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Need to locally reinforce an oil platform deck - 5/8-inch A36 steel plate - with more flat plate welded on top. Span between W14 x 26 supports is 45 inches. Can't weld anything underneath due to access problems and can't use beams on top due to height restrictions.
I'm thinking 1-inch thick x 12 inches wide fillet welded on top of the main deck plate.
Questions:
1 - Is it reasonable to assume if the reinforcing top plate is welded to the bottom plate, the two will act as one plate?
2 - is it reasonable to assume the "beam is fully fixed at each end or should I treat it as simply supported?
3 - how do I work out the shear stress - and thus size - the fillet weld between the two?

Thanks