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3rd floor observatory pier design

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#1 deonb

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Posted 06 May 2021 - 11:31 AM

Currently designing a new house. I'd like to install an observatory on my 3rd floor, so the base of the floor would be around 25ft of ground level.

 

I want a solid pier that goes through both floors below to create a pier-based foundation on the 3rd floor, and then I want to put a metal pier on top of it that foundation that I can swap out as needed.

 

The biggest scope I can see myself put on this would be a CDK-24 at 6000mm, which weighs around 750lbs on a L-600 mount.

 

What kind of pier structure and foundation below it am I looking at for this?

 

My calculations so far: Our soil is rated at 1500 psf. A 25ft long x 30" wide concrete+steel pier would weigh around 18000 lbs above ground. What kind of foundation do I need below ground to hold up such a pier? Will something like a 6ft deep, 10' diameter circular base work (50000 lbs)?

 

That would make it 50'000 lbs of foundation + 18000 lbs for the pier itself, spread over a 78 square foot area = 871 psf. Which seems ok on 1500psi soil? Is this the way I should be thinking about it? Of course I'll get a structural engineer to verify it, then I'll get another structural engineer to verify him, and then I'll get a seismic engineer to verify them both.

 

I just want to see if I'm going down the right path here, or if I shouldn't be thinking concrete in the first place?

 

 

Are there alternatives to a solid pier? e.g. If the rooms below were constructed with steel + concrete walls, and you have a steel+concrete slab on the 3rd floor, would that provide a stable enough foundation for a 6000mm FL? I've used scopes on concrete floor of 3 story commercial concrete buildings, and the floor seemed pretty stable and isolated even if you stomp around. But that was at 1000mm.

 

 



#2 duck

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Posted 06 May 2021 - 12:02 PM

assume the 17.5 cubic yards of concrete in the footing is immovable.  Then calculate the beam bending stiffness of the 25' tall by 30" diameter concrete column cantilevered to the footing.  Consider the 750lb scope and L-500 as a point mass and calculate the natural frequency.  This is a very rough way to get at the dimensions required.  If you get > 20HZ, you'll be fine.

 

Make sure the concrete pour is monolithic.  Don't want any cold joints in the concrete.  


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#3 P_Myers

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Posted 06 May 2021 - 12:11 PM



All this for a location in Seattle???


Not the best weather for something like this... then again if it brings you happiness...that's all that matters.

#4 Bob4BVM

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Posted 06 May 2021 - 01:09 PM

Sounds like a wonderfully energetic idea, but ...

 

-Will the house structure below it be unheated so you are not looking thru a column of warm rising air all the while ?

-What does your building codes say about it ?

-How many clear night per year in Seattle ?

-And the real bomb if all the above works out...Will you be able to get the city and neighbors to turn off every streetlight & yard light within a 10-mile radius ?

 

Not trying to discourage you on this project, I'd love to do the same myself. And certainly not picking on Seattle- i used to love visiting there. But based on a  night view of any large city, i would change location if I were going to build this !

 

Dark skies, Bob

 

 

city sky.jpeg


Edited by Bob4BVM, 06 May 2021 - 01:17 PM.


#5 deonb

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Posted 06 May 2021 - 01:19 PM

All this for a location in Seattle???


Not the best weather for something like this... then again if it brings you happiness...that's all that matters.

I'm actually a bit outside of Seattle in a Bortle 4 region.



#6 robbieg147

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Posted 06 May 2021 - 01:19 PM

750lb is not that heavy really, it is similar to the point load a suspended library floor would be designed to carry?

 

If it were me I would have a reinforced concrete slab on brick load bearing walls, no need for a pier going right through the house.



#7 deonb

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Posted 06 May 2021 - 01:39 PM

Sounds like a wonderfully energetic idea, but ...

 

-Will the house structure below it be unheated so you are not looking thru a column of warm rising air all the while ?

-What does your building codes say about it ?

-How many clear night per year in Seattle ?

-And the real bomb if all the above works out...Will you be able to get the city and neighbors to turn off every streetlight & yard light within a 10-mile radius ?

 

Not trying to discourage you on this project, I'd love to do the same myself. And certainly not picking on Seattle- i used to love visiting there. But based on a  night view of any large city, i would change location if I were going to build this !

 

Dark skies, Bob

 

 

attachicon.gifcity sky.jpeg

1) Thank you, I will take that into consideration

2) You mean why do they same about my "foundation for future elevator shaft". :) ?

3) Actually our summers are great and have more observable nights than not. Right when the milky way is in best view. Sure November to April sucks, but during winter I plan to re-use that pier foundation for a Christmas Megatree.

4) I don't actually live in Seattle. I live like 25 miles outside of there in a Bortle 4 region.

 

Also, I currently do have a scope and observatory. I know what I'm in for.



#8 deonb

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Posted 06 May 2021 - 01:41 PM

750lb is not that heavy really, it is similar to the point load a suspended library floor would be designed to carry?

 

If it were me I would have a reinforced concrete slab on brick load bearing walls, no need for a pier going right through the house.

The issue isn't so much supporting the scope itself, it's not transferring vibration onto the scope when you're moving right next to it. But that would be awesome if that would work.

 

How do I find out if it would?



#9 robbieg147

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Posted 06 May 2021 - 02:46 PM

I would try to have supporting walls as near as possible to the location of your scope, or ideally directly underneath, vibration is linked in with flexure so if the supporting walls are directly underneath or close I don't think vibration will be a issue?

 

It is generally thought reinforced concrete floors are not affected by vibration but your intended use is quite a severe test.


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#10 deonb

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Posted 06 May 2021 - 03:01 PM

I would try to have supporting walls as near as possible to the location of your scope, or ideally directly underneath, vibration is linked in with flexure so if the supporting walls are directly underneath or close I don't think vibration will be a issue?

 

It is generally thought reinforced concrete floors are not affected by vibration but your intended use is quite a severe test.

Ok, so maybe not even specifically build a whole room under it but just use a cross section of two concrete walls and then support a pier on top of those?


Edited by deonb, 06 May 2021 - 03:02 PM.


#11 robbieg147

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Posted 06 May 2021 - 03:16 PM

Yes have two load bearing walls crossing ideally directly beneath the scope then a reinforced slab/plynth onto which your scope is bolted.

 

The load is nothing to worry about and you won't have any vibration.



#12 deonb

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Posted 06 May 2021 - 04:38 PM

Yes have two load bearing walls crossing ideally directly beneath the scope then a reinforced slab/plynth onto which your scope is bolted.

 

The load is nothing to worry about and you won't have any vibration.

That's a really compelling idea.

 

Has anybody here ever done something like this?



#13 Stevegeo

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Posted 07 May 2021 - 11:32 AM

Ever consider a solid pole? Thing telephone pole sunk in concrete . Build house around it.

Cheaper and easier to do. And if you ever sell easier to hide ..

Just a thought . 


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#14 AndrewXnn

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Posted 07 May 2021 - 07:43 PM

The pier (or pole) could be stiffened with the addition of tension wires.  They would be more effective the further out from the center they could be placed.  However, hiding them within your home and isolating them from the structure might be challenging.

 

Alternatively, the structure could be hollow.  That would take more room, but would be stiffer than a solid pier.


Edited by AndrewXnn, 07 May 2021 - 07:57 PM.

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#15 Stevegeo

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Posted 08 May 2021 - 12:23 PM

Another alternative   a block chimney support .  Concrete base, stacked block, this would provide mass and stability.

And if you ever decide to sell...could be converted for a furnace/ fireplace  .



#16 speedster

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Posted 08 May 2021 - 11:50 PM

Your bearing capacity is likely a good deal higher than 1500 psf.  Online soil maps will provide good data without a geotechnical investigation.  Look at Plasticity Index as that is as important as bearing capacity. 

 

Telescope weight is insignificant in terms of bearing capacity but a concrete pier, if you go with concrete, will be in the neighborhood of 24,000 pounds to get a deflection of less than an arc-sec.  In steel pipe, to get the same deflection will be about 30" diameter and weight about 2,200 pounds.  Installed cost of steel solution may be significantly less than concrete solution.

 

Don't overexcavate for a spread footing and then backfill around the column.  It changes things from a desirable cantilevered beam (column) to a simple lever and there is a large deflection difference in the two.  A drilled straight shaft beats the 10' diameter spread footing.  With a spread footing, that Plasticity Index becomes more important as well as where the pier is placed in the house.  Minimum deflection, lowest cost, and speed and ease of installation point to a drilled pier with a steel pipe up through the house and smaller diameter steel pipe when you get to telescope floor.

 

Periodic frequency makes no difference.  You can have a pier with a 10Hz frequency that deflects several inches and you can have a pier with a 400Hz frequency that deflects less than a arc-sec.  As long as deflection is less than what your camera can detect, it can be any frequency it wants.  Sand will tune the frequency but structurally, sand is a fluid so it does little for deflection.  

 

Look for the pier engineering thread here on CN.


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#17 deonb

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Posted 09 May 2021 - 03:44 PM

I did not think a hollow structure would be stronger than a solid one, but seems like several suggestions here would say it would work?

 

Ok, what if I have a small tower (let's say a 10' x 10') with cement+steel walls that is a:

 

a) Root cellar below ground with a solid heavy foundation a few feet deep (so let's say 12' dug out)

b) Pantry on ground level with concrete floor + ceiling

c) Panic room on second level with concrete floor + ceiling

d) The escape room (concrete) ceiling forms the foundation for pier on third level

 

That way it gives me a functional structure on every level and a solid foundation at the top.

 

How do I find out if this will work? (Stable enough, no vibration).


Edited by deonb, 09 May 2021 - 03:48 PM.


#18 archer1960

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Posted 09 May 2021 - 08:05 PM

I did not think a hollow structure would be stronger than a solid one, but seems like several suggestions here would say it would work?

 

Ok, what if I have a small tower (let's say a 10' x 10') with cement+steel walls that is a:

 

a) Root cellar below ground with a solid heavy foundation a few feet deep (so let's say 12' dug out)

b) Pantry on ground level with concrete floor + ceiling

c) Panic room on second level with concrete floor + ceiling

d) The escape room (concrete) ceiling forms the foundation for pier on third level

 

That way it gives me a functional structure on every level and a solid foundation at the top.

 

How do I find out if this will work? (Stable enough, no vibration).

A hollow structure is not automatically stronger/stiffer than a solid one, but *for the same total amount of materials (=weight)", it will be, because the outside dimensions are larger, and the dimensions are the primary determining factor in stiffness. So a 10" square solid column will be stiffer than a 10" square hollow column of the same materials, but you might be able to make a 12" or 14" hollow column with the same cost and weight as a 10" solid one, and it *will* be stiffer.



#19 speedster

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Posted 11 May 2021 - 01:53 AM

The stack of concrete rooms will work fine IF it is actually isolated from the rest of the construction.  That's not hard to do but remember the little things like only flex duct for HVAC and wire loops for electrical.  Saddles at doorways to span the gap between the concrete room floors and and adjacent floor, etc.  This will actually work very well.

 

Regarding hollow vs. solid, the stiffness is a function of the plasticity of the material and it's shape.  Concrete is far more flexible than steel so a solid 10" concrete shaft is more flexible than a 10" pipe even though it has far greater cross sectional area.


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#20 macdonjh

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Posted 11 May 2021 - 07:07 PM

The stack of concrete rooms will work fine IF it is actually isolated from the rest of the construction.  That's not hard to do but remember the little things like only flex duct for HVAC and wire loops for electrical.  Saddles at doorways to span the gap between the concrete room floors and and adjacent floor, etc.  This will actually work very well.

 

Regarding hollow vs. solid, the stiffness is a function of the plasticity of the material and it's shape.  Concrete is far more flexible than steel so a solid 10" concrete shaft is more flexible than a 10" pipe even though it has far greater cross sectional area.

...and, since we're concerned mostly with bending resistance is this situation, the material near the outside of the pier is FAR more important than the material in the center of the pier to resist bending forces.  That's why I-beams work: they do away with most of the steel which isn't doing anything so they're comparatively light and nearly as strong in bending as a solid beam with the same outside dimensions.

 

Of course, I-beams suck in bending if they are oriented as H-beams with respect to the bending force.  Then they don't have their steel where it's needed.


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#21 niteman1946

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Posted 13 May 2021 - 09:05 AM

I've come to this late, so my info may no longer be relevant.

There's  gent by the name of Bob Antol who created a multistory observatory attached to his house.

His Obs is called Stargate, and you should be able to look him up.

His design got me going in the right direction when I was planning mine.

 

Good luck,

 

Mark


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#22 Christopher Erickson

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Posted 18 May 2021 - 01:29 AM

If you go with a steel pier, do a tapered one.

 

If you go with a concrete pier, don't exceed the 4:1 ratio between height and diameter.

 

My favorite pier design is a tapered steel X-pier. Minimal thermal inertia and minimal tendency to "ring." And easier to hide inside the lower room walls. 

 

Hollow steel tubes for piers are awful. They "ring" very easily. And filling them with sand or oiled sawdust doesn't do anything.

 

Filling a hollow steel tube pier with concrete with fix "ringing."  But then you basically have a concrete pier instead of a steel pier.

 

Isolate the pier from the rest of the structure. Both the walls and the foundation.

 

A tapered-steel X-pier can be done with four smaller, isolated, concrete foundation blocks instead of one huge one.

 

I hope this helps.


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#23 555aaa

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Posted 18 May 2021 - 09:50 AM

+1 with Chris - a tapered pier is way better because in a constant section design you have well-defined bending and torsional modes but with a cone shape it tends to 'smear' these out, particularly in torsion.

 

Displacement of the pier is irrelevant in terms of optical performance, it's angular deflection that is critical. You can move the pier an inch and if it's parallel to the ground, that has no effect. But if the angular deflection is an arc second, that's an arc second of resolution lost. The resonant frequency IS important and the worst thing you can have is that you couple between the mount/telescope system and the pier, especially in a direct drive mount where the reaction forces through the pier are an order of magnitude higher than in a geared mount (because you have much higher slew speeds, accel and decel rates).

 

We're also in earthquake country and it's important that the pier doesn't become a toppling hazard to the rest of the house, and this is another benefit for the tapered steel approach.

 

You can certainly make a single pier that goes all the way through all three floors but you could also build off of the first or second floor provided that those are reinforced concrete. The second floor pier can be either "in" the walls of the second floor room or you can box in the pier in some visually interesting feature, such as a round bench or setee.

 

Finally you can also float the pier itself right off of the walls or floor of the top story, provided that it's strong enough, using cable loop isolators. It's tricky but doable. Ignore the haters.


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#24 555aaa

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Posted 18 May 2021 - 09:58 AM

Recommended article

 

https://www.dfmengin...ory_design.html



#25 Christopher Erickson

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Posted 18 May 2021 - 10:04 AM

And in general, an observatory on top of a building or especially a house, usually isn't a good idea. Heat plumes going up from roofs and especially up staircases and access ports could be very problematic for local seeing.




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