Katarina is, i'm sorry to say, not very viable in ranked play. she is super strong and in lower ELO is easy to own with. but in higher ELO play she will be exhausted or cc'd out of her ult no mater what. now that being said she is still very good at shutting people down in lane like a well played AP YI. but somewhat nonviable later game.
now that being said there's not much to say about the build it's a bit basic and gives her a overall good sustain and damage output. and when well played can give you or your team a edge on the enemy's by roaming and shutting down lanes and just destroying your lane.
Don't start a fight with your ult end it you're not an initiate you're a clean up.
good luck and have fun.