Wood Truss Joint Repair Analysis (Statics Review)

[Deadblow]

Hello All,

Please see the two page attachment. I am attempting to analyze the forces needed to design OSB gusset plates to repair metal plated wood trusses. The attached picture is looking at the peak and two web members (tension) shown that pulled out of the connecting plate. When I calculate how much these two web members contribute to the axial compression in the top chord, I don't seem to believe my results. I calculated how much axial force comes from my applied load, 631 lbs, and then I subtracted that from the total axial load of 3,002 lbs. This would mean that I have 2,371 lbs of force that needs to get transferred out of the web member, into my repair gusset and into my top chord. I must be missing something! The original plates, given they failed, were only 8"x10" plates. Please point me to what I'm missing.

Thanks

EIT

 

[SJBombero]

Your numbers don't seem outrageous. Take a look at ESR-1311 for capacities of typical hydraulically applied plate connectors. They are on the order of several thousand pounds.

 

[DaveAtkins]

Each web member transfers 1,096 lbs into the gusset plate. Each top chord transfers 3,002 lbs into the gusset plate.

DaveAtkins

 

[BAretired]

Alternatively,
F_tc*sin(25.29) = 1096*cos(22.442)
F_tc = 1096*2.1635 = 2371#

BA

 

[XR250]

In this configuration, the web members only serve to support the bottom chord of the truss. Probably only a few hundred pounds each max. Look at the attached generic analysis. I have zero load on the bottom chord and there is zero tension in the webs.

 

[Deadblow]

Thank you for your replies! I was not accounting for the axial compression in the top chord members as a result of the truss bending. This lowered my loads that need transferred through the plate significantly.

EIT

 

[DaveAtkins]

I stand by my earlier comment. The 1,096 pound and 3,002 pound forces need to be transferred into and out of the gusset plates (one each side of the truss) in order to maintain static equilibrium.

DaveAtkins

 

[medeek]

With zero load on the bottom chord you still get tensile loads in the center webs:

http://design.medeek.com/calculator/graphs/2016-08-10forces61finktruss.svg

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

http://www.medeek.com

 

[XR250]

With zero load on the bottom chord you still get tensile loads in the center webs:

Not if the truss only has 2 webs like his drawing shows. If it has 4 like yours shows, then yes.

 

[BAretired]

I don't know how many web members the OP's truss has, but it must be more than 2.

BA

 

[XR250]

I don't know how many web members the OP's truss has, but it must be more than 2.

His sketch on page 2 shows only two webs. I guess if the truss is small enough, it would only require 2, but in that case it would probably only need 1 unless the top and bottom chords were different sizes.

 

[BAretired]

I interpreted the sketch on page 2 to show the ridge joint only. The geometry of the truss is not shown. It might be helpful if Deadblow provided a truss elevation.

The compressive force in each top chord member, neglecting the 539# point load at the ridge is 2731# as he calculated.

The force needed to hold each web member to the truss plate is 1096#. Taken together, the force to be transferred from the two web members to the top chord via the truss plate is 2(1096)cos(22.442) = 2026#. That is what failed and that is what the new gussets would be required to carry.

The top chords are in compression and bear against each other, so the truss plate does not need to carry all of the top chord force.

BA

 

[XR250]

After a second, look, I can see how that could be interpreted that way (probably makes the most sense)
However, I don't think your numbers are correct. If you look at the example I gave, I have 530lb of compression in the top chord, but no load in the webs. You have to add in the live and dead load on the top chord which will somewhat subtract from the web load.

 

[DaveAtkins]

The top chords are in compression and bear against each other, so the truss plate does not need to carry all of the top chord force.

I am not sure I agree. How can you guarantee the top chords bear against one another? Shouldn't the gusset plate be designed for the full top chord force, in case there is a gap?

DaveAtkins

 

[XR250]

I am not sure I agree. How can you guarantee the top chords bear against one another? Shouldn't the gusset plate be designed for the full top chord force, in case there is a gap?

MPC Trusses generally do not have gaps at the members as the plates are only designed for shear and tension loads (I am guessing)

 

[KootK]

MPC Trusses generally do not have gaps at the members as the plates are only designed for shear and tension loads (I am guessing)

Substantially correct. The plates suck for compression and the chord splices wouldn't work without assuming contact. There are gap tolerances but they are very small and assumed to close under heavy compression. The style of plate used here, and the sloppy positioning, leads me to wonder if these trusses are field built. If so, there may indeed be large gaps that would need to be bridged by the gusset plating.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

The gap between top chords appears to be minimal in the photo.

It looks to me that the truss plates were placed too high so that they did not lap sufficiently with the diagonal web members to develop a tension of 1096# each.



BA