Baltimore bridge collapse: an engineer explains what happened, and what needs to change

Aerial photo of a large cargo vessel as it crashes into the Francis Scott Key Bridge in Baltimore, Maryland, prompting its collapse.
Aerial photo of a large cargo vessel as it crashes into the Francis Scott Key Bridge in Baltimore, Maryland, prompting its collapse. (Image credit: Maxar / AAP)

When the container ship MV Dali, 300 meters long and massing around 100,000 tons, lost power and slammed into one of the support piers of the Francis Scott Key Bridge in Baltimore, the bridge collapsed in moments. Six people are presumed dead, several others injured, and the city and region are expecting a months-long logistical nightmare in the absence of a crucial transport link.

It was a shocking event, not only for the public but for bridge engineers like me. We work very hard to ensure bridges are safe, and overall the probability of being injured or worse in a bridge collapse remains even lower than the chance of being struck by lightning.

However, the images from Baltimore are a reminder that safety can't be taken for granted. We need to remain vigilant.

So why did this bridge collapse? And, just as importantly, how might we make other bridges more safe against such collapse?

A 20th century bridge meets a 21st century ship

Reconstruction of the bridge will be a long and costly process. (Image credit: Ulysses Muñoz / The Baltimore Banner via AP)

The Francis Scott Key Bridge was built through the mid 1970s and opened in 1977. The main structure over the navigation channel is a "continuous truss bridge" in three sections or spans.

The bridge rests on four supports, two of which sit each side of the navigable waterway. It is these two piers that are critical to protect against ship impacts.

And indeed, there were two layers of protection: a so-called "dolphin" structure made from concrete, and a fender. The dolphins are in the water about 100 meters upstream and downstream of the piers. They are intended to be sacrificed in the event of a wayward ship, absorbing its energy and being deformed in the process but keeping the ship from hitting the bridge itself.

The fender is the last layer of protection. It is a structure made of timber and reinforced concrete placed around the main piers. Again, it is intended to absorb the energy of any impact.

Fenders are not intended to absorb impacts from very large vessels. And so when the MV Dali, weighing more than 100,000 tons, made it past the protective dolphins, it was simply far too massive for the fender to withstand.

Video recordings show a cloud of dust appearing just before the bridge collapsed, which may well have been the fender disintegrating as it was crushed by the ship.

Once the massive ship had made it past both the dolphin and the fender, the pier — one of the bridge's four main supports — was simply incapable of resisting the impact. Given the size of the vessel and its likely speed of around 8 knots (15 kilometers per hour), the impact force would have been around 20,000 tons.

Bridges are getting safer

Guidelines like this have played a crucial role in improved bridge safety. (Image credit: IABSE)

This was not the first time a ship hit the Francis Scott Bridge. There was another collision in 1980, damaging a fender badly enough that it had to be replaced.

Around the world, 35 major bridge collapses resulting in fatalities were caused by collisions between 1960 and 2015, according to a 2018 report from the World Association for Waterborne Transport Infrastructure. Collisions between ships and bridges in the 1970s and early 1980s led to a significant improvement in the design rules for protecting bridges from impact.

Further impacts in the 1970s and early 1980s instigated significant improvements in the design rules for impact.

The International Association for Bridge and Structural Engineering's Ship Collision with Bridges guide, published in 1993, and the American Association of State Highway and Transportation Officials' Guide Specification and Commentary for Vessel Collision Design of Highway Bridges (1991) changed how bridges were designed.

In Australia, the Australian Standard for Bridge Design (published in 2017) requires designers to think about the biggest vessel likely to come along in the next 100 years, and what would happen if it were heading for any bridge pier at full speed. Designers need to consider the result of both head-on collisions and side-on, glancing blows. As a result, many newer bridges protect their piers with entire human-made islands.

Of course, these improvements came too late to influence the design of the Francis Scott Key Bridge itself.

Lessons from disaster

Francis Scott Key Bridge in Baltimore, showing the pier struck by the cargo ship and the sections of bridge, which collapsed as a result. (Image credit: F Vasconcellos / Wikimedia, CC BY-SA)

So what are the lessons apparent at this early stage?

First, it's clear the protection measures in place for this bridge were not enough to handle this ship impact. Today's cargo ships are much bigger than those of the 1970s, and it seems likely the Francis Scott Key Bridge was not designed with a collision like this in mind.

So one lesson is that we need to consider how the vessels near our bridges are changing. This means we cannot just accept the structure as it was built, but ensure the protection measures around our bridges are evolving alongside the ships around them.

Second, and more generally, we must remain vigilant in managing our bridges. I've written previously about the current level of safety of Australian bridges, but also about how we can do better.

This tragic event only emphasizes the need to spend more on maintaining our ageing infrastructure. This is the only way to ensure it remains safe and functional for the demands we put on it today.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Colin Caprani
Associate Professor, Civil Engineering, Monash University

A/Prof. Colin Caprani is a Chartered Structural Engineer (CEng MIEI, MIStructE) and Fellow of Engineers Australia (FIEAust), with considerable industrial and academic experience. He has worked as a design structural engineer on a wide range of projects and his general research areas involve the probabilistic safety assessment of structures, specializing in highway bridge loading for short- and long-span bridges and human-induced vibration of structures.

  • bgaineshunter1
    admin said:
    Why did the bridge collapse, and what can we do to make other bridges more safe against such collapse?

    Baltimore bridge collapse: a bridge engineer explains what happened, and what needs to change : Read more
    I think designing bridges with the largest ships that are likely to be built in mind is a good idea, but I'm wondering how practical it is? My idea would likely be more costly, but it might be more manageable: build smaller ships. Large ships can be unwieldly for not only vulnerable bridges, but for ship crews as well.
    Reply
  • TheBox
    bgaineshunter1 said:
    I think designing bridges with the largest ships that are likely to be built in mind is a good idea, but I'm wondering how practical it is? My idea would likely be more costly, but it might be more manageable: build smaller ships. Large ships can be unwieldly for not only vulnerable bridges, but for ship crews as well.
    I think it is not practical to build a bridge that could stop a ship collapsing it , because of the engineering involved .
    The Baltimore Bridge didn't really collapse , it was destroyed by the force from a large mass colliding with it .
    Road vehicles are nothing compared to that ship , the bridge is made for road vehicles , not ships hitting it .
    Reply
  • TheBox
    admin said:
    Why did the bridge collapse, and what can we do to make other bridges more safe against such collapse?

    Baltimore bridge collapse: a bridge engineer explains what happened, and what needs to change : Read more
    You can't really construct a bridge that can withstand such a great force so like they do on the roads , they needed crash barriers set a distance away from the bridge supports .
    They also shouldn't have a great length between supports because of sectional damage control in case of a failure .
    This is the last free advice I am going to give you people .

    Water foundation

    1.Bore four holes in the river bed in a square pattern . Depth depends to type of bridge .

    2.Insert 4 plastic tubes and bed them in , length dependent to bridge height ,

    3. Pump out access water within the plastic tubes .

    4.Insert steal ''poles'' into the plastic tubes (anti -rust measures )

    5. Mould some concrete ''slabs'' in a square shape with 4 placements of holes

    6.Use a Crane to lower the slabs onto the ''poles'' , lining up the holes .

    7.Add angle supports etc using the same method .

    8. Add cross bars etc .

    Added , 4 holes is for a basic bridge , you could do oblongs if you want more support for bigger bridges and have more holes .
    Reply
  • Giovani
    admin said:
    Why did the bridge collapse, and what can we do to make other bridges more safe against such collapse?

    Baltimore bridge collapse: a bridge engineer explains what happened, and what needs to change : Read more
    Oh, the hubris of men, priding themselves in creating an iconic bridge.
    This substandard design expels any kudos for the creators. In fact, what imbecile designs a bridge where if one stanchion is destroyed, it brings down the entire structure? Idiocy.
    The faulty design of this iconic bridge attributed to destruction and death.
    Reply
  • krishjaga
    bgaineshunter1 said:
    I think designing bridges with the largest ships that are likely to be built in mind is a good idea, but I'm wondering how practical it is? My idea would likely be more costly, but it might be more manageable: build smaller ships. Large ships can be unwieldly for not only vulnerable bridges, but for ship crews as well.
    Where would be the limit if we start designing structures like this . The best is to avoid traffic on such existing structures that could have a potential negative impact.
    Reply