i thought that for example a ballista only get +1 hex for its range in a barrack but it seems that they get +1 whenever they enter a building(any building including drawbridge)!!

Correct , all buildings give extra range, +1 for drawbridges, military academies, training guilds, outposts and barracks, +3 for towers.

owww...okie...doesnt seems to be logic and never was i told that even drawbridges gives that additional range
made me lost hundreads of ballista
..oh well..