If USD falls, its a domino effect to economic system. Therefore, bitcoin would slide down a bit and its very erratic by that time. Btc really depends on the monetary system and dollar exchange and everything that goes and beyond the bank in one way or another.
If USD gets failed then it would make a remarkable effect on the value of bitcoin in the international market zone. Bitcoin is entirely dependent upon the dollars to maintain its value in the monetary matters and this is just going to be limited to a question only as dollar is never going to get down.