I am in process of building a deck over the top of and old concrete patio. I have just enough room to put girder against the concrete and then the joist on top of that. My question is, how do I fasten the gerder to the concrete? I was going to put a double girder in cause of the size of deck I am building with a 4x4 in between each set, normal the 4x4 would be set in to a concrete footing. But I am seting it on top the concrete already there? How do I fasten it to the concrete?

they make a 4x4 post anchor, it's a metal plate you fasten to the concrete and it has 2 lips with holes on each side, this is what you set you post into, and run screw into the metal thus into the 4x4

as far as fastening to concrete you'll need a hammerdrill and anchors, 1/2 anchors