if you really really want this to be done you need to cheat.
the way i got around the problems of getting armour to traverse that particular bridge was to teleport tanks from one side to the other, and then made up the storyline around it
the better way would be to use setvelocity or a setpos loop to capture your groups and move them across in single file by hand.
if the columns are vehicles, use setvelocity. see the
comref for the details.
if you've got infantry, setvelocity doesn't work on them, so i would suggest setting up something like this -
assuming the bridge is exactly horizontal across the map, i.e. west/east
_step = 0.3
#loop
{_x setpos [(getpos _x select 0) + _step, getpos _x select 1]} foreach units bridge_group
?not (loons_across_bridge):goto "loop"
exit
for going across the other way, use - _step instead of +.
it's a very messy way to achieve what you want, and the loons will look like they're moonwalking, but that's ofp for you
EDIT - doesn't work, just tested it and they float across the water.
this does work however -
{_x setdir 270} foreach units bridge1
{_x setdir 90} foreach units bridge2
#loop
{_x switchmove "CivilWalkF"} foreach units bridge1
{_x switchmove "CivilWalkF"} foreach units bridge2
~1.3
?not(loons_across_bridge):goto "loop"
{_x switchmove "stand"} foreach units bridge1
{_x switchmove "stand"} foreach units bridge2
exit
bridge1 are on the left side of the bridge, bridge2 on the right. problem is, after they stop, they continue again.... worth playing with though