Sizing Mitek Truss Plates for Wind Girder 1

[Signious]

Hello,

I have been tasked with designing a set of wind girders to transfer high wind forces for residential tall walls under a gable end roof. The system consists of:

38'-0" long parallel chord truss (46" deep) with 7 panels, designed it as a pratt truss out with MSR chords and spf no.2 webs. Truss sits above ceiling sheathing, underneath the first two gable trusses and butts into the third GE truss back.

My question is how do you go about sizing the appropriate plates when the manufacture will not give the design information for the plates? I feel kind of dirty just specifying shear / tension / compression loads at the joints and leaving it up to the roof truss mfctr to size them.

Are there any good publications with minimum values for truss plates? I have the Simpson catalogue plate information, and CSAS347 (useless) but just don't know how their numbers compare to the Mitek product. OS86 really wasn't much help either.

Thanks!

 

[Signious]

dcarr82775 TXStructural:

Thanks, that is exactly what I am looking for!

 

[Brad805]

For a flat roof why not just extend the wall height 3'-11"? It is far easier to insulate the wall and it is easy if your framer will follow plans.

 

[KootK]

I missed that thread, though I fail to see how adding in a girt truss / rigidity to a gable truss will help to transfer loads to roof sheathing and shear walls. Link?

I don't remember the thread I'm afraid. Just take the second truss from the left in my detail A above and slide it right until it hits the third truss and becomes a double. For my detail, it would mean one plywood box beam instead of two. For your detail it would mean no longer needing a short heeled vertical truss over top of your horizontal truss which I see as being a headache for the truss supplier.

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.

 

[Brad805]

Apparently I need more coffee this afternoon, oh well. Carry on.

 

[KootK]

Like this. It would also allow you to easily field splice your horizontal truss for easier erection.

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.

 

[Signious]

Brad:

My first option is always to ballon frame on trouble Gable end walls as you described, the issue here was with the massive gable hight we would be talking 30'+ studs once you hit the peak.

And yah, coffee would be a good idea, wrong plane haha.

KootK:

I get the idea now. Im concerned about increasing the top chord load from doubling the roof load trib to the 'new' second truss back (48" from gable end wall). You could double that as well, but it would still leave you with a ~42" span for the roof sheathing between trusses. I don't have time (or inclination) to run numbers but at a 26psf snow load + shingles that would be pushing it for deflection & tear out for the screws I think, even if you brought it down to 3" oc. on panel edges.

That second truss with the upset is also pretty important to give some lateral bracing to the web members though.

 

[KootK]

Im concerned about increasing the top chord load from doubling the roof load trib to the 'new' second truss back (48" from gable end wall)

Don't be. The load per truss actually goes down. Before you had two trusses carrying 2' trib each (4' trib total). Now you've got one double truss carying 3' trib total. So the trib load per truss goes down from 2' to 1.5'.

You could double that as well, but it would still leave you with a ~42" span for the roof sheathing between trusses.

Not so. Over the double truss space, the sheathing would span the other way, between the 2x4 outlookers at 24" o/c.

That second truss with the upset is also pretty important to give some lateral bracing to the web members though.

The webs require no bracing over such a short distance. Trust me, I used to be one of the truss guys back in the 90's.

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.

 

[Signious]

Spell my name right Damnit! (kidding)

I did put numbers to the trusses, and did buckling check as per OS86 and midspan bracing on the 46" 2x sticks was an issue unless I butched my math

 

[KootK]

@Sygneyoos: so throw some strong backs on the problem webs as you might a vertical truss. Or tighten up the panel spacing. Stop fighting this and join me in the promised land damn it! Together, we've arrived a detail that solves virtually every problematic issue associated with this condition. Folks on this board have been chipping away at that for months. You should be dancing naked in the street outside of your office, like me.

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.