Questions regarding concrete block pier

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Old 06-15-13, 07:48 AM
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Questions regarding concrete block pier

Hello. I am constructing a 2 story concrete block pier that will support a telescope in an observatory for astrophotography. The pier will be 32x32 on the first floor, made with 16x8x8 concrete blocks. I am planning on filling the holes with concrete and rebar and had originally planned on filling the middle of the pier with sand rock and other rubble. I smashed up a section of the concrete floor and then dug down to the frost line to pour a new pad for the pier to sit on so it's completely isolated from the floor so I have tons of filler around.

When I get to the second floor I planned on making the pier 16x12 the rest of the way up using 6" wide concrete blocks. Since they won't be interlocked like the rest I will be using reinforcing mesh between each layer. I'll also use it on the bottom part of the pier.

The is a massive project for me to under take. It's being done on the isolated northern ontario island st joesph. It is somewhat difficult to get someone to come out and give me advice. Add to the fact the building itself is already over budget (installed dome on the roof ect). My question is in regards to filling the pier vs leaving it hollow and just pouring a new slab on the top.

Is it really that much better to make it hollow? The actual pier will only be supporting a max of say 300lbs however it has to be 100% vibration free. Initially the load placed on top will only be about 100lbs.

If I do make it hollow how do I support the pour of the new slab? How thick does it have to be to support not only the telescope load but another 12 feet of concrete blocks? I believe filling it will be much easier as I already have the filler around the location and can simply build a few layers wait for the mortar to cure fill and then build some more.

A big reason I went with this type of design is it was suppose to be an easy enough job for one person to do. With the advice I am getting to keep it hollow I am somewhat concerned. I should also mention I have to do this the week after next. The building is on my parents property and I have already booked a week off work to do this project. I welcome any advice.

thanks

if you'd like to see the building you can visit wbobservatory.com
 
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Old 06-15-13, 10:18 AM
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Is the scope going to automated or will you be up there with it? If you will be up there with the scope when observing I would isolate the floor from the scope's pier as much as possible, especially if the first floor is hollow.

I'm trying to get my head around what you call a pier and what I think of as a scope pier.

I'm just throwing out ideas here but it would be great if you could get a proper Engineer to comment since you are into the realm of commercial/industrial construction.

If you fill in the base you have to properly compact the fill so it does not settle over time. Next is the big problem of the outward pressure on the walls. The walls must be more than normal walls to mostly carry a vertical load but must also resist the side load of the fill & compaction inside.

If you leave the first floor hollow you have a major structural issue. Corrugated decking is available in various sizes, profiles and thicknesses depending on the span and load which can handle much of the square footage. It's often done in commercial buildings and high rises so that is a well known area and you can probably find standard tables online. The massive load of the second story & dome is where it gets sticky. I can almost guarantee you will need beams if you want to span the full 32'. You could break the first floor up into cells/cubes with interior walls. This would allow you to use steel decking with 6 or 8' spans which is much more manageable and the cells would help provide lateral support to kill vibrations.
 
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Old 06-15-13, 10:55 AM
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32 inches not feet!

The pier will sit on it's own isolated slab. I have already cut 42x42 in the existing slab and dug down to about 48-52" by the end I was exhausted and the deepest parts were more then 48" so it was good enough for me. I've since poured about a 24" thick slab which is what the blocks will rest on. I wanted to make sure the slab was at least 18" thick but I just kept going as it was easier to level out the floor and it will also ensure I have enough blocks to make it to the top (already have the 2 skids of blocks sitting the property).

The main reason the second floor of the pier will be 16x12 is so that it will fit between the floor boards. Thus it's completely isolated from both the ground floor and the second floor. The problem I am having is how do I attach this 16 inch by 12" part to the 32 inch by 32 inch first floor part.

If I fill it with all the old broken up concrete rocks clay and rubble I can then pour another pad near the top say 10" thick. Even if the bottom does sink down a bit the pad will not go anywhere and it will still be 90-100% full under nether. If I leave it hollow I don't know what I do as far as a support for the pour. The other option would be of course to make the slab elsewhere and the mortar it to the pier but it's going to be very very hard to get it up.

The telescope will be used 100% of the time for imaging. Initially I will be going up there to set it up, but while the imaging is taking place I mostly will not be up there and in the future everything will be computer controlled, but like I said steps have already been taken to ensure it is completely isolated.
 
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Old 06-15-13, 02:47 PM
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So, when you say first or second floor you are not talking floors of a building but steps in your base?

Why are you using concrete blocks? Since you mention filling them in solid wouldn't it just be easier to form it up and pour the whole thing with concrete (unless you have to carry everything in and need to minimize concrete use)? You could put in some rebar to help tie everything together but you're mainly looking for mass and there is almost no load on the concrete except to support it's own weight.

Keying your second 16" x 12" layer to the first should not be much trouble. Best probably be to insert rebar into the previous pour and let it protrude up to tie into the second tier. You could also create trenches or divots in the base layer to key the second tier to the first. You would have a "cold" joint between the concrete pours but the keys and the mass of the concrete will keep them locked together even if shrinkage cracking occurs at the joint.
 
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Old 06-15-13, 03:44 PM
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Attaching the second layer to the top layer is what I am interested in learning. I will be using rebar and reinforcing mesh but I'm still not sure what I do to hold the second layer in place as far as pouring concrete and then building the blocks up in the middle of the larger layer.

I am using concrete blocks because they are supposed to be easy to work with. Because I want to do 2 different sizes I can't simply use one giant sonotube without seriously cutting the floor boards and then reinforcing the floor. Also pouring this massive amount of concrete inside would be very tricky. It's not like I can get a truck upstairs. In addition this is a small isolated island getting a truck to come may have been problematic. I bought a mixer which I used to pour the new slab and will again use to fill the block holes. There is also no chance of sonotube blowout using the blocks.



The second floor


The building itself



This is a major major project for me, but for now my pain concern is getting the pier done so I can at the very least start using a telescope up there some time in that not to distant future.
 
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Old 06-16-13, 10:54 AM
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Andrew;
If this is what you're on about, I would think you would be ok filling the core with rubble, if only to provide a temporary support for some concrete. The core will settle over time and, as the previous poster said, on a larger scale there would be the tendency for the core to put outward pressure on the walls. But if they are only 32", and there is some rebar/expanded metal strip, you would be OK if and when it settles a little.

Put some rebar in your poured concrete and build the blocks around this.
 
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