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Re: [TowerTalk] HDBX-48 figures for 90 mph zone

Subject: Re: [TowerTalk] HDBX-48 figures for 90 mph zone
From: Jim Lux <>
Date: Wed, 07 Dec 2011 08:25:28 -0800
List-post: <">>
On 12/7/11 7:56 AM, Gene Fuller wrote:
> Bottom line, if the manufacturer can't, or won't, give you 90 mph data or at
> least the projected tower area and, taking into account the materials used,
> the maximum allowable restraining moment, then they probably know nothing
> about things like section modulus and moment of inertia and never did the
> calculations themselves, in which case I would have second thoughts about
> buying their tower.
> Gene / W2LU
I wouldn't go that far.  It costs money to do the analysis. I can see 
three scenarios:

1) The analysis as it exists was done using guidelines from an earlier 
era.  The published information is sufficient for another engineer to 
redo the analysis for different conditions.

2) A quick informal analysis (or even a thorough one) shows that the 
tower is almost maxed out at 70 mi/hr.  Why publish an analysis that 
shows negative capacity at 90 mi/hr, unless someone asks for it?

3) Changes in regulatory practices since the original analysis was done 
make it less useful for the manufacturer to do the analysis, since each 
local jurisdiction wants local factors considered (e.g. soil bearing 
strength, etc.). (this is sort of like #1)

As a practical matter, for the BX series, all the data you mention is 
published in the manual. Granted, it's from the early 70s, but there's 
not a lot of incentive for the mfr to go through and re-do all the 
assumptions and stuff. You'd need to confirm the materials with the 
manufacturer, and then use updated material properties (the BX manual I 
have has properties from 1968... yeah, steel is steel, but these things 
do change)

Changes in TIA-222 over the last 35 years (latest date on the analysis 
in my copy is 1977) also will change a lot of the assumptions.  It might 
well be that the BX series, analyzed in accordance with TIA-222, has 
negative load capacity.   It wouldn't be the first time that a 
manufacturer uses the "I know nothing, nothing" strategy for a legacy 

  (this is like the installation instructions for my dishwasher in a 
kitchen preparing kosher food.. lots of explicit instructions, but a 
caveat at the end: "Consult your local rabbi", which an Orthodox 
acquaintance says is standard, because there isn't a single standard for 
"kosher".    In the code compliance biz, this is the AHJ (Authority 
Having Jurisdiction) )


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