Previous articles have mentioned the importance of a load's weight and center of gravity. The need for a Rigger to have complete knowledge of these load factors has been stressed. These two parameters fall under the broad classification of Load Evaluation.
Evaluating The Load
In addition to a load's weight and center of gravity location, its balance, stability, and nature must be reasonably determined before the lift proceeds.
The Load's Weight
Guessing the weight of a load is unacceptable. Even an intelligent guess is unacceptable. The following guidelines list in order of ease and accuracy, the ways a load's weight can be established, or if neces- sary, calculated or accurately estimated.
- Inspect the load searching for any identification or marking indicating weight. If found, make sure that it is the weight of the entire load, and not just a single component of an as- sembly of parts. Use judgment.
- Check supporting shipping papers, manifests, bills of lading for load weight
- Contact the load's source (supplier or manufacturer) and request certified weight docu-
- Review any accompanying drawings of the load for a listing of the weight
- While still on the transporting vehicle, and knowing the tare, determine the load's weight via truck scales
- Calculate the load's weight by using available technical data such as materials' tables of weights. This is useful for rolled structural steel sections.
- Determine the weight of a load consisting of identical multiple pieces, such as a bundle of steel pipe, by weighing one piece in order to calculate the total extended weight
- Estimate the raw material's weight from a volume calculation of the load based on the measurement of its length, width, and height. Use the following table:
|Material||Pounds per cu. ft.||Kilograms per cu. m.|
Note: The values in this table are average and may vary depending on the specific class or grade of the indicated material.
The Load's Center Of Gravity
The balance and stability of the load is dependent on the load's center of gravity with relation to the attachment points. This was covered extensively in the article entitled Did You Ever Wonder Why 2 – Center of Gravity. It will be the intention of most lifts that the load remain level when clear of the ground or transporting vehicle. This will only occur if the hook of the hoisting equipment is positioned directly above the load's center of gravity. If lifting points (lugs, eyes, etc.) are apparent, they should be used.
Stability refers to the degree of tendency of the load to rotate about its center of gravity. Inspect the load to estimate the height of the center of gravity above the load's base. Loads with small bases and high centers of gravity need little force to initiate load rotation (instability). A load will be automatic- ally stable if the attachment point is above the center of gravity. Suspended loads otherwise stable can become unstable due to wind and hoist acceleration and braking.
The Nature Of The Load
The nature of the load deals with the configuration of corners and surfaces as well as the load's tem- perature. If the load exhibits sharp corners or is at an elevated temperature then chain slings may be in order. If the load has polished or delicate surfaces then a round sling of synthetic fiber may be more suitable. Under any circumstance, the need for edge protection should be evaluated, both for the load as well as the sling.