# Water Bridge Over Elbe River

**4**Comments

Even after you see it, it is still hard to believe! A Water Bridge in Germany…. What a feat!

Six year s, 500 million euros, 918 meters long…….now this is engineering!

This is a channel-bridge over the River Elbe and joins the former East and West Germany, as part of the unification project. It is located in the city of Magdeburg, near Berlin. The photo was taken on the day of inauguration.

To those who appreciate engineering projects, here’s a puzzle for you armchair engineers and physicists. Did that bridge have to be designed to withstand the additional weight of ship and barge traffic, or just the weight of the water?

Read on to check if your answer is correct

It only needs to be designed to withstand the weight of the water!

Why? A ship always displaces an amount of water that weighs the same as the ship, regardless of how heavily a ship may be loaded.

Obviously, the person who asked the question and provided the “answer” should go back to Physics 101, if he or she ever attended the class. Archimede’s principle has nothing to do with the force exerted on the bridge by the water AND a ship! To convince yourself, Just draw a “force diagram” at the location of the bottom of the bridge!

“It only needs to be designed to withstand the weight of the water!”

Not true. It is true that a floating object displaces an amount of water equal to the object’s weight (Archimedes’ Principle), but the weight of the water and boat together is higher that the weight of the water alone. This is true regardless of whether the object floats on the surface or lies on the bottom. The increase in total weight added by the boat is generally very small compared to the weight of the water. The increase in total weight is significant only if the object causes the water level to rise appreciably. In an extreme case, if the floating boat caused the water level to rise to twice its initial height, the weight of the boat and the weight of the water are equal.

I can’t believe two people have posted claiming the laws of physics are wrong..

“It only needs to be designed to withstand the weight of the water!

Why? A ship always displaces an amount of water that weighs the same as the ship, regardless of how heavily a ship may be loaded.”

as the artical states.. the boat is displacing its weight in water. so if the bridge holds up 1000 tons of water without a boat.. a boat weighing 50tons would displace 50 tons of water, and there would then be 950 tons of water and 50 tons of boat going over the bridge.. there would still only be 1000 tons of total weight on the bridge..

as for the water level rising to compensate for the weight of the boat.. yes it would rise.. but over the entire length of the river.. which would be marginal at best..

Steve,

Where does the 50 tons of water go?

I don’t know how the channel is built, but I have an theory that any boat floating anywhere in the channel will increase the overall load on the bridge. Assuming the channel is ultimately sealed, which is not unreasonable considreing it’s elevated, the displaced water will disperse over the entire area of the channel evenly. For arguments sake, let’s say 1% of the channel is on the bridge. This means 1% of the displaced water will be redistributed over the bridge. So a 50 ton boat anywhere in the channel will add an additional 0.5 tons of load over the bridge span.

It’s purely a theory. I have no way to prove or disprove it. Does anyone else have an idea on this?

Mark.