Except that... then what? That isn't how a coup works. You can't just take hostages and then make demands about who gets to be the President. It has no effect. Official actions taken under duress are invalid. As soon as you leave the building nobody is going to honor anything you demanded.
It's easy to take a building. Just show up with greater numbers than there are cops in the area. But a building is not the government. Physical possession of the Senate floor isn't what grants the power to do anything. It's purely symbolic.
The fact is that law enforcement operates primarily through deterrence. You can install some bulletproof glass and things like that, but the main thing preventing murders and such is that if you do it then you go to jail for a long time after.