>Again the whole concept of one material ("work
>hardening") fatiguing faster than another is
>unfounded, given the load cycles that towers see in
>their lifetimes. Properly designed.
>Any metal that is loaded below its threshold fatigue
>stress level and unloaded can,theoretically be loaded
>and unloaded forever...
>Rule of thumb is to use one half the yield stress of the
>material, this keeps the stress well within the elastic
>range for the material.
>The question of using a steel or aluminum tower of
>comparable load carring capacity is not one of which
>will fatigue before the other but one of enviromental
>corrosion resistance, weight, transportatability,
>absolute strength, size of structure required and cost.
>All of these vary for each material, intended usage and
>total load carrying requirements.
>I have at various time designed aluminum, steel, fiber
>reinforced composite, and timber structures to support
>elevated loads of, freestanding and guyed, antennas and
>other equipment. The decisions were never based on the
>fatigue preformance of the materials but rather on cost,
>availability, transportation, and future expandability
>of the proposed structure.
>As we contribute to this reflector, we as individuals
>need to consider what we are saying and what factual
>basis we are using to support our statements.
>Hank Lonberg, P.E./ S.E., KR7X
Thanks Hank. I see that I've been laboring under some
false assumptions about the concept of metal fatigue,
and regret my post. Some reading is in order.
I'm still pretty sure about relative performance/cost
issues, but for small ham towers, the differential might
be considered reasonable. A 250' self support aluminum
tower designed to handle 4 or 5 full cell/pcs installs
would be an expensive proposition indeed.
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