In last week's competion, my ball came to rest on the bridge that crosses the ditch on the 4th hole. My playing partner was a little suprised when I informed him that I was going to take a drop, behind the bridge and that it would cost me a penalty shot. The reason …. well the part of the bridge where the ball came to rest was over the hazzard and therefore if the bridge had not been there, my ball would be in the ditch.
The full explanation is "The part of a bridge that crosses OVER a water hazard is actually IN the hazard, so any ball lying on such a part of the bridge is considered to be in the hazard. As with any ball lying in a hazard, you may play it as it lies or take relief under Rule 26-1 (more on that in a minute).
If you choose to play the ball as it lies, you get a little break in that you are permitted to ground your club. This is because a bridge over a hazard is not considered to be ground in the hazard, but rather an obstruction in the hazard, since it is man-made [Decision 13-4/30]. While Rule 13-4 prohibits you from touching the ground in a hazard, the Note to that rule allows you to touch obstructions in a hazard.
If you choose to take relief outside the hazard, you incur the same penalty and have the same relief options as for any ball lying in a water hazard [Rule 26-1]. Add one penalty stroke to your score, and drop your ball. This is summarised on the back of the card.