# Thread: Mass of Counterweight (really big object)

1. ## Mass of Counterweight (really big object)

I don't have the faintest idea ow to figure this. It's for a fictional (science fictional maybe) hotel build onto an oil platform.

It's divided into three main sections. The Rig is the fixed base attached to the legs.

The Sail is where the rooms are found. It is 200 feet tall an extends back about 160 feet from the axis, and about 40 feet in front of it. The frame is steel and the outside walls are made up of windows of thick Lucite (maybe). I need the to be unbreakable and capable of being layered with a large LCD array for blocking out light.

As the wind shifts, the sail portion will rotate to put the smallest face to the wind. A set of turbines down either side of the "mast" will be the main source of power generation. 12 total, each 20 feet in diameter and 25 feet tall.

The Base is the link between the sail and the Rig. It rotates with the sail, but also has a counterweight that runs in a channel built into the base to keep everything more or less in balance. This weight will play a part in the story a bit later, but I have no clue as to how big it should be. I've got it about 140 feet from the axis of the Mast, and the Base has bearings on the top and bottom of the rig. I'd like the wieght to be no more than 20 feet wide (from 130 to 150 feet from the axis). Height and length of arc are variable as are the materials. I'm thinking lead or concrete surrounded by a steel shell, possibly with large Teflon blocks for sliding along the channel cut into the Rig. I'm also thinking that a hollow section for pumping in seawater to make a variable mass for any adjustments might be in order.

Any help is welcome.

2. You forgot to mention how wide the sail will be and how tall each floor.

Guessing, I expect you'd want two rows of apartments back to back with access space between them, making for something on the order of 50 feet wide, making for 10000sqf area per floor or in very round numbers about 1000 m^2.
The height makes for about 25 floors, so total floor area would be about 25.000 m^2.
Using the weight/floor area values for the WTC (easiest building to find actual numbers for) that would put the weight at 31,250 tonnes.
Looking at the placement of the pivot for your building, and ignoring the bit symmetrical to the short end means you have 120/200 of the building or 18,750 tonnes placed with a center of gravity 100ft from the pivot point.
A counterweight placed 140 ft from the pivot point has to weigh 18,750*100/140 or about 13,400 tonnes.
Made of lead, (density 11.35 t/m^3) it needs to be about 1180 m^3 or, if a cube, about 10.5 m or 35 feet per side.

You're approximately a factor 8 off in size of the counterweight.

200ft tall, 200ft long, with a somewhat tapered cross section so the leading half of the building is 50 feet, the trailing is 25 feet.
Code:
``` ____________
/            \___________
|                        \
|      ^      ___________/
\____________/```
You can have cheaper/smaller apartments in the trailing end and the pivot point in the center of the thick part, making for perhaps 60/140 instead of your 40/160.
CoG of the asymmetric(thin) load is still 100ft from the pivot, but total off-center weight at CoG only about 4000 tonnes, making for a counterweight of 2850 tonnes or a cube of lead about 20 feet on a side.

Total floor space in this case is about 187,500 sqf, a third of which is in the thin part.
Total weight in very round numbers, 27,500 tonnes, 4000 of which is the counterweight.

This weight does not include the windmills, which I expect to be placed to make the load as symmetrical as possible, so a bit in front of the pivot to counter average wind pressure.

In all an interesting concept, though I suspect not really practical.
For one thing, the wind along the sides of the building will probably have considerable turbulence making the wind turbines less efficient.

3. Order of Kilopi
Join Date
Feb 2005
Posts
11,642
There is always the flip-ship concept scaled way up perhaps A column in the water usually takes waves better when vertical than horizontal.

4. Originally Posted by HenrikOlsen
You forgot to mention how wide the sail will be and how tall each floor.
Bah, and I thought I gave all the important stuff. Yes, the widest point is about 60 feet, which tapers down to about 20 at the small end, before rounding off. There is a total of 10 feet from floor surface to floor surface (based on the hotel where I currently work).

Another thing I failed to mention was that the floors grow smaller with each layer. By the time we get to the uppermost floor, a restaurant, it's 20 feet tall, still about 60 feet wide, but only about 60 feet long total, and still tapered. I have a drawing of it that I made last night, but my laptop sort of died. A lot.

Guessing, I expect you'd want two rows of apartments back to back with access space between them, making for something on the order of 50 feet wide, making for 10000sqf area per floor or in very round numbers about 1000 m^2.
The height makes for about 25 floors, so total floor area would be about 25.000 m^2.
Using the weight/floor area values for the WTC (easiest building to find actual numbers for) that would put the weight at 31,250 tonnes.
Looking at the placement of the pivot for your building, and ignoring the bit symmetrical to the short end means you have 120/200 of the building or 18,750 tonnes placed with a center of gravity 100ft from the pivot point.
A counterweight placed 140 ft from the pivot point has to weigh 18,750*100/140 or about 13,400 tonnes.
Made of lead, (density 11.35 t/m^3) it needs to be about 1180 m^3 or, if a cube, about 10.5 m or 35 feet per side.
That's smaller than I was expecting, and given the changes in the design from above, I think it would be even small than that. That helps out a lot. If I can get the laptop to turn back on, I'll get the square footage of each floor and try to get a figure based on what you did above.

This weight does not include the windmills, which I expect to be placed to make the load as symmetrical as possible, so a bit in front of the pivot to counter average wind pressure.