awesome thanks

that did the trick - blocks marines but not aliens.
I used a trigger push and presence over each other - its ok if triggers are overlapping right?
THe only problem is that the trigger presence is set off by both marine grenades , so vent clearing is a bit tricky - i've just left some areas on the empty with enough room for a grenade to thru but not a player.
Although its pretty funny seeing a grenade rebounding off the trigger push