breakable wood

What propertie do I set my brush so when I shoot it it will break and
the object such as a platform would fall over after I shot that piece
of wood holding it up?

Many entities have a Health property that you can set so they break, and you'd probably do what you're suggseting with the platform by making it a func_physbox and setting its Motion Disabled flag, and setting up the part that breaks to Enable Motion on the physbox when the breakable is destroyed.

Example please

Ok, after a little chatting with Baddog I was able to help him out. He wanted a platform that was held up by three breakable objects to perform realistically. I told him this.

Platform: func_physbox with Motion Disabled.

Breakables: func_physbox with a specific strength and parented to the platform. In their output tabs, they were configured to enable motion in the platform when they break.

I made a tut on glass and breakable wood, etc. check it out.