+1 i have search if someone already report this (and you do) cause inside Building who collapse on us, we don't die or even be hurt.

Reproduce ? : Go inside any house, put an explosive charge and move far enough in the same house, blow up the charge, the building will collapse on you and you wont die.
http://youtu.be/pQ-AkG3kfm8
A video of this bug here.

Edited description and added repro mission from duplicate ticket.

but - using then buildings as defence positions will be unpossible. In fact it isnt as easy to make building collapse in real life by just two shots of rocket launcher
i agree on the op point but arma engine have limitations when it comes to destruction. Charges could effectively take down building - in armaverse as well rockets/tank rounds - normally would have harder times

There should be a small chance of surviving a building collapse, just like some people survive such events during earthquakes. This is apart from any injuries caused by the weapon, that was used to demolish the building. This would be closer to reality and would add a factor of uncertainty (fog of war) to the gameplay.

The easiness of buildings' destruction with weapons is another matter, from actually surviving a collapse. Since I wasn't shooting at the buildings much, I can't comment on how good buildings' strength is portrayed in Arma 3 at the moment.

But if you think, that standard houses are like fortresses and make excellent defence positions, have a look athis Army test video (3 parts):
http://www.youtube.com/watch?v=YSqdTLLZBWw
http://www.youtube.com/watch?v=pSebnSQdJ2k
http://www.youtube.com/watch?v=02ECJ2Lt0cU

Buildings do provide concealment and some degree of protection, depending on construction details, bullet callibre and the angle at which bullet hit, but that's about it.