Fred:
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.
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.
Hank Lonberg P.E. (civil and structural)
KR7X
______________________________ Reply Separator _________________________________
Subject: Re: [TowerTalk] Rohn Tower Designs
Author: k1vr@juno.com (Fred Hopengarten) at ~INETMAIL
To: <towertalk@contesting.com>
Date: 8/19/98 12:17 PM
From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
On Wed, 19 Aug 1998 14:03:14 +0100 k6ll@juno.com writes:
>
>I'm not a PE/ME, but I am a EE. I've tried to reverse engineer some of
>the Rohn configurations in their catalog without any success. I would
>like to see one set of Rohn engineering calculations to see how they
>did the math, and what safety factors were applied. If anyone has
>access to a set of these calculations, either commercial or amateur,
>I sure would like to obtain a set, as a starting point for a
>math model.
>
>Dave Hachadorian, K6LL
>San Diego, CA
>K6LL@juno.com
K1VR: One of the reasons that reverse engineering Rohn specs is
difficult is contained in the following phrase.
See Rohn 55G, drawing C630655 r9 ("Nomenclature") in which compression
capacity, moment capacity, and tension capacity are all specified with
"1/3 increase in allowable stress." Guy wire specifications are at a
safety factor of 2:1.
If anyone can figure out what that means and be confident of the
interpretation, we'd all know more about this foggy business. -- Fred
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