The cargo ship collision that destroyed the Francis Scott Key Bridge in Baltimore on March 26, 2024, is elevating questions on simply how a lot engineers can do to forestall such catastrophes from occurring sooner or later. Right here, Michael J. Chajes, a professor of civil and environmental engineering on the College of Delaware, discusses how bridge design codes have modified over time and the challenges of constructing new constructions, and retrofitting present ones, to allow them to survive excessive occasions
How onerous is it to design a bridge to face up to the drive that took down the Francis Scott Key Bridge?
As soon as engineers perceive the forces {that a} construction shall be subjected to, they will design a construction to face up to them. That mentioned, we all know that every drive has a variety of magnitudes that may happen. For instance, not all vans on the roadways weigh the identical quantity, not all earthquakes are of the identical magnitude, and never all ships have the identical weight. We incorporate this variability in forces into the design.
Even when constructed to a given set of plans, the ultimate energy of the construction can fluctuate. The supplies used have variations in energy. For instance, concrete delivered on two successive days can have a sightly completely different ultimate energy. This variability within the energy of the ultimate construction can be taken into consideration within the design course of to make sure the bridge or constructing is secure. There isn’t any means we may construct two bridges from the identical set of plans and so they find yourself with the very same energy.
Primarily based on the burden and velocity of the ship that hit the Francis Scott Key Bridge, right now’s U.S. bridge design code would name for the bridge to be designed to withstand a lateral drive of 11,500 tons. This implies the bridge has the power to face up to a lateral hit of that magnitude. That’s equal to the burden of about 50 loaded Boeing 777s or the burden of the Eiffel Tower. Whereas this can be a very massive lateral drive, constructions will be designed to withstand such forces. Tall buildings are routinely designed to withstand lateral a great deal of this magnitude that end result from wind or earthquakes. Nonetheless, it’s a matter of how a lot one needs to spend on the construction, and plenty of design objectives and constraints have to be balanced in opposition to one another.
What do engineers do to make sure security in excessive occasions?
Our information of how excessive occasions have an effect on constructions is consistently evolving. One space the place that is very obvious is earthquake engineering. After every earthquake, structural engineers be taught what has labored and what has not labored, after which the constructing and bridge design codes evolve. Infrastructure homeowners additionally attempt to retrofit present constructions that had been designed to earlier codes.
Ship collisions and their influence on bridges are the same space of evolving understanding and improved design codes. There have been over 35 main bridge collapses globally that had been brought on by ship collisions from 1960 to 2015. Engineers consider the failures, and so they replace the engineering codes in order that they higher account for the consequences of ship collisions.
How has bridge design advanced because the Baltimore bridge was constructed?
The Francis Scott Key Bridge was designed within the early Nineteen Seventies. Building began in 1972, and it opened to visitors in 1977. This preceded the 1980 collapse of the Sunshine Skyway in Florida, which was brought on by a ship collision, much like what occurred in Baltimore. That bridge collapse led to the initiation of analysis initiatives that culminated within the growth of a U.S. information specification in 1991 that was up to date in 2009.
Primarily based on that information specification, bridge design codes had been modified to incorporate forces as a consequence of ship collisions. The design of the Francis Scott Key Bridge wouldn’t have been required to contemplate the impact of ship collisions. The present U.S. bridge design code says that:
“the place vessel collision is anticipated, constructions must be:
- Designed to withstand vessel collision forces and/or
- Adequately protected by fenders, dolphins, berms, islands, or different sacrifice-able gadgets.”
Different modifications because the Nineteen Seventies are that cargo ships have elevated in measurement and weight. The ship that introduced down the Sunshine Skyway in 1980 weighed 35,000 tons, whereas the ship that collided with the Francis Scott Key Bridge weighed 95,000 tons.
With the growing weight of cargo ships, probably the most cost-effective design technique to forestall collapse of bridges as a consequence of vessel collision could be to guard the bridge piers from the influence. That is achieved by constructing a bridge collision protecting system, which is usually a concrete or rock construction that surrounds the pier and stops the ship from attending to the pier, as is completed to guard a lot of our nationwide monuments.
A pier safety system was put in when the Sunshine Skyway bridge was rebuilt, and it has been used on quite a few different bridges. The identical strategy is at present being utilized by the Delaware River and Bay Authority at a value of US$93 million to guard the piers of the Delaware Memorial Bridge.
However what about present bridges just like the Francis Scott Key Bridge? Bridge homeowners have an incredible problem discovering the monetary sources wanted to retrofit their bridges to fulfill the most recent design codes and to account for the elevated influence masses anticipated because of the heavier and heavier ships. Each issues occurred right here. That’s, design codes modified and improved, and masses bought a lot bigger. Engineers and infrastructure homeowners strive their greatest to prioritize the place their restricted funds can be utilized to extend structural security and reduce the possibility of structural failure.
What can universities do?
The No. 1 job of structural engineers is to guard the general public and reduce the danger of structural failures that pose a menace to human life. To try this, engineers should be capable of calculate the forces that our constructions could also be subjected to. This contains circumstances the place a big ship by accident collides with a bridge, or a big earthquake or hurricane strikes.
In these excessive circumstances, the construction will virtually assuredly maintain harm, however, if in any respect attainable, it must be resilient sufficient to not collapse. The design codes are regularly up to date to account for brand new information, new supplies and new design methods. The reliability of our constructions is bettering on a regular basis.
Retrofitting constructions constructed to prior codes is an ongoing course of, and one which this catastrophe strikes to the forefront. The U.S. has loads of infrastructure that was designed to previous codes, and we’ve got bigger vans crossing our bridges, and bigger ships passing beneath them.
Engineers can by no means scale back the chance of failure to zero, however they will scale back it to the purpose the place failures occur very sometimes and solely in circumstances the place quite a few unexpected circumstances mix to make a construction weak to break down.
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