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Wood Gusset Connected Trusses

Wood Gusset Connected Trusses

Wood Gusset Connected Trusses

(OP)
I've done some work analyzing and programming a truss calculator for MPC wood trusses however I've put that project on hold and I'm not looking at trusses made with plywood gussets. I'm not finding a significant amount of information online regarding these types of trusses. Does anyone have any links or even books they could recommend. I would like to create a truss calculator for this type of truss that is similar to the previous calculator I started working on, however this time I will incorporate the wind and snow load cases as well as engineer the gusset plates (ring shank nails and glue).

RE: Wood Gusset Connected Trusses

I don't know of any books on the subject offhand. I designed a garage roof using wood gusseted trusses several years ago. I specified nails and glue, but did not rely on the glue to transfer any load.

The amount of labor required to assemble the trusses tended to make the system uneconomical compared to trusses fabricated with light gauge steel truss plates, but the client wanted to do the work himself and was prepared to invest sufficient labor of his own to complete the project. Lots of nails on that job.

BA

RE: Wood Gusset Connected Trusses

(OP)
Can you tell me anything about the nails you used and any calculations you used to specify the nails and nailing patterns?

I'm researching methods used in fabricating with plywood gussets. I'm also of the same thinking with the glue, use it, but do not rely upon it in the engineering calculations, it just adds an additional factor of safety. As far as glue is concerned I'm currently looking at Liquid Nails (LNP-902) Subfloor Adhesive and Loctite PL Premium Fast Grab Construction Adhesive, both glues seem to have the right characteristics for this application.

I'm also trying to determine if using 3" nails clinched on the backside is superior to using smaller 6d or 8d ringshank nails (more fasteners). My thinking is that larger nails would be less in number but they may tend to split the members more than a smaller nail would. The downside of the smaller nail of course is more nails to have to nail to get the required strength, so more labor.

For the gussets themselves I've been reading a lot of interesting information on the Plywood vs. OSB. The one thing that makes me lean more toward plywood is that it doesn't swell when it gets wet which could be a problem for OSB before the roof sheathing is applied. I'm thinking 1/2 DF Structural I plywood.

RE: Wood Gusset Connected Trusses

The truss members were 2x4. The gussets were 1/2" Fir Ply. The nails were 2.5" common wire nails driven alternately from both sides, a code requirement when nails are not clinched. Spacing and allowable load per nail was in accordance with the code of the day, which I believe was CSA086-1980.

You should get a copy of the latest version of CSA086 or other timber code applicable to your area. The latest I have is CSA086-2001 but I am pretty sure there are more recent codes available today.

BA

RE: Wood Gusset Connected Trusses

You have to clinch the nails if you want to utilize each nail as a double shear connection. There have been technical reports on your question. typically for repairing broken trusses in the field. 1/2+1.5"+1/2"=2.5" => use 3" nails, cinched in double shear, both sides, spaced a 2-3" o.c. to reduce splitting of the 2x member.

I second BA, use glue but do not transfer any load to the glue.

RE: Wood Gusset Connected Trusses

The most current CSA O86 is 2009 for those that care. If you are buying it packaged inside the wood design manual (which I highly suggest) the most current edition of that book is 2010. It includes values for wood screws as well (I find sometimes it tough for the guys to swing hammers in some attics but they can get a mini impact driver and some 2 1/2" screws in there easily.

RE: Wood Gusset Connected Trusses

EngineeringEric,
Unless the latest edition of CSA086 has changed, nails may be used in double shear without clinching if they are alternately driven from each side.

jayrod,
The use of 2 1/2" screws sounds like a great idea for repairing trusses in place.

BA

RE: Wood Gusset Connected Trusses

@medeek,
'Structural Timber Design' by Abdy Kermani has a sample question on the design of a nailed-gusset eaves connection: can scan the relevant pages to you if you want. It details the nail spec and design of the ply gusset. butit's a UK book in SI units, and the 'K' factors will probably be different to those in US.

RE: Wood Gusset Connected Trusses

(OP)
Is there anything in the NDS 2012 that specifically addresses this sort of thing?

RE: Wood Gusset Connected Trusses

(OP)
If you can scan the pages from Structural Timber Design that would be greatly appreciated. At this stage I'm compiling every resource I can find (literature search) so that I have something to back up my calcs.

RE: Wood Gusset Connected Trusses

BA

That's what the last carpenter I sent a truss repair to said as well. Easier for him to do on site. It generally means it gets done closer to spec instead of him just putting nails wherever he can.

RE: Wood Gusset Connected Trusses

(OP)
This is kind of what I have in mind however I have not yet determined the proper engineering/research to size the plywood gusset plates, number of nails and correct spacing and edge distances.



I will be using the ANSI TPI 1-2007 to size the members and matrix analysis to determine the moments, axial and shear forces, similar to what I've done here:

Link

RE: Wood Gusset Connected Trusses

Ok, here's what I've normally done. Treat the whole thing as pinned (no moments at the joints). determined the member compression/tension. Then I figure out how many nails I need to transfer the largest members load at each joint. Then, by the time you fit all of those nails in using the allowable spacings as outlined by the CSA or NDS your plywood is sized for you.

RE: Wood Gusset Connected Trusses

(OP)
What do you consider the critical failure mode?

1.) Nails Shearing
2.) Nail pull out
3.) Plywood (tearing, crushing or shearing)
4.) Net section of the member

and would it be prudent to consider all of these failure modes.

Based on my previous research with MPC wood trusses I normally treat web members connecting to chords as pinned connections, however the peak joint, splice joint and the heel joint usually have some moment associated with them. The bottom chord splice joint shown in the diagram above is actually positioned so that it is very nearly at a zero moment along the bottom chord, this was done on purpose, hence only the tensile load is of any real concern. The peak joint can be modeled as rigid, semi-rigid or pinned, for simplicity I can go with pinned.

The main concern will be the heel joint since it is best to model this as rigid. The matrix analysis will give me the moment at this joint. I just need to figure out how to incorporate this additional load into the fastener loading.

RE: Wood Gusset Connected Trusses

I am confused about this exercise. If labor rate > $0/hr there is no savings. What is the goal?

RE: Wood Gusset Connected Trusses

just curious on why you are doing this

RE: Wood Gusset Connected Trusses

All of those things you listed, and some need to be checked. all are critical.

My process.
1) Find the load for all pin-pin, this is when all members are sized and checked
2) size the minimum size of plywood to take the load T/C with a pre-selected thickness (ie 15/32)
3) determine what my nail capacity is for pre selected nail (12d)
4) determine number of nails per 2x member
5) size check for plywood to handle all my nails
6) detail!

And some between the lines work is always required

RE: Wood Gusset Connected Trusses

Rarely do I worry about nail pullout since in general the nails are clinched. Nail shear is the main concern but the maximum load for the nails take into account the plywood and base material they are being fastened into (At least in the Canadian Wood Design Manual).

To be honest, I rarely check the individual truss members unless the loading is changing. If they are only being used for their original design loading then I assume that they were sized appropriately to begin with.

RE: Wood Gusset Connected Trusses

About 40 years back, we did a lot of northern Hudson Bay stores in northern Canada with wood trusses... we used a Cremona diagram to analyze them by drafting and the connections used 1/2" D.Fir plywood gussets. The fasteners used were Stelco hardened spiral truss gusset nails (8ga x 2-1/2" long, I think). I seem to recall we used a capacity of 250 lb/nail, which was very high and they were driven from alternate sides.

CMHC used to have a manual and they had prescribed designs for roof trusses, also. I'll try to upload a file... having problems with it for some reason...

Dik

RE: Wood Gusset Connected Trusses

(OP)
It seems like the Canadians have done a lot more with wood gussets than their neighbors to the south. I'm trying to determine the specs on the special hardened spiraal concrete nails that were used on these plans.

Another point of confusion for me after some further study of the NDS is with plywood. The NDS rates the strength of plywood not by its thickness but rather by the number of plies and by the span rating. What I would like to know is DF EXP1 STRUCTURAL 1 15/32 or 1/2 readily available and can I get it in 5 ply with a span rating of 32/16 or 24/0?

RE: Wood Gusset Connected Trusses

(OP)
After doing my requisite homework (remember I am still quite new to the structural field) I've determined that for double shear I have 4 yield limit equations for the fastener checks at each joint. I'm some previous posts I've seen comments about assigning a certain lateral force per fastener, this seems a whole lot simpler than running a bunch of calculations. Where did these simpler allowable load per nail come from?

RE: Wood Gusset Connected Trusses

Factored lateral resistance for various nails and various penetrations are given in the Wood Design Manual printed by the Canadian Wood Council. The values are based on the most critical failure mode given in the code.

BA

RE: Wood Gusset Connected Trusses

(OP)
After running a few numbers for 10d nails into 15/32 DF Structural 1 plywood (5 ply) with DF truss members it looks like the 10d nails don't have as much capacity as I originally thought. In particular I'm looking at the splice joint in the bottom chord with a pure tensile load of approx. 1800 lbs. It looks like I'll need at least 10 nails on each side in double shear. However the plywood itself is the weak link, 3-1/2 deep gusset on each side of the bottom chord will not be enough for this tensile load. The load is D + S so my Cd is 1.15, which buys me a little but not much.

RE: Wood Gusset Connected Trusses

Not sure what you mean by a 10d nail. The diameter seems to be different depending on which chart you look at. On the attached chart, the diameter is 0.148" and the length is 3". Is that what you are using? If so, you will be clinching the nails.

http://www.thomasnet.com/articles/hardware/screw-n...

Are you using DF truss members? In my area, SPF is more common and much easier to work with for nailing.

It is not necessary for the gusset plate to be 3.5" deep. Make it deeper if required for the sloping chord member.

BA

RE: Wood Gusset Connected Trusses

Check publications from University of Virginia. They are big on this stuff. I built my first house and many garages with the system you are looking at, but I always used ring shank nails, With the modern nail guns you can add plenty more than your numbers call for to be sure. Clinching not needed and most nails won't go much beyond the 2 inch members. Thus, call that extra length as added, not computed for , strength.

RE: Wood Gusset Connected Trusses

Using 1/2" plywood gussets, the overall width of two gussets and a 1 1/2" member is 2 1/2". According to our code, you can use 2 1/2" nails in double shear provided that nails are alternately driven from each side. Alternatively, you can use 3" nails driven from either side and clinched. Nail capacities and minimum spacing are stipulated in our code and presumably also in other timber codes.

BA

RE: Wood Gusset Connected Trusses

@ medeek,
This book -

http://www.scribd.com/doc/211774767/Structural-Tim...

has design details for plywood eaves gusset on pp 204-208, but in SI units to a BS code, so you would need to do some converting. Principles will be the same, though.

RE: Wood Gusset Connected Trusses

(OP)
I think I've figure it out but we will see. I'm working on the code right now to calculate everything. I'll be using the AWC NDS 2012 and the ANSI TPI 1-2007 where applicable. Once I get something together I'll post it and let everyone take a look at it. My hope is that it will help take some of the guess work out of designing wood gusset connected trusses, not just for myself, but others as well.

RE: Wood Gusset Connected Trusses

Here is a fairly good articles on plywood/OSB gussets for 2x wood trusses. As for the size of nails I prefer to use 8d (or 6d) common nails from each face, with 1/2 gussets. The 8d will penetrate both gussets which I feel will add strength if from each face similar to clinching. Of course you can not use the double value for the calculations. Adding glue to the high stress members like BAretired stated also helps.

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: Wood Gusset Connected Trusses

(OP)
Now that I think I have the fastener portion figured out I'm now looking at the failure modes of the plywood gusset plates themselves in tension and in shear. ANSI TPI 1-2007 describes the critical dimension as the gross width of the plate measured parallel to the joint line (section 8.4.3 and 8.5.3). What I've drawn below are these gross widths for tension and shear for each joint.





I am trying to consider all failure modes. In particular Joint (3) has possibly 3 failure modes but tensile case 2 and tensile case 3 may be redundant.

RE: Wood Gusset Connected Trusses

Off the top, your joint 7 has too many nails to the horizontal member. Connection force only has to be for the force difference in the web members, not the summation of the forces.

Dik

RE: Wood Gusset Connected Trusses

Nails are cheap. Piece of mind is not. The number of nails for that bottom chord does not really change the plate size. So you might as well fill the space you've got.

RE: Wood Gusset Connected Trusses

I bet the engineering time spent on this would outweigh the cost of a proper metal plate connected wood truss design

RE: Wood Gusset Connected Trusses

manstrom,

In most cases you may be right, but there may be situations where pre-fabricated trusses cannot be delivered to the site on time.

BA

RE: Wood Gusset Connected Trusses

(OP)
You are correct manstrom. The engineering costs for a one off of this type of truss is too high forcing most ranchers, farmers and DIY'ers to just wing it and forgo the engineering. I'm trying to create a program that automates the engineering of this type of truss so that it is affordable to do the engineering. I do have some experience with MPC trusses and I have done this sort of thing before, look at:

Medeek Truss Designer

However, I've never looked at plywood gusset plates before so I'm in uncharted territory.

Last night I spent some time rereading certain sections of the ANSI TPI 1-2007 and I now think I have a rational approach for dealing with the additional lateral forces on the fasteners due to moments at the peak joint and splice joints.

RE: Wood Gusset Connected Trusses

(OP)
Sorry the link above should have been:

Medeek Truss Designer

To help get this project off the ground I'm listing it at Kickstarter (anyone had any experience with them doing a similar thing?).

Kickstarter Project

I've had some of my engineering friends baulk at this approach to engineering however I think more and more automation of engineering through software is the future of this profession. Most practicing engineers are already half way there since we all use Excel spreadsheets for most of our mundane and typical calculations. I'm just trying to take it a few steps further as it relates to wood gusset connected trusses.

RE: Wood Gusset Connected Trusses

Why not just add plywood gussets to your program (with some CYA notes) with the plywood tension/shear and nails holding values per a psi of plywood? After all an engineer should be reviewing and sealing any engineered design.

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: Wood Gusset Connected Trusses

(OP)
I like your thinking Woodman88. I've made some updates to the current program below:

Updates:

Version 1.0.5 - 07.26.2014

- Added lumber species selection under advanced options.
- Tension perpendicular to grain check added for joint (6) and (7).
- Updated AutoCAD drawing generator so that it now draws the metal connector plates at the correct sizes.
- Limited heel joint plate selection algorithm so that the heel plate is single, symetric and does not exceed bottom chord depth in height.

My next objective will be to create an option for plywood plates vs. metal plates.

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