9.1 Lifting and Rigging
Key Takeaways
- Sling tension increases significantly as the sling angle to the horizontal decreases.
- The center of gravity of a load must always be directly below the hook to prevent swinging or tilting during lifting.
- Wire rope, synthetic, and chain slings each have distinct applications, capacities, and inspection requirements.
- Spreader beams help reduce sling angles and prevent crushing forces on the load.
- Accurate calculation of load weight, center of gravity, and sling tension is crucial for safe lifting operations.
Lifting and Rigging
Why This Topic Matters for the PE Construction Exam
Lifting and rigging operations are critical components of major construction projects. From setting structural steel to placing precast concrete elements, cranes and rigging hardware handle massive loads. The PE Construction exam tests your ability to ensure these operations are planned and executed safely. This involves selecting appropriate rigging hardware, determining the load's center of gravity (CG), and accurately calculating sling tensions. Failure to perform these calculations correctly can lead to catastrophic hardware failure, dropped loads, and severe safety incidents. Understanding the physics of lifting, particularly how sling angles amplify tension, is essential for a competent construction engineer.
Rigging Hardware
Rigging hardware connects the crane hook to the load. Selecting the right hardware depends on the load's weight, shape, and environmental conditions.
Slings
Slings are the most common type of rigging equipment. They come in three primary materials:
- Wire Rope Slings: These are extremely durable and commonly used in heavy construction. They consist of individual steel wires twisted into strands, which are then twisted around a core. They offer a good balance of strength, flexibility, and abrasion resistance. An Independent Wire Rope Core (IWRC) provides greater strength than a fiber core.
- Synthetic Slings: Made from materials like nylon or polyester, synthetic slings are lightweight, flexible, and protect delicate or finished surfaces from scratching. However, they are susceptible to cuts, abrasion, and damage from high temperatures or chemical exposure. They must be protected with edge softeners when used around sharp corners.
- Chain Slings: Chain slings are the most rugged and durable, ideal for lifting hot materials or operating in harsh environments where wire rope or synthetic slings would fail. They are typically made of Grade 80 or Grade 100 alloy steel. They are resistant to abrasion and cuts but are heavy and require careful inspection for stretched or deformed links.
Shackles
Shackles are used to connect slings to the load, to other rigging hardware, or to the crane hook. The most common types are anchor (bow) shackles and chain (D) shackles. Screw pin shackles are used for temporary applications, while bolt-type shackles with a nut and cotter pin are used for long-term or permanent installations. The working load limit (WLL) is usually stamped on the shackle body.
Spreader Beams and Lifting Beams
- Spreader Beams: These beams are loaded in compression. They use a top rigging setup (slings converging to the crane hook) and vertical slings dropping down to the load. Spreader beams prevent crushing forces on the load and keep the lifting slings completely vertical, which maximizes their lifting capacity.
- Lifting Beams: These are loaded in bending. They have a single lifting lug on top that connects directly to the crane hook and multiple lugs on the bottom to connect to the load. They require less headroom than spreader beams but must be heavily constructed to resist bending moments.
Center of Gravity (CG) Calculations
The Center of Gravity is the point at which a load's weight is evenly distributed in all directions. When lifted, the CG will naturally align itself directly beneath the crane hook. If the rigging is not arranged so that the crane hook is directly above the CG before the lift begins, the load will swing or tilt unpredictably once it leaves the ground.
For a composite load made of multiple parts (weights $W_1, W_2, ... W_n$) located at distances ($x_1, x_2, ... x_n$) from a reference datum, the location of the CG ($ar{x}$) is calculated using the moment principle:
If the CG is not perfectly centered between the lifting points, the load will not be distributed equally among the slings. The sling closest to the CG will carry a larger proportion of the load.
Sling Tension Calculations
The most critical calculation in rigging is determining the tension in the slings. Sling tension is influenced not only by the weight of the load but also by the angle of the sling relative to the horizontal.
As the angle between the sling and the horizontal decreases, the tension in the sling increases dramatically. This is a common trap on the PE exam and in the field.
The Sling Tension Formula
For a load suspended by symmetrical slings (all slings at the same angle to the horizontal), the tension ($T$) in each sling leg can be calculated as:
Where:
- $W$ = Total weight of the load (including rigging hardware)
- $N$ = Number of load-bearing sling legs
- $\theta$ = Angle of the sling relative to the horizontal
Note: If the angle is given relative to the vertical (let's call it $\phi$), the formula uses $\cos(\phi)$. Always pay attention to which angle is provided!
Alternatively, using the geometry of the rigging triangle:
Where:
- $L$ = Length of the sling
- $H$ = Vertical height from the load to the hook
The ratio $L/H$ is known as the Load Angle Factor or Sling Tension Factor.
Impact of the Sling Angle
Consider a 10,000 lb load lifted by two slings:
- At 90° (vertical): Tension = 10,000 / (2 * sin(90°)) = 5,000 lbs per sling.
- At 60°: Tension = 10,000 / (2 * sin(60°)) = 5,774 lbs per sling.
- At 45°: Tension = 10,000 / (2 * sin(45°)) = 7,071 lbs per sling.
- At 30°: Tension = 10,000 / (2 * sin(30°)) = 10,000 lbs per sling. (The tension in each sling equals the entire weight of the load!)
Industry best practice recommends maintaining sling angles greater than 60° and strictly avoiding angles less than 30°.
Worked Example: Sling Tension Math
Scenario: A concrete precast panel weighing 12,000 lbs is being lifted using a 2-leg wire rope sling bridle. The slings attach to the panel at pick points that are 16 feet apart. The crane hook is positioned directly over the center of the panel at a vertical height of 6 feet above the pick points. What is the tension in each sling leg?
Step 1: Determine the geometry of the rigging triangle.
- Base of the right triangle = Half the distance between pick points = 16 ft / 2 = 8 ft.
- Vertical height ($H$) = 6 ft.
- Using the Pythagorean theorem to find the sling length ($L$):
Step 2: Calculate the Load Angle Factor.
- Load Angle Factor = $L / H$ = 10 / 6 = 1.667.
Step 3: Calculate the tension per sling.
- The load is shared between 2 slings ($N = 2$).
- $T = (W / N) \times (L / H)$
- $T = (12,000 \text{ lbs} / 2) \times 1.667 = 6,000 \times 1.667 = 10,000 \text{ lbs}$.
Alternatively, using angles:
- $\tan(\theta) = \text{Opposite} / \text{Adjacent} = H / \text{Base} = 6 / 8 = 0.75$
- $\theta = \arctan(0.75) = 36.87^\circ$
- $T = \frac{12,000}{2 \cdot \sin(36.87^\circ)} = \frac{6,000}{0.6} = 10,000 \text{ lbs}$.
The tension in each sling leg is 10,000 lbs.
A 20,000 lb equipment skid is being lifted using a 4-leg sling assembly. The center of gravity is exactly in the middle. The slings form a 45-degree angle with the horizontal plane of the load. Assuming the load is perfectly balanced among all four slings, what is the approximate tension in each sling leg?
When designing a rigging plan for a fragile, thin-walled concrete pipe, which type of lifting device is most appropriate to minimize horizontal compressive forces on the pipe?