Thread by @MattDursh

Bridge design for vessel collision. A🧵

The main span of the Francois Scott Key Bridge is 1300 ft. It also has 185 ft of clearance, making this a massive bridge.

This type of bridge is considered complex.

Baltimore is in for a long haul before replacement. Here is why.

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We design modern bridges for ship impact, but this was not always the case.

In 1980 the Sunshine Skyway Bridge also collapsed from vessel strike.

The photo below is the original Skyway. Similar bridges and identical failure.

The Skyway collapse changed bridge design.

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The Baltimore bridge collapsed because it got hit by container ship. What failed first?

It appears the bow of the ship made contact with the vertical columns that supported the truss superstructure, causing it to have a cascade failure.

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This bridge was going to fail from this event. It simply was not designed for an equivalent static force that is well over 3 million pounds.

The container ship, assuming the navigation channel is centered, veered over 500 feet off course.

Why did the whole thing fall?

4/x

The whole truss fell because this is a continuous bridge.

This means that the 3 span unit behaves as as one. If one span fails, the maximum dead loads redistributes.

This provides benefits to load resistance and is how we design modern bridges for this.

How?

5/x

Modern bridges deal with vessel collision two ways.

The first is to use a dolphin.

This is a mass of rock, sand, and steel that serves to stop the vessel before it makes contact with the bridge.

Likely the new bridge replacement will use a dolphin as one method.

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The second is to design the bridge to take the vessel strike and resist the event.

This is a massive undertaking with a central focus: don't collapse.

We will see localized failures, but maintain global stability.

This load could be well over 3 million pounds.

7/x

To resist that much load to stop a vessel, we need a flexible bridge and a lot of foundations, such as piling or drilled shafts.

Typically this is more foundation needed than to simply resist earthquakes, hurricanes, and every day loads.

So where do we go from here?

8/x

Baltimore is going to be without a critical bridge for a long time.

Tampa's Skyway bridge took 7 years, but this will be done sooner, hopefully much sooner.

What needs to be done? Well, a lot.

Can the approach spans be salvaged?

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The approach spans are likely fine.

But are they tall enough to support the new main span? Does the bridge need more vertical clearance?

Officials will nees to ask... do they fully replace the bridge in full, or attempt to reopen sooner with only the main span?

10/x

The main span is not easy to design or build, unless they make a decision to go to a smaller span bridge (less than 375ft).

But that comes with new challenges particularly with how close the piers will be to the navigation channel.

11/x

There are many things left to talk about here, but on a plane now.

I specialize in bridge design of long bridges over navigable waters.

Thanks for reading and happy to answer all questions.

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