Re: [TowerTalk] Rohn 25G Hardware

[Jim Lux <jimlux@earthlink.net>]

At 11:11 AM 10/20/2003 -0400, Roger D Johnson wrote:
>  After posting my previous message, I decided to do some research
> to determine the strength of the SS hardware I used on my tower.
>
>  It turned out to be more complicated than I thought. I soon got
> bogged down with SAE vs ASTM standards, old bolt head markings vs
> new head markings, psi vs ksi vs kips, etc. I'll try to briefly
> summarize my findings.
>
> 1. There is not much data on the shear strength of bolts. One site
> gave the following figures. For carbon steel, the shear strength is
> approx 60% of the tensile strength. For SS bolts it's 55%.

Shear stresses, at some point, turn into tensile and compressive loads on 
the bolt material, for which there is material strength data. For the 
strength of a real bolt loaded in shear, a lot depends on how it's mounted 
(how tight a fit in the hole, etc.).  Consider that a hacksaw doesn't put 
much load on the bolt, but still cuts through it (an extreme example of 
uneven loading).  Likewise a "bolt cutter", which deliberately loads the 
bolt in shear.  The holes being a bit uneven, or not perfectly parallel, 
etc. can turn that idealized shear loading into a bolt cutter. And, of 
course, you never put a shear load on the threaded part, right?


> 2. Some of my bolts are marked with two lines at 90 degrees. These
> are an example of the "old marking" and are 18.8 stainless (also
> known as 304SS). These bolts seem to vary widely in strength.


Not much about strength because 18-8 (and 304) just specifies the alloy 
constituents, and nothing about the grain structure/tempering/processing 
that went on.  Kind of like the difference between 6061 and 6061-T6 
aluminum... same chemical composition, very different material properties.



> 3. I also have bolts marked F593C. These are cold worked and conform
> to ASTM standard F593. These bolts have a tensile strength of 100 to
> 150 ksi (ksi = 1000 psi) giving them a shear strength of 55 to 82.5
> ksi. This compares favorably with grade 5 carbon steel bolts which
> are rated at 120 ksi tensile strength.
>
> For more info on the F593 bolts see:
>
> http://www.marfas.com/technical.shtml
>
>  Based on the two anecdotes I've read here, it doesn't seem to make any
> difference what bolts are used when the guy system fails! My conclusion
> is that F593 SS hardware is fine for guyed towers. Self supporting towers 
> are a different story.

_______________________________________________

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.

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