Baltimore Bridge Collapse Will Teach Engineers How to Build Safer Infrastructure| Trending Viral hub

Baltimore Bridge Collapse Will Teach Engineers How to Build Safer Infrastructure

The loss of Baltimore’s Francis Scott Key Bridge after a freighter collision will teach engineers how to design structures better able to withstand disasters.

A container ship loaded with red and yellow boxes crashes into a steel bridge.

In this aerial image, the steel structure of the Francis Scott Key Bridge sits atop a container ship after the bridge collapses in Baltimore on March 26, 2024.

Credit:

Michael A. McCoy for The Washington Post via Getty Images

The following essay is reprinted with the permission of The conversationThe conversationan online publication covering the latest research.

The freighter collision that destroyed the Francis Scott Key Bridge in Baltimore on March 26, 2024, raises questions about how much engineers can do to prevent such catastrophes from happening in the future. Here, Michael J. Chajesprofessor of civil and environmental engineering at the University of Delaware, discusses how bridge design codes have changed over the years and the challenges of building new structures and retrofitting existing ones so they can survive extreme events.

How difficult is it to design a bridge that will withstand the force that brought down the Francis Scott Key Bridge?


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Once engineers understand the forces a structure will be subjected to, they can design a structure that resists them. That being said, we know that each force has a range of magnitudes that can occur. For example, not all trucks on the roads weigh the same, not all earthquakes are of the same magnitude, and not all boats have the same weight. We incorporate this variability of forces into the design.

Even if built to a given set of plans, the final strength of the structure can vary. The materials used have variations in resistance. For example, concrete delivered on two successive days will have a slightly different final strength. This variability in the strength of the final structure is also taken into account in the design process to ensure that the bridge or building is safe. There is no way we can build two bridges with the same set of plans and have them end up with exactly the same strength.

Based on the weight and speed of the ship that hit the Francis Scott Key Bridge, today’s result US Bridge Design Code It would require the bridge to be designed to withstand a lateral force of 11,500 tonnes. This means that the bridge has the capacity to withstand a side impact of that magnitude. This is equivalent to the weight of about 50 loaded Boeing 777s or the weight of the Eiffel Tower. Although this is a very large lateral force, structures can be designed to resist these forces. Tall buildings are typically designed to resist lateral loads of this magnitude resulting from wind or earthquakes. However, it is a question of how much you want to spend on the structure, and many design goals and constraints must balance each other.

What do engineers do to ensure safety in extreme events?

Our knowledge of how extreme events affect structures is constantly evolving. One area where this is very evident is Seismic Engineering. After each earthquake, structural engineers learn what has worked and what hasn’t, and then building and bridge design codes evolve. Infrastructure owners are also trying to modernize existing structures that were designed to previous codes.

Ship collisions and their impact on bridges are a similar area of ​​evolving understanding and improved design codes. There have been more than 35 major bridge collapses around the world caused by Ship collisions from 1960 to 2015. Engineers evaluate failures and update engineering codes to better account for the effects of ship collisions.

How has bridge design evolved since the Baltimore Bridge was built?

The Francis Scott Key Bridge was designed in the early 1970s. Construction began in 1972 and opened to traffic in 1977. This preceded the 1980 Sunshine Skyway collapsein Florida, which was caused by a boat collision, similar to what happened in Baltimore. The collapse of the bridge led to the beginning of research projects that culminated in the development of a US Guide Specification in 1991 which was updated in 2009.

Based on that guideline specification, bridge design codes were changed to include forces due to ship collisions. The design of the Francis Scott Key Bridge would not have been required to take into account the effect of ship collisions. The current US bridge design code says that:

“Where a vessel collision is anticipated, structures should:

• Designed to resist collision forces from vessels and/or

• Adequately protected by defenses, dolphins, berms, islands or other sacrificial devices.”

Other changes since the 1970s are that cargo ships have increased in size and weight. The ship that brought down the Sunshine Skyway in 1980 weighed 35,000 tons, while the ship that hit the Francis Scott Key Bridge weighed 95,000 tons.

With the increasing weight of cargo ships, the most cost-effective design strategy to prevent bridge collapse due to ship collision may well be to protect bridge piers from impact. This is achieved by building a collision protection system on the bridge, which is often a concrete or rock structure that surrounds the dock and prevents the ship from reaching the dock, as is done to protect many of our national monuments. .

A dock protection system was installed when the Sunshine Skyway Bridge was rebuilt, and has been used in many other bridges. The Delaware River and Bay Authority is currently pursuing the same approach at a cost of $93 million to protect the piers of the Delaware Memorial Bridge.

But what about existing bridges like the Francis Scott Key Bridge? Bridge owners have a tremendous challenge in finding the financial resources necessary to modernize their bridges to meet the latest design codes and take into account the increased impact loads expected due to increasingly heavier ships. Both things happened here. That is, the design codes changed and improved, and the loads became much greater. Infrastructure engineers and owners do their best to prioritize where their limited funds can be used to increase structural safety and minimize the potential for structural failure.

What can universities do?

The number one job of structural engineers is to protect the public and minimize the risk of structural failures that pose a threat to human life. To do this, engineers must be able to calculate the forces to which our structures may be subjected. This includes cases where a large ship accidentally hits a bridge, or a large earthquake or hurricane.

In these extreme cases, the structure will almost certainly be damaged, but, if possible, it should be strong enough not to collapse. Design codes are continually updated to take into account new knowledge, new materials and new design techniques. The reliability of our structures is improving all the time.

Modernizing structures built to previous codes is an ongoing process, and this disaster is coming to the fore. The United States has a lot of infrastructure designed to old codes, and we have bigger trucks crossing our bridges and bigger ships passing under them.

Engineers can never reduce the probability of failure to zero, but they can reduce it to the point where failures occur very infrequently and only in cases where numerous unforeseen circumstances combine to make a structure vulnerable to collapse.

This article was originally published in The conversation. Read the Original article.

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