Chapter 3: Lifting Operations of Prefabricated Concrete Components
Content Summary
The lifting operation of prefabricated concrete components is essential throughout the construction process of prefabricated buildings. It involves lifting, positioning, and adjusting concrete components to complete their temporary placement. This chapter provides an in-depth overview of lifting operations, covering the types and characteristics of commonly used lifting machinery, selection of lifting equipment, and specific lifting methods for various prefabricated concrete components such as wall panels, floor slabs, beams, and stairs.
Initially, the chapter addresses the selection and matching of equipment and tools for lifting operations, along with relevant regulations. It then offers a comprehensive explanation of lifting processes, operational methods, temporary fixing facilities, and key considerations for different types of prefabricated concrete components.
3.2 Types and Selection of Lifting Equipment
2. Tower Crane Selection and Installation
1) Selection and Positioning of Tower Cranes
Generally, the choice of tower cranes depends on factors such as the building’s structural form, the maximum height for installing prefabricated components, their weight, and the volume of lifting work. The layout of tower cranes is primarily influenced by the building’s plan shape, the weight of prefabricated components, crane performance, and site terrain.
Before selecting a tower crane, calculate the weight of prefabricated components for each building segment to ensure the crane’s lifting capacity matches these weights at various outreach distances, leaving appropriate safety margins. Additionally, determine the actual lifting torque of the tower crane and the lifting height required for the components.
The factors influencing tower crane selection have evolved significantly in prefabricated concrete structures. Since the lifting and installation process differs from traditional methods, the tower crane must coordinate with the division of construction flow and its direction. Beyond general selection criteria, consider the following:
- (1) Choose a tower crane model capable of handling the heaviest prefabricated component at its maximum outreach, based on the component’s weight and approximate installation position.
- (2) The crane’s installation location should be determined by site conditions such as the building plan, structure type, basement design, transportation routes for prefabricated components, and construction flow. The crane should cover the entire site and be near areas requiring heavy lifting. Limit the arm length and mutual interference of multiple tower cranes to balance their operational zones.
- (3) Since many prefabricated components are transported on flat surfaces, plan internal transportation routes carefully, controlling slopes and turning radii. After selecting tower cranes, design road layouts based on the relative weight and installation positions of components, and consider the coverage provided by each crane.
- (4) Verify and compare the maximum weights of all prefabricated components, the heaviest lifting positions during construction, and tower crane lifting performance, ensuring sufficient safety margins to mitigate unforeseen challenges.
2) Requirements for Tower Crane Attachments
In traditional cast-in-place concrete structures, anchor points for tower crane attachments are usually set on beams, columns, or shear walls, reinforced as needed to handle stress after concrete curing. However, in prefabricated concrete structures, rapid construction means the structure may not be fully formed at anchoring time, and exterior wall components might not meet adhesion and force requirements. Unlike wet construction, adhesion and embedment cannot be handled the same way.
To ensure accurate positioning and proper force distribution at anchoring points, maintain the correct angle of support rods, and reduce attachment anchoring time, special attachment tools such as steel attachment beams (see Figure 3-3) must be used to meet force requirements.
3.2.2 Selection of Rigging Equipment
Prefabricated components vary in type, weight, shape, and center of gravity. Therefore, lifting points must be pre-designed, and appropriate lifting equipment selected accordingly. Regardless of the number of lifting points, the vertical line from the hook to the lifting device must always pass through the component’s center of gravity to ensure safe lifting.
To prevent swinging, tilting, rotating, or overturning during lifting, suitable lifting equipment must be chosen based on precise calculations.
1. Selection, Connection, and Retirement of Steel Wire Ropes
Steel wire rope consists of twisted steel wires with specific mechanical properties and dimensions, including a rope core and lubricating grease. It is formed by twisting multiple strands around the core, resulting in a high-strength, lightweight, and reliable rope that is resistant to sudden breakage.
In prefabricated concrete construction, steel wire ropes are primarily used for hoisting components. Proper selection and safe use of steel wire ropes are critical for construction safety.
2) Connection Methods for Steel Wire Ropes
There are two main methods for connecting steel wire ropes:
- Small Joint Method: Combines strands of two ropes within the joint area, resulting in a thicker rope end and shorter joint length.
- Large Joint Method: Involves cutting half the strands from each rope end and interconnecting them, creating a longer joint length.
Insertion methods include one-in-one, one-in-two, one-in-three, one-in-four, and one-in-five insertion techniques. The one-in-three insertion is most commonly used, while the one-in-five is mainly applied for integrated steel wire rope decoration.
Fixed connections at the rope ends typically use one of five methods: braiding, rope clamp fixation, sleeve pressing, wedge fixing, and lead filling, as illustrated in Figure 3-4.
Detailed descriptions of fixed connection methods:
(1) Braiding Method
Hand weaving involves manually inserting and braiding six-strand steel wire ropes with twisted fiber or metal cores to form sling buckles with loops, suitable for loopless buckles.
The rope clamp method is simple and reliable, widely used in practice. When using rope clamps, pay close attention to the number of clamps (see Table 3-5), spacing, orientation, and the strength of the fixing point.
Table 3-5 Recommended Number of Wire Rope Clamps
| Rope Diameter (dt, mm) | Minimum Clamps per Set |
|---|---|
| ≤18 | 3 |
| >18 – 26 | 4 |
| >26 – 36 | 5 |
| >36 – 44 | 6 |
| >44 – 60 | 7 |
Wire rope clamps should be spaced 6 to 7 times the diameter of the wire rope. The proper installation involves fastening the clamp saddle to the working section of the rope and the U-bolt to the tail end. Clamps should not be alternated along the rope.
(3) Aluminum Alloy Sleeve Compression Method
Aluminum alloy sleeves are selected based on the steel wire rope’s specifications. Two main crimping methods are used: force limiting and position limiting. Figure 3-8 shows an example of a steel wire rope end compressed with an aluminum alloy sleeve.
(4) Wedge Block and Wedge Sleeve Connection
Fixation involves wrapping the steel wire rope end around a grooved wedge, inserting it into a cone sleeve, and tightening it securely. This method is simple, firm, and reliable. Wedge cones are typically made from Grade 25 cast steel, while wedges are made from cast iron or regular steel plates. This method is usually used for steel wire ropes with diameters less than 40 mm.
Different fixing methods have distinct safety requirements, summarized in Table 3-6.
Table 3-6 Safety Requirements for Different Rope End Connections
| Connection Method | Safety Requirements |
|---|---|
| Braiding Method | Length should be at least 15 times the rope diameter and no less than 300 mm; connection strength ≥ 75% of rope breaking force. |
| Rope Clamp Fixation | Number of clamps per diameter per Table 3-5; follows GBT5976-2006 standards; connection strength ≥ 85% of rope breaking force. |
| Aluminum Alloy Sleeve Compression | Reliable bonding process required; connection strength equals breaking force of steel wire rope. |
| Wedge Block and Sleeve Connection | Wedge sleeves must be steel; connection strength ≥ 75% of rope breaking force. |
2. Use and Retirement Standards for Shackles
A shackle is a connector linking lifting points to steel wire ropes.
1) Precautions for Detachable Shackles:
- The shackle must bear loads along the axis of its centerline to avoid bending, unstable loads, or overloading.
- The pin shaft should rotate smoothly within the lifting hole without jamming.
- The shackle body must not be subjected to lateral bending moments; load should remain within the body’s plane.
2) Retirement Criteria for Shackles:
- Surface cracks present;
- Body distortion reaches 10%;
- Surface wear reaches 10%;
- Horizontal pin cannot be locked;
- Horizontal pin deformation exceeds 5% of original size;
- Corrosion or slipping teeth on bolts.
3. Selection of Hoists (Gourds)
Hoists are classified into manual and power types.
Manual hoists include hand-pulled hoists (Figure 3-12) and hand-operated hoists. They are lightweight, compact, portable, simple to operate, and adaptable to various working environments.
The hand-pulled hoist operates by dragging a hand chain and wheel, engaging friction plate ratchets and brake seats to rotate. It features a 5-speed long shaft rotating plate gear and a 4-speed short shaft spline gear that drives the lifting chain smoothly. The hand-cranked hoist uses a lever principle to manually generate traction force via a handle, moving loads either vertically or horizontally. Hand-pulled hoists typically lift vertically, while hand-cranked hoists are used for horizontal movement.
Power hoists use either steel wire ropes or ring chains for suspension. Electric and pneumatic hoists (Figure 3-13) fall under this category. They can be installed on monorail cranes, jib cranes, manual single-beam cranes, electric single-beam gantry cranes, and suspension cranes, providing efficient lifting and transportation of goods. Power hoists are characterized by simple structure, easy manufacturing and maintenance, good interchangeability, and straightforward operation.
4. Tool-Type Horizontal Suspension Beam
The lifting tool beam is a versatile, safe, and reliable device designed for lifting prefabricated components. It is fabricated by welding I-beams or similar profiles of suitable size and length. When in use, steel wire ropes are connected to reserved lifting rings on the prefabricated components via shackles, based on the component’s size, weight, and ring positions.
The lifting beam features multiple circular holes, allowing it to lift various prefabricated components by connecting shackles and steel wire ropes through these holes to ensure safety and efficiency. This design replaces traditional single-purpose lifting attachments, enabling one device to handle multiple component types and promoting organized and civilized site operations.
The horizontal suspension beam includes two adjustable hooks with variable spacing, accommodating prefabricated components of different sizes and reducing lifting costs. Since the hooks pass through the steel wire rope and remain perpendicular to the suspended object, the suspension points on both sides are equidistant from the center, preventing tilting and related accidents.
The “mouth”-shaped hanging beam is especially suitable for lifting L-shaped prefabricated wall panels, enhancing the stability during lifting operations.
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