‘Only an old bridge would fail like this’

In 1980, the Sunshine Skyway Bridge in Florida was struck by a ship and collapsed, Ian Firth writes.

Since then, the construction of bridges has changed – engineers place large “dolphins”, or buffers, around the supports.

These concrete structures repel any ship that may run off course.

The disaster in Baltimore stems in part from the fact that the Francis Scott Key Bridge was built in the early 1970s, before this kind of technology was in place.

The Dali container ship drove into one of the supporting legs of the bridge at 5 to 6 knots, imparting a force equivalent to many thousands of tons. Once a central support like that falls it is unavoidable that a bridge of that shape and length will collapse.

The bridge’s supports are about 300m (985ft) apart, offering a fairly wide channel for vessels. One question we are yet to answer is why the Dali had veered so far off its route.

There were some deflector devices in the water, but they do not appear to be very large, and obviously were not sufficient to protect the bridge. The ship missed them and hit the structure itself. What happened after that is not surprising.

When Baltimore engineers come to design the new bridge, they may have to give it a wider span that would reduce the risk of a repeat.

They might think to build spans of 500m or 600m, for example. They might also place the main, most load-bearing supports of the bridge in shallower water so any vessel runs aground before hitting them.

As cargo ships are also taller these days, they may want to increase the height of the bridge from around 55m to about 70m.

Economics will be a factor in the rebuild.

There are British examples of cable-tied bridges that are both attractive and comparatively cheap, such as the Queen Elizabeth II bridge in Dartford and the Mersey Gateway, which could be replicated over the Patapsco river.

Ian Firth is a former president of the Institute for Structural Engineers