Wood truss repair methodology

[charliealphabravo]

Hi guys,

I would like to get your feedback on the appropriate procedure...for example a typical plywood gusset repair for a damaged plate or web. I would usually calculate the T/C loads in the members and then follow something like the article by Fox in Structure Magazine which has been referenced in this forum several times.

On two occasions now I have had supervisors and colleagues question me on this method and ask why I don't just specify something like "5/8" plywood gussets glued and screwed" and abbreviate or bypass the load analysis. I don't want to complicate the matter unnecessarily but the answer seems self evident to me and I'd like to have a reasoned and diplomatic response since I have had this put to me twice now.

Thanks in advance.

 

[msquared48]

Truss plate connections are not necessarily done "by inspection" using "standard" plywood gussets that are "screwed and glued" to H*((. Especially the case with the longer trusses, and to me that's anything over 20 feet or so.

I don't know how you can decide on the number of nails to use without identifying the load conditions, member grades, number of plywood plies, and knowing the forces.

It is true that experience is helpful, but this is not Betty Crocker here... It's supposed to be engineering.

I'll step down now.

By the way, just was asked by a framer if 2X6 @ 16 are ok to span 16 feet for floor load. Now, I will give you that the answer to that is "by inspection".

Mike McCann
MMC Engineering

 

[EngineeringEric]

If we do not have to analyze something and follow a load path and certify that it is adequate... then why are we needed? I see at is if the repair is not in need of analysis and is just boiler plate than MiTEK or code officials would have some detail for the repair. So we analyze to know thickness of plywood, size of plywood, number of nails, double shear crimped nails, and then we through on glue to help and minimize splitting from all those nails.

I follow the same path, as does my supervisor.

 

[Ron]

Agree completely with both of the above posts. I've seen engineers try to justify/rationalize a half-ass approach during depositions or trials and they usually get destroyed!

Never forget that every piece of engineering work you do is open to the scrutiny of other engineers, lawyers and the public. Do it right and you don't have to worry about it. Do it wrong or don't consider it at all (hiding behind the blanket of "engineering judgment" based on nothing) and you'll potentially pay the price for it.


Hey Mike....but what about those of us who tend to look like the Pillsbury Dough Boy?

 

[XR250]

Just looked at a house yesterday with site-built trusses constructed with 1/4" plywood gussets and 6d nails.
They are sagging about 3" over 24 ft.

 

[TXStructural]

Theoretically, you could "design" a standard gusset to transfer the full member capacity. It would not be economical, and would involve bolts and 3/4" plywood. (We designed 100's of truss repairs on ONE PROJECT, so we were looking for economy and safety.) I doubt that you can justify nails or wood glue, since you cannot fit that many nails and using "glue and screw" is like mixing welds and bolts. Also, we had no luck actually being able to rate glues for structural purposes, aside from one, rather expensive and messy 3M tube product that has an ICC-ES report for structural uses.

Not saying it can't be done, only that our experience was that anything other than replacing metal truss plates with larger plates using a field press, or installing large, bolted plywood gussets, was not practical.

Excel: oh, my.
I examined some mobile homes in the wake of Hurricane Charley and found thin lauan plywood gussets glued on 2x2s... many with wallpaper on them. Most of the gussets held, but the 2x2s shattered.

 

[woodman88]

" I don't want to complicate the matter unnecessarily but the answer seems self evident to me and I'd like to have a reasoned and diplomatic response since I have had this put to me twice now."

My answer is that I like doing the numbers. It is why I became an Engineer.

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.

 

[XR250]

@TXStrutural:

Wallpaper - NICE!