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Nick:

Personally, I think people go a bit "overkill" on the base of these piers. I think 2' x 2' x 3' is more than large enough.



I have a porch/deck at the rear of my house, and each post is setting on a mini 'pier'. Basically an 8" diameter sonotube going 4' into the ground (due to frost line), with a 12" diameter footer. These have not budged for the past 15 years.

Compared to a building, astronomical piers have to support almost nothing in terms of weight. Unless your pier is going very high, such as with a roof-top observatory, then you really don't need much in the way of a base.

-Dan

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My base is 18"x18"x3' I mixed the concrete myself. The bottom of the pier flares out to form a footer. There is no rebar in the base. It been supporting the setup shown below for over 10 years with no issues.

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Nick, plan for an earthquake.

If the ground moves from the "big" one, not much is going to stop it.. It's the bunch of smaller ones i'm more concerned about.

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If the earth moves, it isn't going to really matter how big your base is. Entire buildings, roadways, etc. will shift in the right kind of earthquake.

-Dan

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My pier footer is not very big and has not moved in 10 years.

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Mine is moving at over 3cm/year to the Northwest and there's not a darned thing I can do to stop it because the whole Pacific plate in this area is apparently doing the same. It's moved over 126cm Northwest since I've lived here.

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Mine is moving at over 3cm/year to the Northwest and there's not a darned thing I can do to stop it because the whole Pacific plate in this area is apparently doing the same. It's moved over 126cm Northwest since I've lived here.

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Is the pier still on your property?

Gale

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Nick:

Personally, I think people go a bit "overkill" on the base of these piers.

-Dan

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Nick:

Personally, I think people go a bit "overkill" on the base of these piers.

-Dan

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In some cases "overkill" is an understatement, I have spend the last 15 years supervising concrete formwork construction and have built multistorey buildings on footings smaller than some of these pier bases.

The secret is in the base preparation and reinforcement, dig the hole then COMPACT the ground well before laying down plastic, then 'chair up' a sheet of steel mesh about 3" off the ground, another sheet about 3" below the finished concrete level and then fix 6 L-shaped anglebars 1/2" diameter to the bottom mesh arranged so they end up inside the proposed pier with at least 1" of concrete to the face of the pier.

I think a base area of 3' x 3' and 18" deep would be as big as you would ever need. My own pier base will be 2' x 2' x 18".

Just my 2 cents.

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Matt, a footer for a building and a foundation for a telescope pier are two different animals. The former has to accept the load from above, whereas the latter has to do that plus not budge one iota. At high magnification, a small displacement at the scope translates to a significant change in the EP.

Also, it is important to get the base of the foundation below frost line (i.e., well below 18" deep).

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Hey....be nice! We all make typos sometimes.......

I don't pour concrete for a living, I build things..... including not only houses and decks, but also free-standing radio towers and once a 14-meter radio telescope, and I'm siding with Matt, in that the world of amateur astronomy about single-handedly keeps the concrete industry in business.
But, joking aside, much of the ultimate stability of a pier is determined by the soil composition in which the concrete is placed, and much more is determined by the construction of the pier itself and the attention to detail of the backfill placed around it. Wide and flat bottom surfaces poured on undisturbed subsoil and against undisturbed subsoil walls are always the most stable option. Any removed and replaced fill will be less firm than before it was removed, the best possible packing and compacting of backfill will still leave a free-standing pier built in a dug-out hole able to be moved around a small amount relatively easily. For a house, not a problem - all the load is straight down, and as long as the footing can't push downward into soft fill, all is good. For something like a radio tower with a large wind-induced side load, a cube - even a big one - with dirt shoveled back in around it, will soon indicate the prevailing wind direction. Scopes in a small way replicate the loads on towers. A slab of concrete 5 x 5 x 1 foot containing 25 cubic feet with a pier sticking up from it will offer far more resistance to side forces than a 3 x 3 x 3 foot cube containing 27, with a similar diameter pier, because the leverage forces required to lift an edge are far greater.
If you're wondering, my own pier, built to hold a 6" refractor, is a 2' diameter hole dug about 5' deep in firm gravely clay (liberal use of an electric jackhammer was required) filled with concrete directly into the hole about 3' deep, with a 10" sonotube to ground level, and a steel pier above ground. The pier, and the building's corner-piers, all stop at ground level in case of future removal of the building. A hard kick will vibrate the steel pier a bit, but I don't kick the top of it very often when I'm observing.....
I have noticed, though, that the big town snowplow truck, main and wing down, loaded with several yards of road sand, will vibrate the scope visibly when it passes 1/4 mile away, but that thing's pretty much a rolling earthquake, so that's to be expected, I guess.
Russ

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