Blowing up bridges

.....so my question is, can you significantly damage a bridge with 300lbs of explosive [ backpacked in ] ?... damage meaning it can't be repaired [ or fixed to hold vehicles--this isn't where safety groups will be checking it out ] in the ''standard'' amount of time ...time that will significantly alter the battle/etc ....also, will there be troops guarding the bridge?
..time: is there a battle brewing? are troops/tanks/etc going to use it for a retreat? if not, does it matter if the bridge is even blown?
..backpacked in--if you bring in more explosives, that means more men = more chance of being discovered...

...a lot of issues to think about, yes? well, that's reality --not the movies

 

Thank you bronk7, that pretty much sums up my initial post

 

..I did look at some modern examples and it looks like you can do some damage if you hit the right spot ..I am very, very surprised that a truck could possibly cause a bridge problem
however, it does say:

bold mine
...I would think a truck hitting the bridge would be a '''common'''/'''probable'' incident''

I-5 and the Physics of Bridge Collapses | RealClearScience
9 Common Reasons for Bridge Failures - Bridge Masters