CRANE HOT LINE® March 2018 • www.cranehotline.com32

The Proper Way to Set Up Four-Leg Rigging

Spotlight Rigging By Mike Riggs

E arly in my career as a crane and rig-ging trainer and a course developer, I was taught a new (to me) approach

regarding four-leg rigging systems. It required that when a rigger uses four-leg rigging, he or she must down-rate the system to a capacity based on just two legs supporting the load. That was deemed necessary because people believed that even though the load might start with four supporting legs, there would be a point between liftoff and landing where only two legs would support the load. A common demonstration often used to help students visualize the idea is having four students hold up a table, one student supporting each corner. As the table is moved, the elevation of each corner changes. That change causes a shift in load capacity for each corner throughout the table’s movement. That idea and the demonstration are incorrect. Let’s look deeper into the use of four-leg rigging and see what really happens during load movement.

Equal and Unequal Loading It is true that rigging a four-leg sling system incorrectly can create a case in which only two of the slings support the entire load, while the other two slings only help balance it. When that happens, the two slings that support the load are always diagonal from each other. The possibility of that phenomenon leads many riggers to re-rate a four-leg system as if only two legs will support the total weight of the load. Here are conditions that can cause uneven loading in a four-leg rigging system:

A rigid load combined with any of

the following:a. Sling legs of alloy chain, wire

rope, or high-performance fiber roundslings that are not of equal length.

b. Sling attachment points that are not equal distance from the center of gravity (CG).

c. Different sized connecting hardware.

d. Incorrect placement of slings on the hook or the load.

We should teach riggers how to rig a load properly to prevent this prob-lem, not to adjust ratings based on the assumption that it will happen. Once a load is suspended, each sling leg’s share of the load (the percentage of the load weight each sling holds) does not change. It stays the same throughout the entire lift. The only change in loading comes from dynamic loading. By observation, a rigger can determine, with some accu-racy, the loading condition of the sling legs as they are being loaded.

• If all four legs show tension equally, each leg will be lifting an equal 25% share of the load.

• If two legs that are diagonal of each other begin showing tension sooner than the other two slings, those two slings will each be lift-ing more than a 25% of the load.

• If possible, a rigger should touch each leg to feel for tension in the slings before the load is lifted from the ground. When two of the slings are not sharing the load, there will be no tension.

• If you don’t start with two legs unloaded your will never have two legs unloaded.

Once the slings are loaded, the only change in sling tension will come from

dynamic forces as the load is moved. Because the horizontal angle of each sling is less than 90°, horizontal force will not change the load on it. Similarly, if the rigging starts with two supporting legs and two balanc-ing legs, that condition will not change during the lift. Each sling leg will con-tinue to support the same share of the load it had at the beginning.

Ensuring Equal Loading If the sling, hardware, or attachment points require load sharing as near to 25% as possible, use the following methods to ensure equal loading:

1. Chain hoists2. Turnbuckles3. Adjustable hitches4. Two slings of equal lengths attached

to one end of the load and one sling twice that length attached in an inverted basket hitch on the other end. For example, two 10-ft. slings rigged in a straight-line hitch at one end and one 20-ft. sling rigged in an inverted basket hitch on the other end.

5. Four slings and attaching hard-ware of equal lengths correctly attached to the load and a lifting hook

6. Placing slings of unequal lengths on load and hook so that they diagonally equalize length as closely as possible

7. Use of two, two-leg bridle setups rather than a four-leg bridle or four individual legs attached to the crane hook

How Conservative to Be? How conservative should a rigger be when choosing the capacity for four-leg rigging? When a four-leg system is rigged correctly, two legs should never carry the whole load.

Mike Riggs is founder and president of Rigging Institute, which provides rigging application and inspection programs suited to individuals involved in today’s hoisting and rigging industry. He can be reached at mriggs@rigginginstitute.com.

www.cranehotline.com • March 2018 CRANE HOT LINE® 33

Four-Leg Rigging Set-Up

When some details of the load are unknown, a competent rigger should select a four-leg system in which the capacity of three legs can support the load. Another consideration is each leg’s rated capacity compared to the calcu-lated loading on it. For example, if each leg must support 5.78 tons (Fig. 1) and the min-imum size of sling required to support 5.78 tons is a 7/8” wire rope sling (see Table 1) that has a capacity of 7.6 tons, the design factor based on sling loading is 6.57:1. In the same case, selecting a sling one size higher (a 1” wire rope sling with 9.8-ton capacity), increases the design factor to 8.47:1. In that solu-tion, even if each sling doesn’t share the load equally, there is plenty of capacity to support the difference. Selecting rigging one size larger than required is a practical and safe way to maintain a 5:1 design factor on each leg. Here is what we learned from four tests of sling loading. You can see a video at https://www. youtube.com/watch?v=kllHev5f14A.

1. After all sling legs are loaded, theystay loaded

2. When only two slings carry the

load, the other two stay unloaded3. Sling loading increases from

dynamic loading, not a change inthe share of load

4. The share of load will change onlywhen the load’s center of gravitymoves closer to or farther fromthe attachment points

5. The horizontal load force createdby sling angle prevents changes tothe share of load

6. An inverted basket hitch will notadjust unless the D/d ratio at the

hook is large enough and the load is heavy enough to cause the sling to adjust

Now, why is the table demonstra-tion not valid? The table demonstration accurately illustrates share of load for something supported on rollers, skates, or air bearings. It is not accurate for a suspended load because it does not factor in the horizontal load force created by sling angle. For horizontal load force calcu-lations see Table 2.

Table 2

Table 1

Fig. 1