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#226 macdonjh

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Posted 30 June 2020 - 08:35 AM

 

After that - consultations with architecture and constructor of the building. May be I`ll change type of the building footer (from slab to strip)... 

 

But really complicated consultation will be with my wife smile.gif 1 square meter pier is a serious point. Probably she will have much stronger arguments about pier then climate, type of soil and resistance of materials all together smile.gif Then I`ll have to accept solution of domes developer and make a "floating" footer of pier on vibration damper, placed on the mansard floor. We will see. 

 

Thank you a lot again! 

Agreed.  Engineering problems are usually easy to solve in comparison to making the solution aesthetically pleasing.



#227 macdonjh

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Posted 30 June 2020 - 09:16 AM

Using the load conditions at the beginning of this thread, 12" steel pipe deflects about 6 arc-sec.  Too much.  So, you either go bigger or just don't bump it in the first place.  

This has been gnawing at me.  So I redid my numbers.  Hopefully this won't be too boring for the denizens of this thread.  I thought about sending speedster a PM, but then thought some might find this interesting.

 

For a cantilevered beam (or column) with a force applied to the end:

d = PL3 / 3EI  (sorry, no Greek font, that "d" should be delta)

 

With a bit of rearranging, since speedster calculated 6" arc (or 0.007", 240" * TAN(6" arc) ):

P = 3dEI / L3

P = 3(0.007 in)(30x106 lb/in2)(279 in4) / (240 in)3 (note, I= 279 is for 12" x 0.375" wall pipe)

P = 12.7 lb (the load at the top of the 20'-0" tall pier pulling sideways).

 

Since speedster's criterion is 0.5" arc (0.0006 inches) deflection, I reorganized the deflection equation to find I:

I = PL3 / 3dE

I = (12.7 lb)(240 in)3 / 3(0.0006 in)(30x106 lb/in2)

I = 3251 in4

 

The area moment of inertia for a "hollow" circle is:

I = PI(R4 - r4) / 4

 

Since, at least in the US, "standard weight" pipe is available, I'll assume the wall thickness will be 0.375": 

I = PI(R4 - (R - 0.375 in)4) / 4

 

Therefore (I wish I hadn't thrown my scratch paper away, I don't remember how I calculated this without expanding the polynomial above): 

3251 < PI[ (15 in)4 - (14.625 in)4 ] / 4

3251 < 3829; therefore, 30" x 0.375" wall thickness pipe will be suitable.  For reference, the moment of inertia of 24" x 0.375" pipe is 1942 in4, so it isn't stiff enough to meet the 0.5" arc deflection criterion.

 

Those were the calculations behind my recommendation of 30" x 0.375" pipe for a 20'-0" tall pier with a maximum deflection of 0.5" arc under a 12.7 lb side load.  Or, in metric: 762mm x 9.5mm pipe for a 6.1m tall pier with a maximum deflection of 0.5" arc under a 5.8kg side load.


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#228 speedster

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Posted 30 June 2020 - 03:51 PM

macdon,  your calcs are spot on.  The difference is that you are figuring 12.7# force and I'm figuring 5#.  The 5# is an arbitrary number to sort of simulate a bump of the pier and to have some defined load for calculating deflection difference in various pipe and concrete shapes.



#229 jambi99

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Posted 21 July 2020 - 02:03 PM

Howdy GraySkies!

 

Part 9 of the Ontario Building Code states frost depth is 1.2m which is 4' feet.  There is no "wrong" way when the final decision boils down to personal preference.  This thread was started so we all have some accurate info to make informed decisions and get the most bang for the buck.  Ultimately, we are after stiffness which is measured by modulus of elasticity.

 

Steel = 29,000,000 psi

Aluminum = 10,000,000 psi

Concrete = 4,350,000 psi

 

Specific metal alloys make a difference, as does concrete mix design but, a general comparison of structural metals and 3,000 psi concrete shows that Aluminum is 3x stiffer than concrete and steel is 3x stiffer than aluminum.  The shape and length of the piece also affect stiffness so a 16" concrete column is stiffer than an 8" steel pipe even though concrete is 7x more flexible than steel.

 

The 8" concrete pier scheme has a practical problem with embedding anchor bolts in the top of it.  Embedded bolts need to be at least 2-1/2" from the edge of the concrete which really limits your bolt pattern on an 8" concrete pier.  You could do a funnel shaped bolt cage and embed the funnel so there are ways around this if you want an 8" concrete pier.

 

The aluminum option is personal preference but bare in mind that an 8" aluminum pipe is way more expensive than 8" steel and also way more flexible.  Weight is our friend and steel is heavier.  At our thicknesses, corrosion is not an issue for the next 100 years unless aluminum is in contact with concrete. 

 

The one certain thing is that our needs and wants will change over time and the metal pier on top of concrete pier gives you a great deal of future flexibility since it is relatively easy to cut some off or weld some on later.  Also easy to attach stuff to it and run things through it.

 

Looks to me like steel (or Al) on concrete is the way to go.  A local shop can typically weld up a pier to your specifications cheaper than the store-bought ones and you can get exactly what you want.

 

Just to confuse things, since you are going 6' deep (excellent), you could just suspend an 8" steel pipe in the hole and concrete it in like a big fence post.  No bolts, no pier fabrication cost and, if you move, just cut it off and take the top with you.  Then, weld a base plate on it and you still have your pier. 

I'm interested in the funnel type bolt cage setup. My concrete pier is not wide enough to accommodate my steel pier. So, when you are are referring to a funnel type bolt cage you also mean that the concrete shape will have a funnel shape right? Like that (sorry for the quick drawing):

 

funnel pier.png

 

My other alternative( and preferred one) would be to finish the above grade section with concrete block(wider). The only issue with that is that the above grade section will be wider(by 3-4 inches around) than the below grade section. I guess this is not a good design while the frost will push up against the concrete block like if you are trying to remove a nail by its head. Please note at this point I really do not want to dig wider. My current hole is 6' 4" in depth and about 15-16 inches wide.


Edited by jambi99, 21 July 2020 - 02:11 PM.


#230 speedster

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Posted 21 July 2020 - 04:20 PM

concrete shaft is straight but the bolts are bent toward the center.  Our loads are so low we can do some things to make things fit.  Imagine a 1/2" x 12" hex head bolt.  Just below the threads, bend it about 30 degrees and embed it so the head is pointing toward the center of the concrete.  Threads are vertical, the part just below the threads doesn't have much concrete cover but concrete cover increases as you go down the bolt.  Below ground, 2"-3" of concrete cover pretty much eliminates corrosion issues with the rebar.  At bolts above ground, the concrete cover prevents loading in tension of a skinny piece of concrete. 

 

If the funnel shaped bolt cage still doesn't give you enough width for the steel pier base plate bolts, It's common to put an enlarge cap on the top of the concrete, just like your nail head analogy.  The cap part would only be a few inches bigger than the shaft and only needs to be a couple of inches thicker than the embedded length of your bolts.  I wouldn't worry a second about the frost heave acting on the bottom of the cap.



#231 jambi99

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Posted 21 July 2020 - 06:06 PM

concrete shaft is straight but the bolts are bent toward the center. Our loads are so low we can do some things to make things fit. Imagine a 1/2" x 12" hex head bolt. Just below the threads, bend it about 30 degrees and embed it so the head is pointing toward the center of the concrete. Threads are vertical, the part just below the threads doesn't have much concrete cover but concrete cover increases as you go down the bolt. Below ground, 2"-3" of concrete cover pretty much eliminates corrosion issues with the rebar. At bolts above ground, the concrete cover prevents loading in tension of a skinny piece of concrete.

If the funnel shaped bolt cage still doesn't give you enough width for the steel pier base plate bolts, It's common to put an enlarge cap on the top of the concrete, just like your nail head analogy. The cap part would only be a few inches bigger than the shaft and only needs to be a couple of inches thicker than the embedded length of your bolts. I wouldn't worry a second about the frost heave acting on the bottom of the cap.


The thing is that my concrete cap would be 6' tall. So it would be 6 feet underground and 6 feet above grade being slightly larger than in ground. Any issue with that?

#232 outofdark

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Posted 23 July 2020 - 09:02 PM

Hi All,

 

Just got my pier bolted in place today.  Thanks to speedster I designed a pretty stout pier.  It's made of schedule 80 ten inch steel pipe. The base plate is one in thick.  Overall it's 96 inches and just over 600 lbs.  It's bolted to a cubic yard of concrete. 

 

shocked.gif

 

IMG_6281m.jpg

 

 

IMG_6283m.jpg

 

 

IMG_6284m.jpg

 


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#233 macdonjh

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Posted 23 July 2020 - 10:10 PM

Hi All,

 

Just got my pier bolted in place today.  Thanks to speedster I designed a pretty stout pier.  It's made of schedule 80 ten inch steel pipe. The base plate is one in thick.  Overall it's 96 inches and just over 600 lbs.  It's bolted to a cubic yard of concrete. 

 

shocked.gif

 

...and I thought my pier was overkill.  Not that I'm making fun: concrete and steel are cheap,  worrying whether or not your scope is steady enough is expensive.  I'll be interested to see photos of your completed observatory.



#234 outofdark

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Posted 23 July 2020 - 10:19 PM

You make me laugh! One does lay awake at night pondering such thoughts.

 

more build pictures here. http://thestarquarry.com I started work on my obseratory just over a year ago.  I figure I have a few months to go before I'm opperational.

 

 

...and I thought my pier was overkill.  Not that I'm making fun: concrete and steel are cheap,  worrying whether or not your scope is steady enough is expensive.  I'll be interested to see photos of your completed observatory.


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#235 macdonjh

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Posted 24 July 2020 - 07:35 AM

You make me laugh! One does lay awake at night pondering such thoughts.

 

more build pictures here. http://thestarquarry.com I started work on my obseratory just over a year ago.  I figure I have a few months to go before I'm opperational.

That is an impressive project.  I don't think I ever would have built mine if I'd needed a concrete pumper.  The photos near the bottom of your build page with snow on the ground were surprising.  I am always shocked to see snow in what otherwise looks like a desert'y area.  



#236 outofdark

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Posted 24 July 2020 - 07:41 AM

Yes sir, in Salt Lake at 4800'. it's been known to snow in early June some years.  

 

That is an impressive project.  I don't think I ever would have built mine if I'd needed a concrete pumper.  The photos near the bottom of your build page with snow on the ground were surprising.  I am always shocked to see snow in what otherwise looks like a desert'y area.  



#237 dkb

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Posted 25 July 2020 - 12:55 PM

Is there a difference in deflection between a steel pipe compared to a steel square tube made of same material and thickness?

 

Was also wondering how much difference the type of steel would make in a telescope pier.  Since you have access to a lot of different steel I wasn't sure if there was that much advantage of one or another (besides price).



#238 macdonjh

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Posted 26 July 2020 - 12:22 PM

Is there a difference in deflection between a steel pipe compared to a steel square tube made of same material and thickness?

 

Was also wondering how much difference the type of steel would make in a telescope pier.  Since you have access to a lot of different steel I wasn't sure if there was that much advantage of one or another (besides price).

There will be a minor difference in deflection with a square tube.  In some directions deflection will be less, in other directions deflections will be more.  One of the nice things about round sections is their properties are symmetrical no matter which way you try to bend them.  All that said, if you were to do the math, the differences in deflection would be tiny, so I don't think you should worry about it.  If you have access to an inexpensive piece of square tube which meets your needs, you should use it.

 

No, there won't be any practical difference in different grades of steel.  For our purposes they all have a bulk modulus of 30,000,000 psi (that's the property which describes how resistant a material is to deforming).  Some actually have E=29x106, or E=29.5x106, but it doesn't make a practical difference.  You may find your calculations suggest using a 4.1" diameter pipe, but you'll have the choice of "making do" with a 4" or going up to a 6" pipe.


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#239 StarAlert

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Posted 09 October 2020 - 06:38 AM

Shifting gears to the other end of the size spectrum, I’m making an 8” pier extension for my ioptron AZ mount Pro. Like just about everything in astronomy these days, they are on back order and won’t be available for 6-8 weeks. So I decided to make one myself. I’m using two 6” diameter, 1/4” thick steel discs. My question is how thick should the 8” long steel extensions be? I have 5/8” and 1/2”. I was going to use three 5/8” legs instead of four 1/2” legs to allow easier access to the bolt to secure the mount. Would three 1/2” diameter legs be sufficient? 

4E620678 CD95 4574 A0B8 773D7CECD6C1
 
AF71C58F 972F 42E6 95BB ABA82ECD3D63

 


Edited by StarAlert, 09 October 2020 - 03:23 PM.


#240 speedster

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Posted 09 October 2020 - 04:41 PM

Howdy StarAlert!

 

Anything will be sufficient to keep it from falling down.  I think the nut can be tightened with a ratchet with a universal knuckle between 2 extensions so you need enough space between rods the get a socket between them but not necessarily enough to swing a wrench.  My mounts don't have a center bolt so I'm speculating there.  To get a feel for deflection of the options, we can look at the Section Modulus and simplified comparison to a 4" pipe which is a size we have somewhat of a "feel" for. 

 

Sx of 4" pipe = 3.21

Sx of 1/2" solid rod = 0.01225

Sx of 5/8" solid rod = 0.02393

Sx of 1" pipe = 0.13264

 

Options look depressingly flexible compared to 4" pipe but you actually have an assembly rather than just the sum of 3 or 4 members and that's what makes several small members work for you.  Your Sx is the Sx of the assembly rather that just the parts.  Diameter is your friend so the 5/8" beats the half and 1" pipe is much better.  The more members, the better but there are sharply diminishing returns and more than 4 is overkill unless there is something weird going on.  Four 1" pipes laid out on a 5" radius has an Sx of about 1.5.


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#241 StarAlert

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Posted 09 October 2020 - 05:07 PM

Howdy StarAlert!

 

Anything will be sufficient to keep it from falling down.  I think the nut can be tightened with a ratchet with a universal knuckle between 2 extensions so you need enough space between rods the get a socket between them but not necessarily enough to swing a wrench.  My mounts don't have a center bolt so I'm speculating there.  To get a feel for deflection of the options, we can look at the Section Modulus and simplified comparison to a 4" pipe which is a size we have somewhat of a "feel" for. 

 

Sx of 4" pipe = 3.21

Sx of 1/2" solid rod = 0.01225

Sx of 5/8" solid rod = 0.02393

Sx of 1" pipe = 0.13264

 

Options look depressingly flexible compared to 4" pipe but you actually have an assembly rather than just the sum of 3 or 4 members and that's what makes several small members work for you.  Your Sx is the Sx of the assembly rather that just the parts.  Diameter is your friend so the 5/8" beats the half and 1" pipe is much better.  The more members, the better but there are sharply diminishing returns and more than 4 is overkill unless there is something weird going on.  Four 1" pipes laid out on a 5" radius has an Sx of about 1.5.

Thanks,

Wow... there is a big difference going to the 1" pipe. This is going to be used strictly for visual. A 4" f8 refractor for high powers and a 6" f6.5 for low power sweeping. First test will be next week. 


Edited by StarAlert, 09 October 2020 - 05:08 PM.


#242 Gert

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Posted 25 November 2020 - 05:27 PM

Hello All,

 

I posted a pier design question for our club observatory here ..

https://www.cloudyni...question-cdk20/

 

.. and was pointed to this thread.

 

Maybe the experts here can comment.

We are rebuilding our observing site which was destroyed in the CA fires.

https://www.trivalleystargazers.org/

https://charity.gofu...rebuilding-fund

 

Our location is on a hill top with rocky hard soil.

We received donations to build a new observatory with a Planewave CDK20 on a Mathis MI750 fork mount. A sketch is shown below.

 

Maybe the experts here can recommend proper pier design?

 

DomeCDK_Drawing.jpg

 

Here is an unrelated photo that would show the soil material. It was quite hard to dig little holes to place those 3 pavers for tripod legs into the ground.

 

H2O_20081129_05.jpg

Thanks,

Gert

https://www.trivalleystargazers.org/



#243 speedster

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Posted 26 November 2020 - 12:24 AM

Howdy Gert!  Even with 500# of load, weight is not the issue.  Deflection is still the issue and your mount allows larger diameter piers than EQ's so you can get your deflection down to about nothing efficiently and economically.  I think I read something in the other thread about a 10:1 weight ratio.  I would ignore any weight ratios.  It's about shape and depth and a weight ratio just obscures what we are really after.   What you see is only as good as what's in the ground.  My advice:  get an auger mounted on a skid loader or trailer and drill an 18" hole 6' deep.  Bigger and deeper is marginally better but this will do it.  Pour concrete directly in the hole.  Very important to not form anything below ground.  If the rocky soil leaves a rough hole, that's fine.  The concrete will fill all the voids and roughness.  Form the part above ground with 16" Sonotube or spiral duct or 18" galvanized steel culvert pipe.  The base on the mount is 14" so it looks like 16" or 18" diameter shaft will fit up fine.  Four #5 rebar full height with #3 round stirrups at 24" on center vertically.  Considering the load conditions that this thread is based upon, a 16" concrete pier 7' out of the ground has a deflection of 0.125 acr-sec. 

 

If you happen to encounter consolidated bedrock, drill into it and epoxy rebar directly into the rock.



#244 macdonjh

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Posted 26 November 2020 - 10:16 AM

Howdy Gert!  Even with 500# of load, weight is not the issue.  Deflection is still the issue and your mount allows larger diameter piers than EQ's so you can get your deflection down to about nothing efficiently and economically.  I think I read something in the other thread about a 10:1 weight ratio.  I would ignore any weight ratios.  It's about shape and depth and a weight ratio just obscures what we are really after.   What you see is only as good as what's in the ground.  My advice:  get an auger mounted on a skid loader or trailer and drill an 18" hole 6' deep.  Bigger and deeper is marginally better but this will do it.  Pour concrete directly in the hole.  Very important to not form anything below ground.  If the rocky soil leaves a rough hole, that's fine.  The concrete will fill all the voids and roughness.  Form the part above ground with 16" Sonotube or spiral duct or 18" galvanized steel culvert pipe.  The base on the mount is 14" so it looks like 16" or 18" diameter shaft will fit up fine.  Four #5 rebar full height with #3 round stirrups at 24" on center vertically.  Considering the load conditions that this thread is based upon, a 16" concrete pier 7' out of the ground has a deflection of 0.125 acr-sec. 

 

If you happen to encounter consolidated bedrock, drill into it and epoxy rebar directly into the rock.

speedster,

 

Gert's location is in northern California, so seismic activity could be a consideration.  I know astronomy loads a tiny, but should he use more stirrups (say on 12" centers)?  I'm pretty sure in earthquake zones concrete columns are now built using "spiral" stirrups along the whole length of the column.  Of course, that's for "real" loads...

 

Also, if the MI750 has a 14" base plate, shouldn't his concrete be 18" diameter minimum to maintain 2" cover over the anchor bolts and rebar?  Not that it's necessary, but I'd have a stirrup around the embedded portion of the anchor bolts just because.



#245 speedster

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Posted 27 November 2020 - 12:49 AM

More stirrups won't hurt anything and they're cheap so if anyone sleeps better, add more.  Good point about the bolts.  With his mount, it looks like he could go 18".  I assumed if the mount base is 14", the bolts will be inside that.  If the bolts are an inch inside the edge of the plate, he's got his 2" of cover.  Fine going bigger diameter as long as mount clears.  More diameter is stiffer but he's already down to 0.125 at 16" so bigger diameter could be called optional.  



#246 fx4m

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Posted Yesterday, 11:09 PM

Hello pier-heads,

 

Fantastic thread.

 

I am on the team with Gert designing the replacement observatory. We do not have a geologic report and I know very little about geology but here are my observations. Gert's characterization of the "soil" as "hard rocky" may be an understatement (the place were he was digging is a little different than the spot under consideration). The tentative location for our pier and observatory is on, what appears to me to be, a flat-ish solid rock outcrop about 15 feet in diameter on the top of a hill. The surface is weathered, eroded and a bit fractured, (I guess it is sedimentary because of its parallel-ish fractures, if I remember correctly). It has very little soil in the (probably quite shallow) fractures, but I would be really surprised if this was not considered consolidated bedrock at, or an inch below, the surface. Maybe it is tuft. I would think that an 18 inch auger would not be able drill into this (but I don't know what such an auger can do) and that jackhammering would be necessary. The site was graded to form roads and tiers, and there were trees and bushes there (hence the fire) so I guess it is very "hard rocky soil" over most of the hill, but I don't know how deep.

 

Assuming my characterization of this outcrop as bedrock is correct, how would you proceed? Clean up the surface, drill and epoxy rebar into it without sinking the pier below ground? And what about the building foundation being directly on the same bedrock; should we worry about vibrations from footsteps (I'm guessing no).

 

Thanks,

Chuck


Edited by fx4m, Yesterday, 11:11 PM.



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