The consequences would have been even more dramatic in broad daylight.
The Francis Scott Key Bridge in Baltimore (Maryland, United States) collapsed overnight from Monday to Tuesday after being hit by a cargo ship.
Several cars were traveling on it at the time of the impact, and up to twenty people fell into the water.
The “span” of this structure built in 1972, that is to say the maximum space between two pillars, was 366 m.
Bridge and road engineer Michel Virlogeux, who participated in the design of numerous works in France and around the world, explains to us why “zero risk does not exist”.
Does this accident in the middle of the night in Baltimore surprise you?
MICHEL VIRLOGEUX
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Not so much, because a range of 366 m is not colossal and a trajectory error can be enough.
It's always the same problem: the less space between two batteries, the greater the risk of shock.
When a ship collides with a fixed obstacle, it deforms and then, over time, this creates a greater and greater force on the foundations which may not resist.
The kinetic energy of a cargo ship is gigantic, and no pillar of a bridge could withstand such a shock!
Most often, it collapses very quickly or even immediately.
VIDEO.
United States: Baltimore bridge collapses when hit by cargo ship
Is this risk of collision taken into account when designing a bridge?
Of course !
The only way to be sure not to have collusion would be to not have a battery in the water.
At the time of the first Normandy Bridge project in the 1970s, there was a pier in the middle of the Seine, protected by an artificial island.
The project was stopped then it was resumed in 1986, and I became technical manager.
In the meantime, a lot of structures had been destroyed around the world after boat collisions: the Tasman Bridge (Australia), the Tampa Bridge (United States), etc.
We concluded that plans had to be changed.
Ultimately, the span of the Normandy Bridge was 856 m.
If we do not have the means and there is no other choice than to have a pile in the middle, we must limit the risk of shock by taking into account the nature of the passing boats, their capacity to deform, entrained water, etc.
For the Île de Ré bridge, for example, we sized the foundations to take this possibility into account.
But it is obvious that if a 200,000 ton tanker hits it, it will collapse!
And zero risk does not exist: complete fog, the captain may be drunk, etc.
Couldn't a boat deviate from its trajectory to avoid a collision?
A boat cannot be navigated like a car!
For a large cargo ship, it only takes a small navigation error 1 km before passing the bridge for it to be too late.
It obviously depends on the type of boat but, if you take a 200,000 tonne tanker, you don't change its trajectory quickly.
Can the shock also occur at the level of the bridge itself, at height?
No, because we impose a minimum size.
And the upper parts which could hit a bridge, like the mast of a sailboat, are not very strong and therefore aggressive for the bridge.