With the evolution of modern construction, precast concrete elements are assuming an increasingly vital role. Anchoring technology within these elements serves as a critical foundation for ensuring overall structural stability and safety.
Precast elements are factory-manufactured building components subsequently installed on-site. To establish strong and reliable connections—both between precast units and with the cast-in-place structure—specialized anchoring systems are essential. This article examines the working principles, application scenarios, and selection criteria for lifting anchors in precast construction.
How They Work: The Mechanics of Load Transfer
A lifting anchor is an embedded mechanical interlocking device that creates a defined load path from the concrete element to the crane. During casting, the anchor—typically forged from high-strength carbon or stainless steel—is placed within the formwork and encased in concrete. As the concrete cures, it forms a robust mechanical connection with the anchor’s engineered geometry.
Load transfer occurs in three distinct stages:
1. Direct Engagement: The anchor head—whether spherical, eye-shaped, or threaded—engages with the lifting clutch or hook, receiving the direct tensile force.
2. Force Transmission: This force is transferred along the anchor’s shank to its embedded end.
3. Load Distribution: Crucially, the anchor’s widened base or expansion fins transfer the load into the surrounding concrete. This occurs primarily through bearing pressure and the formation of a concrete shear cone, rather than relying on simple friction.
Anchor Types and Their Uses
Depending on the component and lifting requirements, anchors are available in several forms:
- Threaded Inserts: Ideal for thinner elements, offering straightforward installation.
- Lifting Loops/Rings: Provide multi-angle flexibility and are common in wall panels.
- Plate Anchors: Ensure stable load distribution for large-span beams and columns, preventing concentrated stresses.
Each design is precisely engineered to ensure stresses in the concrete remain within a safe range during hoisting.
Key Application Scenarios and Selection Guide
| Precast component types | Recommended main anchor types | Application Scenarios and Explanations |
| Precast concrete wall panels | Pre-embedded lifting nails/nuts, plate anchors | - Standard wall panel: 4-8 pre-embedded hanging nails are symmetrically arranged at the top for vertical hoisting. - Large integrated insulation and decorative panel: Plate anchors are used to distribute the load and avoid localized damage. - Thin-walled decorative panel: Self-tapping screws (post-anchoring) or small pre-embedded parts may be used. |
| Precast floor slabs/stairs | Pre-embedded lifting nails/nuts, and rebar lifting rings | - Large integrated thermal insulation and decorative panels: Utilize plate-type anchors to distribute the load and prevent localized damage. - Thin-walled decorative panels: May use self-tapping screws (post-anchoring) or small embedded parts. - Composite floor slabs: Typically have 4 lifting points, using pre-embedded nuts, resulting in a smooth surface after installation. - Precast stairs: Lifting points are often located near the upper platform, allowing the stairs to naturally form an installation angle. The center of gravity needs to be calculated. |
| Structural components such as beams and columns | Embedded hanger nails/nuts, prestressed tendon ends | - Precast columns: 2-4 symmetrical lifting points are set at the top for vertical hoisting. - Large prestressed beams (double T-slabs, I-beams): Lifting points are usually set at or near both ends; sometimes prestressed ends are used directly. - Pipe galleries and tunnel segments: Plate anchorages must be used to evenly bear the enormous hoisting forces. |
| Irregularly shaped components/municipal components | Combination use, special lifting tools | - Utility tunnels and tunnel segments: Plate anchors must be used to evenly distribute the enormous lifting forces. - Irregularly shaped balconies and bay windows: Based on the center of gravity analysis, multiple pre-embedded hangers should be placed in sturdy areas, and balance beams or multi-point suspension systems may be used to ensure stability. |
| Thin sheet/small components | Self-tapping lifting screws, small embedded parts | - Thin plates less than 100mm thick: Insufficient pre-embedding depth; self-tapping screws are a better choice for post-installation. - Curbs, small paving stones: Specialized clamps may be used instead of anchors. |
Anchor selection depends on the component's type, weight, shape, center of gravity, and production process. However, misuse remains a leading cause of failure. Common errors include:
- Incorrect Type Selection: Using anchors designed for thick sections in thin panels, or for edge/bevel lifts, can cause failure.
- Equipment Mismatch: Employing a lifting clutch not fully compatible with the anchor head leads to partial engagement and uneven force distribution.
- Premature Lifting: Hoisting before concrete achieves its specified strength is hazardous. Inconsistent curing on tight schedules can leave the concrete around the anchor too weak, causing failure before the steel reaches its capacity.
- Poor Placement: Anchors placed too close to an edge, with insufficient embedment, or in poorly compacted concrete create stress concentrations, making the concrete the weak link.
Core Design and Safety Principles
1. Calculated Design: The quantity, size, and placement of anchors must be calculated by a structural engineer, considering the component's weight, a dynamic factor (typically 1.5), concrete strength, and lift angle.
2. Center of Gravity: Lifting points must be arranged so their connection line passes through or above the component's center of gravity to ensure stable, balanced lifts.
3. Preventing Cone Failure: The primary failure mode involves the concrete fracturing in a cone shape around the anchor. Adequate spacing between anchors and distance from edges are required to prevent these cones from intersecting or breaking to the surface.
4. Safe Lifting Angles: When using slings, the horizontal angle must be considered. Smaller angles drastically increase the force on the anchors; angles below 45 degrees are strictly prohibited.
5. Quality Assurance: Only certified, high-strength anchors compliant with relevant standards (e.g., GB/T 37610) must be used. The use of makeshift anchors from bent rebar is unacceptable.
