To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: Re: Question on wood Roof Trusses
From: Monty Hart <montyh(--nospam--at)gci.net>
Date: Tue, 16 Jan 2001 13:04:09 -0900
Good guess Bill. In fact, a number of year ago, when I confronted the chief engineer
with one of the major truss plate manufacturers as to why they did that, he said it was
to be "competitive". No other reason was offered.
Unfortunately, correcting the error on my projects does not really address the problem,
as the manufacturers continue using the primary frames coefficient for 99.5% per cent
of their work and almost nobody (engineer or building department) ever catches it. In
fact, the last "truss technician" that I confronted on the issue, had never designed a
truss with the higher primary frames coefficient. I've seen this now with several
truss manufacturers, in two states, over a period of 12 years.
For the 1/2% of engineers who are knowledgeable and do choose to correct the error,
they are rewarded with less and less work, as the extra costs of building their
projects works in reverse on their competitiveness. Not a very satisfying solution to
those of us trying to feed our families.
If the practice is wrong, it should not be used on any projects. If it is correct, I'm
waiting to hear the justification, and competitiveness is not a justification.
Bill Polhemus wrote:
> Can I venture a guess, and see if I'm right?
> The truss manufacturers are simply looking to save money, that's all.
> They have somehow justified to themselves that the joists are part of the
> "primary framing" (or "Main Wind Force Resisting System" to use the ASCE 7
> terminology) rather than an "element/component" (or "Components and Cladding"
> per ASCE).
> I, too, think they are wrong, and I would simply specify on the drawings or
> specs that the joists are to be so designed, and fix the problem once and for
> Monty Hart wrote:
> > While we are discussing roof trusses. Can someone tell me why the truss
> > manufacturers use the "Primary frames and systems" Wind Pressure Coefficients (Cq
> > in the '97 UBC Table 16-H), instead of the "Elements and Components"
> > coefficients.