Embedded stainless steel threaded rod or dyna-bolts?
I've dug a 600x600x700mm deep hole, right down into clay. I've put around 75mm of course gravel in the bottom which will be tamped down into the clay and I'll also be pouring in one or two bags of cement powder. I've done stuff like this before and know the gravel and cement will prevent the clayfrom drawing the moisture out of my block when I get to concreting.
I have a small section of weld mesh to put in also around mid point of the slab, the slab which will stop 150mm from the top and I'll be putting 3 pre-drilled 600mm x600mm pre-cast concrete blocks down on top after the concrete has cured. This is so if I get complaints from the department of housing I can remove the top 6 inches, grind the threaded rods off and fill with top soil should I ever be instructed to remove the pier (I'd fight that and win but I like to be prepared)
I've also driven 4 x 1000mm lengths of 19mm rebar down, one in each corner of the pit (diagonally from inside corners), radiating out at approximately 40-45 degrees on 2 corners and 60 degree on 2 corners because I know there's an electrical conduit running close though it is at regulation depth of 500mm and these rods are starting out deeper than that but to be safe I drove them in at a slightly steeper angle.
My intent is to use 16mm threaded stainless steel rods to secure the pier and I was going to fabricate a triangular frame out of 2" angle iron and set that in the hole with the rods coming up vertically for the pier to slip over but with different expansion rates of concrete and steel I don't want the slab to crack.
Plan B: I'm contemplating cutting a steel ring matching the piers mount size and hole locations and suspending it over the hole with the rods dropping down into the concrete with retaining nuts and large washers at the bottom to give it a little more surface area for the concrete to cover. To have room to get concrete in pit (hand mixed in wheelbarrow) without disturbing the position of the stainless threaded rod I'll weld up a tripod out of any scrap water pipe I have laying around and with the rods each being 1 metre long I should be able to hang them securely perpendicular and they should remain in the correct position while I pour the concrete (I can weld the supporting top plate to the tripod so nothing moves). I can put heavy scraps of 12-16mm steel at each base to give it weight, I have plenty laying around to use as scrap.
I see many people using dynabolts to do this, were I using solid concrete slab I may consider this but the 6 inches of pre-cast at top layer (3 x50mm x 600x600 slabs). I've used dynabolts and chemsets to secure car hoists (I've installed a lot of car hoists, I used to manage a company which specialised in motor garage equipment) and they are a pain when ANYTHING goes wrong, it's OK in a mechanical workshop when you can move the hoist the few regulation inches from the failed mounting and try again, not so easy on a small slab not that there's regulation on what I'm doing but not a lot of spare room either.
Should I dyna-bolt the 3 slabs together?
I embedded this thing in concrete 15 years ago. My polar alignment has not moved in all that time. I used gal threaded rod and the wooden template you can see in the image to hold it in place while the concrete set.
I'm guessing your concrete cube is larger than mine is going to be James, due to me not owning the house and making it easy to hide the fact it was ever there (I'd never attempt to dig the entire thing out). I have some weld mesh here, I like your idea, make a smaller cube from the weld mesh and a template.
Ah, easier to tack the galvanised rod to the rebar than the stainless steel bar but I'll just use tie wire or buy galvanised rod.
While I mentioned my pit was 600 deep it's actually closer to 750 to allow for gravel and concrete powder to try and prevent the clay drawing the moisture from my concrete too fast as it sets.
James I was just looking at your image, I wasn't going to do anything quite as elaborate as that, just some loose weld mesh separated by spacers for 2 levels. I've noticed you've welded the rebar, did you have any trouble doing the welding?
I guess it's not so much structural and won't collapse or separate once buried in concrete but I do know unless purchasing the rebar with a stamped W (for weld-able) it's not an easy thing to do.
I guess I could tack it together because structural rigidity isn't required, I'll run some tacks on some 19mm stuff I have laying in the back yard and see how it goes, it certainly doesn't have to look pretty and I won't bother with preheating which I'd have to do if it were structural.
Always a pain when I don't do near enough welding despite having qualifications in fabrication engineering and Arc, MIG and AC TIG welders in the shed. It's a health thing and when I do want/need to do nice welds it takes me a while to get the feel, I do a lot of grinding. I never used to have to grind any of my welds but I was doing it frequently.....
EDIT:
Is there anywhere I can go to calculate the best size of reinforcing bar/mesh to use for a 600 x 600 x 600 cube of concrete?
I want to do it once, properly (though I've had the hole dug for over 12 months), Rome wasn't built in a day.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,435
Leo,
I'm sure you know how high the tensile strength of steel is compared to most other materials. You haven't mentioned how big the load is but I assume it's a modest mount and OTA. Looks like you have 16mm rebar there.
The multi-layer inground pier is a great idea. i have thought about doing something like that even on a property I own so that when selling, I can pull out the top layer, throw in some soil and grass it over.
With the loads imposed by most modest amateur gear, 3/8" bolts are more than enough as is thin pathway grade mesh. Let's face it, you ain't mounting the Mt Palomar 200 inch.
Given that you want to make it removable and able to be hidden, less is more. Most people go way overboard with this stuff. The concrete does need to provide a secure anchor to the ground so that the moment of the mount and pier can't tilt the slab. But if your ground plate has say 8 anchor holes (3 per side) 1/4" anchors would be fine, if it has 4 holes in each corner, I reckon 3/8" would still be plenty. Some of the people offering advice, live in corrosive coastal climates. Lithgow is much drier and less corrosive on steel.
When I was 16 yo, I asked my ATM mentor if 1/4" thick steel plate was strong enough to hold the mount head to the top of the tripod. I'll always remember his reply. It went something like this, cleaned up for the forum, "expletive me Joe, expletive expletive expletive expletive expletive. You're not going to expletive pull through one expletive quarter expletive inch of expletive steel plate"
Thanks Joe!
It's a heavy pier made with half inch plate top and bottom and 1/4" wall thickness pipe (I got it used cheap) and only 3 x 20mm x 50 slots/ mount holes on the base which are slots not cut matching the radius but square across the plate meaning it's not easy to get right if I get it wrong. My plasma cutter won't do beyond 10mm steel plate and I don't think the Dremel would be good. I do have a pneumatic die grinder somewhere.
Three 16mm x 1 metre stainless mount rods have long been bought to do it.
As for equipment, 8" GSO f5 Newt, 8" RC carbon fibre, 152mm Skywatcher 1200mm focal length Achromat with 80mm Megrez used for guiding, not excessive weight even with my Nikon D810 and guide camera attached. Though should a 10" Newt ever come up cheap, who knows.
The multi level support isn't going to work the way I'd planned because Bunnings no longer have the 600 x 600 x 50 precast slabs. Some research showed they had no reinforcement and crack easy as I saw myself at the local Bunnings store with a smaller 500mm square plate.
I can either make a frame and use some weld mesh and make a couple of slabs myself or, pour the main slab with reinforcing weld mesh (I have some 6mm wire stuff here) and then do a second slab on top with smaller reinforcing frame and threaded rod coated/covered so it doesn't bond to the thread for the last 150mm ( possibly a plastic conduit sleeve sealed with silastic to the threaded rod) and I should be able to slide that slab off separately if required.
Of course unless I get very lucky with the lotto Gods I plan on dying here and removal won't be my problem.
I've noticed you've welded the rebar, did you have any trouble doing the welding?
My brother did the welding for when we built the obs. It kept the whole cage togther when we poured the concrete and worked well. I'm sure wire ties would be just a good.
Not sure how I would ever remove this thing. Maybe just grow some grass over it.
That all looks amazing Bart and thank you for the amazing process in images (well documented work is hard to find, I like photographing my projects as I go in case I can help anyone else) and while I have plenty of new square hollow section steel laying around I could use I was worried if it rusts and collapsed in the concrete everything just crumbles. Then again I'm days away from turning 65 I don't know how long I imagined I'd be using this stuff.
Your pier looks very similar to mine with the exception mine has a round 1/2" steel base plate but every bit as heavy and solid.
James I can weld the stuff it usually has a high carbon content and not pretty welds but buried in cement it doesn't matter and not being structural, strength isn't an issue. I'm going to pick up the thickest wire weld mesh I can get from the local landscaping supplies and knock something up if this rain ever disappears, we've had maybe not a lot of rain but some huge storms and constant drizzle making it hard to get out and do what I want at the moment.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,435
Quote:
Originally Posted by leon
Gee Joe, he did swear a lot, and to a youngster at the time.
Leon
Leon,
He was famous for it, but never in anger, just his way of expressing himself would make a wharfie blush. He always swore with a smile on his face. He had a heart of gold. Very generous with his time and knowledge. An outstanding optician and engineer and I learnt so much from him. We all used to laugh at his swearing. I still have that 6" reflector and on rare occasions when the seeing is up to it, it will do 500x with a crisp clear image.
I have plenty of new square hollow section steel laying around I could use I was worried if it rusts and collapsed in the concrete everything just crumbles. Then again I'm days away from turning 65 I don't know how long I imagined I'd be using this stuff.
Hi Leo, if you are 65 and are concerned about the internal frame rusting away, it will take a lot longer than you will be using your equipment before it falls into abysmal demise.
Those piers and bases have now been in for seven years, nothing has moved.
