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Response to Roger Turk and Nels Roselund on Trusses by WTCA[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'SEAOC'" <seaoc(--nospam--at)seaoc.org>
- Subject: Response to Roger Turk and Nels Roselund on Trusses by WTCA
- From: "WTCA" <WTCA(--nospam--at)classic.msn.com>
- Date: Sun, 9 Aug 1998 19:49:50 -0500
- Cc: <73527.1356(--nospam--at)compuserve.com>, <NRoselund(--nospam--at)aol.com>
- Disposition-notification-to: "WTCA" <WTCA(--nospam--at)classic.msn.com>
To Nels and Roger,
Please see comments after specific text below.
12 Message:0012 12
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Subject: factory-made roof trusses
Get a copy of MasterSpec Section 06192 (Metal-Plate-Connected Wood Trusses)
from your architect-client. MasterSpec is licensed, otherwise I would offer
to send you a copy.
The Wood Truss Handbook has a sample specification that can be found in Appendix G.
Since most pre-fab truss fabricators use computer analysis provided by the
plate manufacturer, which kicks out *anything* that is permitted by the code,
I would specify that only No. 2 or better grade is to be used. (I have seen
No. 3 called for on their calcs and I have seen No. 3 in lumber yards --- No
way would I ever permit No. 3 to be used on any of my jobs, except as
The only place that you should find #3 is for truss webs. The grade rules just do not allow this material to be reliable enough, particularly for high tension stresses found in truss bottom chords for it to be used in any location other than low force webs. Our industry does however presume that the allowable design values provided by the lumber industry are reliable. Part of the reason you will see a good deal of MSR or MEL used is to enhance this reliability.
Make sure that you clearly specify the loads that are to be applied to the
top chord and the loads that are to be applied to the bottom chord. I
investigated one failure where non-structural wood box beams at 6' centers
were connected adjacent to every fourth truss, causing the web member to
chord connections to fail. (The trusses also lacked lateral bracing of
compression web members --- see next paragraph.)
Your point here is very valid. If the location and magnitude of the load is not well defined for us, there will be an assumption made. We hope the assumption is a good one but it may not be. Most of our problems come from poor communication.
Designs that I have seen typically will not spell out lateral bracing of web members in compression. I specify that *all* bracing that is required as a condition of design is to be clearly detailed on the fabricator's plans.
Our truss design drawings will show the number of lateral braces and their location on all compression webs that need them on the single truss design.
Be aware that truss fabricators are neither licensed, trained nor franchised by the metal plate manufacturers. The metal plate manufacturers merely sell the fabricators the metal plates and the engineering. Therefore, the quality of work produced by different fabricators can vary tremendously.
WTCA has just completed our in-plant quality assurance program called WTCA QC version 3.0. It is a computer database program that compiles information that a plant can use to assess a variety of production attributes. You can certainly specify a truss manufacturer that uses this program to monitor quality. Obviously with a database you can provide graphical trends on quality and take corrective action more easily than what is done now. Right now we are just in the implementation phase so the number of WTCA certified plants is few. We expect this to change rapidly over the next few years. We could use specifiers help encouraging plants to participate in this program.
Additionally, WTCA has a program called Truss Technician Training where we train on load development, trig, plan reading, building codes, etc. Our goal is to have a method to evaluate the competence of those reading plans and doing take-offs. We currently have about 350 level 1 certified and 100 level 2 certified truss technicians. We plan to develop 5 levels of this program.
Kirk Grundahl, P.E.
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