Earlier today I was running around Chicago taking advantage of Open House Chicago where many places were open to the public to tour for free (it is a really great thing and fun…also happening tomorrow (Sunday October 15, 2023) for those in Chicago…just be careful to note what places are open…not all are).
One of the places I visited was the control tower to operate the Michigan Avenue drawbridge in Chicago (see picture below…the towers at each end of the bridge). There are actually four but only two are used for its operation…the other two are for aesthetics…only the fake ones were open to the public.
While in there we could see the mechanisms used to open and close the bridge. I took a photo of a sign in there which said the bridge was balanced so precisely that painting the bridge could screw it up (see pic below). How is that possible? I get that paint has weight but I’d think it is not much compared to the many ton bride and many more ton counterweights.
it is precisely balanced to be used with only a small motor. If you had paint (and with the size it will be tons of it), you had mass and needs more rotational momentum to lift the bridge. That momentum will be provided by more counterweight or by more motor power.
If you paint every surface of that bridge then its a lot of square metres. The paint used for bridges is pretty dense and gluggy and anti-corrosive, and require multiple coats, so you can’t think of it as similar to a quick once-over of the lounge room with a tub of acrylic off-white. It can easily add tons, which matter when it is a counterweight lift bridge. And because the counterweight is so heavy there is a lot of effort required to get that inertial mass moving, hence the bigger engine.
For the first few flights of the space shuttle Columbia, NASA painted the external fuel tank white. After that, they skipped the paint, which is why the tank was always orange (the color of the insulating foam). This saved about 600 pounds of weight, which could be allocated for revenue-generating payload instead.
I don’t know off the top of my head how the surface area of the shuttle’s external tank compares with that of one half of the Michigan avenue bridge. But with all the various structural members of the bridge and their many flanges, webs, and fittings, it seems like it might be in the same order of magnitude. Add in the likelihood that the bridge would probably be receiving a brushed-on coating of thick anti-corrosion paint (as opposed to the shuttle tank’s sprayed-on cosmetic finish), and it seems quite likely that painting the bridge really could add a lot of weight.
They didn’t say it’d be seriously unbalanced. They said it’d be unbalanced enough that they’d want to fine-tune the adjustments to compensate. Because it was engineered well enough that they can do that.
Not to be that guy who just posts a ChatGPT response, but here’s the ChatGPT-4 response:
Weight of Paint:
A gallon of paint typically weighs around 10 pounds, but industrial or marine-grade paint might be heavier due to additives. Let’s say it’s 12 pounds per gallon on average with primers and multiple coatings considered.
Surface Area:
The surface area of a bridge isn’t straightforward because it includes the road surface, sides, undercarriage, pillars, etc. However, let’s assume:
Length of the bridge: 400 feet (approximate)
Width of the bridge (including sidewalks): 80 feet (approximate)
Therefore, the road and sidewalk surface area = 400 x 80 = 32,000 square feet.
Considering the structural components underneath, sides, and all other exposed parts, let’s make an estimate that the total surface area to be painted is roughly three times that of just the road/sidewalk surface. So, 3 x 32,000 = 96,000 square feet.
Paint Coverage:
Industrial or marine-grade paint might cover around 250 square feet per gallon. This is a rough estimate, as coverage can vary based on the type of paint and the surface.
4. Calculation of Gallons Needed:
96,000 square feet ÷ 250 square feet/gallon = 384 gallons
5. Total Weight of Paint:
384 gallons x 12 pounds/gallon = 4,608 pounds or approximately 2.3 tons.
This is a rough estimate, and real-world figures could be different based on the specifics of the paint job, the exact surface area of the bridge components being painted, the type of paint used, and more. Nonetheless, it does give an idea of how much paint might be needed and its potential weight.
What a relief! From the thread title, I got the idea that merely painting a portrait of the bridge on canvas and hanging it in your home could cause the bridge to become seriously unbalanced.
As with any paint, a significant portion of this weight will be the liquid/solvent that’s carrying all those solids. I’d guess once that stuff evaporates, the per-gallon weight might drop by 2-3 pounds.
Great starting point. One thing to adjust is the difference in dry vs wet weight. I don’t know much about paint, so I’m not sure if its drying is more of a curing process (maybe not the right term, but I’m thinking liquid precursors combining into something solid) or an evaporation.
It can be, that’s one reason why these bridges have open grate metal decks. They also weigh less to begin with, so less counterbalancing is needed.
This reminds me of the surprising forces imposed on a canal lock. On the small industrial canals (think the Erie Canal in New York, Miami & Erie Canal in Ohio, and the numerous ones throughout the UK and western Europe that were roughly 50-feet wide), the lock gates were operated by hand. Even children can open them since they have a decently long pivot arm. However, this can only be done when the water level is completely equal on both sides. In a typical lock chamber 15-feet wide by 90-feet long, just one centimeter difference in water level adds nearly 2.5 tons of weight (yes I know I’m mixing units). That doesn’t mean (I think) there’s 2.5 tons of force against the lock gate itself, but it’s still formidable. These bascule bridges in Chicago are the equivalent of a child operating a canal lock, and any change in weight is like adding water to one side of the lick.
I imagine that snow and ice are the reason why they made the counterweight adjustable, but once you have the adjustable counterweight, you might as well also adjust for everything else.
It’s not just the weight you have to worry about, it’s the moment. The cite upthread estimates the length of the bridge at 400 feet. That would make each half 200 feet long. 10 pounds of paint, located 200 feet from the pivot point, would be balanced by a one ton counterweight located a foot away.
If anyone knows the real length of the bridge, or the distance from the pivot to the counterweight, feel free to post them. In general, though, any weight is going to have a greater impact as you get further from the balance point.
