Hank Lonberg wrote:
> The safety factor is typically stated as the value used in the design
> as a ratio of the breaking strength of the guy. Sometimes it is a
> ratio of the allowable strength but not often. Therefore the safety
> factor of 2 means that the maximum guy tension in the design is held
> to one-half of the breaking strength of the guy.
Greetings Hank,Thanks for your comments. I understand completely what you have
said,
but have a question about this one.
Since, I work in a non-code driven world, I may have missed a few nuances of
what is
occuring elsewhere. In our business of designing and building masts and other
guyed
structures for the marine industry, I've found that we always design for a
suitable
safety margins on an allowable load that is 1/2 of the breaking strength of a
wire
rope. The rationale is that steel will yield at less than the rated breaking
strength. A yield will caused the structure to become improperly supported and
fail
due to increased deflections that are combined with the purely compressive Euler
buckling criteria.
I have seen some radio tower analyses that used the same 1/2 breaking strength
values as the allowable.
For the benefit of other readers: The breaking strength of a material is the
stress
level at which it separates, in a complete failure. The yield strength is the
stress
level where it experiences a permanent distortion. In the case of a guy wire,
if we
exceed the yield point, the wire permanently stretches, becoming longer. This
allows
the tower to lean over further and results in increased bending stresses in the
tower. It's result is just like not having it pretensioned (or tuned) properly.
I just made a quick review of common steel alloys, and found that most of them
have
a yield strength that is 70%-80% of the breaking strength.
I've been using 1/2 of the breaking strength as the allowable for my tower
designs.
Maybe, this is too conservative. I also wonder about 10-15 years down the road,
when
the weather has had its way with the steel.
> The 1/3 increase in stress is a design stress allowable increase when
> you are combining loads such as dead load + live load + wind load or
> earthquake load on a structure. What it says is that by including wind
> or earthquake loading you can increase the allowable stress value by
> 1/3 to account for their short term nature and transitory application.
> This stress increase is allowed by the AISC design code for structural
> steel which is adopted by all the building codes used in the US. It is
> also used in other material design codes.
>
> Hope this helps to clarify what is being stated on the dwgs.
Thanks for the explanation Hank! This helps very much.Hoping to make this
clearer to
those who may be having trouble understanding it:
In the case of a tower, buried in a concrete footing, the combined load case
would
be when the maximum vertical compression, lateral bending, lateral shear, and
tower
torque loads all get combined to create the worst case stresses in the
structure.
The statistical probability of all of the worst loads occuring at the same time
is
lower. This includes the transitory nature of each of the load components.
Because
of the dynamic effects of the various loads, it is unlikely that each of the
four
load components will peak in any given time span. Thanks again for your help,
73,Kurt
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