Aluminum Masts:

Aluminum masts can be used safely with caution. I would not use an aluminum mast with a wall thickness of 0.090″ or less. Common wall thicknesses of 0.120″ and 0.150″ are common and work well with OD’s of 1.90″ and 2.0″. 6061-T6 is a good, strong aluminum alloy and has a yeild strength of 35,000 psi. Normally, I would use a 1.9″ OD x 0.150″ wall 6061-T6 mast with the HF beam mounted just above the thrust bearing. If the wind load of the HF beam is 2 to 5 sq. ft. less than the tower’s limit, then you can also stack a small or medium sized VHF beam or omni a couple feet above the HF beam. If the HF beam is 0.5 to 1 sq. ft. less than the tower’s limit, then you can only stack a short VHF or UHF omni just above the HF beam.

Galvanized Steel Masts:

Beware of ordinary steel water pipe! Unless you know the grade of material and it’s yeild and tensile properties, it may be weaker than aluminum and will rust if not galvanized. In fact, the galvanized steel pipe at Home Depot is ASTM A53 seamless, which has a yeild strength comparable to aluminum: 30,000 to 35,000 psi. Most galvanized steel mast material sold by Ham Dealers have a yeild strength of 80,000 psi or higher. With this type of mast, you can stack a medium sized VHF or UHF beam 5 to 6 feet above an HF beam. However, the moment produced by the VHF or UHF beam will be amplified by the distance to the top of the tower, so you cannot just add the wind areas together. This extra moment places more stress on the bottom of the tower legs near where the feet attach. If you are going to stack at this distance, make sure that the total wind area is 5 to 7 sq. ft. less than the tower’s limit OR do the calculations.

Here are some places to find tower masts:

Texas Towers – Texcom Steel masts, galvanized, 87,000 psi typical. A 10 foot long 2″ OD x 0.120″ wall mast is about $129. Other lengths available.
HRO – US Tower M10 , galvanized, reinforced, 10 foot steel mast, 2″ OD x 0.120″ wall is about $129. Other lengths available.
Array Solutions – Mast 2″ OD x 0.22 wall x 22 to 24 ft – 1026 DOM Tube 87 k psi
Array Solutions – Mast 2″ OD x 1/4″ wall x 20 to 24 ft- 4130 Chrome Moly 112-118 k psi
DX Engineering – Heavy-Duty 4130 Chromoly Steel Masts
Texas Towers – Universal Tower AM-216, 16 foot long 6061-T6 aluminum 2″ OD x 0.120″ wall mast is about $109.
Penninger Radio – 6061-T6 Aluminum 2″ OD x 0.250″ wall x 10 feet long is about $100. Other lengths available.
Penninger Radio – 6061-T6 Aluminum 2″ OD x 0.125″ wallx 10 feet long is about $62. Other lengths available.
Online Metals – 6061-T6 Aluminum Schedule 80, 1.9″ OD x 0.200″ wall x 8 feet long is about $47. Shorter lengths and other wall thicknesses available.

Helpful References:

“Match your Antenna to your Tower”, Roger Cox WB0DGF, Ham Radio Magazine, June 1984

“Practical Application of Wind-Load Standards to Yagi Antennas: Part 1”, Stuart E. Bonney K5PB, QEX Jan/Feb 1999, pp 46-50
“Practical Application of Wind-Load Standards to Yagi Antennas: Part 2”, Stuart E. Bonney K5PB, QEX Mar/Apr 1999, pp 44-49
“Tower and Antenna Wind Loading as a Function of Height”, Frank Javanty W9JCC, QEX July/August 2001, pp 23-33

“Tower Tips” a compilation of many tower tips from various authors
K7NV’s Windload equations
EIA/TIA-222-G Explained