why is round pipe used?

[speach]

Why is round tube used as piers and not square? Square is a lot cheaper and if its filled with concrete or sand I can't see that the vibration would be any more.:question:

[sheeny]

Round is obviously the same stiffness in all directions with respect the bending. That's why I think a lot of people choose it over square, however, as an engineer, I chose square because it really doesn't matter that much.

As for filling the pier with sand or concrete - don't do it. The extra mass reduces the natural frequency of vibration of the pier and therefore increases the amplitude of the vibration should the pier be stimulated by an impact or wind. Also when you do the calculations for the stiffness of concrete and steel, steel is of the order of 10x stiffer than concrete.

So for a given diameter, steel is better, and an unfilled steel pier is better becaaue when the pier is stimulated by an outside force the amplitude is much less than a filled one. If the amplitude of the vibration is less than a pixel (say) in you camera, you get away with it... if it isn't, you are collecting rubbish not data.

Al.

[glend]

Yes the strutural engineers regularly put forward the theoretical arguement. I don't know that it matters much on a small static structure like a pier, where wind, especially in an observatory, has no influence. The damping effect of sand or concrete, from accidental bumps, out weighs the other issues in my opinion. Hollow steel piers ring like a bell if you accidently swing a cable connector into them and they resonate for a while. I used a poured concrete pier in a tube form which is integrated into the footing (wet on wet pour), with steel reo right through the pier into the footing supplying the necessary stiffness to the pier. it will outlast me for sure, and pvc form tube (which i retain as a nice exterior surface, damps any bumps and provides a surface which is easy to attach things to with pvc cement).. Simple to construct, all components availabke from Bunnings, low cost compared to having steel fabrication into pier or purchasing a production steel pier. No worries about shiming and bolting down a steel pier.
So i am saying, from a practical point of view, a reinforced concrete pier makes sense.

[julianh72]

As a practising Structural Engineer, I'm going to chime in here and respectfully disagree.

Adding sand or concrete will increase the mass, and therefore tend to lower the natural frequency, but it also increases the stiffness somewhat (negligibly for sand fill, moderately for concrete fill), but importantly, it also increases the damping (significantly for sand, moderately for concrete fill).

Yes, steel is about 10x stiffer than concrete, but there is a lot more concrete than steel when you fill a steel tube, so a concrete-filled steel tube is about 30% stiffer than the empty tube (depending on tube size and wall thickness) - and this is "a good thing".

The stiffness dictates how much the pole will deflect when subjected to an initial disturbing load. A sand filled pipe will deflect pretty much the same as an empty steel pipe, but a concrete filled steel pipe will deflect about 30% less than the same empty steel pipe, when subjected to the same initial load.

Adding mass (by filling with sand or concrete) will NOT increase the initial deflection when the post is disturbed, but it can change the way the pipe vibrates after being disturbed - generally for the better.

Damping affects how quickly the pipe will stop vibrating after being disturbed. Bare steel has very low damping, so will vibrate for a significant amount of time. (The damping of the foundation can be at least as significant as the damping of the post for a bare steel tube telescope pier.) The additional damping provided by the sand or concrete fill can damp out the post's vibrations faster.

And back to the OP's question:

It may be surprising to learn that just as circular posts do, square posts have exactly the same stiffness in all directions (yes, the stiffness in the direction of the diagonal is the same as the stiffness parallel to the side faces) - and for a given dimension "across the flats" and the same wall thickness, a square hollow tube is about 65% stiffer than a circular tube. (However, if you measure the square tube across the diagonal, instead of "across the flats", a circular tube with the same diameter and wall thickness is about 70% stiffer.) [Edited for clarity]

Bottom line: My advice would be, if you have the choice between (say) 150 x 3 SHS (Square Hollow Section) or 150 x 3 CHS (Circular Hollow Section), use the square tube - and fill it with sand or concrete for a bit more damping.

(And yes, a larger-diameter concrete pier may be an even better solution, as it can be cheaper and easier to build, and you can easily build an even bigger diameter than would be practical for a home - built steel pier.)

[speach]

This is what I feel, I didn't know that Square section is stiffer but I wasn't surprised that it wasn't different from round. As for filling it with concrete, the damping of any vibration would decay quicker is what I was thinking. So I'm planning to use 2 meters of 200x200 square sunk 70cm in the ground in a 50cm square pad, then filled with concrete. Of course god help the next owner of this house when they come to remove it!!!!

[billdan]

There was a long thread on SGL about filling steel piers with sand or concrete. The consensus was not to fill the pier more than halfway, this avoids the pier being top heavy and leaves all the mass down low for better stability.

Cheers
Bill

[julianh72]

Sorry - bad wording on my part! When I said "exactly the same stiffness in all directions as circular posts", I should have said, "just as circular posts do, square posts have exactly the same stiffness in all directions" - but a square post of any given dimension "across the flats" will be stiffer than a circular post with the matching diameter.

[g__day]

Also - round hurts your shin a heck of a lot less than the straight edge of a square mount when you accidently clunk into it with a degree of force...

[leon]

When I made my piers they were always round, never really considered square, however I expect there was not much difference in either.
As for filling with sand/cement, absolutely do so, it makes for the most solid an vibration free pier you will own.

Leon