What is the reasoning behind WLL requirements?

First off I know the law. It is here. (393.106 see below)

My question is why is this adequate? Grade 70 3/8 has a WLL of 6,600 lb. Hauling a piece of machinery with four direct tie downs you get 3,300 lb credit for each or 13,200 lb aggregate.The law sates your aggregate only needs to be 1/2 the weight of the machine. So with the four 3/8” chains, one on each corner, you could legally tie down a 26,400 lb machine.

My question is with this setup really only two at most chains or 13,200 lb of WLL are ever keeping the machine from moving in any direction. If it wants to go forward only the back two chains are stopping it.

I know it is legal, but why wouldn’t you want more like each chain ( or multiple chains on each corner) being 1/2 the weight of the machine? This way the two chains in the rear would add up to the weight of the machine and be able to stop it from going forward.

I have been hauling equipment for years and know this is not done but logically I don’t quite get it.
I don’t want to make more work for myself and use eight 3/8” chains to tie down a 20k machine that would be crazy I just am curious why basically a WLL of half the weight of the machine is deemed adequate to restrain movement in a certain direction.




(d) Aggregate working load limit for tiedowns. The aggregate working load limit of tiedowns used to secure an article or group of articles against movement must be at least one-half times the weight of the article or group of articles. The aggregate working load limit is the sum of:

(1) One-half the working load limit of each tiedown that goes from an anchor point on the vehicle to an anchor point on an article of cargo;

(2) One-half the working load limit of each tiedown that is attached to an anchor point on the vehicle, passes through, over, or around the article of cargo, and is then attached to an anchor point on the same side of the vehicle.

(3) The working load limit for each tiedown that goes from an anchor point on the vehicle, through, over, or around the article of cargo, and then attaches to another anchor point on the other side of the vehicle.

 

So in your opinion are more than four grade 70 3/8” chains needed for a 20k machine?

 

Actually that kind of is my question. I know I am legal but in my mind I am not sure it is good enough for anything that might happen.

To take the generality of the question away I frequently haul a small dozer that weighs 19k and a rubber tracked machine that weighs nearly the same. I have been securing both with four 3/8” grade 70 chains. By the law I have over 7k more securement than needed. Still though I can’t wrap my head around how 13,200 WLL of chain holding it from going forward is sufficient if it weighs 19,000 lb.

I recently got a new trailer and am going to make some chains cut to length to secure these things and was thinking of upping them from 3/8” to 1/2”. My trailer has d rings rated at 15 something.