Concrete or steel pier

[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

[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.

[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.

[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.

[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.

[Jeffkop]

Thanks Carl. Time allowed for construction ... :lol::lol: 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]

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]

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]

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

Gotta keep cool somehow!!:):P

[mozzie]

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]

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:D). 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:P) you won't see the difference.

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

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

Al.

[renormalised]

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

[sheeny]

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!:P:lol:

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:screwy::lol:)

Al.

[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

[Jeffkop]

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]

Thanks Brett

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

[Jeffkop]

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]

Just googled it Carl.:D

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


... in theory:P.

Al.

[sheeny]

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.:eyepop:

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!:lol:

Al.

[bert]

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.




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.

[Jeffkop]

Al .. The floor will be about 400mm - 500mm above the ground and so to the top of the footing. I planned to bolt the pier onto threaded rods that were into the footing. Do you think I should be putting the pier into the footing ???

The floor of the Obs will not be in contact with the pier

[Jeffkop]

Sorry Brett. I didnt explain fully what Im up to. See the reply to AL above this post.

[Ian Robinson]

Interesting that you guys advocate supermassive footings for piers (ie 1.5m x 1.5m x 1.5m presumeably only maybe 300mm above ground level).
Is there a good engineering reason for this ? and does this depend on the type of gound you have ?

I have seen similar recommendation by guys at CN, BUT there they have the issue of the ground freezing in winter - and it's done because of this annual freezing and thawing. Obviously not a problem here unless you live above the snow line (in the Snowy Mountains). So is it really necessary to have such massive (AND DEEP) footings ?

[sheeny]

That's not a problem with your floor over the footing (and separated from it):thumbsup:.



For rigidity the preferred options in decreasing order of rigidity of the pier footing connection are:


Embed the pier in the footing
Adequately grout the pier to the footing
Bolt it securely to a smooth, flat footing (with a thick, stubby base plate (distance of bolt from pier member / thickness of baseplate <=2!)
Bolt it to an uneven footing
Bolt it on levelling nuts on the hold down bolts (but no grout)
Bolt it down on a central packer only (no grout)

A well grouted connection is near as makes no difference as good as embedding the pier, and it's easier to do a neat job and get it plumb.:thumbsup:

The "right way" to apply the grout is set you pier up level and plumb on the anchor bolts with levelling nuts underneath, then use a good quality non-shrink grout to fill underneath. If your grout is stiff push it in from one side until it comes out the opposite side so you know the joint is full. If you are using a free flowing grout (epoxy etc) then form up for the grout and pour it in from the one side to make sure there are no bubbles in the joint.

Now if you are a bit lazy like me and think you can do it...:D... (WARNING! There's potential for this to go wrong so attempt this at your own risk!;))
What I did with mine was cast my footing about 20mm lower than floor level. I then made a stiff mortar mix and levelled and smoothed that off to floor level. While it was wet I placed the pier on it and used the anchor nuts to pull the pier down just slightly into the mortar (just enough to make sure there's no air bubbles - not enough to deform the mortar around the baseplate) and adjust the nuts till the pier is plumb and level. Then leave it to cure and make sure is protected from anything that could bump or move it while the mortar sets. After a week of curing tension the anchor nuts up tight:thumbsup:. I used a tension wrench to set mine just because I could:P. Avoid tightening the anchor nuts too soon so you don't crush the fresh mortar/grout.

... and Robert is your mother's brother...:thumbsup:



I could see the need for a wide footing in sandy soil perhaps where there's simply no chance of getting anything too solid, but in reality depth is what you need. The ground pressure increases linearly with depth, so the deeper you go the more secure it is... unless you find rock:thumbsup::D.

Active clays soils will always be a problem with movement unless you can get to bedrock:sadeyes:. In that case, you might have to live (hopefully) with regular polar alignments to correct the movements:sadeyes:).

Al.

[Ian Robinson]

I see.

So a lazybones like me might get away with a footing that is not as deep and if my pier "moves" over time, I can compensate for it by relevelling the adapter plate the GEM will be sitting ontop.

[sheeny]

You might:P... and you might not.:lol: If you don't do your best to make it stable, then you have no one else to blame when you have to do a polar alignment and maybe drift align every time you want to use your scope...:)

Al.

[Jeffkop]

Thanks for this advise Al & Brett .. I wasnt aware of the grouting issues. Looks like Im going to have to box up a section on top of the footing ... less pipe to get then.

My soil here is sandy .. VERY sandy. We live over the top of a huge ridge of sandstone. The underground water tastes absolutely perfect here.

So from your advise you suggest as deep a footing and as wide a footing as is practical. Im thinking 1.5mx1.5mx2m now. Depends on how big I have to get the XY dimensions to allow a backhoe to get down 2m I think. I dont know how far down until I meet stone, but I do know that the sandstone itself is relatively soft here. I guess once we hit a huge stone object I will have to evaluate the depth we are and then make adjustments to the other dimensions to suit. Of course I am limited in that dimension by the size of the room and the position of the post holes too.
In any case looks like a job for a backhoe for an hour or so.

This is a very interesting thread for me, thanks for your inputs.

[gregbradley]

Hi Geoff,

I have just built an observatory and this is the third one I have done and here is a quick rundown of some principles I used that have also been successful on previous observatories plus a few things I did differently based on what I learnt was a problem with other observatories I built:

0. Size - 5 x 3 metres and walls are 2.2mm high, roof overhangs to form a protective eave about 300mm each side, computer room is a kliplok flat roof with insulation under it. Location was carefully scouted and is in a spot (I am on 4 acres) which is close to the house, is private, has no stray light hitting it, is hidden from view, has a clear unobstructed view of the imageable sky, is protected from wind and storm directions and is in a very calm area of the property. Also next to gardens for a pleasant outlook.

1. made 2 pads. One is a pier about 800 x800 x800-900mm deep with a 100mm flange on top about 200mm wide to give about 1m square on top that goes down 800 x 800-900mm. It is lined with building plastic and full of reo steel. It is flat and level on top and about 30mm higher than the paver floor around it and separate from that floor.

If you have a backhoe that is good but digging clay by hand means 900m is REALLY deep! It is probably something like a ton of concrete and steel there - it could be calculated and I'd be interested to know but it is something like that.

The 2nd pad is for a portable pier and is simply 2m x 2m by about 150mm thick with reo.

2. Floor is road base, paver base about 100mm then 40mm thick concrete pavers with pavlock sand in joints. The pavers end a few mm from the slabs.

3. Frame is treated pine 90 x 90 posts dug into the ground about 600mm and set with rapid set concrete bags. Just put the post in, pour in a bag of rapid set and then hose it to make it set.

4. Rest of the frame is 75 x 45 mostly fixed to the posts with 3mm thick 60 x 60mm L brackets from Bunnings and 50mm roofing screws.
The advantage of that is if something needs to be shifted later to make it more square then you can undo the screws and move it and rescrew it very easily. Once nailed it is now hard to change anything. I did have to reshift a few times so it was very very handy to use this and it is quick and easy to use. The brackets though are about $3.50 each.

5. Wall has a layer of Aircell Insulbreak (foam cored aircell bubble wrap type insulation). This is clean (no fibreglass) and effective and quite strong. Roof has it too. I don't want my gear to bake in 40C days.

I sprayed the inside of this and everything else flat black paint.

6. Roof is pitched up and over with gables at each end. Make up pine trusses out of 75 x 35mm treated pine (triangles). Fix them together with a ridge tree beam - 75 x 45mm treated pine.

7. This is all attached to a 100 x 50 x 3mm wall steel section. I drilled holes in the section and got some bolts that act as axles and 4 wheels per side. Wheels are 90mm diameter narrow wheels from Bunnings. Richmond Wheels and Castors though have an incredible range - Bunnings is limited and I went through several types of wheels.

I think garage roller door wheels would be better and their C section track wold be my choice next time. They have strong axles and have a screw on mounting plate which is strong and easy to fix. My wheels lean a bit because the bolt axle is too narrow (its hard to find a bolt that is the right diameter for the wheel and has no thread where the wheel rotates but can be bolted on. Hence the garage automatic door type wheel, axles, mounting plate and track would make it very simple and smooth. Also might make an auto garage door opener an option somehow for the roof (not sure - Brett would know how).

8. Walls and roof were .42mm thick colorbond Trimdek and screwed on.
I have barge cappings to the gables and one is set higher and is not fixed to the roof only the gable as a receiving flashing for the roof to roll under when closed.

9. I intend to install a louvre down low on the walls and install solar powered fans to get an airflow going.

10. My one has a small computer room with a sliding insect screen for summer bug control.

11. I have a steel pier made by Pegasus Piers in Qld and bought though Sirius Observatories. It is super well made and about 800mm tall. It bolts onto the pier with 6 anchor bolts and has slots not holes so I can rotate it a bit if needed to be in the polar alignment range of the mount.
The concrete pier is level and the metal pier has a levelling plate on top for fine levelling adjustment so I plan to simply bolt the metal pier (10inches in diameter and 8mm steel wall thickness and weighs 96kgs) to the concrete pier

12. I may put a whirlybird on the roof to assist in keeping temps down in summer.

That's about it.

A couple of design considerations from previous observatories.

I kept the pier height lower this time. My dark site pier is an 8 inch steel tube stuck in a hole I dug 800mm deep and set in rapid set concrete. It is about 1.2 metres above floor height. After the mount was installed it meant a 12.5 inch RCOS tube stuck out into the windstream about 300mm. That meant unless it was calm you couldn't image with it.

So wall height is 2.2m or so and pier is 800mm high.

I also set my roof trusses so with my new scope mounted it would not hit anything physically no matter roof closed or open. A lot of observatories the scope needs to be parked horizontally before you can close the roof and clearance above the guidescope can be minimal. Considering the cost of gear that opens the door to the possibility of the scope hitting the roof and getting damaged. I think that is important as I have had some mishaps despite being careful in the past.

You want to use materials that don't hold heat. You don't want lots of different temp air masses in your observatory - you want things still and thermally the same as much as possible.

You want the pier isolated so vibrations from walking don't transmit to the scope.

You may also want provision for personal comfort as I find I spend a lot of time in there at night and it can get cold in winter hence my little control room. I also plan to install a wireless hub but have yet to get that sort of thing to work (tried once already).

You want wall height just right so you get maximum imaging angles and views yet wind protection without the scope sticking out above the walls into the windstream. Your BRC250 is quite short and was very handy that way and not windaffected much. Neither are refractors however it is better they are sheltered from wind. So 2 opposing considerations there - wind protection and viewing angle.

However when you consider you really only want to image an object when it is fairly high up you can waste the first 30 degrees of angle as its useless for imaging anyway.

As far as pier design goes I have no data on concrete versus metal. I chose metal pier as it is portable, easier to install,
and I wanted a professional unit so my Paramount bolted straight onto it. I also wanted an adjustable levelling plate and I wanted it strong and sturdy. Diameter of the steel tube is more important than wall thickness. You go for the largest diameter.
I went for around 300mm diameter as that is quite large. My dark site is 200mm and it seemed to handle up to a 12.5 inch RCOS without being a significant factor in imaging (at least as far as I know - but perhaps there was slight deflection - how to do tell?). It seems to me though that once a GEM is setup and the scope on it is mounted everything is beautifully balanced so there is little reason for the mount to deflect as it is not receiving any sideways pressure except from some slight imbalance that can occur at different angles. Is this correct? I think autoguiding accuracy and acccurate polar alignment and an accurate mount are the key concerns there.

So wind, scope clearance, least light pollution, closeness to neighbours and potential noise (you don't want a clunker of a roll off roof closing at 2am right next to your neighbours window!), best viewing angles with best clear unobstructed view, widest possible pier, floor separate from the pier, deep and heavy concrete pier base to minimise movement and form a solid base, thermally designed so nothing is holding heat and loses heat build up fast, well ventilated to help in that regard plus ease of construction at low cost, operator comfort, cable control and adequate surge protected electricity outlets. I think that sums it up.

You also need to plan where you computers will be so they don't get in the road. My dark site observatory is 3.2 x 3.2 with 2.2 walls and I find a smallish table sometimes gets in the way of my TEC180 with all the cameras hanging off it and have to be a bit careful there. Its over a metre of swing room. Allow 1.2 metres swing room for most scopes. Compound scopes are more compact, refractors are longer. 1.2 metres should be enough for either. Go for a pitched roof not a flat roof as a flat roof won't give enough clearance for the scope without hitting it unless the scope is horizontal.

Greg

[renormalised]

Thanks Al....I was thinking about the robustness of a pipe and just wanted to know what the standard weight pipes were like. Sounds OK:)

[Jeffkop]

Some great advise there Greg ... especially good food for thought concerning the wall height making ssure this will protect the scope from wind. The Obs and warm room are going to be 2400mm square, with the scope room sitting about 4 or five steps up from the warm room. (Sloping ground) So the scope roof will slide over the warm roof.

Unfortunately, I cannot locate the Obs in the most protected spots because trees will severely limit my viewing, so it was to go down a hill a bit and will basically sit out on its own. So the wall height is even more important.

The footing size is interesting. 800x800x900 is around about 1/2 of what I considered doing. Of course I havent started digging yet ... 1500 might not be possible, however you definately have got me thinking about whether I'm going over kill on the depth. If that depth works for you with a PME and an Rcos .. (or have you got your new Planewave on the PME ??) then surely that would be OK for my BRC and Titan. All advise is deeper is better though. I really dont intend to be doing this again so I want to take all steps to get the desired outcome.

The structure is along similar lines as yours too ... pine frame, colourbond external, gable ends too. I want to be able to just close the roof no matter what position the scope is in. Dont want to be hustling around with Parking scope etc if it suddenly starts bucketing or something.
One difference is that Im having the pier come through the floor (yellow tongue)

Went and looked at my mates today. He has a 1250mm length of 220mm x 6mm steel pipe that I'm sure fits the bill nicely. Just means I will have to make sure that I continue the footing above ground level about 200mm Id reckon. I need about 1m from floor level.

I'm not concerned about automation of the roof yet, however if want to head that way later I will design the electronics to facilitate it.

Anyway, thanks for your contribution here Greg it has a wealth of information, when it all starts I will post regular updates in the DIY Obs forum.

I cant wait to get the BRC up and running.

Good to hear from you

[sheeny]

Yes. I concur.:thumbsup:

Some good practical tips there too thanks Greg.

As I've just been discussing with Marc via PM, the most obvious shortcoming a lot of people make with piers is to not grout the base plate. Without grout under the baseplate, it only contacts the high points of the concrete and is free to flex in between. It doesn't take much flexure at the baseplate to be amplified to an optical movement at the top.

Unfortunately I think there are a lot of really robust solid piers out there that probably only perform as well as a much smaller one because of lack of grouting... but unless your scope is exposed to the wind, there should be very little to cause a noticeable movement.;) Design by luck perhaps...

:)

Al.

[gregbradley]

That's a very good tip Al. I'll make sure I get some nonshrinking grout to
put my pier on. I did try to level the pier quite thoroughly but as you say there will still be slight high points in the concrete.

Greg.

[renormalised]

That's where a good laser level, or a bubble level, would come in handy. Then you could physically measure the concrete for leveling inconsistencies.

[CarlJoseph]

Sorry to dig up an older thread. I hope people don't mind. :question:

I'm about to embark on putting in my first pier and am looking to do something similar to what's described here. I'm no concreter/builder type person (wood is my construction material of choice) so excuse the basic questions I have ...



I assume in this scenario the levelling threads are embedded into the footing and that the footing is completely cured before you try to apply the grout and level the pier?

I also assume you don't add the nuts to the top of the base plate until everything has set, or do you?

Once levelled and the grout has cured, the levelling nuts are stuck in that position correct?




Similar to above, the main footing should be cured before adding the stiff mortar mix to the top?

What's the stop the weight of the pier squishing the mortar or does this plan also have the same "levelling threads/nuts" embedded into the main footing?


Any pictures would also really help me get this through my skull. ;)

Thanks,
Carl