Press Plate Wood Trusses
Press Plate Wood Trusses
(OP)
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.
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.






RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
The chapter 3 (for ANSI/TPI 1-2007) is a free read only from this link
http://www.tpinst.org/ANSI_TPI1-2007_Ch%203_QC_web...
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.
RE: Press Plate Wood Trusses
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.p...)
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
EIT
www.HowToEngineer.com
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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?
RE: Press Plate Wood Trusses
OK - so the gaps are OK since the plates don't cost much? Not sure I understand you there.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
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.
RE: Press Plate Wood Trusses
See page 22.
http://www.tpinst.org/ANSI_TPI1-2007_Ch%203_QC_web.pdf
RE: Press Plate Wood Trusses
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.