Diaphragm Design for Dutch Gable Wood Roof
Diaphragm Design for Dutch Gable Wood Roof
(OP)
Per the sketch below, I'm trying to work out shear transfer through a dutch gable wood framed roof. I've been considering it as three discrete, connected diaphragms:
1) The hip roof segment at the left end.
2) The main gable roof in the middle.
3) The hip roof segment at the right end.
The trouble is this: conceived this way, diaphragms one and three have aspect ratios of about 8:1 (sketch is poorly scaled). That's a no go per North American wood codes which usually limit diaphragm aspect ratios to around 4:1. I'm having a hard time seeing a away around this.
The only workaround that I've come up with is to consider all three diaphragms, and the vertical plywood at the gable trusses, as a single diaphragm. Basically, I take the five diaphragm surfaces -- three horizontal-ish and two vertical -- and fold them out flat in the east west direction. Then I use that fictional diaphragm for calculating deflections and aspect ratios. My logic for this is that all of the sub-diaphragms are shear connected to one another and jointly span between two drag elements (shear walls) at the far ends. Thus, one diaphragm for all intents and purposes.
Anybody buy this? Got a better idea? Please advise. There is some irony in the fact that, two decades ago, I was the truss guy framing out five or so of these roofs a month. Luckily, I wasn't the EOR and I had no concept of lateral loads whatsoever. A little less lucky for society I suppose.
1) The hip roof segment at the left end.
2) The main gable roof in the middle.
3) The hip roof segment at the right end.
The trouble is this: conceived this way, diaphragms one and three have aspect ratios of about 8:1 (sketch is poorly scaled). That's a no go per North American wood codes which usually limit diaphragm aspect ratios to around 4:1. I'm having a hard time seeing a away around this.
The only workaround that I've come up with is to consider all three diaphragms, and the vertical plywood at the gable trusses, as a single diaphragm. Basically, I take the five diaphragm surfaces -- three horizontal-ish and two vertical -- and fold them out flat in the east west direction. Then I use that fictional diaphragm for calculating deflections and aspect ratios. My logic for this is that all of the sub-diaphragms are shear connected to one another and jointly span between two drag elements (shear walls) at the far ends. Thus, one diaphragm for all intents and purposes.
Anybody buy this? Got a better idea? Please advise. There is some irony in the fact that, two decades ago, I was the truss guy framing out five or so of these roofs a month. Luckily, I wasn't the EOR and I had no concept of lateral loads whatsoever. A little less lucky for society I suppose.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.






RE: Diaphragm Design for Dutch Gable Wood Roof
For that detail I would call up a horizontal truss. Roof trusses in my location are a deferred item usually design by technicians who have no idea about engineering principals. So we normally need to give a design loads and provide schematic details for transferring the lateral forces into the shearwalls.
RE: Diaphragm Design for Dutch Gable Wood Roof
RE: Diaphragm Design for Dutch Gable Wood Roof
@Excel: I see where you're headed and I agree: no failures and lots of data points. Still, I have to find some reasonable narrative here regarding load path. It'll be equally problematic for wind the other way.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
Can you just ignore it and use only the conventional portions (between the gables)? That would likely be my first place to start if it's a capacity issue.
RE: Diaphragm Design for Dutch Gable Wood Roof
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
I didn't pay close enough attention to your detail. A lot of the dutch gables I have done have the main floor wall running in line with the gable wall (truss) above and the lower roof portion is more for show.
I would say your original post seems reasonable. At the interface of the 8:1 piece and the vertical piece I would have a shit-ton of blocking for both pieces to ensure good shear transfer capacity at that joint.
Although my first comment was made in jest, it's not wholly incorrect. The flexibility of wood structures to redistribute load as required astounds me. The issue is it's generally impossible to quantify.
RE: Diaphragm Design for Dutch Gable Wood Roof
I agree with your overall opinion of wood construction. I often feel as though I'm wasting my time trying to design wood systems for lateral in detail. Some prescriptive stuff would suit me just fine.
I'm thinking to taking a swing at this horizontal ceiling truss business. Two options:
1) I align the bottom of the truss with the tops of the walls. The heel heights of the jack trusses, corner sets, and ridge rafters will all be messed with.
2) I align the top of the truss with the tops of the walls. The framing is unaffected but the ceiling will be lowererd by 1.5" and drywall will be affected. It's non-standard wall heights here anyhow.
Which of the two options do you guys think is best?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
RE: Diaphragm Design for Dutch Gable Wood Roof
1 "Diaphragm. A horizontal or sloped system acting to transmit lateral forces to the vertical-resisting elements...."
2 "Diaphragm chord. A diaphragm boundary element perpendicular to the applied load that is assumed to take axial stresses due to the diaphragm moment."
Unless you are designing the ridge blocking to act as a diaphragm chord, without any interior shearwalls you have only one diaphragm.
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.
RE: Diaphragm Design for Dutch Gable Wood Roof
@Woodman: I was hoping that you'd surface for this. I'm not clear on your response however. Are you agreeing with the approach outlined above? My "one diaphragm" would include the main gable roof, the vertical face of the sheathed gable trusses, and the skirts? The 8:1 bits would not be considered separate diaphragms in their own right?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
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.
RE: Diaphragm Design for Dutch Gable Wood Roof
The roof is loaded in the transverse direction (perp. to ridge). Treat the main diaphragm between the two gable/girder trusses as a regular diaphragm, ignore the jack stud corner pieces for simplicity and to be conservative. Then this main diaphragm is transferring all of its shear load into the two gable end girder trusses. The girder trusses are themselves acting as mini shearwalls in a sense. Then somehow the shear force they are withstanding has to be transferred into the shearwalls below, this will be through the hip roof segment. The aspect ratio though would be 1:8 and not 8:1 so your ratio is fine. You should have plenty of strength with that ratio and your unit shear will be okay, even though I don't have the numbers.
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
R2 = R1 = (wL1)/2
R1swl = R1 + wL2
v1 = R1/b1
v2 = R1swl/b2
I think that makes more sense.
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
@ Medeek:
1) If you ignore the jack stud corner pieces, how do you connect the 8:1 diaphragm ends to the shear walls at the north and south sides of the building? Without those connections, I think that the 8:1 diaphragm would not be able to achieve rotational equilibrium.
2) I'd considered the 8:1 vs 1:8 ratio thing too. I find it confusing where the diaphragm is really a transfer element rather than an element receiving directly applied lateral load. I may need to revisit that as I suspect that you're probably right. It'll turn into an 8:1 for wind in the other direction unfortunately. But that's an issue for another day. Or perhaps this afternoon.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
My only concern: your D1/D3 chord forces need to pass through the corner sets as shear to make it back to D1 & D3.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm Design for Dutch Gable Wood Roof
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
A confused student is a good student.
RE: Diaphragm Design for Dutch Gable Wood Roof
FWIW, has anyone EVER observed a failure in ANY roof diaphragm? I have seen sheathing and rafters pulled of the roof, tall walls pushed in, garage doors ripped put, but never and shear failure of a roof diaphragm. I was even doing some work at a large strip mall recently and a roof edge angle did not even exist - no attachment at all from the metal deck to the 50 ft. long exterior masonry wall (one central i-beam diode bear on the wall). I don't even know if the bar joists are fastened down. The building has survived at least one strong hurricane unscathed.
RE: Diaphragm Design for Dutch Gable Wood Roof
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.