The collapse of the Francis Scott Key Bridge in Baltimore on March 26 was a shocking and tragic event. Six people remain unaccounted for in the disaster that sent the world’s third-largest continuous truss bridge crashing into the Patapsco River.
The cause was the Singapore-flagged container ship Dali, which went off beam collision with one of the bridge supportsor piers. When the 300-meter-long ship hit the structure, the so-called progressive decline, where the domino effect results in the collapse of your entire structure. The bridge, built greater than 45 years ago, collapsed into the icy water at 1:28 a.m. EST (5:28 UTC).
But how could one ship demolish this 366 m (1,200 ft) long structure inside seconds of impact?
Progressive collapse involves the failure of a single component, akin to a pillar, and results in the sequential failure of other connected components. These may include the metal truss and deck of the bridge. This type of collapse can have catastrophic consequences in terms of endangering human life, in addition to the realm’s economy and the local environment.
While it’s unimaginable to accommodate every scenario, bridges could be built with inherent features that increase their resistance to progressive collapse. Typically, bridges can withstand a point of damage to a pier or portion of the supporting structure. Depending on the circumstances, the bridge may even remain secure for vehicles.
However, a metal truss was designed in the event of a bridge collapse in Baltimore as one continuous system. The space between each support or pier is named the span of the truss. The collapse of one of the piers effectively doubled the span of the truss to the subsequent pier. This dramatic increase in span exerted a much greater force on the remaining truss structure.
Although continuous truss systems are preferred because they will redistribute weight in the event of failure, in this case the remaining truss members wouldn’t give you the chance to face up to all this extra force once the pier fails.
This caused a complete collapse of the truss section above the damaged pier. However, the autumn didn’t end there. Due to the interconnection of the trusses, the remaining part was initially pulled up. The sudden release of this tension created a powerful dynamic effect, ultimately causing your entire bridge to collapse.
A rare event
It is definitely commonplace for ships to hit bridge supports. On May 9, 1980 a strikingly similar event occurred when the freighter collided with a supporting pillar of the Sunshine Skyway Bridge in Tampa Bay, Florida. As a result, the bridge failed a similar distance to the Baltimore collapse.
Although bridge designers are well aware of the chance of collisions, these events are fairly rare. The impact forces on the support pier are also very variable. Greater speed or a heavier vessel will significantly increase the force on the pier. More vessel traffic on the water increases the likelihood of collisions.
Moreover, the tactic currently used in the USA for calculating the force of a ship collision relies on research conducted in 1967-1976. However, a different method was used for the Key Bridge, which opened in 1977. Needless to say, ships as heavy and fast because the Dali weren’t a common sight in 1977.
In fact, in some scenarios, a collision force is more likely to occur significantly exceeds the strength of the bridge pillars. That’s why bridges produce other safety systems, e.g. dolphins – a group of piles placed in the water near the pier, that are used to deflect the ship or absorb the energy of a collision.
There isn’t any information in regards to the system that was installed when the Key Bridge opened in 1977. Some observers have questioned whether protective barriers across the Baltimore bridge were adequate.
Jon Bilous / Shutterstock
Regular structural assessments and upgrades are critical to making sure the bridge meets current safety standards. Concrete and steel, the fundamental materials of this bridge, are at risk of deterioration resulting from aspects akin to corrosion and other environmental conditions.
Generally, insufficient maintenance or inappropriate upgrades could be aspects contributing to bridge collapse. It should be said, nonetheless, that there isn’t a evidence that this was a factor in this case – or in the case of Key Bridge was said to be “up to code” when the disaster occurred.
We will soon learn more details about this dramatic and tragic event. The findings are sure to influence future approaches to the design and protection of bridges on busy waterways.
