Tutorials
Breakable glass in a rotating door
Start off by making a brush the size of a door and texture it how you want your door to look. Cut the brush into pieces in such a way that a brush is formed in about the center of the door. This brush will be the glass pane.
Now select the brush that will be your glass pane and texture it with a glass texture of sorts.
Now tie the glass pane to an entity (ctrl + t), making it a func_physbox. Set the Prop Data to Glass.Small, the Render Mode to texture, and the fx_amount somewhere around 120. Go to the flags tab and check the boxes "Ignore +use for pickup" And "Don't take physics damage" (if you do not see these options under flags then click apply and the options should come up).
Now select all the brushes that are going to be a part of the door and don't forget about the origin brush which you would also select; DO NOT select the func_physbox you just created. Tie what you have selected to an entity (ctrl + t), making it a func_door_rotating. Select the Force Close property of the func_door_rotating and select yes in the drop down box (If you don't do this the func_physbox will prevent the door from opening).
Now make a new brush on the front of the door. The brush should overlap the junction between the func_physbox and the door, covering the bottom front of the func_physbox by one unit and the front of the door by one unit, ie. the brush should be two units tall. In the top view make sure that the brush is at least two units deep as well (if it is only one unit deep the func_physbox will fall out of the door the instant you load your map).
Now do the same to all the front edges of the func_physbox so that you have created a frame around it. Select the frame you have created and copy and paste it (shift + left mouse button) to the other side so that the func_physbox is sealed in the door completely.
Select both of the frames, the frame on the front and the frame on the back, and also select your func_door_rotating. The tie the selection (you should not have the func_physbox selected) to an entity (ctrl + t). It will prompt you if you want to add the brushes to a currently selected entity, press yes.
You now should have two entities, a func_door_rotating with a hole in it and a func_physbox that fits perfectly within the hole of the door. I advise now that you select both entities and group them (ctrl + g) and now you can move them around easily and copy the group to make another door.
Here is a picture of me opening the door.
Here is a picture of my shooting the glass out, sorry that I don't have good enough reflexes to take a screen shot at the exact moment that the glass is breaking into pieces, but believe me when I say a that a split second later that glass pane was lying on the floor in pieces.
Related:
Example Map
Anchorite