Need comments on Pier Construction?

[wysiwyg]

Hi Guys,

I have attached a diagram of my Pier Plan and I need you guys to rip it to shreds, in other words I need all the advice I can get.

Especially the nature of this construction being so high and all.
Comments on material thickness, plate thickness even the number of bolts needed, dampeners? etc.

I have all the welding equipment I need and can get hold of any steel materials necessary and I am quite capable of welding this thing up myself.

Is there any advantage of having the pier one pieces or two pieces?

Cheers Guys.:thumbsup:

[Astroman]

I'd go larger with the cement slab, at least 1M cube. depending on ground movement etc... you may find the ground will move through the seasons so going deeper reduces the chance of surface movement. Hiring a Dingo or similar with a drill attachment would make this job easier. Nothing wrong with going the two part pier, I have seen quite a few designed in this way, just so long as you have good contact between plates. I would use the same diameter tubing on bottom and top aswell, wont make much difference, but would give you a better footing. Also make sure the bottom half is hollow so you can fill the pier later, a hollow tube is really not that good, you will notice a hell of alot of difference if you fill the pier with foundry sand. Rumples, Peter and I have done this and it makes a great difference. The gussets look okay. :)

hope this gives you a few ideas.

[wysiwyg]

Thanks Andrew,

I think you are right in regards to the concrete slab needing to be 1m cube, Bugger that is a lot of concrete. I wonder how many bags I will need???:P
I might need to order a mini mixer and wheel barrow the stuff in.

When you say fill it with foundry sand, is washed fine sand OK?

[Omaroo]

I'm not a structural engineer, but I'm thinking that 1,800mm for the bottom section is going to be relatively flexible when you have an additional section sitting on top - and then the weight of the mount/scope as well. If you were to extend the gussets to run the full length and then cap it top & bottom & fill it with sand you might be OK. That's a heck of a length given that it looks like you're proposing 300mm tube (based on the 400mm base). How deep are you going to go with the foundations?

Cheers
Chris

[wysiwyg]

Thanks Chris,

As Andrew pointed out I would probably need a 1m cubed foundation, and I like your idea about the gussets running the whole length, at least for the bottom section.

Cheers:thumbsup:

[Roger Davis]

I would make the concrete base 600x600x900 at least and not a cube, but pyramidal.

If you want to keep the two part design, extend the flanges on both tubes down so they overlap by about 5cm and weld them together.

Filling the tube with refractory sand is a good idea. Metal "rings" and will give harmonic vibrations if tapped or moved by low frequencies (nearby truck etc.). Washed sand has a lower density and will not quite do the same.

The top tube, using your height for scale, looks to be around 200mm. What wall thickness are you thinking of using? I would recommend 3mm. That makes it quite heavy, so please consider.

Allow 1 centimetre gap between "future decking" and pier.

[wysiwyg]

Sorry Roger but I used power point for the drawing so its not to scale (not even close).
I am looking to use 150mm to 200mm diameter pipe for the top section, about 8mm in thickness.

Where would I get refractory sand from? Better question what is refractory sand???:shrug:

[wysiwyg]

[quote=Roger Davis;269129]I would make the concrete base 600x600x900 at least and not a cube, but pyramidal.

Roger,

Do you meant like a pyramid (right way up) with the top cut off, or do you mean and upside down pyramid?

[Roger Davis]

5mm would be ample. Strength across the diameter is not a function of wall thickness.
Foundry sand/Refractory sand ,close enough to the same thing.
Pyramid rightway up, but truncated. Picture the Giza pyramid with a plateau or mesa on top.

[rumples riot]

Mark ask Pete on SCP. He has a mate that works at a foundary. That is where we got our sand from.

[rally]

Roger,

The moment of inertia of the cross section of a cylinder is as follows

pi x (OD^4 - ID^4)/64 (in inches)

That is : pi multiplied by (Outside diameter to the power of 4 minus Inside diameter to the power of 4) divided by 64

and Deflection under load is an inverse function of the moment of inertia.

So inside diameter does make a significant difference.

In an approximation of the example given (I used 200mm for the whole length at 8mm and 5mm wall thickness) the deflection increases from 2.8arc secs to 4.25 arc secs - not quite a doubling.
For these particular dimensions in this particular range the relationship is semi linear - ie very approximately - double the wall thickness double the strength

What does occur that is relevant to OD in that if you were to take the same pipe but make it smaller in OD to say 150mm the strength would drop and the deflection is increased by 50%
But by using the same wall thickness pipe but increasing diameter by a relatively small amount you will increase the pier's rigidity by a significant amount.
The amount is a function of the mathematics

Cheers

Rally

[Astroman]

re sand I still have a 20L bucket here if you want that, it wont fill all of the pier but will help if you cannot get your hands on alot of it. Although if you ask pete he should be able to get a couple more buckets. If he needs buckets let me know, I can get heaps of them from work. They came with Castrol white grease but thats easily removed with some hard scrubbing :P

[Aster]

Go To "show us your Equipment Forum" and have a look and read of My Stuff. The Pier and Concrete Block are designed to be used for DSO imaging.