You should probably give examples of champion you think are squishy and you want to build rather tanky items on. I could think of Anivia fo instance as Froggen used to be build her somewhat tanky even in competetive games (Season 4). There a few other AP Carries I can think of you mostly try to build HP on additionally. However, I think it's fairly difficult to define squishyness. Many champions you can build purely damage based can work with only one or two damage/ bruiser itemsI'm thinking of Rengar for instance.
Sometimes I'd get Mercury's Treads but only if they lack an adc, most of the time I don't build this though since Katarina can't make use of the Tenacity.
Sometimes I'd get Mercury's Treads but only if they lack an adc, most of the time I don't build this though since Katarina can't make use of the Tenacity.