A comment was made that steel will break from vibration. While
that is true (if stresses are high enough and/or it is the wrong kind
of steel), steel does not have near the problems of aluminum.
Aluminum "remembers" stress. It is a soft material that was
modified through heat (mostly) to have large internal crystals. The
crystals loose their attachment to each other when "worked" with
vibration or bending.
Steel changes differently. It does not work harden so easily, and it
also stress relieves itself after the stress is removed.
Aluminum "remembers" the stress, and so the damage
accumulates over time until it eventually fails.
The clearest example I can think of is connecting rods is
automobiles, a part subjected to constant vibration. In racing
engines, aluminum connecting rods of the highest quality will last
about 800 seconds of total operating time. At that point, they need
to be removed and checked for hardness and cracks (just like
aluminum airplane skins must be inspected and tested). If you get
40 minutes use out of an aluminum rod, you are on borrowed time.
Into the trash they go.
The same or LOWER tension rating in a steel connecting rod will
last for several hours of total operating time before it is necessary
to check the rod. You can get years of stressful pounding use out
of a steel rod, as long as you never exceed the limits of elasticity
in the material. Not so with aluminum!
I've never in my life heard of a steel tower failing from slight
vibrations, but I know of many aluminum towers that (even though
rated far above the wind load) have cracked and failed. Like my
yagi, the problem is more from constant slight stresses that
accumulate and are "remembered" than large stresses that simply
make the antenna fail.
I'd worry about any vibration or "tune" from an aluminum structure.
Eventually the vibration will cause damage, even if it takes years.
Personally, I care less if my Rohn 25 hums like mad...because
there aren't any aluminum antennas on it.
Steel guitar strings last a long time, aluminum ones wouldn't last a
minute without changing characteristics.
73, Tom W8JI