49 CFR §393.201

1. (a)The frame or chassis of each commercial motor vehicle shall not be cracked, loose, sagging or broken.
2. (b)Bolts or brackets securing the cab or the body of the vehicle to the frame must not be loose, broken, or missing.
3. (c)The frame rail flanges between the axles shall not be bent, cut or notched, except as specified by the manufacturer.
4. (d)Parts and accessories shall not be welded to the frame or chassis of a commercial motor vehicle except in accordance with the vehicle manufacturer's recommendations. Any welded repair of the frame must also be in accordance with the vehicle manufacturer's recommendations.
5. (e)No holes shall be drilled in the top or bottom rail flanges, except as specified by the manufacturer.