Variable Load
In nearly 30 years in the offshore helicopter industry, I haven't heard of integrity issues with the decks themselves.
Most oil companies and rig operators who own facilities with helidecks do or should have annual audits carried out which check deck integrity along with several pages of other items.
The worldwide standard for auditing a helideck is generally the excellent UK CAA CAP437 and most checklists over the years have been constructed using 437 as a reference.