Trusses with Plywood Gusset Plates 4

[medeek]

thread337-18639

I'm now about to embark on the wood gusset stage of my truss designer. I've been reading and searching the internet for as many resources as I can in relation to how to best fasten these plates to the truss members.

As a given I will probably spec. out a glue (ie. LNP-902) which I will not consider in the strength calculations to give an additional level of redundancy and margin to the design.

Looking through a number of documents I've seen clinched 3" nails, and unclinched 6d, 8d and 10d nails used. I've even seen some mention of Simpson SDS screws as well as drywall screws. Most of these documents are 50 years old or older so not a lot of recent data on this topic or so it would seem.

I am curious if anyone has any recent experience with using plywood or osb gusset plates on wood trusses and what was their fastener of choice and why?

I really like the idea of 3" nails that are clinched (less nails = less labor) however would a smaller diameter nail be less prone to splitting the wood members even though there are more of them and they are spaced more closely together?

I'm also considering ringshank nails since the cyclic loading on a truss might have a tendency to back the nails out over time, however the shear strength of these nails is a question needing further investigation.

For the actual gusset plate material I'm considering 1/2" DF Structural I Plywood, however I would also like to have 3/4" PLY, 19/32" PLY and 7/16" OSB as additional options. I've actually see 7/16" OSB used in a lot of home built trusses and I'm actually guilty myself of my own home brewed trusses for a small storage shed using drywall screws and 7/16" OSB (8 ft. span). Other than the strength design values are there any other caveats to using OSB in this application?


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

@anex20

The truss calculator is something I have been slowly working on the last couple of years. It still has a long ways to go before it is really a useful piece of software, the problem of course is finding the time to spend any meaningful time to work on it. I started it due to my frustration with the truss plate manufacturers not offering their software to engineers and architects. The current calculator is located here:

http://design.medeek.com/calculator/calculator.pl

It is free to use but I have limited its daily usage to help curb some overuse and bandwidth problems I was having.

What do you typically see in your line of work as the most frequently used nail size for gusset plates? Also do you use glue in combo with the nails?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[dgapilot]

medeek, Nice web site!

 

[anex20]

@medeek
I generally use 0.131 X 2 1/2" or 3" nails. It depends on what the factory / contractor already has in his nail guns. 0.131 nails are ubiquitous in the factory built structures industry and most factories and set crews have this size nail gun. I don't worry about the 1/2" of nail that sticks out. I always use 2 gussets of at least 7/16" OSB or 1/2" plywood, and alternate the required nails on each side of the connection. I do not use glue at the heel gusset because the top chord of the rafter has to rotate down for shipment of the units. There is usually a bolt placed to allow for the rotation, but it is not included in the final gusset fastening calcs. In a static situation I would use 80% coverage of PVA glue on the connected planes.

BTW, I too am frustrated by the fact that metal truss plate companies don't let us regular engineers use their software....I have talked to reps (as high as VPs) from Mitek and Simpson and told them I would specify their connectors if I could use the software. No go at this point. I design more than 100 houses a year which would equate to many, many trusses, but because of the proprietary software I am not able to spec mitek or Simpson plates and use gussets and other direct framing methods instead....loss for them. I keep trying and maybe I can wear them down.

 

[anex20]

@medeek
I have used some of your tools from your website and find them very helpful. Thanks for sharing, they are a great time saver for finding the correct design loads.

Thanks again, it is nice to "meet" the designer!

I've been designing wood structures for about 20 years now, that is 95% of my work. Let me know if I can help, or if we can just spitball some ideas back and forth.

Rock on.

 

[medeek]

@anex20

Speaking of tools the best single program/tool I've found so far is the Forte app by Weyerhauser. The only major downside I've found with it is the inability to specify wet and pressure treated lumber. Once I realized the load transfer feature was there and figured out how to use it, it has been very handy and saved me a lot of time that I would have wasted manually transfering loads between members.

I'm glad to hear that someone is finding these tools useful. All of them were started at some point or another when I was working on a residential project and fatigue from doing too many manual calcs made me realize there was a better way. Unfortunately, with the Excel spreadsheet tools I have no idea how much they are actually being used.

I've had a lot of interest in this truss designer lately and I would really like to put some more time at it. I actually don't think it will be too much of leap to make it handle the wood gusset plates as it does the metal plates. I'm still undecided on the exact details of how to handle the tensile/shear check of the gussets when the load is not perpendicular or parallel to the "tear line". As you can see in the diagram above the gusset plate is tearing in tension and in shear (approx. 45 deg. angle). The Hankinson formula seems like the best way to handle this.


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[StrucPatholgst]

I haven't read through all the posts yet, but I use structural plywood for gusset replacement. About 6 years ago I ran hand calcs on a bunch of trusses that had gusset plate damage, and came up with a fastener scheme that worked. I used glue and nails.

 

[EngineeringEric]

The clinching to me isn't to save money, it is to save the wood main member. the capacity for a double shear nail reduces the number of nails and reduces the chance of splitting main memeber. You cannot use double shear nails even if your fastener goes through three members unless it is clinched.

Nails are cheap, labor isn't, splitting my truss is even more expensive!

 

[oldestguy]

I've built a few roof systems with plywood gussets and ring shank nails. I got a lot of good information from the US Forest Products lab people in Madison, WI.

One 26 x 30' garage had a large truss built to go from front to back down the center to carry conventional rafters. That also designed to carry the attic loadings for storage of lumber up there. As I recall the gussets were 3/4" plywood, but bolted at the ends. Some douibling up of members also. No problems at all with any of these, including one house. Full Wisconsin snow loads.

 

[medeek]

@oldestguy

Size of nails and were they in single or double shear (clinched). Glue?

Did you do the calcs on them or did you have another structural engineer run the numbers or did you utilize a more prescriptive approach with span tables etc...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[oldestguy]

Medeek:

It has been a long time since most of this, but for the usual 1/2" plywood gussets, the ring-shank nails were just long enough to come out the other side of the 2 x, so other side had to have its nails also. All the scoop on nail capacity came from the US Forest Products publications. I usually staggered all nailing so that they did not tend to split the member.

The one book I still have here is dated 1974, called "Wood handbook", which I recall was free. US Dept of Agricultural Handbook Number 72.a little over one inch thick pages, in a hard cover.

Library of Congress Catalog Card.No. 73-633335

Try thus link. I think this is it, later version.

http://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf

One of the buildings (a tractor and wood shed) had the end truss built so the usual end was used as a support for a beam carrying trusses for the next group in over an entry opening of about 10 feet or so. That end truss cantilevered over an opening of about 4 feet. Reason was this building was parallel and next to a driveway, both on a ridge, so right angle or end entry was not possible. I had to drive in under the corner. Of course the end truss was beefed up accordingly. I don't recall the details. I do recall however that the hanger for the beam along the eaves was two galvanized straps, about 1" x 1/8" salvaged from a old windmill, shaped like a "U" also wrapped around that end truss. Can't recall the detail for the connection, but it probably was a series of nails, at allowable stresses.

 

[oldestguy]

Sorry, but being a civil engineer, I did all the calculations. No glue. For trusses I usually do a graphical analysis, with length of plot as the force in the member as I recall.