What could collapse the Golden Gate Bridge?

Jun 8, 2015

Throughout film history, the Golden Gate Bridge has been leveled in earthquakes, ripped apart by apes, melted, and even bitten in half by a mega-shark.

But how would the iconic span fare in more realistic disaster scenarios? We're going to take a close look at three very real situations – overcrowding, a tsunami, and an earthquake – and find out if those disasters could bring down the Golden Gate Bridge.

Hassan Astaneh is a professor of structural engineering and bridge engineering at UC Berkeley.

Turns out Astaneh, who has been a bridge engineer for over 40 years, is the perfect person to play along.

“God forbid, we don't want anything even related to collapse the Golden Gate Bridge. It's one of the best bridges in the world,” he says. “But you know, question is valid question, so we are in this business to provide you with answers.”

UC Berkeley engineering professor Hassan Astaneh demonstrates his computer-generated model of the Golden Gate Bridge
Credit Isabel Angell

How many pedestrians would it take to bring down the bridge?

Let’s start with overcrowding. When the Golden Gate Bridge opened back in 1937, 50,000 people walked on to check it out. No problem. But in 1987, when the bridge turned 50, 300,000 pedestrians crowded on.

Now, picture the bridge: the roadway gently curves up toward the middle. Well on that day, the bridge actually flattened, losing its curved shape. Astaneh says the load stretched the cables.

“The cable is the weakest link among deck, roadway and towers,” he says. “So if anything is going to go first, it's the main cables.”

But he says back in 1987, it wasn’t actually dangerous.

“Because of 300,000 people being on the bridge, the stresses in the bridge reached 40 percent of capacity,” Astaneh explains. “Under ordinary load of bridge which is cars and trucks, the stresses in the bridge are 25 percent.”

Being a scientist, Astaneh ran a calculation to determine the actual number of people it would take to break the cables. And after crunching the numbers, Astaneh found it would take 900,000 people to bring down the bridge.

“So you have to get the whole population of San Francisco, and perhaps suburbs, on the bridge to have the cable fail,” he says.

Which begs the question: just how would you fit 900,000 people onto the Golden Gate Bridge?

“You have to fill up the whole area of the roadway, and the whole area of sidewalks, both sidewalks, with people such that for each square foot of area,” he says. “So that one square foot of area should hold one and a half person. Which is impossible.”

Unless, Astaneh says, you stack them.

“So if you put two people standing on top of each other, kind of like sitting on the shoulder, you might get one and a half person per square foot,” he explains.

And even with a double layer of people, Astaneh says, you’d have to forcibly shove everyone on the bridge to maximize space.

“I've seen Japanese trains, they do it,” he says. “When the trains stop, there are people that push people inside the train. So you have to do the same.”

So overcrowding is pretty unlikely.

What if a giant wall of water hit the bridge?

What about a tsunami? Maybe the destruction these giant waves recently caused across the Pacific also give us reason to be nervous.

Astaneh says to know what kind of damage a tsunami would do to your structure, you have to know the wave’s speed and the wave’s height. Around here, he says, it would be very unlikely to get a wave faster than 25 miles per hour. So Astaneh ran another calculation, to find out how high a 25 mile per hour wave would have to be to topple the towers.

“You have to have the (wave’s) height equal to about 300 feet,” he says. “And that's impossible. It's impossible to have any tsunami reaching the bridge that probably is even more that 50 feet.”

That’s because we don’t have the type of underwater faults off our coastline that cause big tsunamis. And even if we did, the biggest earthquake-caused tsunami on record was 200 feet tall; according to Astaneh, that’s not high enough to topple the Golden Gate Bridge.

“You know, it’s like if you want to have enough water that you can be toppled in the beach, it has to be up to your neck,” he explains. “That's the same thing with Golden Gate Bridge, but Golden Gate Bridge’s neck is pretty high.”

So tsunamis, nothing to worry about.

How about the Big One?

When the conversation turns to earthquakes, Astaneh becomes more serious.

“Well, that one, of course, is very serious real issue here. It's not like people on the bridge or tsunamis,” he says.

Astaneh says the towers of the bridge are so heavy, the original engineers didn’t bolt them to their foundation.

“So when earthquake comes, the Big One, the real one, the towers of Golden Gate Bridge can actually uplift. I say like a drunk person, if you go, so one leg will uplift. So not completely, but a little bit,” he says. “So what we did actually, we tested those shoes at the base of tower to make sure when uplifts and comes down it doesn't crush the foundation.”

That sounds scary – and Astaneh says bridge officials decided to make sure it doesn’t happen after experiencing the Loma Prieta earthquake quake back in 1989.

That temblor was magnitude 6.9, and the Golden Gate Bridge suffered only minor damage. But bigger earthquakes can happen here; seismologists have determined the largest earthquake the San Andreas fault can generate is a magnitude 8.3.

Astaneh was tapped to be on the Golden Gate Bridge’s seismic retrofit team and make the bridge safe in an 8.3.

“The seismic retrofit that's (being) done now, to the best of our knowledge, is going to prevent serious damage to the bridge,” Astaneh says.

The retrofit, which includes things like strengthening the bases so they don’t get crushed, will be done in 2020, assuming federal funding comes through.

What does it all mean?

Astaneh says the answers he came up with, while mostly hypothetical, proved what he already knew:

“I am very happy to tell your listeners and your station that we are very proud as engineers that we have such a structure, such structures that public use and enjoy, at the same time they are so safe,” he says. “And I appreciate you asking me to look into it. It was a really exciting weekend for me.  

So fear not, Bay Area residents. The Golden Gate Bridge probably isn’t coming down anytime soon. Except, maybe, in your local theater…

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Thanks to recent UC Berkeley Engineering graduate Candace Sy, who was Astaneh’s co-investigator for the story.

This story originally aired on September 14, 2014