I do drag my trailers down Forest Service and BLM roads.   I have cracked a couple of holding tanks and drug the rear of the trailer over a few rough spots.

Aside from damaging the holding tanks,  (which hang down lower) The ony damage I've down was dragging the rear of the trailer through some washes and that broke the last aluminum cross member at the back of the trailer.  The dealer welded that back up in a few minutes when I took in back in for repair. So bent frames are not what I'd consider normal wear and tear for even off road trailers.

I'd think folks must be trying to drag across a pretty big rock to hit the other members that are not at the extreme rear of the trailer.

If the member is bent, then it probably has weaked the floor in that area. Most flooring will be screwed or welded to the cross members.  If the cross member has moved, then those fastners will have lost their hold. This might also show up as cracked wood planks or torn aluminum planks.

Pretty much anything can be repaired, But a bent frame is not something most welding shops are going to be real happy to work on. It's underneath the trailer where you have only 8" to 12" of clearance.  You would be laying on your back working above your face. The cross members are not only welded at the sides, but probably have some form of attachment to the floor from above, that's not accessable from below.  Trailers are big enough that most shops don't have the means to jack them up so they can walk around underneath, nor do you roll them over on their side to work on the bottom. So damage that happens underneath, usually stays damaged.

It depends on the type of damage. The bottom flange of the "I" beam is the most exposed part of the structure, and is the most likely area to show the results of any incidents. Aluminum framing is pretty neat stuff. Depending on the alloy, it can flex without work hardening, has a memory, and can absorb some damage without transferring the energy to others areas.

If you look at an I beam and the bottom flange is bent, but the web and top flange aren't, chances are the damage may be localized. If you were to inspect the areas around that damage, such as the welds to the flooring where they attach to the top flange, or the welds at the end of the joist, where it is attached to the perimeter frame, you may find no additional damage.

 Additional inspections can be made of the impact area to determine if there are any cracks in the extrusion. The metal can be cleaned, and a dye penetrant applied, followed up with a developer. This will specifically show any cracks.

If the welds are intact and there is no visible distortion of any structure, chances are that the frame's integrity has not been jeopardized. However, if the web is crushed, and the flooring above the damaged area is distorted, a new joist will have to be inserted at some cost. In addition, if a joist is bent where the flanges are distorted laterally in a bow shape, this is bad news. The I beam has been caught on something and has been displaced. The flooring in the weld areas will probably be torn, and the ends of the joists may be broken away from the perimeter frame. In a worse case scenario, the perimeter frame may be pulled inward in a coke bottle shape, distorting the outer shape of the trailer.

The common approval for a proper weld of any material, is that the finished joint is as strong as, or stronger than, the base material. All of the statements, "aluminum welds are brittle and will break" are bogus. If the welds are breaking, the welder should be retrained or fired, and replaced with someone who is competent. The welds near any damaged areas, should be as strong as the material itself. They should not break until the frame itself breaks.

The rear bottom frame structure of my larger trailer, has the scars and dents of many steep driveways, gravel roads, and rocky fields. Some we were aware of and heard when they happened, others went unnoticed. A visual inspection, if you suspect any damage, is warranted.

I would not be afraid to purchase a used trailer, with some damages to the bottom flanges. But I definitely would inspect further for any related damages, and reject any trailers that showed visual structural distortions.

BOL   Gard