Tall two story skinny building?

[phalynx]

I am "thinking" about a tall skinny building.  How "safe" or reasonable is this design and what would I need to do to make it safe and or more safe if not safe.  12x12 or close, 2 story building on a concrete slab.  I would like the top roof to be able to open in the middle somehow leaving a stub wall and a large opening (think observatory).

What are the thoughts on how/if this could be constructed.  The goal would be to get a small 1/2 height room about 20' or so in the air.  In the middle of the building would be a concrete pier going from a large base to the top floor.  Probably 20" in diameter or so.

....and yes, phalynx is at it again...   

[dug]

I have no idea how to build it but I can't wait to see it. Judging from your history I won't have to wait long!

Welcome back!

[MountainDon]

Oyjer than buying a dome I saw sliding or rolling roofs someplace. Google home observatory or something like that.

[phalynx]

I have the roof figured out.  It will be a roll-off.  The bigger question is, can I build a 12x12 building that is about 24' tall on a slab?  Would it blow over?  Would it have extra stresses because it is skinny and tall?  etc...

[Don_P]

No doubt on the extra stresses, but they do build tall chimneys, skyscrapers and monopole water towers. It'll take some hefty footings very well tied to good bracing. What is the central post doing?

[MountainDon]

Anchoring a telescope I think. They need a rock solid mount.

[diyfrank]

Tall buildings are usually anchored buy a counter weight type of foundation. heavy or buried deep.
Maybe you just need to have it in the ground a couple feet.

Check out this pour. I was sent this in a email a couple weeks ago.



' San Diego , BAYSIDE condos ($795,000 to $12,000,000.each condo, 282 total.)

These photos show the concrete pour, last weekend, of the south one-half
only of the ten-foot-thick RAT slab, which supports the 35 story tower.

This was a 1,258 truck-ballet in which the contractor placed 11,500 yards of concrete in one monolithic pour that got started at five in the morning and
finished at three in the afternoon.'

1,258 trucks in 10 hours = 126 trucks per hour = 1 truck every 28 seconds

[phalynx]

That's a scary construction site...  Talk about "timing is everything"...  The goal is to get an observatory up where I can have a horizon view that is not obstructed by all of my trees.  I believe if I can get up about 20' in the air, I can clear enough of them to have a nice viewing area.  The concrete pier is for the telescope.  Its a pretty good sized and weighs about 200 lbs on its mount.  Mass is key as vibration is the worst thing in the world.  I will have to pour the pier first and have plenty of mass under it.  I may use 20x20 concrete blocks to build up past the main pier of maybe 6' or so.  I am concerned about how I would pour concrete down a tube 20' in the air....  talk about back breaking.  After the pier is poured, I would then pour the slab around it with foam separating the pier and the slab for vibration dampening.  I was planning on a 5" or 6" slab thickness just for extra mass.  We don't have a frost line so digging down isn't required except for maybe the needed mass. 

Any thoughts on how deep the footings should be?

[muldoon]

I know you are thinking of building a solid structure, but if the desired intent was to have a platform at 20' up; I would look at the deerstand towers that sits about 15'-20' foot up.  Sure your going to have different "box" but there are plenty of places in Texas that can provide 20' very stout platforms likely cheaper than you can build it. 

Check out the tower stands from this deer stand window place.  They sell it 20' with substantial ladder and steel framing.  You just frame out your obersvation box on top of it. 

http://texasdeerblindwindows.com/tower-stands/

Or if you want to get silly,
http://www.texaswildlifesupply.com/tws/monster.htm

[glenn kangiser]

Drilled piers might use less concrete than dead weight, but likely you would need an engineer to tell you sizes.  I'm thinking about half the side pressure from wind - 30 psi at 100 mph I think, would go toward tipping it over - on a 12x24 that could be close to 5000 lbs.  Deeper piers or footings would gain leverage advantage.

...but then again, this works just fine as I recall.......

[phalynx]

Muldoon, a deer stand wouldn't come close to working.  I need room to move around and the scope is quite large.  Mass is key to the stability of the scope and the controlled vibration.  I assume mass is key to the tall building as well.

Ignore the giant ugly guy standing next to it but this is the scope.  The tripod won't be on it but everything on top will be.  Keep in mind the big ugly guy is 6' 5" tall.



Here is an example of what I want to do but I need to be taller.  Also, I would like to completely enclose it and build like a normal building so I can use the downstairs space as extra room.

[firefox]

At UC Berkeley a very long time ago they had a laser

lab. The lab was in the basement of a building

sitting on bedrock, but the road that circumscribes

the campus went around the rear of the building

about 500 yards away, Trucks that used the road

would cause vibrations large enough to mess up their

experiments. This was their sollution:

They started wit a granite slab about 4' x 8' by 10

" thick. Under the slab was the following support:
A sandwitch of 1" plywood followed by (going down)
a matrix of blocks rouhly 2' x 2' x 6" layed out

under the plywood, then a second sheet of 1"

plywood.

Each block had a cutout in the middle as if you took

a giant router and cut a donut shaped hole throug
the block. Then they put an inner tube in the space
that was cut out. Leaving a lateral hole for the

inflation tube.

This way you have a matrix of 8 tubes under this

huge slab of granite.Probably inflated just enough

to lift the slab a faction of an inch.

You might be able to use this tecqnique undr the

base of your center support the following way.
Think of the base as looking like a daisy. Each

petal is joined to the center through a reinforced

concrete beam of the right size. The petal is a

circular block similar to the granite slab.
You will want the bottom of this block sitting

slightly above the surface so that you have complete

access to the supporting structure underneath so

that you can replace inner tubes as needed by

putting a floor jack in place to support that

pettel. The rest should be obvious, using truck

inner tubes and some kind of wood/fiberglass block

to support it sitting on concrete pads.

You will need to do the engineering calcs to size

it, but it should give you a vibration free

platform.

I would build the structure around this system so

that it does not touch it and defeat the whole

purpose of it.

Bruce

[JRR]

There are a couple of wooden towers not far from the Atlanta airport.  Each has a footprint of 12'x12', maybe 14'x14'.  I think each is about 4 or 5 floors, possibly more.  Plywood sheathing.  Looks like standard framing.  One is on a testing facility owned by a technical school ... this being at an off-campus site.  The other is owned by a firestation ... I assume for cadets to run up and down the staircases as training ... maybe they do "rescues" on the various floors ... I don't know.

If I really knew how to use this iPhone, I would post some photos.

[MPalange]

Years ago, the forestry service used fire towers to get above the trees. Maybe a fire tower would work?

http://www.fs.fed.us/database/acad/lt/

Marty

[phalynx]

Oh now Marty, that is just too cool.  I have been looking for fire tower designs for a LONG time!!!!!!

[Squirl]

I know this is off topic, but thank you marty.  I have been looking for firetower designs for a while.

[MikeOnBike]

I have the roof figured out.  It will be a roll-off. 

If it doesn't roll off symmetrically that will put additional tipping forces on your structure.


Very nice scope!  I'm still hand guiding my 12" dobsonian.

[Don_P]

Basically, turn it on its side to visualize it. It is a uniformly loaded cantilevered beam. Now turn the flagpole back upright. The anchorage needs to be sufficiently strong and heavy to prevent sliding or overturning, the bracing needs to prevent it from racking. I quickly sketched a schematic of a truss, could be a way of looking at the building, with a 3:1 ratio and pushed on the top with a 50 unit load to show the forces. Notice the reactions, there is an equal and opposite horizontal sliding force of 50 units at the base. The left vertical reaction went off page the arrow should be pointing down. There is a 150 unit overturning force in this example so the connection to the base and the weight of the base must resist this. Internally the brace forces are shown... this can be handled by your sheathing.


The truss program is here and is fun to play around with;
http://www.jhu.edu/~virtlab/bridge/bridge.htm

[considerations]

Some old farms had a 2 or three story accessory building/addition.  The top floor was a water cistern so the house had some water pressure.  The ones I've seen appeared to be about 12' x 12' and were finished on the outside so they looked like the farm house. There are still a few around here.

[upa]

I have the roof figured out.  It will be a roll-off.  The bigger question is, can I build a 12x12 building that is about 24' tall on a slab?  Would it blow over?  Would it have extra stresses because it is skinny and tall?  etc...

I have built a fancy kids playhouse about 12 years ago at our old place in pretty much those exact dimensions. Wish I took pictures now.
We live in tornado alley. It was built on a concrete slab and is still standing, 24 feet is not really all that tall up until you start roofing

[phalynx]

"24 feet is not really all that tall up until you start roofing"

So very very true...

[phalynx]

For those of you keeping score.  Here is what I came up with and what it looks like now.  I only went 17' in the air and I am not enclosing the sides below so it won't have quite the wind load of a fully enclosed building.  It is 12x24 and there will be a building that is 12x12 with 6' high walls.  The roof will roll off onto the other 12x12 section. 

This is a picture of construction so far.  Very sturdy.  Wiggles very little.  Everything is double bolted.

[nathan.principe]

Man! I thought I built my cabin high off the ground! looks pretty cool

[Squirl]

Have you tried a telescope on it yet?

[phalynx]

Have you tried a telescope on it yet?

I have to construct the pier first.  That will be a large undertaking requiring a lot of concrete....