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The collapse of Baltimore’s Francis Scott Key Bridge on March 26 was a devastating event resulting in six workers still unaccounted for. The cause of the collapse has been identified as a collision with the Singapore-flagged container ship Dali, which struck a critical bridge support. Here’s a breakdown of the events and the engineering concepts relevant to understanding this tragedy as explained by the Conversation.
At 1:28 am Eastern time, as the 300-meter-long Dali veered off course, it struck a support pier of the bridge. This triggered a catastrophic chain reaction known as a progressive collapse, ultimately leading to the downfall of the 45-year-old bridge.
Understanding progressive collapse
A progressive collapse occurs when an initial failure (in this case, the pier) sets off a cascading failure of connected structural elements, such as the metallic truss and the bridge deck. The potential consequences can be devastating – a great danger to human life and significant economic and environmental disruption.
Bridges often have built-in measures to combat progressive collapse. They’re designed to remain functional after a certain amount of damage to structural elements. In the case of the Francis Scott Key Bridge, however, the continuous truss design meant that the loss of one pier significantly increased the load on the surrounding structure. This unsustainable strain ultimately caused the entire truss system to buckle.
Ship collisions: A rare but devastating risk
While uncommon, ship collisions with bridge supports are an ever-present risk for bridges over waterways. The forces involved are significant, and factors like ship speed, size, and traffic volume all increase the chances of a disaster.
Engineers use calculations based on research from the 1960s and 70s to assess potential collision forces. However, the size and speed of vessels like the Dali were less common when the Key Bridge was constructed, raising a key question: are ship collision calculations due for an update?
Protection systems like ‘dolphins’ – clusters of pilings meant to absorb or deflect impact – are employed as an additional safeguard. There is ongoing debate about whether the protective system around the Francis Scott Key Bridge was adequate.
Maintenance and bridge safety
Regular assessments and updates are vital to keeping bridges structurally sound. Materials like steel and concrete deteriorate over time, and insufficient maintenance can contribute to failures. While no evidence has surfaced of maintenance issues with the Key Bridge, this is a standard area of investigation in such events.
The full analysis of the collapse of the Key Bridge will shape future bridge design and protection measures. This tragedy highlights the delicate balance between functionality and resilience engineers must strike when building critical infrastructure over busy waterways.
