the sides of some semi- trailers have a regular grid of flat circular disks (they look to be oh… 20 cm in diameter). What are these?
My guess is they are damping weights to keep the thin walls from vibrating.
They are mounting points for one type of strap used to tie loads down with.
They have a bracket on the inside a clip recerver sort of thing that the strap end lock into.
Found your post when my coworker and II were wondering the same thing luckily for us we work at a freight yard and just had to snag a driver to find out (after google faild us)
8years later
Wow. Is that a record for us from question to answer?
Maybe not the record to be proudest of.
“Post your burning question on SDMB. We’ll find your answer, even if it takes 8 years.”
I am still impressed, even if the answer is eight years late.
Poor Hooleehootoo will probably never see this. 
There’s hope; he’s posted here twice this month.
Sorry, I must have been out the day this question was posted. My bad.
It’s taking longer than Hooleehootoo thought.
He should have added Need answers fast!
Without the 20cm qualifier the answer could have been “the wheels”.