[sheeny (Al)]

I did some calcs a while ago comparing steel and concrete piers. They are here on IIS somewhere - I'll try to find the thread. Basically for any given diameter, a steel pier is 10x stiffer than a concrete one. I had to make some assumptions about the grade of concrete used as that affects the young's modulus of concrete, but regardless no concrete mixture comes close to steel.

Now that said, remember the stiffness of a columm or beam in bending is proportional to the fourth power of the diameter. The fourth root of 10 is 1.78, so a 178mm diameter concrete pier is as stiff as a 100mm steel one. But... there's another part to the equation. The amplitude of vibration for any given energy (size of bump/knock) is inversely proportional to the natural frequency of the pier. The natural frequency increases with stiffness and decreases with mass. So a concrete pier of equivalent stiffness to a steel one, will have a lower natural frequency and therefore a higher amplitude of vibration if you bump it.

Don't get too hung up about it though. Unless you are in the habit of tapping your pier in time with the music, you don't need a super stiff pier once you're out of the wind (in your obs!), as long as its strong enough (minimum 100mm steel / 178mm dia concrete IMO at <1000 tall - I'd say at least 150NB steel @ 1800 tall).

The most important thing to worry about IMO is the size of the top of the pier so you have maximum / good scope clearance in all positions.

Al.