My earlier reply seems to have gone...
Checkered plate is often used for moveable covers and would be considered pinned all around. Sometimes they are loose plates with a 'handle or ring' for lifting, sometimes they are hinged on one side, again with a 'handle or ring' for lifting. It's not often that the edges are fixed. Permanent covers can be welded in place and the fixity caused by the distance from the edge of the opening to the location of the weld. This may be an inch or two and for a 1/4 checkered plate, this would create a relatively fixed edge condition. Also if the checkered plate is part of a floor and has a series of channels or angles in a grid pattern as support... the interior portions would be almost a fixed end condition.
Since checkered plate covers are nearly always not fixed at the perimeter, nearly always they fall into the pinned condition. The corner lever moments are nearly always small and negligible.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
