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Pier engineering

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#76 Steve Haverl

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Posted 09 August 2019 - 04:51 PM

Scott:
1. That’s called honeycomb and should be minimized with vibration. If you have any, use sakrete mortar mix to patch (use a flat trowel to apply, lightly mist the pier first). The light misting improves the bond by helping the mortar to not flash set.
2. Use a paint/ coating specifically formulated for concrete. HD is your friend.
3. Use a angle grinder with a corundum concrete disc.
4. I personally think the tube looks like caca and will deteriorate further in the weather. I’d strip and follow above.

Best, Steve
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#77 speedster

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Posted 09 August 2019 - 08:20 PM

Howdy Scott!

 

Congrats on the pier. 

 

+1 to Steve's reply.  Don't worry about honeycomb yet.  Get the tube off and then see what you have. 

 

You can use latex paint but wait about a month to paint it.  It takes that long for the moisture content to go down enough for a good bond.  Will it stick before then?  Yes.  But, if you wait, it will stick better.  When we are doing something critical, like gluing a synthetic gym floor to a slab, it takes months for the moisture content to fall below adhesive manufacturer's spec and that is in dry west Texas.  Here is what we do for gloss on concrete with Sherwin Williams products:

 

1. Latex Systems
a.  High Gloss Finish 
  1st Coat: S-W Loxon® Exterior Acrylic Masonry Primer, A24W300  (8 mils wet, 3.2 mils dry)
  2nd Coat: S-W All Surface Latex Enamel High Gloss, A41 Series
  3rd Coat: S-W All Surface Latex Enamel High Gloss, A41 Series  (4 mils wet, 1.6 mils dry per coat)

 

If the top is rough, grinding is not fun.  You might grind the worst of the high spots and then cover the whole top with surface bonding cement.  Pretty much every manufacturer in the concrete business makes a surface bonding cement in bags or pails.  The big box stores will have it.  If you wind up with honeycomb on the sides, this product also fills the voids and then trowels over the whole pier for a uniform finish. 

 

I would definitely take the paper tube off.  It has a light wax coating and won't paint well.  The tube will eventually delaminate and be ugly.  Better to get it off now.  You may find that getting the tube off is the worst work of the whole pier.  Hopefully, yours will pop right off.  If the inside layer of paper is bonded to the concrete, saturate with water and use a wire brush. 
    
 


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#78 speedster

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Posted 09 August 2019 - 09:27 PM

Howdy Darkskyrancher!

 

Sand is a problem much like water.  Structurally, sand is a fluid.  You can have 2 situations:  a layer of loose sand somewhere down the depth of the pier, or loose sand from the surface down to the intended depth of the pier (like you had at the solar farm).  If a loose layer is encountered, the pier is cased with steel pipe to depth just below the sand (or water) and then the hole is advanced to the required depth.  The pier is then poured and the casing is immediately removed.  I can't see this happening in shallow telescope piers. 

 

When there is loose sand all the way down from the surface, you can clean the hole and accept the sloped sides and fill the whole thing with concrete or you can set a form, like sonotube, and fill the tube. 

 

If you just fill the oversized hole, you are pouring against undisturbed soil, even though that soil is sand, and the key word is "undisturbed".  We can then assume the underground part of the pier is fixed and the above ground part is a cantilevered beam sticking out of the ground.  Let's say it is 36" high.  Our length in deflection is then 36".

 

With the sonotube scheme, let's say we are 60" deep.  Since we can't take credit for undisturbed soil since we backfill around the tube (even if we compact the backfill), our length in deflection is  96".  Further, the round pier is a structural hinge at the bottom.  Let's ignore the hinge issue for the moment. 

 

Since deflection is a function of the cube of the length, the tube version is deflecting 19 times as much under the same lateral load. In reality, it won't be quite that bad because there will be some lateral support from the fill even though we can't count it in our calcs. 

 

So, when we can't go deep for lateral support, we do a spread footing.  Not because we need to spread out a load but because we need to stay shallow to shorten the deflection arm.  Like the horizontal part of a retaining wall.  In this case, wide and shallow is better than deep and skinny.  We don't get below the most active surface soils and that is is price we have to pay unless we dig a wide and deep hole.  We still need to be below frost depth so we may have a wide deep hole anyway.  The deeper we go, the longer the unsupported pier and the wider we have to be to approach the same deflection. 

 

Some of the wind turbines use a very shallow foundation like a wagon wheel on its side with a beefy concrete ring beam about 100' in diameter and "spokes" of concrete beams from the ring to the center.

 

The way around all this is driven piles and those are what we use in loose soils and water.  Steel piles last 70+ years in salt water and concrete piles could last about forever.  Stiff, fast, strong, economical - except for a little one-off telescope pier where we have to mobilize the equipment (which won't fit in the typical back yard) and then use it for 10 minutes to drive one pile.  In the case of your solar farm, you'd drive a steel or concrete pile to a specified depth depending on how much one blow moves the pile and then cut the pile off at the proper height.


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#79 speedster

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Posted 09 August 2019 - 09:43 PM

Don't forget what I said earlier, we are likely splitting hairs for most astronomers with this thread.  A telescope will work on a stack of concrete blocks glued together with a can of foam and that can be better than a tripod.  This thread is about likely getting less than 0.5 arc-sec deflection and having it stay that way pretty much forever.  More than one way to skin that cat.


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#80 Scott123

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Posted 10 August 2019 - 09:08 AM

Argh, it didn't turn out well. You can see where the concrete changes. This is when one of my helpers dropped, it was close to noontime. 

 

I cut a milk jug to exactly 3.8 gallons, as directed by the bag. Looks like I should have added a little more water. I'm on my way to the hardware store for Sacrete.

 

post.jpg

 


Edited by Scott123, 10 August 2019 - 06:14 PM.


#81 darkskyrancher

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Posted 11 August 2019 - 01:17 AM

Hey Speedster

 

First, my apology to Scott.  I am new to the forum and did not know how to start a new thread to Speedster.

 

Scott, our passionate pursuit of this hobby referred to as amateur astronomy is fun partly because of what we learn in the process.  Even though we may seek out the best advice the theoretical and practical aspects of our endeavor need not coincide, and the fun is doing new and unfamiliar things even when the results are less than what we expect.  The best part is that we always get "do-overs"  if we want them. 

 

In my case, I am about to try to build a pier in the desert sand and I most assuredly will scew it up.

I will do the best I can knowing that worst case I will become very good at polar alignment.  I will probably never get close to .57 arcsecs of deflection.  I have sampled the sand by virtue of a previous project and there are voids everywhere.  Our ranchita is on an active earthquake fault and minor treamors are routine.

 

Speedster,  your articulation of the moment arm in going deep suggests that to be the salient point here.

I can go shallow and spread out.  Plenty of room and I own the backhoe.  Pile driving seems cost prohibitive and unnecessary for the reasons I mentioned to Scott.  Still, I want to do the best I can and frequent alignment procedures may be required.  I like the windfarm technique you describe.  Maybe, I can scale it down and I would like to learn more.  We have huge windfarms nearby and the forces there must be astronomical (no pun intended).  Same sand, same desert.

 

I think the real test might be the degree and frequency I am required to realign.  That test presupposes a pier of some kind be built.  If my results are better than I would achieve should I choose to place a dome on the second level of the house that would define success.  Since it is assumed we are located on sheets of sand subject to flashflood and earthquake spreading out the "footprint" of the pier construct might well be advanced by using the house, footings and slab themselves to mitigate to some degree many geophysical factors.  Then, simply overbuild the substructure of the shortest pier on the house second level. 

 

Your thoughts would be greatly appreciated given what I have shared.  You guys can reach me by email if you wish at dsrborrego69@gmail.com

 

Regards, Darkskyrancher


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#82 speedster

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Posted 11 August 2019 - 12:08 PM

Scott, no structural problem and that will clean up just fine with a thin layer of surface bonding cement troweled on. 


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#83 speedster

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Posted 11 August 2019 - 12:31 PM

Howdy Darksky,

 

Your house vibrates a lot more than you think and that can be a problem.  Heat plumes coming off the roof can also be a problem.  You might consider some thing like a 5' diameter slab, 12" thick, flush with the top of your obs floor, with an 8" steel pier bolted to it.  1/4" plate on the bottom of the pier with a nut on both sides of the plate holes for leveling.  That will get your deflection to 0.5 arc-sec.  Isolate that concrete base from the rest of the slab with a 1/2" air space.   That should be great for visual and, IF you get vibration when imaging, just set it all up and then stay off the 5' diameter base while the cam is running.



#84 darkskyrancher

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Posted 11 August 2019 - 03:08 PM

Thanks, Speedster

 

Yep!  I assume the diameter of 5 feet is optimal given the sand below.  Is there any practical benefit to increasing beyond 5?  Is there any benefit to using any horizontal steel BEYOND  best practices?

 

My interests include imaging and spectroscopy so there is ample opportunity to avoid vibration due to walking around. 

 

Now, I need to calculate the cost of all that concrete.

 

Thank you again for your valuable contributions to our forum.

 

Darkskyrancher


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#85 Scott123

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Posted 11 August 2019 - 05:36 PM

Scott, no structural problem and that will clean up just fine with a thin layer of surface bonding cement troweled on. 

Hi Speedster, thank you, I was rather worried about it!

 

I checked both Home Depot and Lowe's, they have surface bonding cement listed on their websites, but nothing in stock, and it can't be delivered to their stores in Bangor or Waterville Maine. (These are the closest stores to me). Very odd! I'll keep checking around.

 

I had scheduled Friday to set the mount and scope on the pier, of course it rained. And we had a thunderstorm.

 

Scott



#86 speedster

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Posted 11 August 2019 - 11:23 PM

Darkskyrancher -

 

5' is a number I pulled out of the air.  That puts your horizontal arm about the same as your vertical arm and seems, to me, to be about the biggest diameter I could stay off of while the scope is working.  Bigger obs gives you more room to move around while staying off the 5' disk.  5' diameter x 12" thick is 20 cf or 30 #80 bags.  About $150. 

 

As for reinforcing, for comparison, we use 6" slab with #3 at 16" for concrete paving for bus traffic.  I'd call that minimum for the disk, with a ring around the circumference.  I'd probably put a mat top and bottom.  Overkill, but it just looks better in the form to me.

 

Here's an 8' disk with the pier going through it.  It's for a Skyshed Pod.  It's a 4" slab with a 12" deep beam around it because the slab is elevated 12" above the subgrade.   Form material is 5mm plywood held by 3/4" ply rings.  Pier is wrapped with blue "sill-seal" foam strip for isolation.  Various trade names but it is foam strip designed to go between slab and bottom plate of wood frame walls.  It's common and cheap.

 

IMG_2413sm.jpg

 

 



#87 Steve Haverl

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Posted 12 August 2019 - 08:41 AM

Scott
That’s unfortunate, but should be cosmetic and not structural.
You don’t need to use bonding cement to surface the column, sakrete mortar mix will work fine if properly applied. Mix it stiff (slightly less water than instructed) (slightly!), mist the column and use a flat towel to apply it and press it into the voids. You could then use a rubber float to finish it.
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#88 darkskyrancher

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Posted 12 August 2019 - 09:33 PM

Hey Seedster

 

I assume I am looking at an example of  "spread footing" of 12 inches with #3@16.  What I do not understand is what is below the pier tube in the image and what bonds the tube to the concrete over a relatively small surface area of the lower 12 inches of the tube.  Am I missing something that has yet to be completed, or do I simply misundertand the concept?

 

I introduced my concern of sand because of it's fluidity and yet as I re-read all your comments of why not to use a spread footing that reasoning also seems to especially apply in the sandbox.  Yet, your recommendations for a spread footing in my sandbox seems intuitively valid given the choices. 

 

I want to entertain the Wagon Wheel with #3 connecting all member beams and hub at 12 inches depth.

Maybe overkill, but intuitively the pier supporting the telescope would need the full benefit of the wide footing through the beams and steel connecting to the hub..  The weakest aspect of the image you show seems to be the relationship of the pier tube to the concrete supporting it.

 

Finally, do I entertain pouring concrete into the 48 inch tube for rigidity?

 

The load in my case will be greater than most with a C14 and additional gear including a spectrometer..

 

Still in the fact gathering stage so it will be awhile (maybe October) before I build.  It would great if I could contact you along the way.  Will you continue this thread?

 

darkskyrancher



#89 John Fitzgerald

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Posted 12 August 2019 - 11:12 PM

Scott
That’s unfortunate, but should be cosmetic and not structural.
You don’t need to use bonding cement to surface the column, sakrete mortar mix will work fine if properly applied. Mix it stiff (slightly less water than instructed) (slightly!), mist the column and use a flat towel to apply it and press it into the voids. You could then use a rubber float to finish it.

I would get a small sack of portland cement, and mix it 1:2 with fine sand, a little thick, then press that into the voids.  Or add part portland to 3 or 4 parts of the sakrete sand mix, and do the same as the quoted post.  Adding portland will make it smoother and less "sandy".  The quickrete "vinyl concrete patcher" that comes in a bucket works well also, and it has bonding agents.


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#90 speedster

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Posted 14 August 2019 - 02:35 AM

Howdy Darksky -

 

I should have explained better.  In the pic, the pier is already poured.  12" hole 5' deep and the exposed part you see wrapped in black plastic is the formed part above grade which is a 10" PVC pipe with pier concrete and reinforcing continuous through it.  It's wrapped because it is clean and pretty and the plastic simply keeps sun off of it and concrete from  the floor pour off of it.  The blue foam isolates it from the floor.

 

The 8' disk you see formed is just the floor and not attached to the pier.  It's similar to what you are doing but in your case, your pier is attached to the disk rather than just passing through it like this one.

 

Vertical load can be ignored since deflection drives the sizes of things.  Your concrete is good for about 3500 psi and your soil will be something like 2500 psf so we'll never get close to having an issue with weight and bearing. 

 

We are after minimizing deflection.  The concept of your disk is to couple the pier to the ground horizontally since there are problems doing it vertically in your case.  If the base is stiff enough and wide enough, we can consider the pier to be a vertical cantilevered beam fixed at it's base and then, for example, the deflection of a 12" concrete pier 48" tall is only 0.074 arc-sec.

 

Since your are in sand, soil moisture will make little difference and the disk concept won't be subject to surface soil movement since there basically is no surface soil movement.  The sand that messes up a vertical pier concept is the same sand that makes the horizontal concept work. 



#91 darkskyrancher

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Posted 14 August 2019 - 09:35 AM

Hey Speedster

 

I understand and simply wanted to clarify earlier thoughts on the subject in sand.  I will design such that the pier is tied into any and all horizontal members of the disk.

 

Grateful for your comments.

 

Darkskyrancher



#92 Scott123

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Posted 24 August 2019 - 01:27 PM

I used surface bonding cement to finish the pier. I'm not a mason, and knew I couldn't make it as smooth as I wanted. I used a grout float, giving the final product a kind of stucco look. The bottom will be hidden by the platform, when I build that. I might grind the bevel a bit, my wife said it looks fine, and I'll likely grind off the whole pier (she knows me too well!).

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Edited by Scott123, 24 August 2019 - 01:29 PM.

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#93 Mel M

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Posted 05 September 2019 - 03:10 PM

If you have decided to use a vibrator get a pencil vibrator for the 12" piers. Over vibration could separate the aggregate. One plunge should do it. Insert quickly and remove slowly. I'd take about 6 seconds to remove it from a 3 or 4 foot hole, depending on slump.


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#94 Scott123

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Posted 26 September 2019 - 07:10 PM

I finished the pier as best I could, now I want to pour a slab. Considering the time of year, I am putting that off until Spring.

 

A ten by sixteen foot square slab four inches deep requires at least two yards of concrete, not including the footing. Sounds like I should have a truck come in (and pay for the unused yardage/short load fee).

 

How difficult is it to properly make a slab? I intend to insulate the slab a little by laying down 1 inch sleepers with 1 inch foam insulation between on top of the finished slab. Then top with 5/8" or 3/4" plywood for the floor. 

 

Here I am waiting for the sun to set. The banding is there for visibility, some friends have ridden their ATVs or snowmobiles through this area.

 

september 21 2019.jpg


Edited by Scott123, 26 September 2019 - 09:09 PM.

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#95 VVObserve

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Posted 27 September 2019 - 06:34 PM

In my experience building a roll off roof observatory the two concrete piers I installed are very inadequate.

The pier shifted when the whole setup frost heaved during one of our central Pennsylvania below zero F winters.

 

Pier 1 for my heavy 12 1/2" Cave and Astrola Mount is 10" diameter concrete with rebar and concrete footer buried 4 feet deep.  I failed to backfill properly in the clay & rock stuff they call soil around here.  The coup de grace occurred when my whole building frost heaved through my own dumb move leaving an extra "brace" in place  during one of our Appalachian winters.  The building moved enough for the pier floor opening to push against the pier and so the pier also shifted.

 

Pier 2 in my observatory is an 8" diameter concrete pier for planetary scopes in the 8" refractor range.

I am in the process of re-digging and re-pouring concrete to enlarge this pier.

 

Foundations and piers are pretty well established sciences.  

Avoid any short cuts or experimenting.  

And definitely watch out for frost heave.

 

1 - dig a proper hole to below the frost line.

2 - pour an inch or two of gravel in

3 - set the rebar into the hole with supports to keep the rebar positioned for the concrete pour

4 - pour a footer that is wider than the pier to distribute the weight of the pier

5 - position and secure a large tube  (from 8" to 12"+ depending on the size of the OTA) and do single concrete pour

6 - make sure to position and secure your stainless steel mounting bolts properly 

7 - finish off the pour 

8 - let cure

9 - backfill with crush gravel

 

My mistakes -

I did not backfill with crush gravel...I merely packed the dirt I had dug up back around the pier.

 

I should have built a 12" form and pier for my 12 1/2 inch Cave Astrola mount.  The 10" is ok but 12" would be much better.

 

The 8" pier is really too small.  I should made this one 10".

 

Best of luck and go big,

 

David

 

 

 

 

 

 


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#96 Scott123

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Posted 28 September 2019 - 07:46 PM

Hi VVObserve, thank you for the advice. I already have the pier set, and have been using it for a while now. It is seven feet deep, and is fourteen inches in diameter, necking down to a ten inch construction tube above grade.

 

Next step will be the slab. Since I am going to cover it with a layer of insulation and plywood I shouldn't have to get too critical on the finish. (Screed and bull float?)

 

But that will be next Spring's project!

 

Scott



#97 Cliff Halliwell

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Posted 28 September 2019 - 09:56 PM

I finished the pier as best I could, now I want to pour a slab. Considering the time of year, I am putting that off until Spring.

 

A ten by sixteen foot square slab four inches deep requires at least two yards of concrete, not including the footing. Sounds like I should have a truck come in (and pay for the unused yardage/short load fee).

 

How difficult is it to properly make a slab? I intend to insulate the slab a little by laying down 1 inch sleepers with 1 inch foam insulation between on top of the finished slab. Then top with 5/8" or 3/4" plywood for the floor. 

 

Here I am waiting for the sun to set. The banding is there for visibility, some friends have ridden their ATVs or snowmobiles through this area.

 

attachicon.gif september 21 2019.jpg

When I did a pier base and observatory deck post bases I discovered that the cross-over point between the supposedly ‘cheap’ option of buying sacks and mixing one’s own concrete and the supposedly ‘expensive’ option of getting a cement truck delivery comes quickly I recall needing under 1 1/2 cubic metres and it turned out delivering mixed was cheaper than just buying the sacks.  So, I had my concrete delivered in the cement truck.  It poured out fast (used a spare ‘Big Foot’ as a funnel) with a little help and saved tons of labour, of the kind a lifetime of office work had not prepared me for.  Plus, you can order extra ‘features’ like ‘air entrainment’ or the addition of small fibres to the mix to help keep micro cracks from forming.  So, bottom line: always check the cost of getting the cement truck to come.  It may pleasantly surprise you.  


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#98 mmalik

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Posted 29 September 2019 - 03:40 AM

I have been thinking about a backyard pier for imaging and have glanced through most of this thread. What are your thoughts about a prefabricated solution of following kind where you don't have to pour concrete.

 

 

I'll explain in more detail in coming posts how I am envisioning such a solution over pouring (pier) concrete. Your feedback will be appreciated in the meantime on such an idea.

 

 

I am basically looking for an idea that will save me from pouring concrete and something that will eventually save me the hassle of dismantling many year from now. Taking down a set concrete pier as you can imagine can be a daunting to impossible task. Think backyard scenario as you provide your feedback. Thanks in advance. Regards

 

 

Note: Prefabricated sections can be bolted together with the central long bolt

 

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#99 speedster

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Posted 30 September 2019 - 03:39 PM

How well the EZ Tube prefab sections work depends upon your expectations.  If you want to set your PA and seldom have to adjust it, other solutions may be more likely to achieve that.  The EZ Tube themselves are only a little more money than concrete in a pier drilled with an auger but they are a lot more work and not nearly as stiff.  Certainly doesn't mean they won't work for you.  The spread base does work against you.  No need to spread the load as our loads are tiny in terms of bearing capacity of even poor soils.  The spread base requires over-excavating the hole to get it in.  You also have to figure out how to level a #100 base in the bottom of the hole.  Gravel and sand are, structurally, fluids and don't count in stiffening the pier.  If you level that base on gravel, it will move over time.  If you back-fill around sections with sand, gravel, of soil, it will compact quite a bit over time.  If you compact the back-fill in layers as you place it, it's better but it is still going to further consolidate over time.  It takes about a decade for the loose material to get back to maximum density and actually hold the pier against vertical displacement.  Crisis?  No.  You just reset your PA as needed.  Even when you get as much movement as VVobserve above, you just reset your PA and keep on going.

 

If you have a concrete shaft, poured in place against the soil, as in an augered hole, A steel pier on top is a cantilevered column and the deflection is entirely in the steel pier (plus bolts which can generally be ignored).

 

If you have a concrete column in an over-excavated hole with backfill against the concrete, with a steel pier on top, you have a column that is structurally hinged at the bottom of the concrete.  A spread footing adds a small lever arm  which helps but it can't compete with an augered shaft.  The backfill does give some lateral resistance, to be sure.  It also increases over the years.  But, deflection is a function of the cube of the length so that hinged column might be 7' as opposed to a 3' steel pier on top of a concrete shaft.  5x more deflection (actually a little less because the back-fill does contribute something).

 

This thread started as a description of how to minimize deflection.    In DYI astronomy, many foundation/pier arrangements can work well even though they may be doing some things that are actually working against them.  If my piers have a deflection of 0.4 arc-sec and my neighbor's are 10x worse with a deflection of 4.0 arc-sec, on a calm night, if nobody is stomping around, and nobody is bumping the mount, there is no difference.  Except that mine may be better due to sound system and minibar.


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#100 mmalik

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Posted 30 September 2019 - 05:58 PM

Here is a great idea; please let me know your thoughts? If viable, please comment on the kind of wood that will be best suited for such an application (...the one that doesn't rot, or warp, etc.)

 

...heavy wood pier, eg 4 4x4 or 6x6 PT bolted together and sunk into the ground / concrete

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  • Timber.JPG

Edited by mmalik, 30 September 2019 - 06:10 PM.



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