Rigging Fundamentals

Rigging Fundamentals

Determining Load Weight

Weight estimation is nothing more than figuring out how much material you have and then- multiplying that amount by what the material weighs. Many riggers have learned through experience how to estimate weight to within a few pounds of an object's actual weight.

Estimating an object's weight should always be a last resort. Everyeffort should be made to discover an object's actual weight through consultation of engineering diagrams, bills of lading, placarding or catalogs.

REMEMBER: The Total Load is the weight of the load plus the weight of the rigging.

Pounds per Cubic Foot / Pounds per square foot
Steel / 490 / ¼” Steel Plate / 10
Aluminum / 165 / ½” Steel Plate / 20
Concrete / 150 / 1” Steel Plate / 40
Wood / 50 / Pounds per gallon
(7.5 gallons per cubic foot)
Sand, Gravel, Dry / 105
Sand, Gravel, Wet / 120 / Diesel Fuel / 7
Soil, Dry, Loose / 75 / Gasoline / 6
Soil, Wet / 100 / Water / 8.3

All weights shown are approximate

PROBLEM #1:

Estimate the load weight for a bundle of lumber 4' high x 4' wide x 16' long:

PROBLEM #2:

Estimate the load weight for a concrete pipe that has a 4 foot OD, is 6 inches thick and 12 feet long.

PROBLEM #3:

Estimate the load weight for a ½”steel plate that is 4 feet long and 3 feet wide.

Rigging Angles and their Effect on Sling Capacity

As the horizontal angle between the legs of a sling

and the load decreases, the load on the sling legs

increases. The effect is the same whether a single

sling is used as a basket, or two slings are used in a

bridle configuration.

The farther the sling legs get from vertical, the more tension is introduced into the sling legs. The tension

decreases sling capacity because the sling is applying crush force to the load.

The crush force applied to a load can pose a significant problem or no problem at all; it depends on theload and the amount of tension. The crush force can manifest itself in damaged loads or damaged slings - care must be exercised in determining sling tension and selecting the proper rigging equipment.

Tension

Anytime pull is exerted at an angle on the leg of a sling, the load in each leg can be calculated using the following formula:

Step 1: Divide the total weight of the lift by the number of legs to be used - Also known as the Vertical Load. This gives you the Share of the Load for each leg.

Step 2: Measure the length of each sling leg from bearing point to bearing point.

Step 3: Measure the head height in the sling from the load hook to the top of the load.

Step 4: Divide the sling length measurement by the head height. This gives you the Load Factor

Step 5: Calculate the sling tension using this formula: Share of the Load x Load Factor =Tension in each leg.

For Example:

1) 8000 Lbs ÷ 2 sling legs =4000 pounds per leg

Share of the Load

2) 6 feet (sling length) ÷ 4 feet (head height) = 1.5

Load Factor

3) 4000 x 1.5 = 6000 pounds tension in each leg.

The total load weight is 8000 lbs. which will be carried on a 4 legged chain bridle. Sling lengths are 8’ and head height is 6’