Kind: captions
Language: en
- [Narrator] March 26th, 2024.
A heavily loaded container
ship careens out of control
and heads straight toward
one of the supports
of the Francis Scott
Key Bridge in Baltimore.
The ship's data recorder
reveals that the Dali lost power
at 1:25 in the morning
and started to drift
rudderless at nine knots.
At that time, the vessel
was sailing closer
to one side of the shipping channel,
possibly causing pressure
differences along the ship's hull
that may have pushed the bow to the right.
At that precise moment,
the ship passes the mouth
of a river tributary
whose currents may have
pushed the stern to the left
and set the ship on a collision
course with the bridge pier.
With no propulsion to correct its course,
the 124,000-ton juggernaut was
only seconds from disaster.
- If they lost power 30 seconds
earlier, 30 seconds later,
you probably don't have the
collapse of the Key Bridge.
30 seconds earlier, the ship
may have sideswiped the pylon.
It may have gone aground
inside the bridge.
30 seconds later,
it may have coasted under
the bridge and not hit it.
- [Narrator] The critical
structural element
that allowed the Key Bridge
to reach all the way across the
river was its massive truss,
a lattice of steel beams
arranged in triangles
that made it light yet strong.
Spanning 1200 feet, it
was the third longest
continuous bridge truss ever constructed.
But at this extreme length,
it would have buckled under
its own weight without support.
It needed two large,
reinforced concrete piers to hold it up,
dividing it into three shorter spans.
The Key Bridge was a feat
of structural engineering,
but it had a fundamental weakness.
- If you take away a critical element,
you lose that equilibrium,
and the structure is gonna collapse.
- [Narrator] With one of the
main support piers destroyed,
the bridge cannot span such a distance
and begins to break apart.
- [Abi] You can see separation
happening at the bottom.
The collapse is progressing, actually,
and you can see it's breaking
off because of that tension.
- [Narrator] Without the rest of the truss
to hold it in tension, the
remaining span becomes unbalanced
and collapses under its own weight.
- A typical progressive collapse scenario.