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Truss plate sizing design for prefab trusses

Truss plate sizing design for prefab trusses

Truss plate sizing design for prefab trusses

(OP)
Has anyone else wondered how truss plates are tested and rated for certain loads (say 150p/si of plate)? I get the feeling that lab tests are absolutely straight pulls on the plate, yet the plates at actual joints are pulled in several directions at once. How do truss engineers justify slapping a 6x10 heel plate on a joint between a 2x8 topchord and a 2x4 bottom chord? The plate has to work as a cantilever off the 2x4 and has to rotate in order to transfer tension loads. Yes - I know that factors are applied for perpendicular and parallel stress - but the forces are offset, yielding a moment force on the teeth. Does the truss supplier software take these factors into consideration when they size a plate?

RE: Truss plate sizing design for prefab trusses

ANSI/TPI 1-2002 Chapter 5 specifies the standard tests for plates.  Our software does model heel joints to account for partial rigidity.  It uses combined wood/steel property members to model the heel joint to account for the moment.       

RE: Truss plate sizing design for prefab trusses

(OP)
CJS: Could you be more specific please: is the Chap 5 standard test a 'straight pull' or is it a moment plus force test? I did ask how these ratings of (say 150p/si) are obtained.
OK - you've got some accurate modeling program. But my basic question was how do you convert all those moments and forces into a 6x10 plate?
A final question arises when the fabricator tries to accurately position this 6x10 plate. What is the methodology for locating this plate for most efficient load transfer to all those finite teeth elements?

RE: Truss plate sizing design for prefab trusses

Plates are typically designed with standard positions that the guys in the shop are familiar with.  If the designer is clever, he can sometimes get a smaller plate to work by repositioning the plate from it's "normal" position, but then a plate placement diagram is generated which shows the fabricators where it is supposed to be placed.  Tolerance for placement is built into the calcs.

I don't think Chapter 5 gets into determining moment capacities of the plates, only how to test the specimens and what minimum tests are required.  They show straight pull tests and also require 6 tests where the plates are skewed at varying angles across a shear plane.  I believe the moment capacities are determined by the manufacturers and used proprietarily (if that's a word).

RE: Truss plate sizing design for prefab trusses

Agree with ctcray.  The standard I don't think has any tests for moment.  The programs are proprietary and have built in checks for moment.  The grip ratings are based on a pull test for different orientations of load with respect to grain.  Based on those tests you can calculate allowable loads at various angles.  There are also tests for tension (of the plates only) and shear (of plates only) for various angles.  How the forces are converted to the plates is proprietary and depends on the software.  You'd have to look at each software individually to answer that question.
We use self calibrating laser jigs to set the plates.  They project the outline of the plate on to the truss.  They are tied into our software and are very accurate.         

RE: Truss plate sizing design for prefab trusses

CJSchwartz:  What Cq factor does your plant use for roof trusses?  We use 1.0 so we only have to do Plate Placement Method checks.  Of course, that knocks 20% of our allowable plate values straight away, but avoids the cumbersome QA requires with higher Cq values.  The proposed changes to the next TPI looks like it will have better (or more reasonable) QA requirements.  

Another question:  I can imagine the laser helps with accurate placement on the top side of the truss, but what about the back side?

RE: Truss plate sizing design for prefab trusses

We also use Cq of 1.0 and use the PPM for quality control.  I haven't really noticed a problem with the Cq of 1.0.  I actually prefer the 20% reduction in allowable plate values as I've seen some plate coverages that are pretty slim and still work.  

The back side of the truss requires good judgement on the truss builders part.  Larger plates aren't a problem.  The smaller plates seem to be the ones that are harder to locate on the back side.  

RE: Truss plate sizing design for prefab trusses

(OP)
Sorry folks, but due to some kind of computer glitch I haven't been able to view your replies from my computer (I have to open the site from another machine!).

Based on replies I have read: it seems that tests on plates do not mimic real-time forces on plates or time-dependent creep and deterioration of holding strength. The 'black box' proprietary solutions for plate design mentioned should be transparent to outside engineers.

It is surprising to me that code authorities even allow plate manufacturers (who have a vested interest in minimizing plate sizes to reduce costs) to claim that gang-nail plate behavior is 'proprietary'. It's one thing to have a different tooth design and protect it by patent, and quite another to claim that shear, tension and moment forces can not be revealed because they are a 'secret'.

And I'm not referring to secondary moment forces; only those caused by forces not acting on the same line of action.

Is anyone aware of any load tests performed on full-scale prefab trusses? It would really seem like the time has come to compare actual truss failure loads with 'proprietary' black box solutions.

RE: Truss plate sizing design for prefab trusses

WOw, you seem so skeptical!  I think plate manufacturers have a vested interest in their product not failing, foremost... AND they make more money by selling LARGER plates, not smaller.  So I think the conspiracy is non-existent.  


Why don't you call Mitek or Alpine and talk to their engineers, I don't think they're hiding from you.

ALso, I think the SBC just built a facility to test trusses full scale.

http://www.sbcmag.info/Archive/2007/may/0705_sbcri.pdf

RE: Truss plate sizing design for prefab trusses

(OP)
Thank you for the link to SBC truss testing facility - those photos say it all and I can't wait to see results of load testing.

In regards to the subject of vested interest on the part of a material supplier, if you consider that various fabricators will try to submit lower bids in order to get the job you will understand the reason for using smaller plates once you do get the job.

I still have no understanding of what *methodology is used by 'black box' decisions to use a 6x10 vs a 4x14 at a heel joint. Anyone?

*or should I call it faith-based

RE: Truss plate sizing design for prefab trusses

Plate cost is typically a small percentage of the total job cost (~ 5%)  Decreasing plate sizes isn't going to result in a significant savings.  When we try to reduce costs on a job, we typically try to see if we can alter the layout to remove trusses, decrease girder plies, and minimize webbing.  I never try to reduce plate sizes to save cost.  The software will design the joint for the minimum plate size.  Typically, you can not reduce it.  If we alter plates, it is usually to increase them.      

The methodology used by 'black box' decisions is no secret.  It is simple matrix analysis.  The proprietary part of the programs are how they choose to model the various configurations.  For example the location of the nodes at a pitch break may be slightly different from one software to the next.

The decision to use a 6x10 vs. a 4x14 is fairly complicated and would take too long to describe here.  If you are interested in learning, I would suggest contacting one of the plate manufacturers and talking with them to see if they have any information they could give you.  Good luck.  

Until then, faith based may be your best bet.        

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