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It depends on how the deckis "engineered". Some could be built that are barely able to support their own weight. Others could be built strong enough to park your car on them. It depends on the type and dimension of wood used for the joists, the spacing between joists and the length of the span.
Typically you would not put hardwood flooring directly over the joists. You should first put down a subfloor.
That depends upon the size of the joists & grade/species of wood
That depends on how well supported the floor is and that depends on how far apart the joists are placed. The closer they are, the more weight can be supported.
120 pounds
Depends on the structural integrity of the deck.
How long are the 2x10's, what is supporting the deck and how is the deck attached to that support. A lot of factors go into figuring load limits.
not much
Depends on the area above the ceiling joists. Is it a live load (living space) or dead load (just the joists with maybe an attic above)? Also, the length of the ceiling joists from the last bearing point is a factor. How much weight is bearing on the beam is the critical issue.
it can hold as much weight as it can handle
4.3 Lbs. That's based on the length/width of the deck being no more than 6". A suspension bridge will support more... Go find some string!
Yes , you can skate with a cracked deck as long as the board has enough strength to have the weight of the person on it without bending too much