Pier shape

[planetdnb]

I am looking into putting a pier into my scope shed and I have a friend who does steel fabrication and he has loads of square shaped steel tubes.

Would it be a problem using a square tube instead of a circle?

I notice that generally all piers are circular, is this for a reason? Vibration perhaps?

[Bizibilder]

I suspect that folk use round tubes because they are commonly available in the sizes required. A thick walled square tube should work OK - If in doubt fill it with concrete!!

[Peter Drew]

I think most metal piers are round because round tubes are more readily available. Given the choice, I would always go for a square one, a 6" square tube is more stable than a 6" round one for instance. A square tube also makes it easier to fit other components. It's also better to have a large, light guage tube than a small heavy guage version.

[planetdnb]

ok thats quite cool. whats the minimum recommended diameter?

I have a SW200P with an EQ6 pro mount. I think fully loaded the scopes and accessories are about 15KG.

Could i get away with 3 or 4" of square tube or would it need to be bigger?

I'm thinking 2M long buried 1M deep in concrete and then filled with sand.

[JamesF]

Yup, can't see why you shouldn't use a box-section tube. I'm fairly sure that for a given mass of metal or wall thickness then a cylinder is the strongest possible shape, but that doesn't mean that a square section pier can't be sufficiently strong to do the job.

James

[Bizibilder]

The weight of the scope is not the deciding factor. The rigidity is a function of diameter. So the larger diameter the better (within reason of course!).

[planetdnb]

lovely thanks guys!

[JamesF]

I guess there are two things you need to worry about when designing a pier. First, how much it deflects for any likely load, and second how fast it will damp that movement. I believe sand might help with the damping, but it won't help with the rigidity. How rigid you need it to be depends on how much deflection you might be willing to accept when you're imaging and you should be able to work back from there to suitable pier dimensions given a specific wall thickness. My (ex civil engineer) brother could probably do chapter and verse on cantilevered box section steel, but he doesn't appear to be online right now. My suspicion is that unless you're using very thick walls, a 4" square section box is unlikely to give you as much rigidity as you'd really like. You might get away with it, but a 6" or 8" section is probably a better bet. The overall dimensions are far more important than the wall thickness, as I understand it.

James

[yesyes]

I would also think that 4" square section is not rigid enough. Not even if it was round. The more diameter the better. But don't make it too big as you don't want the back of your scope to hit the pier.

If you have 4" square section available, why not weld 2 together and have 8" x 4". That might be enough....

[opticalpath]

There is a good article on pier design here that explains the various parameters and the effect that they have on rigidity and damping.

Pier Design Fundamentals - How To

As has been said earlier, diameter is far more significant than wall thickness for rigidity.

Adrian