seige equipment and arrow throwers

[Liberator]

How to take out seige equipment?
I thought that I could take out seige equipment like towers and battering rams with burning arrows, but arrows seem to have no effect (or at least damage shown remains at 0%), but I have noticed that towers actually do collapse and rams actually bust in flames at times. When and why?
I also thought that I could destroy sap points by attacking them (With cavallery, for instance), but there seems to be no effect, either.