Sizing Mitek Truss Plates for Wind Girder
Sizing Mitek Truss Plates for Wind Girder
(OP)
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!
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!






RE: Sizing Mitek Truss Plates for Wind Girder
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Sizing Mitek Truss Plates for Wind Girder
Already talked to Mitek and you are correct in saying they will NOT give out any design information for the plates.
RE: Sizing Mitek Truss Plates for Wind Girder
RE: Sizing Mitek Truss Plates for Wind Girder
Can't you tell the truss supplier that you want a truss and give them a design load and let them design the entire thing? It would likely end up being more economical.
If you detail your connections and orientations effectively they'll provide you with exactly what you need for the lowest price possible.
Once you start telling them exactly which members to use, they'll likely charge you an increased amount.
A few people have noted in other threads on this sort of topic that the erection of this truss may be difficult as the plates may not survive the out of plane bending/prying.
Would it be more constructable to detail a built-up (2 or 3 ply) 24" deep LVL on the flat?
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
@Signeous: you'll want to check out this thread if you haven't already: Link.
I've been thinking about this. I wonder if one could make the truss in, say, eight foot segments and field splice them together with nailer plates.
Go Signeous go!
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.
RE: Sizing Mitek Truss Plates for Wind Girder
Keep in mind also, even if you provide a detailed plan specifying all the requirements you may find the truss plant will turn around and do whatever they want anyway. We find the wood truss guys are more tuned to working with residential clients and they are far less likely to follow drawings than steel suppliers.
RE: Sizing Mitek Truss Plates for Wind Girder
I followed that thread for a bit Kootk, but missed the last half or so - thanks for the link. Just for the sake of asking - anyone know what software Medeek was using for his FEA? Looks useful for this scope of work!
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
RE: Sizing Mitek Truss Plates for Wind Girder
https://res.cloudinary.com/engineering-com/image/upload/v1429722352/tips/WIND_TRUSS_vtfdxa.pdf
RE: Sizing Mitek Truss Plates for Wind Girder
I have actually done that once.
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
RE: Sizing Mitek Truss Plates for Wind Girder
First nice thread and solid sketches and details all around.
@Signious What is the DSP for? I mean why is it required? This is like a hold down for the double top plates?
@Kootk -> in your sketch the "4x8" shear panel is really more like "2-2x4", right? as it is is "interrupted" at the first truss?
Thanks!
EIT
www.HowToEngineer.com
RE: Sizing Mitek Truss Plates for Wind Girder
Yesir. 22.5" I suppose. Thanks for the fact check.
Although... in another thread someone had the clever idea of making the first truss a double and spacing it 4' in. That might be a good deal simpler for both my detail and Signious'.
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.
RE: Sizing Mitek Truss Plates for Wind Girder
The DSP is to assist transferring overturning moments to the set of double vertical studs, and thru to the foundation thru hold down anchor.
When I ran loads prying action between top plates and the vertical studs was a failure point with LS50 angles, and any kind of nailing or simple framing angle connection was disallowed as nails in withdrawl.
@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?
RE: Sizing Mitek Truss Plates for Wind Girder
Thanks, that is exactly what I am looking for!
RE: Sizing Mitek Truss Plates for Wind Girder
dcarr82775TXStructural:Thanks, that is exactly what I am looking for!
RE: Sizing Mitek Truss Plates for Wind Girder
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
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.
RE: Sizing Mitek Truss Plates for Wind Girder
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'.
Not so. Over the double truss space, the sheathing would span the other way, between the 2x4 outlookers at 24" o/c.
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.
RE: Sizing Mitek Truss Plates for Wind Girder
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
RE: Sizing Mitek Truss Plates for Wind Girder
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.