Press Plate Wood Trusses

[dcarr82775]

I have a project with press plate wood trusses. TPI-1 gives allowable gaps between the wood and the plates (lack of full penetration of the plate). I see rampant issues where the gaps greatly exceed these allowable gaps. Both the framer and truss supplier say this gaps are typical happen all the time and are of no concern. Gaps routinely are close to 1/8" over a fair portion of the plate.

I don't deal with a ton of this type of truss so I wanted to ask some folks who deal with them on a more regular basis if these large gaps are the norm and left alone. Obviously it depends on if the member is in tension or compression, how heavily loaded it is, etc. I am just looking for a general feel if I am making a mountain out of a molehill.

 

[Ron]

dcarr82775...Yes, the framer and the supplier would say that. Once they deliver and install the trusses they are done within them. They never see the result of failures, mostly because buildings never see the loads for which they are designed. We have a different obligation and the guidelines of the TPI should certainly be followed. The observations of the framer and supplier are anecdotal....if those defective trusses were tested, there is a high likelihood of failure.

The penetration points on a truss plate are triangular. They achieve their highest resistance when the whole triangle is embedded in wood. As the triangle becomes less embedded, the resistance drops off quickly...remember, the greatest area of that triangle is at its base.

 

[woodman88]

It really depends on the method they design the joint plates by. Are you quaified to do this type of inspection? Also do you have the design information to do an inspection properly?

The chapter 3 (for ANSI/TPI 1-2007) is a free read only from this link

http://www.tpinst.org/ANSI_TPI1-2007_Ch 3_QC_web.pdf

Other places for some information are.

http://sbcmag.info/Archive/2008/may/0805_tpi.pdf

http://www.tpinst.org/

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[TXStructural]

1/8" is a big gap. Teeth of plates should be pressed fully into the wood UNLESS the plate manufacturer allows otherwise and the designer took it into account. Failure to control quality of plate-connected wood trusses is a problem, and failure to adhere to good practice is a problem in the industry. Properly designed, manufactured, and installed, these trusses provide good value to owners, so long as they are properly made and braced, and NEVER overloaded or to exposed to fire.

You can also obtain the ICC-ES report for the manufacturer's plates, which specifies minimum truss manufacturing parameters. (for example,

http://www.icc-es.org/Reports/pdf_files/ESR-1118.pdf)

 

[concretemasonry]

It is fortunate they are press plate trusses because the are made in accurate jigs and usually have plates on both sides. The low cost systems using the older roller pressure were not as reliable.

Usually, the plate requirements and designs are engineered by the plate manufacturer for the specific truss run. The possible gaps are usually compensated for in the minor cost item of the plates.

Often, the gaps can come from drying shrinkage of the lumber over time when restrained by the structural connectors.

If you dealing with and are really concerned, turn to the truss supplier (and also probably the plate supplier). A reputable truss supplier should be able to demonstrate the typical performance of trusses designed for the typical designed loads supplied. - I don't think you would want to go through the design and detailed certification and make sure the final trusses are made to the generally accepted standards.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RFreund]

I've put up quite a few trusses (from the framing end of things) and I don't remember routinely seeing large gaps like that. Although I was not paying much attention at time... I'd be curious to see what my father has to say about it, they've been doing it for a long time (not that it should really influence the decision either way).

EIT

http://www.HowToEngineer.com

 

[dcarr82775]

It is a building under construction, I have the piece drawings, and yes I can look at a truss and tell if the plate is embedded per TPI1-2007. I can look at a truss and determine compression/tension members so I have my own opinion on if the 'damage' at a given joint matters. Some might/might not matter, others surely matter (tension chord splices, diagonals at bearings, etc.). In my mind the decision on the acceptability of the truss is best made by the truss designer since it is an engineered component, although ultimately I have to sign off on the building. With over 100,000 sqft of floor it is an issue.

I have asked the truss manufacturer to either design/specify repairs to the plates, or the truss engineer can look at them and decide if they are acceptable being out of spec. My reading of everything says the plates have to be embedded withing the TPI-1 tolerances, I have seen no exceptions. The truss company will not bring their engineer out, and said even if they did, they say he would not write such a letter....very comforting.

 

[woodman88]

"... The truss company will not bring their engineer out, and said even if they did, they say he would not write such a letter....very comforting."
Actually, it should be comforting. Having an engineer who knows that they are not qualified to do wood truss plate quality inspections is a good thing.

The real question is how you know that the trusses are "...being out of spec..." from the information you have? Can you to upload the information, from the manufacturer, on one of the trusses so that I can review it?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dcarr82775]

I actually don't find it very comforting that the designer of the truss may not think he is qualified to speak to its as-built adequacy. But anyway, attached is an example of a random truss with various identifying bits of info removed.

My opinion is based on TPI-1. I can't find anywhere that permits reductions in the plate strength based on reduced embedment depth. Is there such a thing?

 

[JAE]

The possible gaps are usually compensated for in the minor cost item of the plates.

OK - so the gaps are OK since the plates don't cost much? Not sure I understand you there.

 

[TXStructural]

The ITW/Alpine and MiTek ES reports both state: "All truss plates are pressed into the wood for the full depth of their teeth..."

Many truss manufacturers actually do not have engineers on staff. These folks layout the truss using proprietary software, press a button to have the design sent to engineers with the plate manufacturer, and the sealed drawings are sent back. In my experience, the people who visit job sites from these suppliers, to evaluate problems, seldom know enough about engineering to provide proper guidance. Any reputable truss company will stand behind their work and will provide field support as needed.

You are making a mountain of a mountain, not a molehill.

 

[woodman88]

"dcarr82775 (Structural)
2 Aug 13 9:50
I actually don't find it very comforting that the designer of the truss may not think he is qualified to speak to its as-built adequacy..."

It is good that you have all the required certificates for the "special inspections" for the jobs you design. For I do not have the time or money to do so and so I would not take the liability of doing the inspections.

So I am just like the "designer of the truss" who will state that as designed the structure/truss will work. But if you want the as-built to be inspected, than you need an inspector that is certified for that type of inspection.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[woodman88]

TXStructural

The ICC report in the link in an above post, also states that the must be assembled per the tolerances in the ANSI/TPI 1-2007 which does allows gaps. You might want to read the complete section of the reports you are quoting.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dhengr]

Dcarr82775:
I think TXStructural’s 2AUG, 12:24 post pretty well sums up the situation as I’ve experienced it, w.r.t. truss manufacturers. Most of the designers of trusses are just computer operators with a few basic rules to follow, and know nothing about the real engineering of the trusses. I’ve gone directly to their truss plate/software provider/supplier with serious engineering issues. And, their engineering people tell the truss manuf’er. how to fix their problem, or design that special truss. If this defect/deficiency is common and can be readily documented, and the truss manufacturer won’t address it and write a letter that it is o.k. and should cause no long term problems, and will still be covered by their warrantee, etc., then just reject the trusses. Instead of sending out an engineer or knowledgeable company rep., they can send out a truck to pick up the trusses, and be back charged for costs and delays. This might be one time when the local bldg. inspector is your friend, he could reject them if properly educated; and he could be involved in the inspection meeting at the site too. At this stage, given what you’ve said, it sounds more like a Q.C. problem at their plant, than a special inspection or as-built issue. I’m not usually in favor of making a big issue or legal matter out of something which can be handled diplomatically, but sometimes there is no other alternative.

 

[dcarr82775]

woodman88,
I know a gap of 1/32" is allowed, I have been through TPI-1 and the commentary. But what happens when the gap is bigger than 1/32"? I can't find anything on that. I only raised the issue because the gaps are routinely much much larger than that. I understand the 'dilemma' of the truss engineer. But there has to somebody in the darn chain that can make a call on the acceptability of their engineered component. If not the guy who designed it, than who? A 'special inspector' who took an 8hour class (maybe it is 16 hours I don't know) to get a certificate?

I attached a page of a sample truss, hopefully with everything that identifies any parties removed.

 

[NITTANYRAY]

I am not sure which edition of TPI 1 you have.
In the 2002 edition there is a table for effective tooth embedment (A3.4-1)
0" <G< 1/32" 100% effective
1/32" <G< 1/16" 60% effective
1/16" <G< 3/32" 40% effective
G> 3/32" 0% effective
If you have 1/8" gaps those trusses should be rejected.

 

[NITTANYRAY]

I down loaded chapter 3 from TPI-1 2007 and the table on effective plate values is the same.
See page 22.
[URL unfurl="true"]http://www.tpinst.org/ANSI_TPI1-2007_Ch%203_QC_web.pdf[/url]

 

[woodman88]

dcarr82775
The above post effective "tooth" values are correct. But they are applied to the teeth and not the plate. So a plate with more than the 3/32" gap will be good at the teeth locations that are 3/32" or less inch gaps.

Now I have been out of wood truss design for ten years. But for the truss drawing you uploaded. The FT (Fabrication Tolerance?) values appear to be 10%F, 10%E. These % will cause the program to oversize the plates. With a plate grip of 220/195 psi per plate. The two main forces of concern are web W2 B-N and B-M of 1825 lbs and 652 lbs. Web B-M 652/2(220) = 1.5 sq.in. per side of good plates at 100%. Web 2 (B-N) 1825/440 = 4.2 sq. in. per side of good plates at 100%. Now base on this information do you still see a problem with the trusses?

Now when I was designing wood trusses the compression members required plate area could be reduced for certain conditions. Whether that is still true or not I do not know.

As for "...But there has to somebody in the darn chain that can make a call on the acceptability of their engineered component..." Yes the manufacturer is responsible. Their contract/warranty should state this. And since they have already given written "proof" of their trusses, they probity are wondering why you need more.

Now if you really want someone to approve/look at the trusses. I would take a picture of two of the critical joints with gaps. Do a quick hand calculation of the plating. Send it to the truss manufacturer and ask if your calculations are correct or not. If they don't give you a acceptable answer, send a copy to the truss engineer. Hopefully you will get an acceptable answer and be done. If not you will have shown that you tried to address the situation this way before going farther.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.