steel strand passages, ensuring channel curvature matches the tower's cross-section. Tension Control: After concrete strength reaches design value (typically ≥85%), use jacks to synchronize tensioning of steel strands (dual control of stress and elongation) for uniform force distribution. Anchoring & Protection: Post-tensioning, secure with anchoring devices (flange-type or pier-head type), and fill channel gaps with high-pressure grout to prevent corrosion.
4. Performance Assurance Fatigue Resistance: Steel strands must undergo 2 million cycles of alternating loading tests (with a stress amplitude of 120 MPa) to ensure fatigue fracture resistance over a service life exceeding 20 years. Prestress Loss Control: Through measures such as low-relaxation steel strands, super-tensioning (1.03 times design stress), and delayed tensioning (to reduce concrete creep loss), total prestress loss is controlled within 15%.

IV. Application Advantages

Compared to pure steel structures or conventional concrete towers, the core advantages of prestressed steel strand hybrid towers are: Lower Cost: For tower heights above 120 meters, prestressed concrete sections save 30%-50% material costs compared to steel structures (concrete unit prices are significantly lower than steel). Enhanced Durability: The concrete-clad and anti-corrosion treatment extends the lifespan of steel strands to over 25 years, eliminating frequent maintenance (steel structures require regular painting at high costs). Wider Adaptability: Can support taller towers (160 meters+), adapt to low-wind regions (higher elevations provide more stable wind speeds), and improve power generation efficiency.

V. Case Studies Widely adopted globally:

For example, onshore wind projects in Germany and Spain (towers 140-160 meters high), and low-wind areas like Inner Mongolia and Gansu (using "concrete lower section + steel upper section" hybrid towers 120 meters high) have all achieved safe and stable operation through prestressed steel strand technology.
In conclusion, prestressed steel strand solves the core problems of "crack resistance, stiffness, fatigue and cost" of wind power hybrid tower by optimizing the stress state of concrete. It is the key support for the development of wind power high tower technology. In the future, with the expansion of wind power to high altitude and low wind speed areas, its application will be more extensive.