Pier dimensions?

[MartinFransson]

Hi guys!

I wanted some input before I make my plans come true and make a pier in my garden I´ve been browsing through a few DIY pier descriptions but wanted some extra input before I press GO.

What do you think of the following:

- Concrete base, approximately 80x80x50 cm, lower part 1 meter below ground surface. Will use rebar, both net and rods. 3-4 rods will be achored in the base, sticking up into the pier

- Concrete pier, 19 cm diameter

- 4 M16 threaded rods, lowered about 40 cm into the concrete pier

- 1 or 2 steel top plates, 20 cm diameter, 10 mm thickness. The lower one will be almost flat against the concrete, to stiffen up the rods, the top one will be about 5-6 cm above to allow for a M10 bolt to be inserted into the mount from below

- Will use an aluminum mount head adapter later on, starting with a dummy one made out of MDF or similar

 

How close to the pier edge can the threaded rods be located without the concrete being too thin? I´m thinking I don´t want them too close to the center.

Would 3 rods be better?

I was thinking of placing the rods to the SW, NW, SE and NE so they are not in the way of the polar scope, adjustment knobs etc. Good?

[pete_l]

I think you would want to make the concrete base and the concrete pillar for the pier as a single operation, so that the base concrete does not set before the pillar is poured.

Also, remember (given your location) that it is unwise to pour concrete when the temperature is below 2°C.

I would have the threaded rods about two-thirds out from the centre of the pillar.

[MartinFransson]

13 minutes ago, pete_l said:

I think you would want to make the concrete base and the concrete pillar for the pier as a single operation, so that the base concrete does not set before the pillar is poured.

Also, remember (given your location) that it is unwise to pour concrete when the temperature is below 2°C.

I would have the threaded rods about two-thirds out from the centre of the pillar.

Thanks! The two thirds thing, how fixed is that? Is there a minimum thickness? I would like to be as close to the edge as possible.

[pete_l]

The two-thirds thing is merely a guideline.

If you want the rods closer to the edge, you can make a more crack-resistant concrete by adding fibres to the concrete mix.

https://www.travisperkins.co.uk/TDP-20mm-Fibres-For-Screed-and-Concrete-900g/p/495654

[MartinFransson]

OK, that sounds like a good idea, with the fibre mix.

Let´s say I do that, how close to the edge would you say is the limit? Will 1 cm of concrete between the rod and the edge do?

[Adam J]

2 hours ago, pete_l said:

I think you would want to make the concrete base and the concrete pillar for the pier as a single operation, so that the base concrete does not set before the pillar is poured.

Also, remember (given your location) that it is unwise to pour concrete when the temperature is below 2°C.

I would have the threaded rods about two-thirds out from the centre of the pillar.

I decided not to do this as you would need to contain the whole cast with a form including the top of the base  or the concrete in the pillar would just sink into the wet concrete of the base and raise the base level. Its a fluid when mixed after all. Also by the time you have reinforcement bars (i used 3 x 1cm) linking the two pours and you add exterior PVA to the pillars concrete mix to aid bonding between the pillar and base and you score the base to allow better keying.....please take my word for it...its not moving unless you hit it with a car and I would not want to be in that car. 

By doing the base and the pillar together you are also looking to kill yourself unless you are going to have the concrete delivered. At that point you had better hope that the form is sturdy as its would be a disaster if not. 

The size of the base is mainly down to your environment and soil type. But mine was 70 x 70 x 60 as I hit gravel and the water course 60 cm down, I made  a 25cm pillar and I can kick it without seeing any wobble through the scope. 

39 minutes ago, MartinFransson said:

OK, that sounds like a good idea, with the fibre mix.

Let´s say I do that, how close to the edge would you say is the limit? Will 1 cm of concrete between the rod and the edge do?

As for adding fibre to the mix what are you planning to mount on this thing, a good quality mix should not crack unless its very hot and prone to drying too fast? 

In the end if you are planning on using a HEQ5 pro on this peir then you are going full overkill in my opinion. From the whole thing the main source of vibration will be the exposed section of the M16 threaded rods not the concrete. 

Now if you are intending to mount a 16inch scope on this pier disregard what I just said and make everything much bigger. Over engineering is all well and good, you cant make something too well, but it might hurt your wallet and your back in the process. 

 

 

[MartinFransson]

6 minutes ago, Adam J said:

I decided not to do this as you would need to contain the whole cast with a form including the top of the base  or the concrete in the pillar would just sink into the wet concrete of the base and raise the base level. Its a fluid when mixed after all. Also by the time you have reinforcement bars (i used 3 x 1cm) linking the two pours and you add exterior PVA to the pillars concrete mix to aid bonding between the pillar and base and you score the base to allow better keying.....please take my word for it...its not moving unless you hit it with a car and I would not want to be in that car. 

By doing the base and the pillar together you are also looking to kill yourself unless you are going to have the concrete delivered. At that point you had better hope that the form is sturdy as its would be a disaster if not. 

The size of the base is mainly down to your environment and soil type. But mine was 70 x 70 x 60 as I hit gravel and the water course 60 cm down, I made  a 25cm pillar and I can kick it without seeing any wobble through the scope. 

As for adding fibre to the mix what are you planning to mount on this thing, a good quality mix should not crack unless its very hot and prone to drying too fast? 

In the end if you are planning on using a HEQ5 pro on this peir then you are going full overkill in my opinion. From the whole thing the main source of vibration will be the exposed section of the M16 threaded rods not the concrete. 

Now if you are intending to mount a 16inch scope on this pier disregard what I just said and make everything much bigger. Over engineering is all well and good, you cant make something too well, but it might hurt your wallet and your back in the process. 

 

 

Thanks for your input! I thought it would be overkill too

I will be using the HEQ5, possibly a somewhat heavier mount in the future but probably not. Focal length - not above 1000mm.

I will not be doing the base and pillar as one. If I let rebar from the base go up through the pier it will be strong enough. It worked for my front stairs to the house so it should work here.

[Astrokev]

8 hours ago, MartinFransson said:

How close to the pier edge can the threaded rods be located without the concrete being too thin? I´m thinking I don´t want them too close to the center.

 

Are you planning on having a permanent external tube for your pier, or removing the form after drying, leaving bare concrete?  The former may allow you to have the rods closer to the edge, as there'll be additional support holding the concrete together with less risk of cracking.  This is what I'm planning for my pier - a 28cm air-con duct. 

https://stargazerslounge.com/topic/295916-astrokevs-ror-the-build/

 

[MartinFransson]

9 hours ago, Astrokev said:

Are you planning on having a permanent external tube for your pier, or removing the form after drying, leaving bare concrete?  The former may allow you to have the rods closer to the edge, as there'll be additional support holding the concrete together with less risk of cracking.  This is what I'm planning for my pier - a 28cm air-con duct. 

https://stargazerslounge.com/topic/295916-astrokevs-ror-the-build/

 

No tube. It will be removed, so only concrete. 

[Adam J]

10 hours ago, Astrokev said:

Are you planning on having a permanent external tube for your pier, or removing the form after drying, leaving bare concrete?  The former may allow you to have the rods closer to the edge, as there'll be additional support holding the concrete together with less risk of cracking.  This is what I'm planning for my pier - a 28cm air-con duct. 

https://stargazerslounge.com/topic/295916-astrokevs-ror-the-build/

 

Something to remember is that concrete shrinks by a percentage. As such the bigger the volume the larger the reduction in size in terms of length / width. With a very large pier like your 28cm air duct you may find that it separates itself from the form as the diameter of the concrete decreases during drying. I made a square pier of 25cm x 25xm x 90cm from ply wood and exactly this happened. After two weeks of setting the form was no longer in contact with the concrete by a fraction of a millimetre as it had shrunk. Hence you will probably end up removing the form once it has set as it will no longer be doing anything to enhance the strength of the structure, not that you have to remove it mind. 

[ollypenrice]

For the first time I recently used concrete for a pier. I bought four pre-cast concrete cylinders of 30cm diameter which I epoxy bonded onto an existing concrete floor with rebar also bonded into it and sticking up into the stack of cylinders. This was a breeze because, unlike temporary shuttering, the cylinders just stood there obligingly while I filled them with concrete and rubble. The cylinders give a smooth and stable outer finish and mean that the threaded bars can be close to the edge without any danger of the concrete flaking when drilled.

I don't think you need two metal top plates. I was careful when drilling for the threaded bar to get the holes in exacly the right place in the concrete and then I assembled the threaded bar to the top plate and tapped them all in together with a mallet, tapping the plate level at the end so there was little need to adjust for level anyway. (This was for an alt az mount so levelling was important. It isn't important on an EQ though.)

The result is very stiff and is comfortably over-spec for the 14 inch to ride on.

Olly

 

[spillage]

I have just completed a concrete pier. 

I decided not to use a rat cage to reduce vibrations.

A plastic pipe that was reinforced with ribs and is black so look okay left as it is.

My base was only about 300mm deep but what I did was to drill four 200mm diameter holes in each corner and these went 800mm down plus the central one to hold the pipe. Effectively casting a concrete table into my garden. 

I used brake discs that I will bolt together. I bolted a block of wood to the bottom disc that stuck out past the pipe and bolted threaded rod lengths to where the wheel nuts would have gone. Then I cut out a slot from the pipe. When filled with concrete I sat the bottom disc into the pipe. When it had dried enough I undid the wooden block and pulled it out.

This then gives you access to bolt down your mount.

My only mistake was due to being rushed I was 20mm out for polar and the bottom disc was not central in the pipe. But it seems to still work okay.

[MartinFransson]

6 hours ago, spillage said:

I have just completed a concrete pier. 

I decided not to use a rat cage to reduce vibrations.

A plastic pipe that was reinforced with ribs and is black so look okay left as it is.

My base was only about 300mm deep but what I did was to drill four 200mm diameter holes in each corner and these went 800mm down plus the central one to hold the pipe. Effectively casting a concrete table into my garden. 

I used brake discs that I will bolt together. I bolted a block of wood to the bottom disc that stuck out past the pipe and bolted threaded rod lengths to where the wheel nuts would have gone. Then I cut out a slot from the pipe. When filled with concrete I sat the bottom disc into the pipe. When it had dried enough I undid the wooden block and pulled it out.

This then gives you access to bolt down your mount.

My only mistake was due to being rushed I was 20mm out for polar and the bottom disc was not central in the pipe. But it seems to still work okay.

You wouldn't by any chance have photos? I have a hard time visualising the construction (except for the base, that was a smart solution with drilling the holes). 

[spillage]

@MartinFransson yep sure. I have just been out and lifted up the cover so not the best photos in the world. I sprayed the wood with lithium grease and covered it in plastic to make sure it would easily come out and also counter sunk the bottom nut so that it would not stick out and also not spin around when it was undone.

I used araldite and stuck two flat metal bars on the underside of the bottom disc and although you cannot see there is a large square washer for fixing down the mount.

Just noticed that the photo of the wooden block near the pillar is actually upside down and the extra wooden parts fitted against the hub part of the disc.

If you really wanted to you could undo the bolts used to fix the two discs together and put in meta strips between the two to level up but as long as its near enough it will be fine.

Let me know if you want any more info.

 

[MartinFransson]

Thanks!

I think I will go with a simpler solution than I first thought. As @ollypenrice suggested, I will lose the bottom plate and only use one plate, about 5-6 cm above the concrete. It will be held in place by 4 M16 threaded rods.

Here is a scetch from what it will look like from above. The concrete pier is 190 mm and the M16 rods will be about 20 mm from the edge and they will be bent, curving inward further down the pier. I can´t get the rods closer to the center or the bolts will reach the mount adapter plate. The base of the HEQ5 is 100 mm.

[Astrokev]

12 hours ago, Adam J said:

Something to remember is that concrete shrinks by a percentage. As such the bigger the volume the larger the reduction in size in terms of length / width. With a very large pier like your 28cm air duct you may find that it separates itself from the form as the diameter of the concrete decreases during drying. I made a square pier of 25cm x 25xm x 90cm from ply wood and exactly this happened. After two weeks of setting the form was no longer in contact with the concrete by a fraction of a millimetre as it had shrunk. Hence you will probably end up removing the form once it has set as it will no longer be doing anything to enhance the strength of the structure, not that you have to remove it mind. 

That's a good point, which I hadn't really considered. If the concrete does shrink away from the tube a little, I don't think this will be a major problem with. I intend to leave the tube in place - if it doesn't add much structural support then at least will look quite attractive!

Kev

[MartinFransson]

For the base, I was thinking of something like this. In all 4 corners, I will place concrete plinths (prefabricated, with fixed post fittings). The post fittings will be joined with the rebar net in of the base. The rebar net will be joined with rebar sticking up into the pier.

See my scetch, looks like it will be solid enough? There will actually be more concrete than in my scetch as it will pour down into the holes I dig for the plinths.

[Astrokev]

Hi

If you are digging holes for the plinths, why don't you just fill these with concrete? I'm struggling a bit to see the advantage of using prefabricated plinths.

[MartinFransson]

7 hours ago, Astrokev said:

Hi

If you are digging holes for the plinths, why don't you just fill these with concrete? I'm struggling a bit to see the advantage of using prefabricated plinths.

It's simply an easy way of reducing the amount of concrete to mix. It will be hard enough anyway and the cost is comparable to the amount of concrete needed to make the plinths. 

[Astrokev]

Ok thanks. That seems fair enough !

[tooth_dr]

On 20/07/2017 at 10:25, ollypenrice said:

For the first time I recently used concrete for a pier. I bought four pre-cast concrete cylinders of 30cm diameter which I epoxy bonded onto an existing concrete floor with rebar also bonded into it and sticking up into the stack of cylinders. This was a breeze because, unlike temporary shuttering, the cylinders just stood there obligingly while I filled them with concrete and rubble. The cylinders give a smooth and stable outer finish and mean that the threaded bars can be close to the edge without any danger of the concrete flaking when drilled.

I don't think you need two metal top plates. I was careful when drilling for the threaded bar to get the holes in exacly the right place in the concrete and then I assembled the threaded bar to the top plate and tapped them all in together with a mallet, tapping the plate level at the end so there was little need to adjust for level anyway. (This was for an alt az mount so levelling was important. It isn't important on an EQ though.)

The result is very stiff and is comfortably over-spec for the 14 inch to ride on.

Olly

 

Hi Olly

Sorry if I misread, are you saying you drilled out the concrete for the threaded bar after it was set?

[ollypenrice]

14 minutes ago, tooth_dr said:

Hi Olly

Sorry if I misread, are you saying you drilled out the concrete for the threaded bar after it was set?

Yes, that's right. I had wondered about trying to embed the threaded bar prior to pouring the concrete but it struck me as adding a lot of complexity. Besides, tapping the threaded bar in with the addition of epoxy resin makes for a better bond, I think. There will be no shrinkage of the concrete and the threaded bar will be protected against corrosion by the epoxy. I'm confident that the result is an order of magnitude stronger than it needs to be.

BTW, if you go for four threaded bars the trick when levelling the top plate is to work in diagonals. (You probably realize this already but I mention in just in case.) So get one diagonal level with the other loose then do the other diagonal. We have four pier tops like this in the robotic shed and this methods works well.

Olly

[tooth_dr]

30 minutes ago, ollypenrice said:

Yes, that's right. I had wondered about trying to embed the threaded bar prior to pouring the concrete but it struck me as adding a lot of complexity. Besides, tapping the threaded bar in with the addition of epoxy resin makes for a better bond, I think. There will be no shrinkage of the concrete and the threaded bar will be protected against corrosion by the epoxy. I'm confident that the result is an order of magnitude stronger than it needs to be.

BTW, if you go for four threaded bars the trick when levelling the top plate is to work in diagonals. (You probably realize this already but I mention in just in case.) So get one diagonal level with the other loose then do the other diagonal. We have four pier tops like this in the robotic shed and this methods works well.

Olly

Thanks for that. I've made one with three and one with four, no bother with either. I welded bits of metal to ends of the threaded bar and bent it so it can't rotate in the concrete. Good to know either way works. 

[ACross]

I used a 300mm dia air-con duct with a 900mm deep base, approx 400mm diameter. It is rock solid and with no internal shrinking. I also fitted 40mm waste pipes into the duct, before pouring, to use as a cable conduit. 

Agree with pouring for the surrounding feet, hire a mixer for an easy time of it.

 

[Adam J]

On 21/07/2017 at 22:45, MartinFransson said:

For the base, I was thinking of something like this. In all 4 corners, I will place concrete plinths (prefabricated, with fixed post fittings). The post fittings will be joined with the rebar net in of the base. The rebar net will be joined with rebar sticking up into the pier.

See my scetch, looks like it will be solid enough? There will actually be more concrete than in my scetch as it will pour down into the holes I dig for the plinths.

Huuum have you actually dug a test hole yet to see what the ground is like? You could end up digging 40cm down and finding gravel and that would mean that additional depth is unlikely to make your pier more stable. What you have in your diagram seems like over kill. Also dont disturb any soil that you are not intending to remove and fill with concrete as it will just make the ground unstable.