I had tried a lot to do this (for my Oct level design contest!), but failed
In the end I had to make do with spawning a small vial (with strength=1,explodable=yes,damage=100) a little above the floor. And it should fall close to a barrel (or whatever you want to to explode!), and set that strength to around 10 (just so that it doesnt break otherwise) and set the damage to around 100 or so.
Let me know if you have problems with that.
-filya
p.s: it would be better to make that vial invisible, but if the explosion is far away from the trigger zone, it wouldnt matter.
-- n00b at playing games...and now at making em too :p --