The second deck however is a totally different animal. The ACTUAL structural is 51 lb STS and would be perfectly sufficient as a strength deck ONLY without the addition of armor.This decking runs athwartships from the back of torpedo bhd 3 (which supports the 12.1" Class A armor OUTBOARD), It is completely WELDED in place.
But the 4 3/4" thick Class B armor on top only runs between the Port & Stbd Holding Bhds 4. AND, it is NOT welded (sort of). It is secured to the 3rd deck by "Quilting Pins". These are a version of a "Cold" rivet. It's hard to describe by words only as it is a unique design and when I first reviewed the plans as to how they are made and installed gave me much more respect for the engineers that designed them and shipfitters who installed them back in those early post-depression times.
But, in a nutshell, it was determined that Class B armor was not a necessary weight to add on as the past the P&S Holding bulkhead. But it is interesting (though logical) that holding bhd 4 is only 1/4" thick M.S. between Main and 2nd Decks. But below 2nd deck it is 5/8" thick STS.