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Ready for the answer nobody likes...? "Well it depends,..." There are several pieces of information missing from your question, like the following:
1. How far down are you going?
2.How many bays are attached to the leg attached to the clamp?
3. How many plank levels are there?
4.What capacity are you trying to get? i.e.Light, medium or heavy duty?
These are just a few questions that should be answered before you can pose the very broad question above. I'm sure someone can give you an answer that they heard one time from that one guy on that one job somewhere else, but I believe anyone who could actually answer that question, i.e the qualified person, is not going to give you an answer without more information. Helpful? Not really, just honest. Sorry
The only way you can get a more definite answer would be to get one from the manufacturer of the tube showing the load bearing capacity of a tube in a cantilever of 12", or of course you could consult an engineer, but theyre going to ask for many more answers before they give you one:) Sorry I couldn't be of more help.
Incidentally where did the 3 1/2" specification come from? .
If it is over 12 inches must have extra support or 45. to hold the weight