Brad,
No, thank you for the offer on the spreadsheet, I have myown, and
thanks to some very patient structural engineers on the hanging and
bracing committee, and the Eighth Edition of the Manual of Steel
Construction, I have all the formulas I could ever need or use. And,
because I felt so unprepared at the first ROP meeting I attended in
2008, I spent that summer annoying those same structural engineers to
help me understand those formulas, what they meant, and how to interpret
them for the work I do relative to fire sprinkler systems. Even thoughI
think I have a fairly complete understanding of the formulas, and their
results, I would never dream of giving someone those formulas and
sending them out on their own to use them correctly. I'm not a
structural engineer, and I don't pretend to be one.
Yes, you did hit a nerve for me, in that that the explanation contained
a significant error, and anyone taking that guidance as factual was
going to get out on a limb where they would be in serious trouble.
Perhaps to the point of failure of the trapeze to hold the load, despite
the safety factors built in (5x weight, 250#, 15,000 instead of 30,000),
and all because they followed the path you described.
I do completely understand your need to understand the things in a
code or standard, and I applaud that, and encourage that. The danger in
that is when you, or me, or anyone elsefor that matter, describes that
understanding to others without first checking the language and
documentation to make sure there was no significant error or omission,
because others may be tempted to act on that information. An example is
the reference to half the weight and half the length, neither of which
are even remotely applicable. Errors of that nature get people sued, or
worse.
The tables are in NFPA 13 for a reason, and to me that reason has
been so that I can do my work in designing systems without being a
registered engineer with experience in any of the myriad of topics
covered by tables in that document. For example, in the last cycle, the
trapeze tables were proposed to be extended tocover spans up to 20' in
length, in 6" increments. The substantiation for that included the
following text: "For spans more than 20 feet. the trapeze should be
looked at further by an expert as many scenarios would then involve a
joint in the trapeze which could create the need for additional analysis."
The committee accepted that proposal, in principle, however it limited
the table to 16', and provided the following change: 2) Add annex
material as follows: A.9.1.1.6.1(a)The table values are based on the
trapeze being a single continuous member." In support of that statement,
the committee wrote: "The TC feels that for spans in excess of 16 feet,
the values should be reviewed on a case by case basis."
In what you wrote, I took it to encourage any reader to simply
double the section modulus required for a 9'-0" span, and that was good
enough. That's contrary to the intent of the committee, and the
published information on how that table was developed.
The above is myopinion only, and does not reflect that of the committee
on hanging and bracing of NFPA 13.
*
PARSLEY CONSULTING
*
Ken Wagoner
760.745.6181 voice
760.745.0537 fax
website: www.ParsleyConsulting.com <http://www.ParsleyConsulting.com>
On 6/3/2013 9:48 AM, Brad Casterline wrote:
Ken,
Sorry if I touched a nerve or upset you. I am nothing but a designer with a
trust issue; I simply do not trust anything in any code or standard until I
can explain it to myself like I'm 6 years old.
May I send you a spread sheet I made many years ago? It will allow you to
quickly calc the req. sec. mod. based on any pipe type (not just S10 or
S40), any span, any length of pipe being hung, no matter 'where in the span'
you are. It calculates the amount of steel based on 490 lbs/ft3 and water at
62.4 lbs/ft3. It also calculates the Available Section Modulus for pipe
trapeze based on type (i.e. not just s10 or s40). You will see it matches
the table perfetctly.
If you still say Gibberish I will print this post and eat it :)
ps, the spreadsheet is rather ugly and not real user friendly because it
takes after me
On 6/3/2013 7:51 AM, ParsleyConsulting wrote:
Brad,
Part 1 - Gibberish. Pure and simple.
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