Quote:
Originally Posted by 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 square posts also have exactly the same stiffness in all directions as circular posts (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.)
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.)
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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!!!!