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[TowerTalk] Building Code Units of Measure

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Subject: [TowerTalk] Building Code Units of Measure
From: (Bill Coleman AA4LR)
Date: Thu, 11 Sep 97 12:28:42 -0400
On 9/8/97 8:24 AM, Fred Hopengarten at wrote:

>For example, Rohn uses a safety
>factor of 3, while Cushcraft uses a safety factor of 1.25.  My friends
>who are mechanical engineers tell me that 1.25 is closer to
>"appropriate," and comment that the real reason for Rohn to use 3 is to
>sell Rohn 55 when Rohn 45 would do.  No serious mechanical engineer has
>ever told me to use more than 1.5.  Ever.

Depends on a number of factors. In the aviation world, where weight is 
much more important than the tower world, design factors of 1.5 over 
ultimate design load seem to be standard for metal designs. (My Cessna 
mentions these design factors in the owners manual)

For other types of materials, larger design factors may be appropriate. 
For kit-built or plans-built aircraft, the construction process can't be 
precisely controlled, so larger design factors are the rule.

For example, Burt Rutan's moldless composite aircraft used design factors 
of 2 to 3. The main reason for this was the inability to control the 
construction process or to inspect the structural components once 
assembled (since they are just fiberglass, foam and epoxy). Even so, some 
of the more poorly-built designs have failed in flight.

Molded composite kits are partially constructed at the factory, and have 
lower design factors, but they are still closer to 2 than 1.5.

I suspect Rohn uses large design factors because of the history of poor, 
improper installations. While Rohn probably isn't assailed with lawsuits 
the way aviation companies are, they still have quite a large liability 
trailer. I don't blame them for being conservative.

Bill Coleman, AA4LR, PP-ASEL        Mail:
Quote: "Not in a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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