[bung]

Someone mentioned in a comment recently on the Venezuela protests, how some of the highways we saw in pictures packed with people were not necessarily designed to support the standing weight of people shoulder to shoulder.

And yet, in so many movies, bridges in NYC are filled with people and/or military vehicles going one way or another. I wonder if they were tasked with actually making sure the bridges could support a city evacuation or massive protest.

[arjunvpaul]

Short Answer: Don't worry about people's weight. Invite as many people and cars as you like.

Bridges are designed and built with enough redundancy that if you fit all the people you can find on them, they can comfortably handle the weight. One should be more worried about not how many folks are there but HOW they are moving. Remember if you are in a protest and on a bridge don't "walk in step" like this https://goo.gl/dXIvBN . If you are interested in why this happened watch this https://goo.gl/ZOlzyx (skip to 2:42)

Forget the modern concrete and steel ones, when we built small bridges for villagers in Rwanda and Central America, we would ask all the villagers and their horses to come and fill them (just like in the protest pictures). This is basically done to address the concerns of some folks like you who may have to see it to believe it :-) See picture here: https://goo.gl/RIixT0

Source: Personal experience being a construction engineer who has built cable stayed bridges and the ones in the picture.

[bung]

Very cool. I suppose in hindsight I think the guy was talking about those elevated highway overpasses and/or on/off ramps, not exactly nice new suspension bridges :) Anyway thanks for the details.

[ricardobeat]

That doesn't sound right. You need 15-20 people to make up the weight of an average city car, nevermind a SUV or truck, plus 4+ passengers and load. That's a crowd of 30 for every car the bridge can support.

[ghaff]

Yes, but you can pack the bridge more completely with people.[1] It's apparently an urban legend that the Golden Gate was in any danger of collapsing when people were allowed to walk on the roadway for its 50th anniversary--but it may well have been a peak load.

“It was probably the biggest load the bridge had ever seen,” said Mark Ketchum, a San Francisco bridge engineer who studied the Golden Gate Bridge from 1989 to 1991. “But it did not exceed the design load capacity of the bridge.”

[1] http://www.mercurynews.com/2012/05/23/the-day-the-golden-gat...

[ComradeTaco]

Large bridges and elevated roadways generally have a factor of of at least Safety 2 to 3 times their maximum expected loads. Everything these days is, for good reason, overbuilt.

Even the Brooklyn bridge, partially because structural mechanics wasn't quite as well developed at the time, has a factor of safety of 6.