Pier Onto Existing Concrete Slab?

[kbrown]

Hi, 

We've recently moved into a house with a back garden where I'd eventually like to build a RoR obsy at some point. There is an existing concrete slab at the location where I'm thinking to put it in. It's what's left of a garage that used to be there ages ago. 

I know it's not and ideal foundation as the concrete absorbs heat during the day and releases it at night causing heat turbulence. I'm thinking of decking the slab with deck risers to alleviate this but the bigger question is how to erect a pier on the slab? Has anyone done anything like this? Would be shame not to utilise the existing foundation for the building itself. 

Been thinking maybe bolting down a large brake disc on it with some steel rods poking up from it and then cast a concrete pier over and on top of it. Does this sound feasible? 

Not sure if I want to use a commercial steel pier bolted onto it as they all seem quite pricey for what they are. 

[symmetal]

You could drill holes in the existing base and fix the steel rods in the holes with 2-part concrete anchoring adhesive. You could then spread concrete bonding agent on the existing concrete before pouring the new concrete pillar on top.

Alan

[Astrokev]

One thing to consider is - if you use the slab as foundations for the future shed, and have your pier bolted to the same slab, there's a risk of vibration being transmitted to the pier when you walk around the observatory.  Ideally (and it may not be feasible) it's considered good practice by many to try and isolate the pier from the obsy foundations. At the very least, just be aware of the risk so you can make an informed decision on the way forward that works best for you.

[Swoop1]

It may be worth trying a bit of drilling to establish how hefty the slab is. You could also later consider cutting out a squarer from the slab and excavating a pier foundation hole.

[Hawksmoor]

11 hours ago, kbrown said:

Hi, 

We've recently moved into a house with a back garden where I'd eventually like to build a RoR obsy at some point. There is an existing concrete slab at the location where I'm thinking to put it in. It's what's left of a garage that used to be there ages ago. 

I know it's not and ideal foundation as the concrete absorbs heat during the day and releases it at night causing heat turbulence. I'm thinking of decking the slab with deck risers to alleviate this but the bigger question is how to erect a pier on the slab? Has anyone done anything like this? Would be shame not to utilise the existing foundation for the building itself. 

Been thinking maybe bolting down a large brake disc on it with some steel rods poking up from it and then cast a concrete pier over and on top of it. Does this sound feasible? 

Not sure if I want to use a commercial steel pier bolted onto it as they all seem quite pricey for what they are. 

Hi

I've cast a reinforced concrete pier onto existing concrete paving slabs sitting on top of a 100mm concrete slab. As my wife had agreed to me installing this 'sculpture' in our tiny backyard I decided digging a big hole in the recently installed patio was pushing my luck! 

I drilled through the paving slabs into the concrete and then used specialist adhesive to anchor mild steel reinforcement bars into the existing concrete. I then wired and welded a cage of mild steel reinforcement for the column to the bars anchored in the existing concrete. To give the whole construction additional mass for increased stability I cast an integral cube of concrete directly on top of the concrete slabs and at the base of the pier. This cube had continuity steel drilled and bonded through the paving slabs and into the existing concrete. Basically, instead of having a big lump of concrete under ground as a foundation I cast a big lump of concrete on top and anchored it to the existing concrete slab with steel and adhesive.  There is a image on my SGL Blog somewhere I think named Lowestoft Pier.  This isn't the prettiest piece of construction but seems to work fine. I haven't got an observatory as the garden is too small but you could use a similar pier in an obsy, particularly if you raised the floor level to the top of the base cube. This would enable easy isolation of the floor structure from the pier to minimise vibration.

Hope helpful George

[kbrown]

Well I'm glad I asked. Thanks for the responses! Definitely gives me some ideas to think about. 

I'm not too concerned about the possible vibrations caused by walking in the obsy as most likely I'll be controlling everything remotely from inside the house. I'll probably isolate and lift the obsy floor using decking risers which should help a little but mainly for keeping everything a bit drier as well.

 

[Anne S]

My observatory is sitting on paving slabs laid by a friend who works for the council. It is very stable and has a Pulsar dome sitting on it. I use an Ioptron tripier and a CEM60ec with refractors on it. No problems with walking gently past it when I'm imaging. I also didn't have any problems with my eq6 which was on the standard tripod. The garden used to be a car park so is really difficult to dig down any depth.

i would imagine you'd only have an issue if you mounted a very long focal length telescope.

[ollypenrice]

A lot depends on the mass of the slab but I have the following observatories, all built on single concrete slabs carrying the walls and to which piers are directly bolted using threaded bar bonded into the slab.

- 4 scope robotic shed, 3M x 4M, 6 tonne base.

- Single mount dual scope imaging observatory. Base mass roughly 3 tonnes.

- Single scope robotic shed. Base mass 1.5 tonnes.

These all work perfectly and show no need whatever for the isolation of the pier slab.

- Single scope imaging observatory built on two-part concrete slab.

I can't remember why I laid this slab in two halves but it was a silly idea! The shed works but heavy-footed moving about inside it does show very briefly on the guide trace. It simply means that I have to be careful when I move, at least when imaging at fine pixel scales.  A single casting of the slab would have worked fine.

Conclusion: I would expect a former garage base to work perfectly well with a pier attached to it. Peter Drew invented a cheap and easy pier known as the 'Todmorden Pier.' https://www.google.com/search?q=todmorden+pier&rlz=1C1CHBF_enFR821FR821&tbm=isch&source=iu&ictx=1&fir=a71unkkhOJN1kM%3A%2CWl7whdD2DAJ3LM%2C_&vet=1&usg=AI4_-kSIKSVnn4aUcVIZO2xahGyDNMbp0w&sa=X&ved=2ahUKEwj639qcjNjnAhXo4IUKHXpZBEEQ9QEwAXoECAcQBg#imgrc=a71unkkhOJN1kM:

I came up with a slightly harder-work concrete pier alternative using pre-cast concrete cylinders placed over vertical rebar epoxy-bonded into the slab and then rubble-and-concrete filled. We've made two of these and they work well.  There's a picture on this thread. 

Olly