thanks!
I made it! from another forum(ofp)...
Crate a trigger for each bridge section,and put
damage (object 167669) >= 1 in the condition field.
where the 167669 is the object id of the first bridge section (you change this for every section,you can click the button "show ID" to see their ID in the mission editor.)
and in the Activation field of each put:
bre = true;
then create another trigger and in the Condition field put:
bre
in Type select End
Do not forget to initialize bre to false first.
you can initialize this in player's init field!