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Truss Repair Question

Truss Repair Question

Truss Repair Question

(OP)
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!

RE: Truss Repair Question

It sounds fine to me, conceptually.  I agree that the chord splice is probably at the worst location.  Though truth be told, it probably now more closely matches the assumptions used in design.  

RE: Truss Repair Question

Right now, since a portion of the press plate was cut and removed, the load from the left diagonal is transmitted through the press plate to the right half of the bottom chord, not as originally constructed.  

That needs to be remedied with plywood scab plate(s) of suficient size to avoid splitting the members.  Do the numbers and figure it out.  I do not think that just straps will suffice here.

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

RE: Truss Repair Question

(OP)
Thanks for your responses.

Mike - I do plan on using a plywood gusset plate (on both sides).  Since the bottom chord is in tension, my thought was to use the straps to help transfer the tension load across the splice.

RE: Truss Repair Question

Go ahead and use the strap, but just be mindful of splitting the members with too many nails.  No worries.   

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

RE: Truss Repair Question

(OP)
Agreed - splitting does concern me in this case.

RE: Truss Repair Question

If you want to use a strap, try one MSTI72 - a longer one - and only use half the nails, or only nail further away from the plywood gusset.  You will have to reduce the capacity of the strap accordingly, but it may help with the splitting issus.  

You couls also use a longer section of a CMST coiled strap and do the same thing.   

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

RE: Truss Repair Question

(OP)
Thanks for your suggestions Mike.

RE: Truss Repair Question

Ya velcom.

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

RE: Truss Repair Question

(OP)
I have a question about the OSB gussets.  The contractor would like to use 12"x12" gussets (see attached jpeg).  I've calculated allowables for loading perpendicular and parallel to the panel strength axis.  I was going to conservatively use the perpendicular allowable.  My question is what I should assume for the effective width of the gusset.  I was going to use the width of the truss member, but that seems overly conversative.   

RE: Truss Repair Question

Use 24 X 24 CDX PLYWOOD gussets, not OSB.   

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

RE: Truss Repair Question

Mike, what is wrong with OSB?  I believe it is at least as strong as plywood and is cheaper.  So what gives, old buddy?

BA

RE: Truss Repair Question

I just don't trust it as much - seen too much lousy OSB.   

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

RE: Truss Repair Question

(OP)
Mike - I was also going to ask about using plywood over OSB.  I'm getting my allowables from NDS Table M9.2-2.  OSB is as good or better depending on the span rating/# of plys for the plywood.  I've attached the table I'm looking at for reference.

On the 24x24 size - is that what you typically use for this type of gusset?  Is there some rule of thumb that you're going by?  I was trying to figure out what I should use for the effective width.

RE: Truss Repair Question

There is no rule of thumb.  You must use engineering judgment.

BA

RE: Truss Repair Question

(OP)
Thanks BAretired...that's kind of what I thinking it was going to be. smile

RE: Truss Repair Question

I was concerned with the nail spacing, trying to limit any splitting of the members by using a larger gusset.  

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

RE: Truss Repair Question

(OP)
That's a good idea on the gusset. For the nailing I was going to specify 10d nails and use the Table C11.1.5.6 in the NDS as a guide for the spacing, etc.

RE: Truss Repair Question

(OP)
Update.... I'm using 8d's with the nail spacing recommendations in NDS Table C11.1.5.6.  I'm spacing the nails 4" o.c. on each side, so I'll have 2" o.c. in the truss members.  In order to accomodate that spacing, I'm getting gusset sizes in the neighborhood of almost 3' wide.  Does that sound reasonable?

RE: Truss Repair Question

Yes.  Remember to stagger the nails too.   

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

RE: Truss Repair Question

(OP)
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.

RE: Truss Repair Question

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

RE: Truss Repair Question

(OP)
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.  

RE: Truss Repair Question

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

RE: Truss Repair Question

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.

RE: Truss Repair Question

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.

RE: Truss Repair Question

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.

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