Bridges have very high armour values, bt can still be destroyed with satchel charges. I believe you need around 5 in total, or maybe 6, play around to find out how many.
I would suggest either using a backpack addon to carry the satchels, or a vehicle loaded up with satchels, unless you want a script to destroy it, which would use ID numbers, for example :
(object #####) setDammage 4
Well, I think that's the right syntax IIRC.