Jump to content


Photo

Help with Pier on a Patio

  • Please log in to reply
10 replies to this topic

#1 barbarosa

barbarosa

    Viking 1

  • -----
  • Posts: 863
  • Joined: 11 Apr 2010
  • Loc: "lamorinda", CA

Posted 04 April 2013 - 03:20 PM

I’ve finally decided that I need some sort of observatory and I need advice about a pier. Currently I do mostly video but the plans include a wedge and (if fortune favors) a camera capable exposures longer than 17 seconds. The scope is a CPC1100 carrying a piggyback 85mm refractor.

The building will be a metal shed/roll off conversion, probably using this Australian model rather than a less expensive Arrow product. I will use a locally fabricated pier. The only aesthetically acceptable site is to one side of a large concrete patio area. The shed will go on a site built wood framed deck spanning parts of two slabs that are separated with a 2”x3” wood strip. At least but likely two deck posts will rest on the slab under the steel pier. That slab is about 12 x12 and 3.25”+ thick at the outside edges. The patio is 40 or 45 years old. The underlying soil is probably a mix of graded adobe and sandstone.

I want to do as little damage to the patio as possible, so cutting an opening and pouring a deep footing for the pier is out. This leaves two alternatives I think.
1. bolt the pier to the patio.
2. pour a pier block of say 2’x2’x1’ on the patio slab. The idea being that if necessary we can hammer the block loose without damage to the patio slab.

Will I gain anything from alternative two? Is there something else I should be looking at? I can use your help. :help: :bow:

#2 Gastrol

Gastrol

    Apollo

  • -----
  • Posts: 1215
  • Joined: 04 Nov 2011
  • Loc: los angeles

Posted 04 April 2013 - 05:00 PM

I did not want to dig through a part of my asphalt driveway so I poured the base and pier on top of the asphalt.
Lay a thin plastic barrier like a large trash bag over the patio surface and pour the base on that so the concrete base will not adhere to the patio.

Posted Image

#3 barbarosa

barbarosa

    Viking 1

  • -----
  • Posts: 863
  • Joined: 11 Apr 2010
  • Loc: "lamorinda", CA

Posted 04 April 2013 - 09:47 PM

Gastrol-

What a neat solution- a stand alone concrete pier. :grin:

Did you discover any problems caused by walking on the near the pier? Did you use this pier for imaging?

From what I read, the plastic might not be necessary pouring new concrete on old. The bond is weak. But perhaps safe is better than sorry and plastic is cheap.

I'm guessing that your base and pier weigh a 300 to 400 lbs. I think between the limits of space and my back about 200 lbs is the limit. I might do more if I can do it in two or three pours.

That brings me back to asking if what advantage, other than not having to drill holes for the anchor bolts, do I get from putting 200 lbs or so of added concrete?

#4 Gastrol

Gastrol

    Apollo

  • -----
  • Posts: 1215
  • Joined: 04 Nov 2011
  • Loc: los angeles

Posted 04 April 2013 - 10:35 PM

Gastrol-

Did you discover any problems caused by walking on the near the pier? Did you use this pier for imaging?


I built an 8'x10' obs around it with the floor joists running above the base. I don't image, but the base and pier are rock solid just sitting there on asphalt.

#5 Mary B

Mary B

    Vendor - Echo Astronomy and Electronics

  • *****
  • Vendors
  • Posts: 3067
  • Joined: 21 May 2010
  • Loc: Minnesota

Posted 05 April 2013 - 03:03 PM

Drilling the existing slab would be just as stable as pouring a bunch of concrete on top.

#6 barbarosa

barbarosa

    Viking 1

  • -----
  • Posts: 863
  • Joined: 11 Apr 2010
  • Loc: "lamorinda", CA

Posted 07 April 2013 - 02:04 PM

I think I get this. Gastrol’s concept works for any surface that supports the pier without shifting, but adds nothing to the stability of a 20’x14x3.5” / 11,000 pound concrete slab. The only thing is might do is give me an option to raise the pier if I need it.

At an engineering consulting firm’s site I found a paper on protecting lab instruments from vibration by placing them on "islands&qu...separated from the surrounding slab floor. The study used a 36” deep “island” separated from a surrounding 6” thick slab and with a flexible sealant filling the gap. It concluded that this does not reduce vibrations from electrical and mechanical sources in the building. In fact, at some frequencies vibration from local sources increased on the "island".

I did some internet searching on the topic of piers and pier bases before I posted and I have to admit I was a bit confused. I am still confused. There are many anecdotal posts and blogs, some data, and conflicting advice from commercial vendors and DIY sources. I figured that I was not the first person to want to put a pier on a patio slab (even if it is not the best choice), but it does not seem to be a poplar topic.

The fabricator who will make my pier does mostly larger scale commercial work, but he has done other and larger piers. I showed him some plans culled from the net and most had gussets or fins, running from the base part way up the side. His take is that it is better to use the correct diameter pipe. In addition, he said that there was no point in filling the pipe with sand or concrete, unless I was planning to hit it with a hammer.

#7 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 11218
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 07 April 2013 - 04:28 PM

Keep in mind that fabricators are not usually astronomers and don't give a lot of consideration to the tiny vibrations that show up in a big way through the scope. Sand will definitely help in damping those vibrations.

By "using the right size pipe", I assume that he means using a larger pipe as opposed to a smaller pipe with gussets. While a larger size pipe, and especially a heavier "schedule" pipe, will be stiffer and reduce the vibrations, it is not the same as using gussets. The problem with a very smooth, regular shape like a cylinder, is that it has a very strong resonant frequency and will easily "ring" at that frequency for a good while. That's why they make wind chimes out of smooth tubular shapes! The gussets improve stiffness, but they also break up the regular shape of the tube and thereby reduce its ability to resonate so easily.

-Dan

#8 Mike E.

Mike E.

    Vanguard

  • -----
  • Posts: 2379
  • Joined: 26 Jan 2010
  • Loc: Moonstone Observatory

Posted 08 April 2013 - 03:26 PM

Food for thought ?

Have you considered cutting the concrete slab in the form of an artistic design, in keeping with the ambiance of your back yard ?
It could then be dug out, an isolated concrete footing poured few inches below the slab, and a pipe pier bolted to it.
In the future, you could remove the pier and fill in the design with colored cement, and or perhaps with a metal inlay of some sort. A design showing north, south, east & west; as an example.

#9 barbarosa

barbarosa

    Viking 1

  • -----
  • Posts: 863
  • Joined: 11 Apr 2010
  • Loc: "lamorinda", CA

Posted 09 April 2013 - 03:43 PM

I would never have thought about a pier as a decoration. :shocked: I would, no doubt, make a terrible landscape artist.

After reading the article on Isolated Concrete Slabs I have to wonder if is any benefit to cutting the existing slab and pouring a deeper isolated base.

#10 Alex McConahay

Alex McConahay

    Vanguard

  • -----
  • Posts: 2337
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 09 April 2013 - 04:56 PM

>>>>>if is any benefit to cutting the existing slab and pouring a deeper isolated base.

The point of a pier is stability. THere is a lot more stability from a deep and isolated pier than there is from anything done on top of a slab. Attach it to the slab, and you are no longer isolated. Attach it to something on top of a slab, you are no longer isolated, nor are you stabilized necessarily.

I would strongly suggest that you cut at least a 1 by 1 foot hole in the slab. That will give you access to the dirt below. Then dig that dirt down as far as you can (below the frost line if that is a consideration. But make it at least three feet if you can at any rate). Flare it at the bottom if possible. THen fill the thing in with concrete to three or four inches below the slab surface. Set three large J bolts and some rebar into the concrete to act as pier anchors. When all is set, you have a pier base as solid as any. THe top of the concrete base concrete is four inches below slab grade. Overfill that hole with the kind of sealant made for that kind of thing so that you do not have a puddle there. THen put a steel pier on top.

When you need to move, remove your pier, remove the filler material, and cut off the bolts. Fill the hole with surfacing material so it matches the slab. Nobody will know the difference.

Don't look forward to the work of cutting off the bolts and filling the hole? Just put a birdbath on the top of the pier and tell the new owner it was meant to be that way.

(Or, better yet, sell the place to an amateur astronomer. )

Alex

#11 barbarosa

barbarosa

    Viking 1

  • -----
  • Posts: 863
  • Joined: 11 Apr 2010
  • Loc: "lamorinda", CA

Posted 10 April 2013 - 02:20 PM

Alex, what you suggest seems reasonable and clearly many piers are on sizeable concrete plugs. I have yet to see a post by anyone complaining about the result after pouring a ton of concrete.

After reading three articles addressing vibration in electron microscopy labs (link 1, link 2, link 3) I think that the data presented suggests that even relatively massive concrete structures, (both slab on grade and buildings on piers) do not by themselves reduce the effects of vibration transmitted through the ground. Two of the studies make the point that the best outcome is that the structure does not amplify the vibration.

Mike Weasner, who has a good reputation in the Meade community, recently wrote, “That's my plan whenever I get a pier. My concrete slab is so thick (8-12") that I see no vibrations in my eyepiece. So I'm hoping that a pier can be bolted down securely with the same results.”

The wooden pier fraternity includes those who anchor the pier in a sizeable concrete base and those who do not. Both groups seem satisfied.

I already know that dropping a five-pound sledgehammer from ~5” onto the slab causes a small quick jerk in the image of both a C5 on a Nexstar 6SE mount/tripod and a CPC1100. This is not a problem for visual or video (17-second max exposure). If a pier bolted to the slab is not worse than a tripod on the slab it would be good enough, unless there would be a problem with longer video exposures.

I am going forward with the pier on the slab. If that is a disaster and it might be, I will report and very humbly on plan B.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics