Colin, I've done it that way and ultimately didn't like it. I don't like that there's nothing other than friction holding the spanset on the fork.
What I've started doing when I span off the Sumners is to choke the corner of the fork, instead of choking the fork itself.
Here's one for you statics guys:
This weekend we put up a square grid for an event that had to move inside. 40x40 box on six Sumners.
The tour rigger came back behind us and added steel cable "Cross bracing" because "all lift structures have to be guy wired down, no matter what."
The red is the cable he added:
To me, this accomplished exactly two things, neither of which was desirable, without adding any safety:
1: Horizontal Vector: side load on the top of the lifts, which is horrible.
2: vertical vector: additional load, reducing weight capacity of lifts.
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He also claimed that the lifts had to be derated 75% because the load center was 20' out from the forks, and the lifts specifically say "650lbs at 16" load center." What is the load center of a truss or square supported evenly at each end/corner?
The final bit of enjoyment was the point in the day where he decided to call Sumner to explain what we were doing and voice his concerns. He told the client if Sumner deosn't sign off on it, he wouldn't sign off on it and the show would be cancelled.
Sumner, of course, wouldn't say a word either way on the phone. I asked the guy what his take on the first page of the CM manual (Not for use for overhead lifting) is, and he had nothing.
So. Is my 40x40 rig unsafe, or was this guy wrong, or both?