first, make a func_chair, with the Leg property set to "Dislodge on trigger", then make a func_noose and have it target the func_chair.
Or you could just do what JB said...
HL has about 10 different enitities that you could use to simulate collapses of various natures. Read about all the func entities in the
HL Entity GuideP.S. no, there isn't a func_chair or noose...that was a joke