Baltimore's Francis Scott Key Bridge: a look into struck overpass's design

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The Francis Scott Key Bridge, once a vital artery connecting Baltimore City to its surrounding areas, tragically collapsed on March 26, 2024, after a large cargo ship, the MS Dali, lost power and struck one of its support columns.

 

The critical impact caused the bridge’s continuous truss structure to fail, sending a section crashing into the Patapsco River below. 

 

But before its untimely demise, the bridge stood as an icon of both innovative engineering and a bygone era of infrastructure development.

 

Bridging the Gap for A City in Need of Expansion

 

Baltimore’s harbor has always been its lifeblood. However, by the 1960s, the Baltimore Harbor Tunnel, the primary route for east-west traffic, reached capacity. The Maryland Department of Transportation (MDOT) recognized the need for a new crossing and envisioned a bridge as a more cost-effective and efficient solution compared to a second tunnel.

 

 

 

Trusting in the Truss

 

Construction began in 1972. The winning design, chosen for its functionality and affordability, was a continuous steel truss bridge with a suspended deck. The now-defunct Greiner Engineering Sciences Inc—which had a strong reputation for infrastructure projects on the East Coast, including the Chesapeake Bay Bridge—was sought as the project’s engineering consultant.

 

Baltimore was facing a growing traffic problem, but a tight budget limited options. This design offered several advantages. Firstly, a continuous truss design offered a strong and durable bridge at a generally more manageable price point compared to alternative designs like suspension bridges.

 

Artistic rendering of a truss bridge

 

The modular nature of the arch also facilitated faster on-site assembly and easier future maintenance. This was crucial for minimizing project delays and keeping ongoing costs in check.

 

Additionally, unlike simpler beam bridges, the continuous truss distributed weight across multiple connected segments. This efficient load-bearing system allowed the bridge to handle the heavy traffic Baltimore envisioned. Furthermore, steel offered a good balance of strength, weight, and affordability. The truss design minimized the amount of material needed while maintaining structural integrity.

 

The bridge’s main span measured a staggering 1,200 feet, ranking it as the third-longest continuous truss bridge globally at the time of its opening in 1977.

 

 

A Bridge Named for a National Symbol

 

Originally dubbed the “Outer Harbor Crossing,” the bridge was renamed the Francis Scott Key Bridge in 1976. Key, a lawyer and amateur poet, penned the Star-Spangled Banner after witnessing the bombardment of Fort McHenry during the War of 1812. And the linkway’s location, near Fort McHenry, solidified the connection to the national anthem's author.

 

 

 

Where it Went Downhill

 

Today, design considerations for waterway traffic control are likely more thought out than those of historical solutions like the Baltimore bridge. Back then, incorporating features like protective structures around piers to deflect large vessels might not have been a priority.

 

The biggest drawback of such constructions is their vulnerability to catastrophic failure. Damage to a single critical support pier, as seen in the Baltimore bridge collapse, can have a cascading effect, causing the entire interconnected structure to fail.

 

According to Dr Andrew Barr, Research Fellow in the Department of Civil and Structural Engineering at the University of Sheffield:

 

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This is an example of what engineers call progressive collapse, where the failure of one structural element leads to the failure of neighboring elements, which can’t support the new loads placed on them.

 

In this case, the collapse of the pier caused the now unsupported truss above it to buckle and fall. Because this is one continuous truss, the loads are redistributed—the truss pivots around the surviving pier support like a seesaw, temporarily lifting the northern span into the air before the high tension forces cause this to fail too, and the whole truss collapses into the water.

 

A Legacy of Progress, Marred by Tragedy

 

 

The Francis Scott Key Bridge served Baltimore well for four decades as a critical passage. It facilitated trade through the Port of Baltimore, eased commutes, and became a recognizable symbol of the city. Alas, all it took was a single point of impact for it to make its descent.

 

 

• https://www.baltimoresun.com/2024/03/26/key-bridge-collapse-what-we-know-about-the-bridge/
• https://www.nbcnewyork.com/news/local/what-to-know-francis-scott-key-bridge-baltimore/5261444/
• https://www.sciencemediacentre.org/expert-reaction-to-baltimore-bridge-collapse/

 

 

Images: Charlie Floyd and Eugenesergeev | Dreamstime.com