Sources for Effects of Ancient and Medieval Throwing Artillery in Sieges?

Mar 2016
A complicated title, but my question is for hints to ancient and medieval sources or maybe clear archaeological proof of effects of heavy stone throwing machines (torsion, tension or gravity, like ballista, onager, trebuchet, such stuff) against stone walls during sieges. I know they were effective anti-personal as wall cleaners and could wreak havok to buildings and weaker top structures. I got however the impression of them not being very effective as wall breakers, not at all, but I'm curious for examples to read.