Truss Repair Question

[Cap07]

I've been asked to look at a residential truss repair. The truss spans about 20 ft. The contractor has replaced the damaged members of the truss and is proposing using 12"x12" OSB gusset plates where the members connect. At one connection where the web connects to the bottom chord, the bottom chord has been spliced. See the attached jpeg. Right now there is only a Simpson tie plate holding the chord together.

I don't like the splice being under the web members (at the joint), but at this point I don't think moving it is an option. My thought was to replace the tie plate with a couple of Simpson straps - e.g. an MSTC along the side (where the tie plate currently is), and maybe an MSTA strap along the bottom edge. And then tie the whole joint together with the OSB gusset.

I haven't calculated the loads yet, but I would appreciate anyone's initial thoughts or opinions about how I'm thinking on tying this together.

Thanks!

 

[Cap07]

In each row I have the nails spaced at 4" o.c. each side, staggered so there's 2" between the nails in a row. Between the nail rows I have 1.5". The attached .jpg illustrates this - the solid o's are nails on one side, the +'s are nails on the opposite side.

 

[msquared48]

You need to stagger the nails both horizontally and vertically. Right now, in your sketch, you have rows of nails that line up. You need to stagger the nails up and down to limit the tendency to split.

Assuming the grain is straight in the member, you do not want adjoining nails in the same grain line for that will encourage splitting. Hence the staggering vertically.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Cap07]

I see what you're saying about staggering helping with the splitting issue. I was using Table C11.1.5.6 in the NDS for my minimum spacings (see the attached jpeg). For the spacing between rows of fasteners, they list 2.5d for staggered fasteners and 5d for in-line fasteners. I think what I have right now would be considered the in-line configuration and my spacing between rows is greater than 10d.

 

[AZengineer]

This is a great article on wood truss repairs:

http://www.structuremag.org/article.aspx?articleID=797

 

[Cap07]

Thanks for posting the link AZengineer - that is a great article!

 

[TXStructural]

After designing repairs for a couple of hundred trusses (on a single job), we found that 3/4 B-B with bolts (not nails) is the most cost effective, most reliable repair. Nailing introduces too many unknowns, and the only adhesive I found that would not creep and has a rated structural strength is made by 3M for about $10/tube, wholesale.

Another option is to support the member for actual loading, and against deflection, remove the bad plates and replate with larger plates. They make portable presses which work quite nicely. The local truss manufacturer should be able to provide that repair in place.

 

[woodman88]

Basically a good article (the Repair of Wood Trusses, by Steven Fox PE), my personal recommendation on the plywood/OSB gusset in figure 5 would be to radial cut between the sloping bottom chord and the lower diagonal web (instead of the angle cut shown at the bottom chord to vertical) to decrease the possibility of causing the gusset to start ripping at the point of the angle.
Also, when using clinched nails at the plywood or OSB face I use a decease value (usually 66% for the clinched side) for the connection. This is due to the damage I have seen done to the plywood and OSB when the nails are clinched.

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.

 

[bragee]

too much?

 

[AZengineer]

Agree with TXStructural. I have used bolted plywood gusset plates on many truss repairs because the number of nails required can become excessive, requiring the plates to become quite large. You can still end up with a lot of bolts, but it makes the plate sizes smaller.