First of all, what is the subject of the sentence? What had kept?
Second, while "plywood pieces" is perfectly acceptable, I would say "pieces of plywood."
Finally, the tubes (or pipes) don't come on the floor, they meet the floor.
"Its pipes" is still a problem for me. The individual pieces of metal used in scaffolding are called "tubes". If you mean "where the scaffolding pipes/tubes meet the floor" then I think that's what you should say. You could omit "scaffolding" there and just leave "the pipes/tubes", I suppose. However, when most people see "pipes" or "tubes" they tend to think of drainage.
Are you trying to say, "They had only put pieces of plywood under the feet of the scaffolding" ?
Edit: I think you may be saying that instead of having proper welded plates on the bottoms of the scaffolding tubes/poles making stable bases, they had merely put pieces of plywood. Thus making the scaffolding dangerous.