Here comes the bride.
Big, fat and wide.
Indeed! IIRC Chicago has more drawbridges than any other city in the world (maybe just more bridges period…I forget). I always loved the commercial below from United Airlines which shows off a lot of those bridges in operation in a choreographed manner (pretty cool).
ETA: Seems most drawbridges (of some sort) is true:
You’re ready to defend from the barbarian hordes!
I measured on Google Maps from the seam at one end of the bridge to the seam at the center. Looks like each half of the bridge is about 165 feet long. However, the axis of rotation for each half may not be directly below the proximal end of each half, as this video shows (seen more clearly in this closeup at one end of the bridge). It looks like it’s maybe 25 feet out from where the bridge deck separates from the landing.
Well worth a look:
And this is just fabulous (also linked to in the above page.)
tl;dr:
Spans are 128 feet centre to centre of trunnions. Each leaf weights about 2800 tonnes including a counter weight of 1400 tonnes. Each leaf is powered by two 100 hp electric motors. (The documents above use short tons, which is most annoying.)
Whoever wrote the sign in the OP’s pic has a very rose coloured view of what a 1950’s VW could do. Nevertheless, it is not much power. The trick is perhaps that when in motion there is a goodly amount of kinetic energy. It wouldn’t matter if the thing was perfectly balanced to within a micro-newton. The time required to get the bridge to rise dictates the power needed, and the moment dictates the torque. Electric motors are perfect for the job.
I though maybe it was some old-timey weight like, “One moderate-sized hog”, but it turns out, that’s almost exactly the weight of a cubic foot of steel, so I suspect they just used a cubic foot of steel as a convenient counterweight add-on.
How much does that piece of steel stock weigh?.
Fact: Most steel weighs about 489 pounds per cubic foot.
That sounds likely, but it is a bit odd.
In my truck driving days, I did some work for a company that made weights for testing weighbridges and commercial scales. All the heavier ones were made of cast iron with handles or lifting rings. They came in exact fives and tens though.
I too was afraid that the drawbridge might go on a rampage.
I was wondering that too. We know that there’s a liquid in there, but is that water-based? If it is, then presumably it evaporates, but what if the liquid is oil-based? Does that evaporate? Plus, the solids that are dissolved in that liquid, to make it paint. The goal is to have those solid stay on the bridge, so there must be weight there, even if lower than the liquid base.
Someone needs to tell the people who wrote the plaque in that room of the Michigan Avenue bridge. As you can see in the pic in the OP they claim the bridge leaf weighs 4,100 tons and the counterweight weighs 12,000 tons. Whatever definition of “tons” you use (short, metric) doesn’t account for that difference.
That said, it is quite possible they had an intern write that and no one ever bothered to check their work.
I find it quite likely that the counterweight weighs more than the leaf itself, given that it is much closer to the pivot.
You joke but the last mayor lifted the bridges more than once as crowd control to mitigate a perceived threat of rioting / looting.
Hard to tell if it actually succeeded in preventing anything in particular.
I saw a documentary once that showed that it’s not hard to get past an open Chicago drawbridge. https://www.youtube.com/watch?v=QTOg4aYGtdY&t=20s
Without question, the best documentary on Chicago…ever. The best documentary on anything ever.
Just…“Fix the cigarette lighter.”
From this quote, I’m picturing the Mayor doing it by hand, with a crank. Which would require a very well-balanced bridge indeed.
I thought that was the point, to illustrate that thanks to the help of the counterweight and the magic of reduction gears, the bridge can be raised using a not very powerful motor.
It’s all magic as far as I know.
That might be the bridges named after Donald Trump or Shirley MacLaine?
I agree. But it is a pet peeve that rather than stating a value, a near useless supposedly meaningful common metric is used. Width of a human hair, Olympic sized swimming pools etc.
The bridge uses two 108 horsepower motors on each side. A 1950’s era VW made 36 horsepower. Not only is the metric near useless, for a start it is torque that matters most, but it is just plain wrong.