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[TowerTalk] Universial Freestanding Tower

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Subject: [TowerTalk] Universial Freestanding Tower
From: (
Date: Thu, 14 Feb 2002 22:19:15 +0000

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 
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 6061-T6 
and 6062-T6 and Suggested Specifications for Structures 
of  Aluminum Alloy 6063-T5 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 

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 T-6 )

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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