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RE: C & C pressures, trusses[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "seaint" <seaint(--nospam--at)seaint.org>
- Subject: RE: C & C pressures, trusses
- From: "Andrew Kester, P.E." <akester(--nospam--at)cfl.rr.com>
- Date: Mon, 12 May 2008 13:20:40 -0400
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.
Andrew Kester, P.E.
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803
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