Those are called "puddle welds" and they were part of the roof diagram.
Sounds like those washers were used because the steel decking gauge was too thin to meet whatever uplift calc the engineer had to use.

It's unusual to see washers used like that.
Any pics ?

So these puddle welds are probably in every down flute ?

Without seeing a pic, I would want to destroy the washer and the decking while leaving most of the puddle weld.
Cutting between the beam and decking with a Sawzall sounds brutal.

I would lean more toward a peanut grinder (4-5") or even a 7" with a metal stone to go after the washer/weld.
It would be extremely useful to have a scissor lift, 1.) For the safety of working off of and 2.) To utilize its uplift power to push up on the deck panels after the welds were weakened.

A short 2x4 or 4x4 post from the scissor deck to the roof panel would be how to apply some push to the decking.
A gas powered scissor lift has a big deck and has lots of power compared to a skinny electric scissor, if you have the room and ventilation.

Another useful tool would be an air chisel like you would see in a muffler shop to go after those washers.

Edit:
Not knowing how clean you need to get the beams, a faster method would be to take a peanut grinder with a cut-off disk and simply do a 4-sided cut around the washer/puddle weld. I think that this would be the fastest way to get the decking off, even if you had to come back and touch up the welds.


I swear grinders cause more injuries than anything else so make sure they know what they're doing.
If they hit the work wrong the grinder can walk onto the user.

Edit:
A tool that we use for cutting deck panels during install is what's referred to as a "Hot Saw". This is a chain saw motor fitted with a large circular Carborundum blade.
They are not for the uninitiated, especially on a roof but they make short work of cutting up pan decking.

Every once in a while a tool comes along and I say "Holy Schitt".
Just recently I used a good bunch of these metal-cutting in a Skil Saw.
They look like wood blades from a distance but they sail through steel beams....big schitt.....freakin' amazing.
Got 'em at the Depot and it seems that they were a Makita product.

These would be very good for cutting up the deck panels if necessary for handling.
I have never seen a better metal cutting for a Skil Saw.
They're almost unbelievable. Not cheap, magic costs money.