How to show a "Session Expired" whenever a 30 mins passed without user activity

I put the following XML statement on the web.config file of my project
<sessionState timeout="30"/> However, even if I left the screens for more than 30 minutes, I can get back normally into the screen and resume the work without "Session Expired" message.
I want to see all the open running screens automatically redirected to "Session Expired" screen whenever the 30 minutes passed.
How to achieve that?