Concrete or steel pier

[Jeffkop (Jeff)]

Hi All.

The time has finally arrived to start my observatory and I thought that I would throw the question of which type of pier out to the community ... I know there will be more considerations put up than I can think of in this regard ... not just to do with the materials, but hopefully from the perspectives of practicality, ease of manufacture, stability. etc. I'm not too concerned about cost, Im certain that a concrete pier would run out cheaper than a steel one but also reasonably sure that the difference is not going to be significant enough for it to be a major consideration.

Hope this thread is best placed in this forum than the Observatory one ... if not please relocate it mods.

Thanks

[Ian Robinson]

Originally Posted by Jeffkop

Hi All.

The time has finally arrived to start my observatory and I thought that I would throw the question of which type of pier out to the community ... I know there will be more considerations put up than I can think of in this regard ... not just to do with the materials, but hopefully from the perspectives of practicality, ease of manufacture, stability. etc. I'm not too concerned about cost, Im certain that a concrete pier would run out cheaper than a steel one but also reasonably sure that the difference is not going to be significant enough for it to be a major consideration.

Hope this thread is best placed in this forum than the Observatory one ... if not please relocate it mods.

Thanks

I opted for a steel pier , yet to be bolted to a concrete footing.

Wanted something I could move if I needed to.

[renormalised (Carl)]

Well, a concrete pier will be less prone to vibrations than a hollow steel pier, if it's made properly. No matter how thick the walls of a steel pier are, the simple fact it's hollow will allow it to resonate if bumped into. However, if you fill it with something...like sand, that will dampen the vibrations. A steel one is easier to construct and fit in place...you don't have all the form work to create the pier to contend with, and waiting for the concrete to cure etc.

One good thing about a steel pier, you can route your electrical and other cables through it, if you so wish and that'll get them out of the way.

I suppose what your choice is will depend on cost and just how enthusiastic you are about knocking one up. Also, the time allowed for construction will be a factor.

[Bassnut (Fred)]

A steel pier is far more expensive, can be removed, and has a lower thermal mass to reduce thermal currents at high FLs, but youd have to be fairly anal to see the diff IMO.

A PVC tube inserted in a concrete pier before poring concrete solves the cable problem easily.

A concrete pier is a far cheaper, subbie-free, same-result solution IMO.

[Jeffkop (Jeff)]

Originally Posted by Ian Robinson

I opted for a steel pier , yet to be bolted to a concrete footing.

Wanted something I could move if I needed to.

I thought the same thing Ian but I also realized that the huge lump of concrete in the ground wasnt so portable and that seemed to negate portability as a consideration to a point. Thanks for your input.

[Jeffkop (Jeff)]

Originally Posted by renormalised

Well, a concrete pier will be less prone to vibrations than a hollow steel pier, if it's made properly. No matter how thick the walls of a steel pier are, the simple fact it's hollow will allow it to resonate if bumped into. However, if you fill it with something...like sand, that will dampen the vibrations. A steel one is easier to construct and fit in place...you don't have all the form work to create the pier to contend with, and waiting for the concrete to cure etc.

One good thing about a steel pier, you can route your electrical and other cables through it, if you so wish and that'll get them out of the way.

I suppose what your choice is will depend on cost and just how enthusiastic you are about knocking one up. Also, the time allowed for construction will be a factor.

Thanks Carl. Time allowed for construction ... Ive been waiting to start this for a good while now mate ... So Im not concerned about how long any part of the process takes to come together ... everything that is done is one step closer to operational !!!

Good point about the cure time though. Super critical that nothing is allowed to come into contact with it. I guess that the pier would have to be done at the same time as the footing to give it the best overall structural strength too. I mean if it was poured after the footing there would always be a layer of weakness at the join wouldnt there, even tho there would be reinforcing coming up the pier from the footing.

[Jeffkop (Jeff)]

Originally Posted by Bassnut

A steel pier is far more expensive, can be removed, and has a lower thermal mass to reduce thermal currents at high FLs, but youd have to be fairly anal to see the diff IMO.

A PVC tube inserted in a concrete pier before poring concrete solves the cable problem easily.

A concrete pier is a far cheaper, subbie-free, same-result solution IMO.

Thanks for the input Fred. I couldnt come up with any of my own reasons why one was better or worse than the other and all suggestions so far do not alter this logic. Seems that its really going to come down to which is going to be easiest depending on ones resources, both practically and financially. This is good and a welcome change from many of the other considerations in astro photography where there are many differing opinions.

[Ian Robinson]

Originally Posted by Jeffkop

I thought the same thing Ian but I also realized that the huge lump of concrete in the ground wasnt so portable and that seemed to negate portability as a consideration to a point. Thanks for your input.

Point taken.
But --- most of the concrete footing is under the ground so it's really never going to be in the way for me or some one else if I ever move, unlike a monolythic concrete pier which also has a big footing, which would certainly would and isn't that easy to demolish.

[renormalised (Carl)]

Originally Posted by Bassnut

...subbie-free...

Might be subbie free, Fred, but it can never be "stubby free"

Gotta keep cool somehow!!

[mozzie (Peter)]

hi jeff
i opted for a concrete pier for me its easy to do im a bricklayer and have all the nessesary gear to do this as well i ran flexible condut through it for my power before pouring as for removal ive a jack hammer and if i sell just bust it up.id do another one for my next house.im buying a new scope at end of year and will probably have to cut 100mm of it easy ive all the gear to do this as well,steel i dont work with much and dont have the gear to do it

[sheeny (Al)]

I did the calcs a while ago Jeff.

For any given diameter and length, a steel pier will be 10x stiffer than a concrete one - simple ratio of Young's moduli for the two materials (I know... Young's Modulus for concrete varies with the mix to some extent but not enough to get it out of the 10x ball park). That means 10x less deflection, more than 10x less amplitude should it vibrate (steel will be 10x stiffer but a fraction of the mass, so the natural frequency is higher, therefore the amplitude for any given energy of vibration is less). Of course a concrete pier will dampen any vibrations quicker than a steel one (unless you dampen the steel pier with concrete or sand internally) ... etc etc etc...

In reality though, Jeff, it makes no difference. You build your pier as big in diameter as you can within cost and physical constraints, and unless you are a clumsy so and so, or you use the pier to pull yourself up out of your chair, or you are a habitual toe tapper (on the pier) you won't see the difference.

Minimum 100NB Standard weight pipe or 100SHS, but bigger is better.

There are a lot of over engineered piers getting around... but that's OK.

Al.

[renormalised (Carl)]

100NB Standard weight pipe or 100SHS

How thick are the walls on that pipe, Al??

[sheeny (Al)]

Originally Posted by renormalised

How thick are the walls on that pipe, Al??

I don't have my tables at home ATM but its about the 5 to 6mm mark. However, the wall thickness is of little concern as the section modulus is proportional to the diameter to 4th power, so there's little difference between two pipes of of different wall thickness but the same outside diameter. Obviously you don't want a paper thin pipe that has the section modulus but not enough local robustness to stop you poking holes in it with your fingers!

If I was making a concrete pier I'd make it at least 200mm diameter, simply because you want about 30 to 50, of cover over your reo to prevent it rusting, so that only gives about 100 to 140mm between reo bars. Anything smaller than that in concrete is getting impractical IMHO. A steel pier of equivalent stiffness is about 133mm diameter according to my quick calcs... (I hope I haven't made a silly mistake)

Al.

[bert (Brett)]

Further to what sheeny has said, another consideration is the size of the pier width. If you make a pier from concrete thats around 30cm wide, which you pretty much need to, you will lose a lot of real estate around the zenith (before a meridian flip is required to miss the pier) with longer scopes. My vote goes to steel.

I personally have a 150x 150 shs pier that is around 7 foot (diagonally braced to 4ft) tall and I'm not having any probs with stability.

One thing I always say (and usually proptly ignored) when you do your footing go deep!!!! Dont put a square of concrete near the ground surface, use a thinner pier but go down at least (for you titan) 1500mm below ground level, but the deeper the better. Your polar alignment will thank you.

Brett

[Jeffkop (Jeff)]

Thanks Pete and thanks Al.
I have decided today to go with a steel one. It will need to be about 1500mm long MAX which puts about 1000mm above the floor and something like 400mm to 500mm below as Im building on slightly sloping ground. By the look of Als figures if I opt for something say 150mm-200mm in diameter and about 10mm thick as I can get that kind of stuff for nothing from a mate. I dont want to go too big and have problems with the camera snagging on anything going thru the zenith.

Will post pictures like everyone does during construction .. so lookout for those.

Thanks again

[Jeffkop (Jeff)]

Originally Posted by bert

Further to what sheeny has said, another consideration is the size of the pier width. If you make a pier from concrete thats around 30cm wide, which you pretty much need to, you will lose a lot of real estate around the zenith (before a meridian flip is required to miss the pier) with longer scopes. My vote goes to steel.

I personally have a 150x 150 shs pier that is around 7 foot (diagonally braced to 4ft) tall and I'm not having any probs with stability.

One thing I always say (and usually proptly ignored) when you do your footing go deep!!!! Dont put a square of concrete near the ground surface, use a thinner pier but go down at least (for you titan) 1500mm below ground level, but the deeper the better. Your polar alignment will thank you.

Brett

Thanks Brett

Plan is to have 1.5mx1.5mx1.5m footing. I think this fits in nicely with your recommendations.

[Jeffkop (Jeff)]

Thought just occurred to me Bert. Are you suggesting that the Pier itself goes into the footing ??? I was considering bolting it (onto threaded rods that are in the footing) at ground level.

Thanks for the input

[sheeny (Al)]

Just googled it Carl.

100NB standadr weight pipe (Schedule 40) is 114.3mm OD and 6.0198mm wall thickness...


... in theory.

Al.

[sheeny (Al)]

Originally Posted by Jeffkop

Thanks Brett

Plan is to have 1.5mx1.5mx1.5m footing. I think this fits in nicely with your recommendations.

Are you building a false floor over the footing or will the top of the footing be at floor level?

If its at floor level, have a think about the size in plan of that footing, I suggest Jeff. The edge of your footing will be about 600 to 700mm from the pier, so if you are in the obs moving around you will be changing the load on the footing... it probably won't make a difference if your footing is solidly keyed to bedrock, but I'd hate to go to the trouble of building an obs only to find the scope moves when you walk around it.

In my obs, the footing is level with the floor, so I kept my footing size down to 350 x 350 x 1000 deep (but in reality I hit bedrock at 700mm so I keyed it in 50mm and I was more than happy!

Al.

[bert (Brett)]

Nah, just bolt it to the concrete. And I prefer to use nylon anchors and coach bolts in the concrete vs dynabolts. If you get the anchors wrong you can pull them out and it doesnt leave anything sticking out of the concrete. Embedding it in the concrete gains nothing.

Originally Posted by bert

One thing I always say (and usually proptly ignored) when you do your footing go deep!!!! Dont put a square of concrete near the ground surface, use a thinner pier but go down at least (for you titan) 1500mm below ground level, but the deeper the better

1.5 X 1.5 x 1.5 is a square (I should have used a cube, my bad) that I recommended you DO NOT do. If you want to use that much concrete go a meter by a meter, by 2 deep. Then you can walk close to the scope without walking on the isolated pier, and far stronger. I'm sure sheeny can explain the math, cause I sure as hell cant....

My concrete pad is only around 1/4m3 (compared to your plan of using 3.3m3)of concrete. It is a 300mm round hole down 1200mm slightly flared at the top and bottom. The secret is go deep. Sheeny was lucky that he hit bedrock, he effectively used the rock as an extension of his footing.