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
product.
(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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