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Re: C & C pressures, trusses

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This kind of e-mail is why I read this list.  Well said, and instructional.

Thanks Andrew.


On Mon, May 12, 2008 at 10:20 AM, Andrew Kester, P.E.
<akester(--nospam--at)> wrote:
> Conrad,
> I read most of your post, time allowing. It is well thought our and
> researched, but with all due respect, I must kindly disagree with the
> overall assertion that trusses are MWFRS. And I do read most of your posts
> because I find you very knowledgeable on the subject of wind pressures.
> ASCE and Florida Building Code (FBC) define MWFRS as "An assemblage of
> structural elements assigned to provide support and stability of the overall
> structure. The system generally receives wind loading from more than one
> surface."
> Components and Cladding are basically everything else. A truss and plywood
> system is a MWFRS when it is part of the shear diaphgram, and I use MWFRS in
> computing the shear force in the diaphragm. But in regards to uplift loads
> on an individual truss, I do not see a way that a standard pre-eng roof
> truss could ever be a MWFRS. Also, in an official FBC wind course they
> clearly indicated trusses and roof joists as C and C. So with this
> information, I would fear Board action if it was ever found that we were
> using MWFRS for roof element design.
> You have mostly reinforced my reasons for continuing to use C and C
> pressures for truss and other component design per your discussion on
> localized pressure effects. It is the localized effects that make truss
> systems so vulnerable to wind pressures. Overhangs, jack trusses, hip and
> ridge trusses, etc. all may receive non-uniform pressures. Some of these
> trusses are 1'-6" long and 2' o.c., this is a very small trib. Sure your
> truss girders may get up there in TA but they too may have areas of loading
> vulnerable to local effects.
> It takes one little jack truss to come apart and the wind can get its claws
> into a roof system and wreaks havoc. I have seen it in Mississippi after
> Katrina and in Florida after hurricanes and tornados (I have done lots of
> storm forensics luckily). It is my professional opinion that trusses and
> plywood, along with garage doors, doors, and windows, are the main factor in
> many wind load failures as they are the weakest points (and their connecting
> elements of course). So I believe in erring on the conservative side of
> things in this regard, if I am making an error.
> Cost VS Benefit
> The cost of a truss package and the connections are a small percentage of
> the cost of your average house or multi-story building. The average uplift
> strap is less than $1. A slightly higher wind pressure in truss design means
> a couple more webs, maybe a couple of bigger mending plates. Since your
> average kitchen cabinets will cost as much as all of the roof trusses and
> connections, this is money well spent as it may save your life or your
> house. So even if it were found to be technically OK to use a (lower) MWFRS
> pressure, I would advocate using a slightly higher pressure due to the
> importance and vulnerability of these elements.
> Kindest Regards,
> Andrew
> Andrew Kester, P.E.
> Principal/Project Manager
> ADK Structural Engineering, PLLC
> 1510 E. Colonial Ave., Suite 301
> Orlando, FL 32803

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