I'm beginning a 13' pier and was surfing the net and found many people who sincerely want to do a good pier, some who's piers work for them but for the wrong reasons, some real mistakes, some folks completely off base, and few youtube vids that beg for the presenter to don his tin foil hat. I had to run some numbers for my tall pier and, being architects and engineers, we thought others might be interested in what is really going on in piers.
The first misconception is the load involved. They are tiny but, so is the acceptable deflection. A 4,500 pound car hitting a 4" curb at 20 mph experiences a force of nearly 500 pounds. A C8 on a tripod in a 10 mph wind sees 0.84 pounds and 3.36 pounds at 20 mph. The first question is what load do we design for? Someone else used a 5 pound horizontal load at the top of the pier. His numbers seemed off but his logic is sound. 5 pounds is a good wind gust or a bump. Using 5 pounds, let's limit the angular deflection to 0.5 acrsec which is acceptable for imaging and beyond the resolution of many scopes. With a deflection that small, vibration and dampening almost become mute points. Do we care if it's vibrating as 57 hertz if the amplitude is too small to detect? No.
Here is the angular deflection at the top of the pier for various pipes and concrete piers of different lengths with a 5 pound horizontal load at the top:
PIER 1.pdf 12.93KB 1220 downloads
Shaded cells are most economical pipe sizes for deflection less than 0.5 arcsec at each length. Concrete section modulus assumed to be 2,465 ksi, varies with aggregate mix properties and varies over time. Can range from 2,030 ksi to 5,946 ksi. If you don't like a force of 5 pounds, divide the table values by 5 and then multiply by whatever force you want (in pounds). Filling steel pipe with sand or concrete will change the harmonic frequency but not the deflection under load. The sand is a mass fluid and the concrete is more flexible than the pipe that contains it. Concrete may stiffen thin wall tubing but it won't help much on these pipes which are garden variety A36 schedule 40 structural shapes. Masonry piers of concrete block or brick, with the same cross section size as the concrete piers, are about 10x more flexible.
But, pier flexure is only a part of the whole pier. There are also connections to deal with. So, let's look at a whole pier with foundation:
PIER 2.pdf 100.99KB 830 downloads
Starting at the top, this drawing does not show a bolt cage. This can add very little flex or can add a lot. Having that cage is very handing when mounts have to be attached from below and when there is a possibility of future equipment changes. If they are 3/4" bolts and rigidly attached, even 5" of bolt height might add only 0.25 arcsec of deflection. The bigger issue is things not being rigid and the whole cage wiggling. Attach the mount to the top plate and then adjust the plate down as far as it will go and then tighten everything up to minimize deflection at this point.
The next thing down is the 6" pipe which has a deflection of 0.108 arcsec. Then 2 plates bolted together which adds 0.023 acrsec of deflection. This could be eliminated but this connection allows a smaller diameter pier where the scope may hit it and also allows changing the height by replacing this short piece while leaving the bigger stuff below alone.
Next is the 10" pipe with a deflection of 0.404 arcsec and the bolted connection to the foundation which adds 0.036 arcsec of deflection. So, our total, with no cage on top, is 0.571 arcsec of deflection. Pretty close to our max target of 0.5 arcsec of deflection.
If you don't agree with the numbers, that's fine. It's just plain old physics and pretty simple stuff. If you just want to shoot from the hip based on your intuition, that's fine too. This is just for those who want to understand the mechanics and what can be done to improve their designs. Structurally speaking, there is some crazy stuff out there and pushing the envelope is great but some want to push it in the right direction. If you glue concrete blocks together with foam in a can and strap a $1,200 mount to it and your happy, I don't understand you but I'm happy too.
Now, some details of the pier before looking at the foundation. Personally, I like the top of my pier level. It doesn't need to be. All we are after is the polar axis being truly polar. The base of the mount can be stuck to a wall and the thing will still work. I just bugs me when things aren't level and plumb.
Rocket fins on the tube. We put gussets on lot's of things but not telescope piers. The little short ones do virtually nothing unless you just like the look. If you pick the right size pipe or concrete diameter, fins are just decoration. If you skimp on the diameter, long wide fins will help but why go to the trouble and cost? Most fins I see are a gimmick.
Bolts. Don't waste your money on Grade 8 or stainless. All steel bolts have virtually identical stress/strain curves independent of alloy or heat treatment. The more expensive bolts just make it farther up the curve and stretch more before the fail. Avoid Chinese bolts. They are notorious for failure. Always use galvanized bolts in concrete. Drill your bolt holes 1/16" over.
Aluminum pieces. If you just like aluminum, use it. You are saving weight where we don't want to save weight and introducing a dissimilar metal but it will certainly work. Never put aluminum in contact with concrete. If a part is steel, weld it. If aluminum, braze it. Brazing aluminum is easier than welding steel and can be done easily with a cheap MAP gas bottle torch from a big box store.
The foundation is the weakest link in what I see being done on the net. Throw enough concrete in a shallow hole and it will probably work but here are some things that can be improved.
Never embed your steel pier shaft in your foundation. It will corrode later, if not sooner and the expansion will break your foundation.
Yes, you do need vertical reinforcing. It only controls failure in tension and above grade there may never be tension unless the weight of your mount is not centered on the pier or you hang something heavy off the side of the pier. You will definitely have shear in the underground portion and it can be quite large. Best to reinforce the whole thing. Four #4 vertical bars in piers 12"-16" in diameter is a decent guide. Always keep reinforcing at least 2" from soil and from the face of forms. Never stab your bars in the dirt. That will break your concrete. Stirrups (those ring things) are only needed to keep the vertical bars in position. Never weld reinforcing - only wire tie them or zip tie it. If you have construction joints, reinforce continuously to tie the pours together. Fibermesh is not structural reinforcing and has no place in a foundation. If your concrete is coming from a ready-mix plant, add 5% air. It will better resist freezing and thawing. If you want to pour soupier mix, do not add water. Add plasticizer (if coming from a plant). It keeps your strength up even though it makes the mix runny and easier to place. Your slump can go from 3" to 9" with no loss of strength.
The shape of the foundation. Bearing capacity of poor soil is 2,000 psf so 1 sf is plenty of area for nearly all piers. Shallow spread footings (often seen under telescope piers) are designed to spread the load so the soil bearing capacity is not exceeded or to prevent overturning. We don't have a load that needs to be spread and we don't have any loading that is trying to overturn anything. There is no reason to have a spread footing but there are reasons not to have a spread footing and they have to do with soil movement. You an look up your soil properties at https://websoilsurve...s.usda.gov/app/. Look at your plasticity index. It is an indication of shrink/swell potential. Soil pressure can be as high as 15,000 psf both vertically and horizontally and your foundation WILL move. It moves more in shallow soil. Plasticity below 17 is pretty easy to deal with. My site has a PI of nearly 40 and is moving up and down almost 4" per year, with moisture content. If the whole neighborhood moves together, no one notices. If there is differential movement, such as watering a flower bed on one side of your pier foundation but not the other, differential movement occurs and the foundation tilts. In your house, it is not unusual for some doors to stick in certain seasons and not in others. This is differential movement caused by moisture in the soil.
Freezing also causes soil movement. Put the bottom of your foundation 12" below your frost line. In central Nebraska, that's 5' deep. But, that's a good thing because soils get more stable as you go deeper. The moisture content is more constant as you get below the active surface zone.
Instead of a shallow spread footing in the most active zone, go vertical with no bell at the bottom. In the diagram above, the foundation is 60" deep. That is the deepest a Bobcat auger will go without an extension. Oh, but renting equipment costs money! Yes, but you make it up in less concrete, less labor, and you get a better, more stable foundation. The pier example above is only putting 0.046 psf of horizontal load against soil that will take 2,000 psf before deforming. Very strong and out of the active zone.
If you still want to do spread footings, why dig 2' deep and fill the hole with concrete? You could just as well place a 3' x 3' footing 8" thick right on the surface and get the same result.
With drilled piers, always clean out the bottom of the hole to get rid of loose material. With spread footings, always compact anything you disturb. Rent a vibratory plate and greatly reduce the effect of settling. Whatever you leave fluffed up will take years to consolidate on its own.
Don't form concrete below grade. We want the concrete bound to the soil. If you form below grade with wood and leave it, it rots, forms a void, and the foundation easily moves. If you Sonotube below grade and don't remove it, it rots, forms a void and the foundation easily moves and you also create a path for water to expand soil adjacent to the pier and things move. On larger drilled piers, the skin friction can take more load than the bearing at the bottom of the pier. There is good reason utility poles, signs, and parking lot lights are bolted to drilled piers: it's much more stable and moves less. The little drilled pier in the example above is not much smaller than what we put 40' light poles on and expect them to survive 100 mph wind. Would the typical shallow telescope pit full of concrete do that? And, the drilled pier is cheaper and faster. Think vertical.
Some parts of structural engineering don't always make intuitive sense and there is,unfortunately, plenty of misinformation on the web from really good people with really good intentions. Keep on experimenting! That's one of the beauties of this activity. You know, I didn't notice anyone driving piles. You can put a 500 pound hammer on a tractor PTO and bang a pipe down for an excellent foundation with no digging, no concrete, and only a little welding. Hmmm.