don't use rub rail for securement, but what is the underside called?

You are going to get answers all over the place on this. I've owned trailers where the underlip was just as scary as the rub rail. I've had trailers with no rub rail at all and the under lip would kick the strap hooks out at a weird angle. My current trailer eats straps when strapped to the under lip, right where the strap is sewn to the hook. I've had this trailer for 12 years now and the only way I hook my straps is to the rub rail. It's also a 96" trailer that has the winches permanently welded to the side of the trailer with rub rails between the winches. The trailer has a sticker that says the stake pockets and pipe spools on the rail are rated to 6000# each. This has been good enough for every scale guy I've encountered when hooking on the rub rail near them.

My little hot shot trailer (25K gooseneck behind an F450) also has issues hooking 2" ratchet straps to the under lip and running behind the rub rail. Pipe spools and stake pockets are rated on this trailer as well, so we just hook to rub rails.

I DGAF what other super truckers and flatbedders think about hooking to rub rails. I've hauled thousands of loads of lumber this way with zero issues over the last 20 years hooking straps to rub rails.

 

Some trailers have a piece of flat bar, steel or aluminum bolted or welded to each crossmember. It is called an "under frame tiedown flat"..



Otherwise you are usually just going to hook to the deck frame outer rail, or lace rail, which is usually C-channel. On lower end gooseneck's, some lace rail is just 6"-8" flat bar..

PJ's in particular used to, might still use tube for the lace rail. In which case you can either spec the under frame tiedown flat, or you got to hook to the rub rail. A few chain tail straps come in handy so you can grab pockets & spools as well..

 

If the rub rail has a listed wll on your data plate then use the rail

 

Anchor points are not required to be rated. Where ever a hook can fit, a hook belongs. Never had an issue.

 

§ 393.104 What standards must cargo securement devices and systems meet in order to satisfy the requirements of this subpart?

(c) Vehicle structures and anchor points. Vehicle structures, floors, walls, decks, tiedown anchor points, headerboards, bulkheads, stakes, posts, and associated mounting pockets used to contain or secure articles of cargo must be strong enough to meet the performance criteria of § 393.102, with no damaged or weakened components, such as, but not limited to, cracks or cuts that will adversely affect their performance for cargo securement purposes, including reducing the working load limit.​

 

Unrated and Unmarked Anchor Points

FMCSAs cargo securement rules do not require rating and marking of anchor points. While the agency encourages manufacturers to rate and mark anchor points, the new rules do not include a requirement for ratings and markings.

 

I would ask you for the cite, but I know where you got it from, and it's from the old text from 2002 currently outdated. Last updated: Monday, March 3, 2014 which you can read here:
Cargo Securement Rules | FMCSA


The cite I gave you, is current:

Displaying title 49, up to date as of 12/23/2022. Title 49 was last amended 12/22/2022. view historical versions​

Which by the way, supercede any and all prior references, when it is stipulated, and a codified section overrules a general description section. They may not have to rate and mark, but as the driver, we are required to know if the structure can meet the requirements of the section referenced by my previous reference which I suspect, for reason of merchantability and reduction of liability, I have seen manufacturers increasing the documentation and marking of tiedown/anchor points with ratings.