Baltimore’s Francis Scott Key Bridge collapsed on Tuesday after a container ship smashed into it, plunging cars into the water. It is a major traffic and transportation hub for the U.S. East Coast and the entry point to the Port of Baltimore. Here’s what we know about the collision:

BRIDGE HISTORY: The four-lane steel bridge opened in 1977 after five years of construction and spans 1.6 miles (2.6 km). It crosses the Patapsco River, where U.S. national anthem author, Francis Scott Key, wrote the “Star Spangled Banner” in 1814 after witnessing the British defeat at the Battle of Baltimore and the British bombing of Fort McHenry. It has 185 feet (56 meters) of vertical clearance.

Built at an estimated cost of $110 million, it allowed for more traffic lanes and carried lower operating and maintenance costs than a tunnel.

BRIDGE STYLE: The metal truss-style bridge has a suspended deck; a design that contributed to its total collapse, engineers say. The ship appeared to hit a main concrete pier, which rests on soil underwater and is part of the foundation.

“This type of bridge is not designed to redistribute loads in the event of a main pier collapsing, and therefore videos show there is a progressive collapse of the bridge” in which one element fails after another, said Marina Bock, a structural engineering lecturer at England’s Aston University.

PLANNED UPGRADES: Starting in the summer of 2025, the Maryland transportation agency planned to replace the bridge’s deck and install a “fiberglass jacket protection system at the water pier columns,” an agency report says.

TRAFFIC AND TRANSPORT INFO: The bridge carries 11.3 million vehicles a year, the Maryland Transportation Authority says, on the I-695 highway that circles Baltimore, also known as the Baltimore Beltway.

GATEWAY TO BALTIMORE PORT: The bridge leads to the Port of Baltimore, the deepest harbor in Maryland’s Chesapeake Bay. It is the busiest U.S. port for car shipments, handling more than 750,000 vehicles in 2023, according to official data. It is also the largest U.S. port by volume for handling farm and construction machinery, as well as agricultural products; and a cruise terminal, with operators Norwegian, Carnival, and Royal Caribbean, all using the port for Caribbean, Canadian, and other Atlantic destinations.

Other bridges impacted

Other structures along the route include a 0.64-mile dual-span drawbridge over Curtis Creek and two 0.74-mile parallel bridge structures that carry traffic over Bear Creek, near Bethlehem Steel’s Sparrows Point plant.

COMMUTE DISRUPTIONS: Maryland’s Transportation Authority called the incident a “major traffic alert” and redirected cars to the I-95 or I-895 highways. Large trucks are prohibited from using the 1-95 tunnel route that goes under the Baltimore harbor.

BALTIMORE PORT TRAFFIC: Because of the bridge’s collapse, ships are not leaving the Port of Baltimore. More than 40 ships remained inside the port including small cargo ships, tugboats, and pleasure craft, data from ship tracking and maritime analytics provider MarineTraffic showed. At least 30 other ships had signaled their destination was Baltimore port, the data showed.

—Jyoti Narayan, David Shepardson and Nina Chestney, Reuters