Regarding the design weight for bridge: The weight of a container of water increases when a floating object is added to the water, because you are adding more mass to the previous single unit of mass. The water level rises in the container (an amount relative to the volume of the floating object); however, this results in an increased weight equal to that of the added floating object. Don’t believe this, then prove it to yourself by using a digital postal scale, placing on the scale a partially filled bowl of water, record the weight of the bowl and water, place a floating object in the bowl and notice that the weight will increase the exact weight of the floating object. The only way your theory works is if the water level does not rise by venting off the water (opening the ends of the bridge or container) to remove the excess volume of displaced water which resulted from introducing the floating object. So the old fly-in-the-bottle high school physics doesn't apply.... if the water rises in the bridge... it will weigh more!