New obs build

[The_bluester]

So, I am finally making a start after years, a pretty basic start.

On the weekend I made a final decision as to where my proposed RORO shed will go and used that to pin down the spot for the first pier block to go in. Taking advice received here on IIS and elsewhere I am aiming at approx 1 cubic meter of concrete in the ground. Thankfully I have been able to give myself a head start as I have a post hole auger on a tractor so I bored a number of holes down to about 900mm (The limit of where the auger will plunge to) so I can then break out the soil and clay between them rather than having to manually do the whole lot.

It has led to a couple of questions in my mind. I have tossed the idea around for a long time and decided to set threaded rods in when casting the block (Three, not four unless someone can give me a good reason why not as three mounting points makes for far easier leveling by shims as required) What material are people using for the threaded rods, gal or stainless?

The second was how extensive a reo mesh cage are people putting in? I was thinking it does not need to be all that extensive as the purpose is really to make sure the block stays together in the ground, rather than trying to make it strong enough to hold up a bridge. I was thinking of a pretty basic light reo cube some 400mm or so smaller than the hole, with the threaded rods attached to the top of it, and the whole lot supported by a form that the threaded rods will be bolted to so that it is all suspended away from the bottom and walls of the hole during the pour.

If all this works out then with the currently planned shed size I might end up with anything up to 5 blocks, 3 for piers and two that will be double poured (Leaving the ability to remove the top section) and brought up to finished floor level to have somewhere stable to plonk down dobs or visiting tripods.

Based on previous advice a wooden floor will be built over the blocks and cut in around the piers with a space for some foam rubber seal to avoid the thermal mass of a concrete floor.

[The_bluester]

Day 1 progress. Hopefully the bit of rain we have had will soften things up a little without leading to problems later. I would want a nice dry hole before the pour to make sure I was not going to see any shrinkage in the ground later.

Do people enlarge the hole at the bottom to create a locking profile where the hole tapers down in size to the top?

[The_bluester]

So, posting in the hope of seeing what others have done in terms of reo and mounting bolts.


Unless someone says something otherwise I plan to build a rough cube of light reo mesh about 700 mm on a side. I reckon 12mm bolts should be plenty to attach the steel pier and I am going to weld them to a plate which in turn will be welded into the top of the reo cage. Then to pour the block I will make a timer frame that will screw to the boxing up for the pour and will support the reo cage via the mount bolts so it won't come out of the concrete anywhere except for the pier mount bolts. The hole has ended up just under a cubic meter, I ran into a rock in one corner so I am stopping there!

I am tossing up if I should use four bolts or three, three should provide plenty of strength and makes for far easier leveling via shims than four bolts do.

[pfitzgerald]

Hi Paul

Sounds good so far. My ROR Obsvy. is 'still' in the planning stage. From the research I've done over the last several years it sounds like you're on the right track. Your idea for the cage inside the concrete plinth is sound based on what I've read. The advice I got from Steve (he of the SVAC spit!) was to go for three bolts for mounting the pier - for the same reasons you give. I actually have six holes drilled into the base of my pier - if you like I can post a picture. The holes are 25 mm in diameter. The top of the pier has no rat cage - instead it has three steel blocks that have been cut and drilled to take the top of a HEQ5 Pro tripod head.

Hope that this all helps - but keep in mind that although I have the pier made (and it's sitting in the garage), it doe not yet have a proper home!

Paul

[The_bluester]

I made up a pier with materials gifted to me by Andrew C so that is ready to go. No rat cage here either. I michined (Which huge effort as it is a bit big for my lathe) a round base big enough to put an EQ8 plate on and bought an EQ6 pier adapter that I will bolt down once it is all in place and I can do a solar noon to align the plate pretty accurately. I welded the round plate on top and cut an access hand hole in the side of the pier to get at the central bolt for the EQ6 mount then fitted a wooden panel inside the pier so I can't drop the bolt to the bottom.

I was originally going to use four bolts to mount the pier to the block but was thinking better of it for leveling reasons, four mount bolts is a pain and three is easy! There is space on the base plate to make it three and for subsequent piers (Andy is going to want one too) I will get a triangular base plate cut.

I can see the point of up to five pier blocks and in a 6M X 6M roll off section (With a 3M X 6M warm/store room) I reckon I can make space for that many, not that I would expect to ever see that many piers in place at once, but three would be easy to see with a concrete base near the warm room to plonk a dob on a movement free surface.

[Sunfish]

The function of a concrete pier is to get a footing down below the influence of weather on the ground . So deeper, within reason is always better.

Cantilevering out of the ground to resist lateral live loads like a telescope mount , again , deeper offers a larger resisting arm. Deeper is still better given the same weight on flat ground.

Creating a bell on the bottom would only be useful in soft ground to spread large vertical loads as a little area makes a big difference.

Having said that the cantilever arm can also be horizontal , so widening the top of the pier with a small slab level with the ground is an easy way of adding more overturning resistance and weight in solid ground with less digging. It would also make use of your reinforcing which other wise is simply stopping the pier cracking around your bolt plates.

[Sunfish]

Ps ,
Check Marcus pier here:
https://pbase.com/gailmarc/image/169751663

http://www.iceinspace.com.au/forum/showthread.php?t=178080&page=6

The widened top with a 45 degree edge is very good I think.

[The_bluester]

I probably can’t make a batter and flat top work for my planned obs build. I intend to put in a raised timber floor so the block needs to allow for footings to support the joists. By the time I add another 400mm or so to the top of the block I might be getting a bit wide?

I might just go a little deeper. Some movement over time might be inevitable given about 700 of the 900mm or so is in clay so probably reactive soil. Although once an obs is built over it there should not be much variation in moisture to contend with.

[muletopia]

Paul, I think you are optimistic about the ob increasing clay stability. I think lateral water flow is quite substantial. My situation is on deep clay.


My pier sits on somewhat more than 1 cubic metre of concrete and the ob on a 3 metre by 3 meter by 100mm slab with plenty of reo. The pier has a 25 mm gap between it and the slab.
Four observations about this set up.
1) When it used to rain here moisture came up the gap and flowed out drainage slots cast in the floor, not much but just detectable.
.
2) due to the above I had to place interlocking plastic grating beneath the foam tiles on the floor


3) The slab and the pier move differently


4) the pier moves by a couple of seconds over a couple of months and I use Sharp Cap to re-align the mount. After a few years the floor had developed a noticeable

slope.



Chris

[The_bluester]

I am planning on adding some drainage around the obs to help combat that sort of issue. More or less a 500 or so deep aggregate and slotted pipe drain around three sides out from the walls to intercept water coming from the high side and keep it away as best I can. A couple of meters on the low side is a dam so the water will end up in there.

Beyond that I will just check PA periodically and adjust as required and shim the pier on the block if needed. I am planning a suspended wooden floor for the obs, so no slab as such to worry about differential movement.

[Sunfish]

Chris has demonstrated the reason for piers in clay. His slab moves around as it is not founded on deep piers but in the top layer. Not a good base for any building you would like to stay level.

Irrespective of the size and weight of the pier, one pier by itself will heave and move around with the soil over time. You can only found it deeper or add a number of piers anchored into rock, but as you say , it is not a bridge.

The idea of drainage well away from the pier may help isolate the top layer of wet soil outside from the inside dry material as long as it is sealed and does not work in reverse and let water deeper into the top layer. 500 sounds pretty deep. For that amount of digging you may as well replace the soil under your observatory with a 200 bed of hardcore which is unaffected by water or fill the slot with concrete and put your observatory on it.

[The_bluester]

500 deep for a drain is just a guess, it might end up shallower but I don't plan to be digging that by hand that is for sure. So long as I get the fall right so that any water entering drains away it should help at least to some degree. As much as anything else I would be aiming to help keep the under floor dry, along with ventilation.

Hopefully the idea pilfered from someone else to drive star pickets well down (Think in terms of a meter driven in) inset from each corner by maybe 250mm and use them to hang the reo off will help to some degree by keying the block into the subsurface, because the only way to avoid clay around here is to hit rock! I have plenty of both. Even driving the pickets may be pot luck between driving in easy and hitting rock after six inches.

Either way, I can only do what I can do and deal with any movement later by monitoring PA and adjusting as required.

[The_bluester]

Turns out the rock I found in one corner extends under almost all of the current hole, so I will get a rotary hammer and bore half a dozen holes in it to pin the block to it with reo. There are plans for more pier blocks, hopefully the rock does not interfere with those! Or maybe it would be good if there was a house sized monster under there to pin them all to.


For the sake of simplicity, unless someone convinces me otherwise I am going to build a reo cage, supported from the top The support I will remove just before it is covered, after there is no chance of the reo moving down and coming out the bottom of the block) and I am going to drill and grout unithread in with concrete epocy for the mounting bolts to save the inevitable chance of the bolts moving around during the pour.

[Sunfish]

A giant floater increases the depth and weight of your footing. As you only have one pier it is not a bad thing. Chemset anchors are great. Good enough for that bridge if the holes are blown clean. You could chemset a couple of short deformed bars into your floater.

[The_bluester]

I had a couple of lengths of the right diameter galvanised unithread on hand so I used them and will tie off my reo cage to those. I bought a nylon brush and dragged my compressor out to clean the holes properly too. I have no idea if it is a little floater or something the size of the house under there. It extends more or less under the entire hole and I did not push through the bottom of it anywhere when going the full depth of a 250mm masonry bit.

Only question left in my mind is what mpa concrete to order up. I am really hoping to get it poured on Saturday so I can leave it sit for a few weeks to cure well before I go boring holes in it and loading it up. Apart from anything else I don't like leaving holes open even though I am covering it when I am not actively working in it.

[Sunfish]

I always order the highest standard strength as I think you may as well go for a little strength. 20mpa or 25mpa standard and is ok. As long as the concrete does not have too much water added and is properly compacted with a bar when placed. You can keep it covered and wet.

[The_bluester]

Yeah, I will work it into the corners with a shovel to make sure it gets out there and also has a decent chance of expelling the air, I just need to buy or find a bit of steel to use as a screed for the final surface. I am bringing it up about 100mm above ground level to give me a bit of freeboard to level and gravel the ground around it.

The plan is still to have multiple piers over time so I will use a string line from the top of this one to make sure they all end up at the same height.

[Sunfish]

Great. Sounds like you will have yourself a solid pier. I only mean that with multiple piers for the same beam , a floater can get in the way and lead to unequal bearing. With a single pier for telescope mount it will be a bonus in any case.

[The_bluester]

I do plan for anything up to five pier blocks (The rest I will hire an excavator to dig, but we lost my young blokes terrier recently to a snake bite and I needed something suitably menial to do to clear my head) but I don't plan for them to be connected to each other, hopefully this rock is as it looks, only under that first hole so I don't end up with any issues by tying half of them to what turns out to be the same floater.

[The_bluester]

No pic update. Concrete pour is booked for Saturday. It will be really nice to have a pier after many years.

[appiice]

Hello Paul

Good luck with your build, some comments if not to late,

3 or 4 bolts, 4 is no harder to plumb the pier with than three, just think of adjusting the bolts on a diagonal pattern. Bottom right, top left as one adjustment pair, bottom left top right as the other pair.

Mpa, not really an issue, you are going to mount, what, a mount and scope weighing, say 100Kg's and your mass base will be about 2.5 tons, the bolts won't pull out of poorly batched 15Mpa concrete. if you are worried 35Mpa in theory is water proof.

I am a licensed builder and built a dome and Obs 2 years ago, your mass base will be fine, don't over think it.

[The_bluester]

Actually, the block was poured on Saturday, I went with 25Mpa in the end and a light reo box to help avoid cracking. I have decided to use four bolts as the pier base I have is square and that allows me to spread the bolts as far apart as possible. This will be an AP pier so precise level won't matter that much.

I am waiting another two weeks for close to final cure then drilling holes to chemset the bolts in. I could probably do them this weekend given it is not really stressed much, the nuts more or less only need to be snugged up rather than torqued down to hold the Sydney Harbour bridge, but the last thing I want to do after digging that hole by hand is pull chunks out of it by being impatient!

[muletopia]

Paul,
Somewhat late but I just found an old print kicking around in our implement shed so the pic is a scan complete with grot. It shows that all is not hard labour digging the hole for the concrete.


Chris

[The_bluester]

Ha, I never had a cup of tea down there!

The concrete is curing away, I am going to drill and chemistry set some bolts and mount the pier next weekend. It will be a week or so away from final strength, but it is not like this has to hold up a bridge or anything.

Then Andy01 will be keen for me to start digging his!

[The_bluester]

So. First light for the pier was on the weekend. Setup is so much easier than before without needing to mess around with getting a tripod aligned close enough to the pole.

[Outcast]

Nice one...

When are you putting the 'building' around it...?

You know you want to... :)

[The_bluester]

In Current circumstances, who knows. But if we see a lockdown coming a quick trip for some RHS, some quickset for footings and Tracks for the roof and I am set. I have everything else including plenty of corrugated iron to clad it as a temporary building.

[Sunfish]

Excellent work of concrete and steel. A pier is a great thing.

[The_bluester]

It is funny, I never really though that the concrete would feel different to a regular slab (Due to the depth and mass) but it does. If you jump on it it actually feels more solid and "dead"

That will be an experiment for the next night I suppose, get it set up and the guider running then jump up and down on the concrete to see if it shows up in the guiding.

[Sunfish]

I have administered construction of 3m concrete cantilevers and one surely does feel vibration from a heel drop. Bridges can be the same with traffic. You could dance around on your pier block and not notice a thing in the imaging.

[The_bluester]

People don't realise how flexible concrete is. There is a substantial bridge I cross regfularly and when you get stopped on it you soon realise that it is gently bouncing up and down. By the feel of the way it moves I reckon has a gentle sine wave of movement bouncing from end to end.

[muletopia]

Not to forget the seasonal thermal expansion of concrete arch dams. The centre may move up and down stream a metre or so during the course of a year.

[The_bluester]

That much?

How does concrete compare with steel regards thermal expansion? I assume it would have to be similar or the reo inside would lead to some crazy stresses.

[muletopia]

Sorry Paul,
A bad head moment here, :screwy: the documented thermal effects on concrete dams are cracks in the down stream face. When I can contact my dam engineering friends I will find the actual excursions.
It is large clay core earth filled arch dams the have large excursions, as surveyed here in WA.

[The_bluester]

Still, regardless of what is inside the shell, that is an amazing amount of movement. I suppose though that the actual expansion ratio though is probably expressed as a fraction of a percent?


All the things that go into engineering things. Like people not liking the old noise on trains as they go over the gaps in the rais for expansion (ANd them not being suited to high speed use) so now they weld them, and in the summer the risk is of railse expanding and going too far out of alignment (I have to use the term buckling)


Or just getting the cant of a bend wrong so that trains do not go through it at the natural speed of the bend and they either ride up or fall down the cant as they go around and wear out the wheel flanges on one side or the other. Which happened in Melbourne where both high speed commuter and slow freight trains use the same line, if you recall the wheel wear issue on the Ballarat trains.

[The_bluester]

Well, zero progress toweards putting a shed over this, but I have found that even though I need to break everything down to carry it out 25M away where I was cheating before and simply brute force hauling the whole kit (Scope, cam, mount, tripod etc) out in front of my garage, it is taking less time to set up as the polar alignment is so much quicker. Octans is ALWAYS visible via the Polemaster software as soon as it is dark enough instead of trying to get the tripod aligned the same and spending time hunting around for Octans.

[Sunfish]

A pier is great thing. I have two of differing heights now made out of leftover 6mm 102 pipe on my slab looking North and I am thinking a third in the garden looking South could be good.

[The_bluester]

While it will be really nice to have some sort of shed over it, but I am loving having a pier there.