Talkians:
Sometimes I just can't believe the amout of
misinformation and absolutes that get pawned off as
verifiable truth on this website. Ordinarily it does no
harm or potential harm. However this discussion of the
aluminum vs steel towers and reduction of strength due
to "work hardening" is in need of some scientific and
engineering basis.
The first order of business is the posting of an
individual who relates that he installed a 75 foot
cantilever aluminum tower without a concrete base in a
hole that is 12" in diameter by 6 feet deep. Sir, if you
installed that tower in the dirt then you are at risk of
a failure of the soil at some time in the future due to
the soil getting saturated with ground water or by the
interaction of the soil and the aluminum. At 12 inches
in diameter it must be a round section and not a
latticed type tower.
The next item is the term "work hardening." This maybe a
term used in the welding and fabrication shops but the
accepted terminology is Fatigue and Fatigue stress.
"Fatigue, as used in this Specification, is defined as
the damage that may result in fracture after a
sufficient number of fluctuations of stress. Stress
range is defined as the magnitude of these fluctuations.
In the case of a stress reversal, the stress range shall
be computed as the numerical sum of the maximum repeated
tensileand compressive stresses or the sum of the
maximum shearing stresses of opposite direction at a
given point, resulting from differing arrangement of
live load.
1. Loading Conditions; Type and Location of Material
In the design of members and connections subject to
repeated variation of live load, consideration shall be
given to the number of stress cycles, the expected range
of stress and the type and location of member or
detail. ........"
(Manual of Steel Construction, Allowable Stress Design 
9th Edition, AISC)
Two things are important; the concept of cycles of
loading and a stress limit value to eliminate fatigue.
".... Aluminum, like other materials, may fail in
fatigue after large numbers of applications of load.
Fatigue failure is almost always traceable to a stress
raiser such as a notch, hole, or sharp reentrant corner,
or to local bending resulting from joint eccentricity.
Concentrations of stress and local bending can
frequently be alleviated by proper design, fabrication
and maintenance, resulting in greatly improved fatigue
life.
It is not possible to design precisely for a specified
fatigue life, even in cases where a constant, know load
cycle is applied to the structure, ..........
Nevertheless, approximate rules for design of structures
subjected to repeated loads have been developed on the
basis of fatigue tests of structural joints."
(Task Committee on Lightweight Alloys: Suggested
Specifications for Structures of Aluminum Alloys 6061T6
and 6062T6 and Suggested Specifications for Structures
of Aluminum Alloy 6063T5 and T6, ASCE)
For both materials and all structural metals the design
for the elimination of fatigue failure involves the
concept of number of cycles of stress reversal and the
magnitude of the stress range.
Example time:
Cantilever Tower for Amateur Radio: Design Life 25
years, Daily cyclical stress reversals say 5, that is 5
maximum wind load occurances every day. Total cyclical
loading = 5 x 365 x 25 = 45,625 cycles for the design
life.
Both the AISC for steel and ASCE for aluminum give max
stress range based on cycles of loading below which
fatigue is not a factor.
for aluminum : 100,000 cycles Stress range if max
stress is larger than 1/2 max allowable, = F, the max
allowable for static loads.... i.e., no reduction in max
allowable stress due to fatigue. ( 6061 T6 )
For steel: 100,000 cycles, 63,000 psi stress range.
i.e., max stress = 63,000 / 2 = 31,500 psi > .6 Fy
=21,600 psi... no reduction in max allowable due to
fatigue. (A 36)
Neither material has a greater propensity for fatigue
than the other.. properly designed.
Other comments:
No such animal as aging of steel or aluminum due to
flexing... if properly designed.
I would like to hire any individual who can; by eyeball
and gut feel tell me the stress values in a structure
without calculating them from the loading conditions and
structural geometry.
Most catastrophic tower failures occur due to
compression buckling of either a tower leg or brace.
There is no progressive derating of any structure.
There is, in my experience, no such animal as:
Coefficient of Flexation. Maybe the Modulus of
Elasticity is what the person meant.
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
________________________________________________________________________
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