The spiderweb strategy that allows building catastrophe-proof bridges

A container ship as large as a building collided on March 26, 2024, with one of the two piers of the Francis Scott Key , and the largest bridge in the U.S. city of Baltimore collapsed. The impact of the ship brought down a work of modern engineering that was the city's pride, with its 2,632 meters of lattice-work steel. However, the bridge collapsed like a toy. It carried 33,000 vehicles every day. What could have been a major catastrophe fortunately resulted in only six deaths, but it left a gaping wound in the city, serious economic losses, and communication problems.

Could the collapse have been avoided with better construction design? A Spanish study, led by the Polytechnic University of Valencia and the University of Vigo, has found six resistance mechanisms that can help design safer bridges and minimize damage in the event of an event like the one in Baltimore, an earthquake, or any other catastrophic event. The study's findings will allow for further design optimization without increasing costs or materials. The details of this work are published this Tuesday in the journal Nature.

By looking at the work of spiders and the structure they weave, they have shown that just as these arthropods adapt and continue to catch prey after damage to their webs, steel truss bridges are able to withstand even greater loads than they would normally endure when damaged.

The comparison with spiderwebs arose by chance, José M. Adam, a researcher at the Polytechnic University of Valencia and lead author of the study, told ABC. "While we were writing the article, we came across another study also published in the journal 'Nature' on the strength of spiderwebs. We observed several points in common with the strength of steel truss bridges," he explains.

Like spider webs, these bridges are made up of many linear elements connected to each other, and the function of the failing element determines the impact on the integrity of the whole. In both the structure woven by spiders and these engineering works, "when one element fails, there is an extraordinary capacity to transfer the load to others, so that the structure can continue to support similar loads, as if nothing had happened. Even with the failure of an a priori critical element, collapse-rupture can be avoided," Adam explains.

His research has focused on engineering structures designed with steel trusses, such as the Baltimore Bridge . It was known that these structures were more resistant to impact or other catastrophic events, but it was not clear why in some cases the damage spreads disproportionately while in others it barely affects the bridge's functionality.

The answer lay in the six hidden resistance mechanisms identified by engineers from Valencia and Vigo. "As with spider webs, if the bridge is able to activate these mechanisms, the failure of one element does not propagate to the rest of the bridge. However, if none of these mechanisms are activated, a collapse will almost certainly occur," says the researcher from the Polytechnic University of Valencia.

To find the six key factors that provide greater safety, the Spanish researchers built a scale model of a real bridge span in their laboratory, inducing failures in specific areas. They devised a structure composed of triangulated beams in which all the bars act in tension or compression. They gradually eliminated bars until they identified six different ways in which the remaining bars of the bridge are able to withstand the loads to which it is subjected.

This showed how a structure changes its behavior, going from being a beam supported by two supports to deforming in order to continue transmitting loads to the supports without collapsing. "In my opinion, this is the greatest contribution of the article," José López Cela, professor of Applied Mechanics and Project Engineering at the University of Castilla-La Mancha, told SMC.

Cela, who is not involved in this study, raises doubts about how it would behave in a real catastrophe. "Local failures in the laboratory are easy to cause (simply by cutting the bar in question); it's not easy to imagine how they might appear in a real structure. However, these considerations do not invalidate the interest, quality, and scientific rigor" of the research, he insists.

Adam argues: "The results are completely applicable to real bridges. In fact, we were inspired by one in the province of Alicante. We applied similarity laws, which makes the results directly applicable to reality."

The researchers behind this study believe that their findings will not only allow for the construction of safer bridges, but will also help reinforce existing structures when their useful life is to be extended. Between the late 19th century and the first third of the 20th century, numerous steel truss bridges were built, and many of them are currently in operation, particularly for railways. "Our work will help define guidelines for their inspection and repair, which will contribute to the preservation of a valuable civil engineering heritage," the Valencian engineer argues.

The collapse of the Baltimore Bridge, however, seemed impossible to stop. Little could be done against the ramming of a ship of that size. Adam believes the lack of defenses on the pillars was the main weakness of the infrastructure that brought down the freighter. "Our work is not directly applicable to the case of American infrastructure, because the problem with this bridge was much more basic, and the impact almost certainly caused the collapse." However, it is applicable to other situations that ended in tragedy and are now part of our recent history, such as those in Minneapolis and Mount Vernon.