are there FMCSA rules governing trailer modifications?

[sriley]

Where are the rules concerning modifying your own car carrier? Is it in the FMCSA websight? I have been looking and can't seem to find anything on the subject. The issue is, I have a 4 car hauler. I want to make it into a 5 car hauler by adding a hydraulic 4 post lift to the bed right over the axles. That's a lot of welding and it modifies the condition of the original trailer. The weight factor I am not concerned about. I would add another axle for that. But what about design modifications? Are there any rules against it? And if so, where are they located?