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!

 

[Lion06]

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.

 

[msquared48]

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

 

[Cap07]

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.

 

[msquared48]

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

 

[Cap07]

Agreed - splitting does concern me in this case.

 

[msquared48]

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

 

[Cap07]

Thanks for your suggestions Mike.

 

[msquared48]

Ya velcom.

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

 

[Cap07]

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.

 

[msquared48]

Use 24 X 24 CDX PLYWOOD gussets, not OSB.

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

 

[BAretired]

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

 

[msquared48]

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

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

 

[Cap07]

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.

 

[BAretired]

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

BA

 

[Cap07]

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

 

[msquared48]

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

 

[Cap07]

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.

 

[Cap07]

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?

 

[msquared48]

Yes. Remember to stagger the nails too.

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