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
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
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
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
I second BA, use glue but do not transfer any load to the glue.
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
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
'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
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
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
A nice reference
RE: Wood Gusset Connected Trusses
http://www.fpl.fs.fed.us/documnts/fplrp/fplrp444.p...
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Wood Gusset Connected Trusses
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
RE: Wood Gusset Connected Trusses
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
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
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
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
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
Dik
RE: Wood Gusset Connected Trusses
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
RE: Wood Gusset Connected Trusses
BA
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
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
RE: Wood Gusset Connected Trusses
BA
RE: Wood Gusset Connected Trusses
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
RE: Wood Gusset Connected Trusses
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
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
Dik
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
RE: Wood Gusset Connected Trusses
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
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
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
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
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.