Portal Frames and Nailed Moment Connections
Portal Frames and Nailed Moment Connections
(OP)
I'm doing an investigation on analyzing portal frames and so far my best resource has been "Analysis of Irregular Shaped Stuctures" by Malone. Specifically I am trying to figure out how to calculate the moment capacity of a group of nails (grid @ 3" o/c spacing) at the connection between the shear panel sheathing and the header of a portal frame. Any suggestions, resources or opinions would be appreciated.






RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
DaveAtkins
RE: Portal Frames and Nailed Moment Connections
I think that chapters 13 and 14 of the book that you mentioned does the best job of explaining design of portal frame design.
For more information check out the "References" at the end of those two chapters in the book. The APA information is all available on line, for free, at: www.apawood.org.
RE: Portal Frames and Nailed Moment Connections
I only use portal frames that are prescriptive. Anything beyond that, I use steel moment frames.
RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
The prescriptive bet is the safe bet, I agree. However, I would like to be able to actually engineer one of these with some confidence. If I increase the strap capacities, holdowns, anchor bolts and sheath both sides of the portal frame with 15/32 Structural I Ply/OSB where does that put me? I should be able to get a little more than the prescriptive IBC/IRC portal frame. Consideration for the concrete foundation stiffness and the header stiffness also factor into the design as well.
RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
Many designers object to three sided buildings wholesale due to their poor seismic performance history. I'm not one of them. I simply take a great deal of care when designing lateral elements for three sided buildings, particularly with respect to drift.
Diaphragm rigidity is about the relative stiffnesses of the vertical and horizontal shear resisting elements. Viewed in that light, an assumed rigid diaphragm over a spongy moment frame makes sense. A true three sided building diaphragm is rigid by virtue of equilibrium as it is statically determinate.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Portal Frames and Nailed Moment Connections
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: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
However, I didn't trust the number I was getting for the total moment of the fasteners from the elastic method so I created a spreadsheet which calculates the moment contribution of each fasteners and then sums them up:
The problem I am having is the two numbers for the total moment capacity of the nails don't agree and I haven't been able to get them to agree. Maybe someone can tell me what is wrong with my calculations in particular the equation M(nails header) = Z'J/(spacing)2rmax.
My feeling is the correct answer is the 17,724 in-lbs given in the spreadsheet, correct me if I'm wrong.
Once I have fully researched this example and possibly a few other configurations I hope to make a comprehensive online calculator that goes through all the tedious calcs and even programmatically determines the nail spacing on the header among other things. I don't think such an app currently exists, but there are a few white papers out there that provide pretty clear guidance.
In addition to the wood engineering other items that should be considered is the deflection and sizing of the concrete stemwall or thickened edge slab footing. The foundation portion should be fairly easy given the direction of the Malone and Rice's text however deflection is not so clear cut.
RE: Portal Frames and Nailed Moment Connections
Then the V(nails header) = 2 X M/lcr = 383.8 lbs since both sides are sheathed.
Note that this value is conservative since I am ignoring the contribution of the nails into the sheathing above the header (into the pony wall), actually probably too conservative for some. This value is only taking into account the nails that are nailed in a grid pattern into the header.
I could theoretically include all of the nails that are in the panel above the header and to the side however the calculation of the center of rotation would become quite complex and I think a conservative approach is warranted where the quality control in the field might be somewhat suspect.
RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
The tensile capacity of a DF No. 2 2x6 is fairly decent especially with a CD of 1.6 applied for wind / seismic loads. However, my engineering mentor pointed out to me that the nails connecting this king stud to the rest of the framing and sheathing will probably fail in shear before the member will actually fail. So then my thought was to just increase the strap on this side to equal the strap on the other side of the portal frame (ie. MSTC52 strap on both sides, interior and exterior, 4 straps total). However, if you look at this the outside strap would mostly be nailed into perimeter king stud and most of its developed strength would rely on this king stud shouldering the tensile load. In other words even with a larger strap the point of failure would still be the nails shearing that connect this king stud to the rest of the portal frame (header, framing and sheathing).
Perhaps I am over thinking this.
RE: Portal Frames and Nailed Moment Connections
1) Where does the outer strap deliver its tension load to in the end? To the header? To the outermost stud? I would have expected the path for that portion of the of the tension load to extend right to the top of the portal frame. You could make it do that easily enough by extending the strap over the cripples above.
2) You've got a metal strap and a wood stud working in parallel to resist a common tension load. There may be some issues with stiffness compatibility. Suppose the stud absorbs all of the load initially and snaps, then sheds its load to the strap which then yields or fractures. Or vice verse. I'm not clear how much ductility can be relied upon in this kind of system. This probably is over thought as we seem to disregard such issues in wood construction routinely.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Portal Frames and Nailed Moment Connections
RE: Portal Frames and Nailed Moment Connections
To address item #2, I'd take all of the load across the strap.
To be honest, I'm not sure that anything needs to be changed so long as you've addressed your load paths thoroughly, As I mentioned above, it's not clear to me where the outer strap tension ends up at the end of the day. It appears to deliver its load into the header.
Certainly, if the outer king stud and it's connections can deal with the entire tension load, I think that it's fine to leave it at that.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.