FROM GALLOPING GERTIE TO WHISTLING GOLDIEJune 8, 2020
I knew, when V.T. emailed this story, that I'd have to blog about it, because it's about another one of my "favorite things," and not because it has any special major news worthiness; it's not about riots, burning cities, Baal and Malicious Gates, faulty virus studies, or odd tan lines because of wearing surgical masks. It's not about anything important. It's more in the line of my fascination for engineering "stuff," and hence you can file this one in the same category as my fascination for steam locomotives (and why "they" are restoring and actually using them), for "really big artillery", for roller coasters and so on. Well, another one of my "favorite things" are bridges, especially big ones, and especially suspension bridges.
It seems that a recent "refit" of San Francisco's famous and iconic Golden Gate Bridge has caused it to "whistle" in high winds:
Ok, so what? Now the Golden Gate "whistles" when the wind is right. Big deal.
Well, it may really be a big deal, if we explore a few questions, and do a little background digging. The Golden Gate is a marvel of engineering, especially if you consider that its graceful lines, its massive central span, and its status as an icon of the state of California, were all done in the 1930s, with 1930s technology. The bridge was outfitted with new panels along the sidewalk and bike paths to either side of the main roadbed of the bridge, which slats in turn, with their holes, turn the span into a big whistle when wind conditions are right. So here's the explanation for the retrofit from the U.K.'s Guardian:
City officials offered an explanation for the sound, which can seemingly be heard several miles away.
Paolo Cosulich-Schwartz, a Golden Gate Bridge, Highway & Transportation District spokesperson, said the sounds stemmed from long-planned wind retrofitting.
“The new musical tones coming from the bridge are a known and inevitable phenomenon that stem from our wind retrofit project during very high winds. The wind retrofit project is designed to make the Bridge more aerodynamic under high wind conditions and is necessary to ensure the safety and structural integrity of the Bridge for generations to come,” Cosulich-Schwartz said.
“We knew going into the handrail replacement that the bridge would sing during exceptionally high winds from the west, as we saw yesterday. We are pleased to see the new railing is allowing wind to flow more smoothly across the bridge.” (Italics added)
When the Golden Gate was designed, its architects deliberately had to make the bridge flexible and aerodynamic enough to "bend" laterally with high winds. When traveling across it, one may seem like one is traveling straight, but in reality, the roadbed deflects in an eastward direction by several feet, depending on wind conditions. Hence, it's that little statement, italicized in the quotation above, that makes me wonder: if the bridge was originally designed to deflect laterally, what made the retrofit necessary? Had decades of use required the bridge to become more flexible? And if the bridge is now whistling in a way it did not before, could resonance factors eventually occur that the bridge might not be able to damp? Probably not; after all, it's stood this long through all sorts of weather conditions. But, the question is there, hovering rather uneasily in the background.
Now, you may have noticed that the title of this blog mentions another famous, or rather, infamous suspension bridge, the first Tacoma Narrows Bridge of 1940, a span known now by her nickname, Galloping Gertie. When she was completed, Gertie was the third largest suspension bridge in the world, and also one with an enormous central span. During her construction, the crews noted that the roadbed of the bridge gently undulated in the breeze blowing down the narrows. During her grand opening, for example, the parade across the bridge had to contend with gentle waves moving down the roadbed as it bobbed up and down in the wind.
On November 7, 1940, a strong gale moved through the Narrows, however, which caused the roadbed to twist and oscillate, and the bridge, unable to damp these oscillations, eventually collapsed that day. You've probably seen the videos:
Now I include this video for a very important reason, for you will note that the panels along the sidewalks on either side were solid, there was no way for air to move through those panels; it could only go over or under the panels. In mild wind, the was not a problem, the roadbed gently rose and fell; but during the gale that eventually brought the span down, the oscillating movement was set up, and the bridge was unable to damp it; the resonance grew, and the central span collapsed. The causes of the collapse are debated to this day. (Q.v., Tacoma Narrows Bridge (1940)
Fortunately, the Golden Gate was not originally designed with solid side panels... but the question nonetheless remains: why was a new retrofit of the side panels of the Golden Gate recently done? Was it necessary? Or just a precaution? Or, to put it country simple, what, if anything, are we not being told?
See you on the...
...oh, wait, I forgot to mention. The chief architect of Galloping Gertie was Leon Moisseiff.
Oddly enough, his theories were applied in the design of the Golden Gate bridge, on which project he served as a consultant.
Have a nice day.
See you on the flip side...