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Roof Truss Failures
August 25, 2009
7:09 am
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architect1281
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Con Arb
Very correctly so (and to the wailing and moaning of many a builder) most of the RI local jurisdictions AHJ''s REQUIRE the submission of FULL TRUSS ENGINEERING AND LAYOUT prior to permit issuance. AT the state level I require them prior to even foundation permitting

(IT" THE LOAD PATH THING)

AS a RDP I include in the plans a required shop review of truss layout because Girder Placement and the increased loads they apply are CRITICAL. particularly when hip arrangements may vary based on supplier, pitch , or lumber grade.

It ain''t easy its engineering

August 22, 2009
4:07 pm
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george roberts
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What I find bothersome about this thread is that the AHJs seem not to know about the prescriptive methods of bracing.

(Of course Uncle Bob points us in the right direction. 🙂 )

August 22, 2009
2:16 pm
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rosso
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Struc,

"2006 IRC, R802.10.3 Bracing. trusses shall be braced to prevent rotation and provide lateral stability in accordance with the requirements specified in the construction documents for the building and on the individual truss design drawings. In the absence of specific bracing requriements, trusses shall be braced in accordance with the [b]Building Component Safety Information (BCSI 1-03) Guide to Good Practice for Handling, Installing & Bracing of Metal Plate Connected Wood trusses.[/b]"

So, if the construction documents do not have the proper bracing specification; use BCSI 1-03.

In this little booklet; you will find [b]BCSI-B2 TRUSS INSTALLATION AND "TEMPORARY BRACING"[/b]; that will provide "step by step instructions" on erecting and temporary bracing [b](with pictures)[/b].

It includes everything you need.

What the wonderfulness of herself, Peach; was refering to ("You need to also look at the bottom chord bracing") is shown on page 15 (BCSI-B1, 2) BOTTOM CHORD TEMPORARY/PERMANENT BRACING.

Hope this helps,

Uncle Bob

August 22, 2009
11:58 am
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rickastoria
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CA: Henceforth, what deferred submission is about.

The building designer / architect''s job is not done. He needs to remain to prepare these final elements before the trusses are made and installed.

August 22, 2009
11:54 am
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rickastoria
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If the Building designer designed the truss then yes, absolutely.

Whenever there is truss being involved, this needs to be done before "plan submittal" or deferred submission is involved. The designer needs to make all these matters addressed before the trusses are installed.

The principle act here is - coordination and cooperation. The truss designer/manufacturer is acting in many regards as a consultant to the building designer (or architect) who is the project leader.

As for X braces, I''m not all that aware of "X" bracing for trusses. There is diagonal bracing. Maybe we are talking floor trusses and "box trussing".

struc does bring a good point but that can be addressed by good project planning, coordination and team work with the consultants and outlining responsibility. The truss manufacturer/designer needs to coordinate the details and explain the details to the building designer so he/she can make the proper design decision throughout the building.

The truss designer/engineer of the truss manufacturer needs to work with the building designer (or architect) much like a consulting engineer (or any other consultant). This is a team, matter.

August 22, 2009
11:51 am
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constructionarbitrator
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[QUOTE] [b]"Struc said:"[/b] It is difficult for the designer to call out bracing for a product when they have limited control over the design of that product. When plans are produced and submitted, the truss manufacturer hasn''t designed the trusses yet, so web configurations aren''t known yet, the stresses aren''t known yet, so the bracing can''t be properly designed-[/QUOTE]This has always been a problem with lightweight trusses that aren''t even designed until after the job is permitted, theoretically it''s the responsibility lies with the "Design Professional of Record", yet many times there is no real DPR after the job starts, most plans state that "Truss drawings to be supplied by truss manufacturer'', and many times the builder doesn''t even know who that''s going to be until well into the job, the field inspector just picks up copies of the truss designs and calculations at framing inspection and puts them into the file. There is a huge disconnect here.

August 22, 2009
11:25 am
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rdahlmann
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Peach,
How have you seen this diagonal bracing called out on plans? It is difficult for the designer to call out bracing for a product when they have limited control over the design of that product. When plans are produced and submitted, the truss manufacturer hasn''t designed the trusses yet, so web configurations aren''t known yet, the stresses aren''t known yet, so the bracing can''t be properly designed- but it is the building designers responsibility??? How does that work!

This has been a point of conflict between the design industry and the truss manufacturers for a while. Designers think the manufacturer should provide this since they have all the information, but the manufacturers don''t want to take this responsibility. The truss people have put it in writing, so they have the edge at the moment- but this discussion isn''t over yet.

August 22, 2009
6:56 am
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peach!!
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Remember that the truss bracing shown on the truss shop drawings is not the only bracing required. The "X" bracing, etc. is left up to the building designer.

You need to also look at the bottom chord bracing.. if there isn''t a ceiling attached, the rat runs are even more criticial (trusses topple and bend when they are over loaded laterally)

August 21, 2009
12:58 pm
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glenn roy denman
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I always verify the size of the plates per the specs. I''ve run accross undersized plates, missing plates, etc. The local truss manufacturers here provide a couple of one inch wide (or so) staples on each side of the member being joined, through the plates. I guess this keeps them from working their way out like conarb''s pics show.

August 15, 2009
1:10 pm
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drp
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It might be worth reviewing AITC 104-2003.
http://www.aitc-glulam.org/sho.....xam-oregon itc_104_2003.pdf
Truss connections begin on page 20, Also note detail A16.

August 15, 2009
10:23 am
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constructionarbitrator
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These truss plates won''t twist out or collapse. This is why truss identification should be limited to lightweight trusses and not all trusses.

August 14, 2009
5:38 pm
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rosso
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Boo,

I agree and if more Building Officials would require a qualified truss engineer to inspect and approve the installation it would be a major improvement. We did it on post-tension foundations; there is no reason not to do it on Truss systems.

Uncle Bob

August 14, 2009
2:19 pm
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rickastoria
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What else is an engineering department for?

😀

August 14, 2009
1:15 pm
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boo1
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sp_Print Print Post Post #14

lol

Back to the subject, MPC wood trusses are an engineered product. But we don?t have engineering inspections (except 60 ft and greater) requirements.

I feel few residential inspectors are qualified to conduct MPC truss inspections. ?Engineering judgment becomes critical in the determination of what is acceptable.?

August 14, 2009
9:23 am
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rosso
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Boo,

Whew, for a second there I though you were going to give number 3. away.

Uncle Bob

August 14, 2009
9:20 am
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boo1
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Permit Placement:
1. Building permit shall be placed six (6) inches from road way. Permit location shall providing drive up access.
2. Permit shall be place with the signature block 48 inches from road way or at window height of F100 truck.
3. Donation envelope shall be located...

August 14, 2009
7:48 am
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jdrobysh
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sp_Print Print Post Post #17

Boo - You''re supposed to slow down, silly!

August 14, 2009
7:42 am
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rosso
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sp_Print Print Post Post #18

Boo,

LMBO, thanks for brightening my morning; don''t you have field glasses. 🙂

Uncle Bob

August 14, 2009
7:30 am
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boo1
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But they are so hard to inspect as we drive by.

August 14, 2009
6:39 am
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rosso
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Rick,

I am by no means or stretch of the imagination; an expert on metal plate connected truss systems; however, with my limited knowledge, experience and training; I could write a book on your lack of knowledge conserning wood truss installations.

You don''t seem to understand that metal plate wood truss systems are a specialty area of Engineering; which is not part of general structural engineering. No person, without certified, specialized training in metal plate connected truss systems has any business [b]attempting[/b] to design the systems.

My only area of knowledge is limited to proper installation; with a smidgen of how they are manufactured; which also has provided me with an ample amount of observation of their weakness''.

You don''t sit at a drafting table in your study and design truss systems, and tell the framers how to build them; they are [b]manufactured[/b]; not stick built.

They are not manufactured to withstand muliple re-roofings, with laborers clomping about, tossing bundles of shingles on them.

As John stated; "that just standing there, trusses have all the structural integrity required by the code"; and code adequacy does not take into consideration the pounding they will take during re-roofing.

In Central Texas the "real" lifespan of shingles is from 3 to 6 years; due to the numerous incidents of hail; and truss systems [b]are not[/b] designed to sustain multiple stacks of shingles being tossed on top of them.

When laborers (roofers) stack (toss) shingles on roofs; they don''t open BCSI 1-03 to BCSI-B1, page 18 and BCSI-B4, page 57 through 59; and take the necessary precautions to protect the integrity of the truss system!

Example; maximum staking height for asphalt shingles is "two bundles"; [b]fat chance of that happening[/b].

Truss systems are not part of the plans on home designs; they are seperate and require [b]a separate[/b] set of Truss Design Drawings prepared in conformance to Section R802.10.1.

My disdain for truss systems comes from my experience inspecting their installation and studying their manufacturering. They are the least structurally sound product you can legally use.

Uncle Bob