Suspension bridges are far and away the target of choice in America’s action blockbusters. In just the past three years, the Golden Gate Bridge has been destroyed by a Kaiju, Godzilla, a Skynet-initiated nuclear blast, and a tsunami. Americans don’t build real bridges anymore, or maintain the ones that we have, but we sure love to blow them up in movies.
There is logic here: A disaster scene involving a famous bridge serves both to root the film in the real world and to demonstrate the enormity and the immediacy of the threat. The unmaking of these huge structures shocks us because many bridges have gained an aura of permanence in our collective consciousness. Although we know when the Brooklyn Bridge was built and who built it, we feel like it has always been there and always will be. The destruction of our familiar human topography is even more disturbing than the deaths of the CGI victims, and I’m not just saying that as a misanthrope who loves bridges.
However, in all of the planning, storyboarding, rendering, and compositing of these special effects shots, nobody pauses to consider how suspension bridges actually behave. I can accept messianic alien orphan superheroes and skyscraper-sized battle robots, but I will not stand for inaccurate portrayals of structural mechanics. It’s fine to bend the laws of physics if the plot warrants it, but most suspension bridge mistakes are so needless and stupid that their only function seems to be irritating engineers.
How Suspension Bridges Actually Work
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. I’ve taken a look at some notable suspension bridge disaster scenes and listed them from least bad to most bad:
Final Destination V
The most realistic bridge collapse sequence somehow comes from a film franchise in which people regularly get cut in half by errant kites. In this chapter, Sam Lawton is stuck in traffic on the North Bay Bridge in western New York (played by Vancouver’s Lions Gate Bridge, the only non-iconic bridge on this list) when he notices that cracks are splaying out from a point where some workers are cutting up the road. These cracks widen, and soon the vertical cables start to pop out of their connections to the main suspension cables and to the beams under the road deck. A progressive collapse ensues, as each vertical suspender fails to compensate for the loss of its neighbor.
The origin of the structural failure in this situation is pretty absurd because the asphalt driving surface on a traffic bridge is non-structural. The road itself rests on a steel structure, which would probably not be seriously compromised by some sawing and jackhammering on the asphalt. Further, it’s hard to invent a scenario in which any of this could cause a failure at the top of a vertical suspender. But who knows, maybe there had been some plot-friendly corrosion in the steel. Regardless of the initial cause of failure, the collapse progresses in a halfway believable manner: The road deck falls, but the main catenary cables and the bridge towers remain. With no road to support, the vertical cables swing dumbly over the void.
On a pedantic note (as all best notes are), the collapse shown in the film does not accurately reflect the change in shape that the real parabolic cables would take. Without the dead weight of the main span, the main cables would sag noticeably less between the towers because they would only be carrying their own weight. Since the cables themselves cannot appreciably change in length, they would droop much more at the outer spans where the load is still applied. In the movie, the intact portions of the road deck should be sagging much lower. This is a rare fictional collapse scenario for which we have a real world comparison, the 1940 Tacoma Narrows Bridge collapse.
I Am Legend
The government blows up the Brooklyn and Manhattan Bridges in order to head off an outbreak of the movie’s mysterious virus. The explosions sever the main cables, and the center spans of the bridges drop into the East River while the towers remain standing. This would be a fairly accurate representation of the physics of a suspension bridge, except the spans between the towers and the shore are shown to be unaffected.
However, the loss of tension capacity in the main cables would cause a total collapse of the entire bridge deck, not just the central span. The road deck at the outer spans is still pulling downward on the main cables, which would normally go into tension and resist the load. But since those main cables are severed, there is nothing to pull against, and the necessary structural capacity cannot be developed. (Pacific Rim makes the same mistake, showing a flash of the destroyed Golden Gate Bridge with the center span destroyed and outer spans still present.)
X-Men: The Last Stand
Magneto, whose mutant power is dominance over all things metal, rips the Golden Gate Bridge off of its foundations, flies it over the San Francisco Bay, and places it between the city and Alcatraz Island. (This insane act of destruction serves only to transport his army of mutants from point A to point B, a basic logistical task that he could accomplish by lifting a couple of buses instead). Magneto pulls up the bridge towers and severs the suspension cables directly at their anchorages, effectively removing any means by which the structure can transfer load into the Earth. However, if we grant Magneto the ability to lift the bridge in the first place, then can also assume he has the ability to resolve all the structural forces within the bridge as well. So far, so good.
However, when Magneto drops the bridge between Alcatraz and (what looks like) the Marina District, the same structural problems are still present even though he isn’t actually supporting it anymore. The road deck forms an arc from shore to shore, the towers hover bizarrely over the water, and the main suspension cables still carry load. We could convince ourselves that Magneto is graciously holding up the bridge during the climactic battle, but that logic unravels when Magneto is “cured” of his mutation and therefore loses his ability to control metal. The whole bridge apparatus should crash into San Francisco Bay in a tangle of useless rubble. (Not that it really matters because apparently nothing matters, but the Golden Gate Bridge isn’t long enough to span the distance depicted even if the structural forces could be resolved.)
An errant army missile completely severs one of the Golden Gate Bridge’s two main cables. The loss of that cable, which had been supporting the right side of the road deck, should result in a catastrophic collapse: Without one of its two vertical supports, the road deck would hinge downward like a trap door, dropping all the tanks and school buses into the water below.
However, in the film world, bridge suspension cables are purely decorative elements that can be destroyed without consequence. The soldiers continue to battle Godzilla and the drivers motor across the bridge to safety, all oblivious to the physics errors that spared their lives. Godzilla even tugs at the remaining cable, which should at least cause the impossibly floating road deck to sway, but it doesn’t. Even when Godzilla busts through the center of the main span, the function of the bridge is unaffected.
The Dark Knight Rises
Bane blows up a series of bridges (actually some New York City bridges with some fictional CGI additions) to cut Gotham off from the world. Explosions sever the suspension cables and the road decks all at once, and a neat section of each bridge falls into the river below. Nothing else happens. That’s it. Completely cutting the main structural element on a suspension bridge has no effect on the structural integrity as a whole. Movie magic. (I must note that the bridge featured in the movie’s climax is a cantilever bridge and therefore outside the scope of this article.)
Imagine stringing a clothesline between two buildings and putting some shirts out to dry. Now, cut the line in the middle. In our world, the line loses all its capacity and the shirts all fall to the ground. In Christopher Nolan’s world, the clothesline is unharmed and, who knows, may actually be stronger.
I consider this the worst suspension bridge destruction scene in motion picture history. The Golden Gate Bridge collapse in The Core is somehow more realistic than this. Nolan, who famously hired astrophysicist Kip Thorne to advise him on black holes for Interstellar, failed to hire a sophomore engineering student to explain regular gravity here on Earth.
A Bridge Too Far
There are a few reasons why these mistakes keep happening during America’s annual summertime bridge demolition derby. I grudgingly concede that a director’s top priority is to tell a story rather than present a technically accurate demonstration of bridge mechanics. Also, suspension bridges don’t fall down very often in the real world, which means that the general public has no actual collapse imagery to use as a reference point. Finally, these movies are made and marketed for teenage boys and young men who want to see stuff explode. The bigwigs at Warner Brothers aren’t really aiming for that big structural engineer dollar.
However, since we’re in an era when many filmmakers are producing grittier, more realistic action movies, I still hold out hope that someone will get this right. If there are any Hollywood screenwriters, directors, or special effects artists inexplicably reading this post, please do me a favor: The next time you blow up a bridge, spare a thought for the people who actually build them.
Alex Weinberg, P.E. is a structural engineer living and working in New York City. You can e-mail him at firstname.lastname@example.org.