Discuss Bob Suffolk's answer to: Suggest me any calculator for hollow beams and round tube beams?

Basically, concrete tube beams (AKA Lally Columns) are called out on four criteria.

1. Exposure.  Smog poses one challenge to concrete and salt water another.  Although we get a stronger bond with bare rebar, salt water will rust it away in short order leaving us with a brittle structure.  That leaves us with coated rebar but it doesn't have as great bond strength.
2. Root/ratio.  3:1 columns are considered "short" regardless their actual height.  4:1 columns are "intermediate."  5"1 or greater are "tall" columns.  Depending on the load we will need thicker walls for greater loads.  However, the weight of the column itself can exceed the compressive strength of the concrete.
3. Aspect.  Vertical structures (columns) face different load dynamics from box girders.  Here we need to be rather astute concerning bending moment, compression strength of the top chord, and tensile strength of the bottom chord.
4. Shift.  Shifting weight, resonance, wind, live loads, and seismic loads, play a part.  Refer to IBC for details.

Here is a little more detailed description.

http://www.mhprofessional.com/downloads/products/0071638342/fanella_0071638350.pdf