At 04:36 PM 7/9/2003 -0400, Pete Smith wrote:
>At 11:00 AM 7/9/03 -0400, you wrote:
>>In short, "bigger" is not always "better" when it comes to choosing guy
>>wire diameter or breaking strength.
>A question, though. One of the beefs with Phillystran is that it
>stretches about twice as much as EHS of the same approximate breaking
>strength, for a given tension. This implies to me that using bigger
>Phillystran might make for less deflection of the tower under load, and
>result in a tower that would be somewhat stronger than otherwise. Am I
>missing something?
Stiffness isn't necessarily related to strength. Properly engineered
structures have some amount of flex built into them. Changing the relative
stiffnesses of structural members can change the distributions of the
stress, and one of the challenges of structural engineering is distributing
the stress appropriately. You'd much rather have the strain energy in
something that is designed to take it with elastic deformation (i.e.
springiness).. the tower in this case, which takes the strain by deforming
(bending).
The guy wire is really there to limit the sideways loads, which will cause
premature buckling (a non-elastic deformation) or exceeding the elastic
limit (when it's bent "too far" and the compressive load on the tower gets
too high). As it happens, a guy wire/cable can only work in tension, so
the only way to get a "sideways" force at the attachment point is with a
vertical force, by tensioning the cable to put a down force on the top of
the tower. The saving grace is that the tower (a column) is quite strong
in compression. Presumably, the tower designer has allowed for some amount
of deflection in the tower, and trying to reduce the deflection might imply
that you are putting more load on the guy (and the tower).
Thought experiment: A big strong coil spring with a thin piano wire strand
down the middle. Load it up... the stiff part (the piano wire) fails,
because it takes all the load.
Another analogy.. Flex in structural members is like capacitance.. more
flex(compliance, inverse of stiffness) is like more capacitance. (mass is
like inductance, friction is resistance) Put several capacitors of
different values in series and apply a voltage. The smallest capacitor will
have the largest voltage. In a structural design, the loads are
transmitted through the various structural members (i.e. in series). The
stiffest member tends to have the highest load.
>73, Pete N4ZR
>The World HF Contest Station Database was updated 17 June 03.
>Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
|