Currently designing a new house. I'd like to install an observatory on my 3rd floor, so the base of the floor would be around 25ft of ground level.
I want a solid pier that goes through both floors below to create a pier-based foundation on the 3rd floor, and then I want to put a metal pier on top of it that foundation that I can swap out as needed.
The biggest scope I can see myself put on this would be a CDK-24 at 6000mm, which weighs around 750lbs on a L-600 mount.
What kind of pier structure and foundation below it am I looking at for this?
My calculations so far: Our soil is rated at 1500 psf. A 25ft long x 30" wide concrete+steel pier would weigh around 18000 lbs above ground. What kind of foundation do I need below ground to hold up such a pier? Will something like a 6ft deep, 10' diameter circular base work (50000 lbs)?
That would make it 50'000 lbs of foundation + 18000 lbs for the pier itself, spread over a 78 square foot area = 871 psf. Which seems ok on 1500psi soil? Is this the way I should be thinking about it? Of course I'll get a structural engineer to verify it, then I'll get another structural engineer to verify him, and then I'll get a seismic engineer to verify them both.
I just want to see if I'm going down the right path here, or if I shouldn't be thinking concrete in the first place?
Are there alternatives to a solid pier? e.g. If the rooms below were constructed with steel + concrete walls, and you have a steel+concrete slab on the 3rd floor, would that provide a stable enough foundation for a 6000mm FL? I've used scopes on concrete floor of 3 story commercial concrete buildings, and the floor seemed pretty stable and isolated even if you stomp around. But that was at 1000mm.