Thursday, January 27, 2011

Know when to say when on the Golden Gate Bridge

By Alaina G. Levine

Next year is the 75th anniversary of the Golden Gate Bridge. So it was fitting that while in San Francisco this week I took a look at this modern marvel, but more importantly, learned a little about the physics associated with keeping it intact.

My guide was Paul Doherty, Senior Scientist at the Exploratorium in San Francisco. I recently wrote about Paul and his museum-working comrades for APS News, so it was an especially sweet coup to not only visit him in his native habitat, but also get a behind-the-scenes tour of one of the most famous science museums in the world.

The Exploratorium is located a stone’s throw from the bridge and its staff have developed a number of very interesting exhibits relating to its dynamics over the years.

I learned that they have a telescope at Fort Mason, which is about 2 miles east of the museum, which is pointed at the center of the Bridge. The telescope allows one to see the height of the center of the Bridge as a function of temperature. As the temperature decreases, you can actually see the Bridge contract and lift up. Paul told me that generally this amounts to + or – 10 feet over a year, although during a day it might only be a few centimeters.

Another team of Exploratoriumers is building a GPS system to be installed on the Bridge, which will monitor its height due to weather (wind, etc.). The contraption, seen below, will provide a 3D position of the Bridge at anytime. So when the wind starts pushing and the vehicles start depressing the structure, you’ll be able to know how much the Bridge has moved, within 5 mm accuracy -- you’ll have the actual global position.



This prototype was just assembled this week, so I guess I got the scoop for PhysicsBuzz readers. The scientists and exhibit builders at the Exploratorium spent the week testing it, and after it is installed, Paul suggests that a next phase might be the creation of a mobile app, so your phone call tell you the location and orientation of the Bridge from anywhere in the world.


Paul and his colleague Dave Fleming, a mechanical engineer who’s been with the Exploratorium for 25 years, took me to the area of the museum where they design and construct the exhibits. Currently, most of the museum’s staff work in a series of old Barracks just off Crissy Field at the northern edge of San Fran, right along the bay, These barracks were used by the Presidio, and parts of them were also used to store bodies of soldiers returned from Vietnam. In fact according to Paul, “The Cooler”, where these corpses were placed, is now used to conduct sound-based experiments by the museum staff, on account of the thick walls.

Here's Dave explaining the forces that cause the Bridge to move but not buckle:

http://www.youtube.com/watch?v=Z4oGwg--KnY

By the way, the official name of the museum is the ExplOratorium, and as such the employees’ business cards reflect this soft-spot for circles:


This quite possibly could be the best business card I have ever seen, because it is not just a cleverly designed graphic, it’s a puzzle: although the diameter of this circle on the card is smaller than that of a quarter, there is a way to get the quarter through the hole. It’s an enigma, get it? Can you figure it out?

2 comments:

  1. I think one must fold the card and then slip the quarter through the slot. it would be harder to fold the quarter.

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  2. I'm surprised that you didn't mention the events at the 50th anniversary celebration, when an estimated 250,000 people ended up stuck on the bridge for hours, and the bridge flexed so much under the weight that it actually became concave upward. The human load was something like 3x the usual load due to vehicle traffic, and bridge engineers were sufficiently concerned to calculate the safety margin.

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