Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Pier engineering

  • Please log in to reply
328 replies to this topic

#326 macdonjh

macdonjh

    Fly Me to the Moon

  • *****
  • Posts: 6,859
  • Joined: 06 Mar 2006

Posted 13 September 2021 - 05:30 PM

Great thread! I very much appreciate the engineering and pragmatic approach, with math and physics to back it up.

 

If this detail was in buried within, I missed it: As to finish leveling the concrete pier top, a quick check shows some self-leveling epoxy resins show compressive strength ranging from 3900 psi to 7500 psi depending upon compound. That's JB Weld territory at the lower end. This should be sufficient to resist (significant) deflection in the mount, yes? I realize the mount needn't be level to achieve PA, and a proper GEM won't care much.

Short answer: yes.

 

I have reasonably big gear: a Mountain Instruments MI-250 (65 lb), a 14" classical Cassegrain (55 lb), counter-weights (63 lb).  Even if my pier plate was 1" x 1", that's only 183 psi...

 

210612 Plot 6 PIer small.jpg



#327 speedster

speedster

    Viking 1

  • *****
  • topic starter
  • Posts: 832
  • Joined: 13 Aug 2018
  • Loc: Abilene, Texas

Posted 13 September 2021 - 06:30 PM

If anchor bolts go through the plate, just double nut it with the leveling (bottom) nuts spun down to the concrete before you start leveling. 

 

If you can't level with double nuts, I would avoid epoxy and use ordinary non-shrink grout.  Cleaner, cheaper, and still way overkill.  Big box stores all have it.



#328 astronut17

astronut17

    Sputnik

  • -----
  • Posts: 35
  • Joined: 28 Jan 2020
  • Loc: MA

Posted 18 September 2021 - 11:44 AM

Hey Speedster,

 

I really appreciate the detail and logic in your analysis.  I have a question.  I am planning a backyard observatory for imaging - likely an 8' dome about 50' from the house.  I will not ever be in the dome while imaging and would really like to put it on a slab since I think that would be better for critters as opposed to a deck.  If I go the slab route and since vibration near the scope will not likely be an issue.  What are the best options for bolting a steel pier directly to the slab?

 

The slab will be about 10' x 10' 

 

How thick?

Thicker around the pier location?

 

Is this even a good idea, or would I get equal results with a tripod?  If I have to PA a few times a year, I am ok with that.  The pier would give me more room in the dome but again I will not be in the dome while imaging.

 

...and for the slab, you mentioned something before about clearing out 6" of top soil and replacing with sand.

 

TIA

Carl



#329 speedster

speedster

    Viking 1

  • *****
  • topic starter
  • Posts: 832
  • Joined: 13 Aug 2018
  • Loc: Abilene, Texas

Posted 18 September 2021 - 05:22 PM

Howdy Carl!

 

Yes, thicken the slab at the pier so that the anchor bolts have 3" of concrete cover.  Tripods obviously work but a reasonable pier beats even a very good tripod.  Tripods have the advantage of portability so it's not an apples-to-apples comparison.  If you're happy on a tripod now, you'll be even happier on the tripod in the dome.  One solution is to thicken the slab but not embed anchor bolts.  Use the tripod (free) and if you later feel the need for a pier, install epoxy anchor bolts.

 

Removing 6" of topsoil gets rid of organic stuff that will rot and also gets rid of the looser soil in the root zone.  The sand is just to bring the grade back up to the bottom of the slab.  You can also use any acceptable soil material with a plasticity index of about 14 or less.  Sand is just convenient and available everywhere..




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics