Hollywood isn’t exactly known for the most accurate depictions of science and scientists — hence the long tradition of nerd gassing over the details any given film gets wrong. Add one more disgruntled engineer to their ranks who takes special issue with the way bridges are depicted on film.
Over at Hackaday, NYC structural engineer Alex Weinberg joins the many physicists, chemists, neuroscientists and biologists who have long bemoaned the many groan-worthy on-screen depictions of their respective scientific fields. He looks at several blockbuster films featuring lots of rampant property destruction, and finds the treatment of collapsing bridges to be particularly wanting. Here’s the basic real-world science involved, per Weinberg:
The construction of a suspension bridge is a huge, challenging undertaking, but the basic physical principles are fairly simple: First, two towers (or sometimes more) are built. A pair of thick steel cables (made of thousands of smaller strands) are strung from shore to shore, resting on big saddles atop each tower. These main cables will form a parabola and support a series of vertical suspender cables, which in turn support the road deck. At each shore, huge anchorages keep the main cables in place.
When you drive across a suspension bridge, the weight of your car pulls down on the vertical suspender cables, putting them in tension. That tensile force is then transferred into the main cables, also in tension. The main cables pull downward on the towers, which resist the vertical compression force by carrying it into the foundations. Finally, at the ends of the main suspension cables, the massive anchorages resolve the tensile force.
On the big screen, these basic concepts don’t apply. Most of the structural elements have no purpose, and bridges are instead supported by a mix of perplexing whimsy, directorial ignorance, and nothing.
Remember when that army missile severed a cable on a suspension bridge in 2014’s Godzilla, and nothing happened? Yeah, that thing should have come crashing down. Suspension wires aren’t just for decoration, people. I am Legend and X Men: The Last Stand also come under fire for their inaccurate structural mechanics. As for that scene in The Dark Knight Rises where Bane blows up all those bridges to cut off all access to the island of Manhattan, Weinberg declares it “the worst suspension bridge destruction scene in motion picture history.”
So there you have it, future filmmakers: be more like Final Destination V when it comes to the willful destruction of city infrastructure.
Weinberg isn’t above taking a few liberties with other disciplines, mind you. Bend the laws of physics if you must, and he will happily suspend his disbelief. “I can accept messianic alien orphan superheroes and skyscraper-sized battle robots,” he writes. “But I will not stand for inaccurate portrayals of structural mechanics.”
[Via Boing Boing]