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Roof Truss Failures
August 14, 2009
5:21 am
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jdrobysh
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conarb - How about ''teeth'' or ''pins''?

Rick - I think U B understands the extended span aspect of truss construction. As a code purist he is generally uncomfortable with the reductions in fire, life and (yes even) structural safety caused by the use of ever-more lightweight construction materials and methods.

Before you ''enlighten'' me on the structural adequacy of trusses - I too understand that just standing there, trusses have all the structural integrity required by the code. Even under normal environmental loads they are good. In a fire or other extreme circumstance they just don''t hold up or perform as well as traditional framing.

''Economic use of materials'' does not necessarily mean ''safer''.

August 13, 2009
7:19 pm
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rickastoria
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conarb, thanks for the clarification about what you meant.

I agree with you on that.

August 13, 2009
7:18 pm
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rickastoria
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UB: Not entirely true.

The idea of trusses is also about providing adequate structural framing for large spans without resorting to massive timber/steel members to achieve span. It is about economic use of material to achieve the required unsupported spans. Say you want a 40''x75'' little store with completely open interior so instead of having columns wasting up store fronts, trusses often provide an economical solution to addressing completely full span solutions with open interior without the requirement of posts located 12'' - 13'' into the floorspace.

Alternatives to the truss is Massive steel beams, Glulam, large timber beams, ect, Truss provides a means of achieving such with smaller wood members.

I''ve seen the use of multi-ply 2x10 members used in larger trusses at roughly 12''6" o.c. spacing.

August 13, 2009
7:14 pm
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constructionarbitrator
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By "tangs" I''m referring to the bent prongs in the plates that go into the wood of the trusses, if those were longer the plates wouldn''t pop loose as easily.
[QUOTE]The slender projecting tongue, or prong, forming part of one object that serves to secure it to another, as the projecting tongue on a chisel that secures it to a handle.?[/QUOTE]? http://www.answers.com/topic/tang

August 13, 2009
6:58 pm
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rosso
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ConArb,

The whole idea of using trusses is to provide the cheapest product possible.

Uncle Bob

August 13, 2009
6:46 pm
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rickastoria
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CA, can you illustrate what part of the truss that you are referring to as "tangs". The term is throwing me off. Just for clarity of what you mean by "tangs".

This might help:
http://www.wheeler-con.com/tru.....ltrus.html

or

http://www.wheeler-con.com/tru.....cab01.html

The main parts in the truss is called "web members" or "joint members".

The diagonal parts were known to be called "struts".

Other terms for the diagonal pieces inside a typical truss is called "brace". The lowwer/bottom chord is also called the "Tie Beam" and the upper chord is sometimes called the "rafter" or "principal" The vertical members in the truss are sometimes called "rods" (or "posts"). Like "King rod" or "king post" is used.

Then there is the purlins (horizontal member running along the roof plane / upper chord planes from truss to truss. Sometimes there is a diagonal brace along the plane of the upper chord.

There are all sorts of terms used. You probably know most of the terms used.

August 13, 2009
10:29 am
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boo1
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Lowest grades of lumber and lightest guage plates allowable lead to the cheapest truss package.

August 13, 2009
9:33 am
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constructionarbitrator
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Boo:

It seems to me that when everyone buys trusses they are always looking for the cheapest option, why doesn''t the truss industry use the "good, better, best" marketing approach? Offer trusses with heavier chords and plates and with longer "tangs" or whatever they are called. Car manufacturers (not a good example today) have always offered quality upgrade options.

August 13, 2009
9:10 am
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boo1
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Rick, my bet is that 95% of our industry doesn''t have ANSI/TPI 1-2007 in there office, read it or even know what it is.

Homes today can have 50 to 200 MCP trusses. If we take two minutes per truss to find it, inspect the truss reaction, strapping, bracing, metal plates, stud supports, uplift connection to foundation, load path, strongback bridging, interior wall connection, truss spacing, nailing we can spend several hours to inspect just the truss installation.

When reviewing truss installations, do inspectors review the truss program inputs; wind speed, exposure, classification, allowable bearing (you need to use the top plate lumber species, not the trusses lumber species), creep factor, dead loading (tile flooring require a higher dead load), open, partially open or enclosed design? We have found errors in the technician?s inputs before (garbage in garbage out).

How many BO require Registered Design Professional (RDP), to prepare erection and bracing plans for 60 foot and greater trusses? You need to review ANSI/TPI 1-2007 2.3.1.6 Long Span Truss Requirements.

The 134-page Commentary and Appendices of ANSI/TPI 1-2007 is available free on the web for review:
http://www.tpinst.org/TPI1-200.....es_web.pdf

August 13, 2009
8:48 am
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fredk
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We also had access to WCTA training and I learned how to properly inspect Metal Plate Connected Truss installation.

Can''t stress trainging in all areas of inspection should be taken advantage of. Call an most groups in what ever area ones want info on would be happy to talk with you or schedule a class. WCTA is one example.

August 13, 2009
12:01 am
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rickastoria
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I''m already got the ANSI/TPI 1-2007 edition that I''m basing on.

August 12, 2009
2:49 pm
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boo1
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UB, we do have good personel are in our industry. I like to learn from guys like you and your BO.

But, some lack the skills, education and training in some areas they inspect. Some who want to provide the best professional service, some who dont becouse of politics, time restraints, lack of knowledge or desire. It has been my experience most inspectors dont understand truss installation, load paths, QA issues, loads , bracing (temporary and permanant) or even reading the truss sheets.

ANSI/TPI 1-2007 will be the referenced standard for metal-plate-connected (MPC) wood trusses in the new 2009 International Residential Code? (IRC) and International Building Code? (IBC).
See chapter 2 for the responsibilities in the design.
http://www.tpinst.org/publicat.....-tpi1.html

August 10, 2009
8:39 am
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rosso
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sp_Print Print Post Post #13

Addendum to my last post;

I know I was the first; and, still may be the only inspector to join and become a member of the SBCA (Structural Building Components Association);

http://www.sbcindustry.com/ins.....7t77if4in3

I did let my memebership expire; but, would recommend it to at least the Inspections Departments (maybe one inspector for training purposes).

http://www.sbcindustry.com/education.php

They also have training for Fire Service Personnel;

http://www.sbcindustry.com/firepro.php

It''s a whole new world when you know what your looking at.

Uncle Bob

August 10, 2009
8:27 am
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rosso
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Boo,

When I was inspecting; there was not a minimum time limit on inspections; and, because I normally had to reinspect frames several times; I had ample time for the truss inspections.

I have to give my Building Official the credit. He also conducted inspections; and especially when I first started; he and a super knowledgeable fellow inspector would check my work periodically; which at first I didn''t appreciate; but, later found to be invaluable training.

We also had access to WCTA training and I learned how to properly inspect Metal Plate Connected Truss installation.

By the time I had conduct 50 inspections; I could pretty well walk through the house and find most violations very easily and quickly; one of those practice makes perfect rules.

I am also grateful to the great Truss Manufacturer''s Engineers; who would allow me to call them from the inspection site and then explain anything I wasn''t familiar with; and/or even come out to the site sometimes.

However, I doubt that even one (1%) percent of inspectors have any training or knowledge; or even know what they are looking at; on inspecting these truss installations.

Uncle Bob

August 10, 2009
8:06 am
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jdrobysh
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Boo1 -

Some of us do look at them. We have to check for the hurricane clips anyway, so spot checking plates for obvious defects is no big deal.

It''s the not-so-obvious defects that become problematic...

August 10, 2009
7:32 am
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boo1
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Do any residential inspector look at the truss nailing plates?

I bet you can''t see very many in the hour allowed to conduct the dry in inspection.

August 9, 2009
8:45 am
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rickastoria
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quote:What''s that round thing near the corner at the joining part?

A sch 40 "crosspipe". This is a heeljoint being researched and tested at Purdue. The bolts simply keep the wood in contact with the "crosspipe". It''s doing the same thing as the gang nail.

This is an older way of accomplishing the same thing;
-

Think about the forces, see the similarity? Not relying on sideplates and dowels in shear. Just another way of resisting the forces.

A nail plate is good for about 80-100 psi depending on wood density. A .120 nail through 1/2" ply in single shear is good for about 50 lbs. One crude way to reverse engineer a truss would be to allow 2 nails per square inch of nail plate. The nails cannot damage the wood so it''ll take more area to accomplish the same goal. There is alot more penetration so I would assume the withdrawal numbers will be better. Add glue and it should shine.

Table R802.5.1(9) is yet another way. The rafter/ceiling joist is really a truss. The table specifies the heeljoint connection. How many times have you seen 2 nails there? Kinda makes me think the truss isn''t that bad by comparison.

[ 08-09-2009, 05:52 AM: Message edited by: DRP ]
--------------------------------------------

Thanks for the info. I personally haven''t seen that method used but I can understand it. I''m not sure how it keeps the pieces together against forces in the direction perpendicular to the direction of the overall truss.

I agree 100% with you that a conventional roof is a truss. In fact it is called a simple truss compared to a compound truss. The rafters are the top chord and the ceiling joists is the bottom chord.

Great info.

August 9, 2009
7:55 am
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jdrobysh
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DRP - Much appreciated.

I noticed the pin for the post, just wasn''t sure if there was an unpinned tenon coming down.

It is increasingly difficult to find this type of craftsmanship in the real world these days. That''s a shame.

August 9, 2009
7:21 am
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drp
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There can be, in this case no, I extended the post tennon up through the bottom chord and rafter. There is also a hidden threaded rod through the top and bottom chords to keep the top chord seated in uplift. The design you mention is/was used but has fit up challenges which affect bearing so is not often used. The design takes into account the tension force in the tie at design load and the allowable horizontal shear in the area beyond the notch, also the compressive strength of the wood in the notch at that angle (it is neither perp nor parallell so is an interpolation between the two, Hankinson formula in the NDS).

In situations where the bottom chord extends out further the toe of the rafter can be buried rather than the heel. Typically that would be a major truss, purlins over, common rafters over the purlins situation. You''ll sometimes see an iron "shoe" used in this highly stressed joint as well.

C.A. Hewitt''s "Historic English Carpentry" is a great resource for joinery design as is the free Truss series and Historic American Timber Joinery, available on the TFGuild website.

We evolved through these methods, the stepped lapped rafter seat then in light frame the ceiling joists were cantilevered over the balloon framed walls, a "raising plate" was nailed across the cj''s and level cut rafters were nailed on top of that flat plate. Then to the modern birdsmouth. The modern cantilevered truss looks like the raising plate technique without the raising plate in between chords.

Mainly I was trying to show that there are not one or two options, there are many ways of skinning the cat. With proper design the notch or some form of developing shear resistance along the mating edges could absorb much of the force and the plates would simply hold the thing together. We tend to get "stuck" in mindsets real often.

August 9, 2009
6:33 am
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jdrobysh
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sp_Print Print Post Post #20

DRP - Thank you for the information.

My experience with heavy timber/post and beam types of joinery is somewhat limited. The photos you provide are excellent. Is there a motise/tenon along with the notch cut on the rafter?

Would that be effective with a 2 by rafter as well?

One of these days I''m going to have to take a serious hands-on in HT. They have some great programs that I have seen - spend a week working on an actual building going up.