Portal Frame (PFH) Double/Triple Sill Plate?

[JungleJoe]

Hi all,

I've got a question about the Portal frame with hold-downs (PFH) detail in the 2015 IRC (R602.10.6.2) and 2018 IBC (2308.6.5.2). I have seen many versions of this detail drawn and many of them mention a double, or even triple sill plate being required. In the language of the IRC and IBC I cannot find anything talking about this sill plate requirement, yet many of the details I see out there specifically mention that the codes require the double or triple sill (see image below for an example). Do any of you know where this requirement is mentioned?

 

[jayrod12]

I'd bet from a wood perspective, you'd need more lumber just to make the shear transfer from the portal to the concrete work out. a single bottom plate doesn't have a ton of capacity when you really only have room for a single anchor bolt.

 

[JungleJoe]

jay,

That was my thought as well. It's just odd that that detail above says "Double sill (2015/2018 IRC)" and then right below it says "Triple sill (2012 IRC". I can't find either the double or triple plate mentioned in any of those code books. I'm sure they're in there somewhere...

 

[phamENG]

That is odd. The term "Double sill" doesn't appear in the IRC at all. Or so my CTRL-F tells me.

 

[JungleJoe]

lol, I have scoured the web and all codes using every CTRL-F combination of "double", "double sill", "triple sill", "portal double", etc. that I can think of and can't find anything!

 

[dhengr]

JungleJoe:
The rigid frame gets it primary strength, at the base, from the straps which are embedded in the conc. found. wall. But, the sill pls. (bot. pls.) have a lot going on too. They are the edge chords for that narrow (small width) vert. shear panel (something of a box beam/beam column) which has several lines of nailing, which just won’t fit in one or two sill pls.; and they also distrib. the vert. gravity loads and moment loads to the found., and of course, the straps do not contrib. to this load distrib. function, except for the tension component in each direction. Sometimes, in EQ design, we will use 3x4’s, 3x6’s or 4x’s for these edge members to provide enough lumber for the panel edge nailing, that is row and line spacing, and edge dist. and nail size and to avoid that third edge in the middle of a double 2x member.

 

[jayrod12]

dhengr may have figured it out. Depending on the required nailing at the base of the panel, you may need more material.

 

[JungleJoe]

I do agree that that's what the extra sills are there for. What confused me is why these details specifically call out the number of sills and provide a reference to a code. It's especially confusing when it says the requirement in 2012 IRC was for a triple plate, but it was then reduced to a double plate in 2015 and 2018 IRC. I don't see any specific sill plate verbiage in any of these code books, and I can't find any documentation for why the supposed requirement for the triple plate in 2012 was allegedly reduced to a double plate in 2015.

 

[phamENG]

I totally agree with dhengr's reasoning, but simply from a "code compliance" standpoint it's tough to figure out where they're coming from. NDS limit for nails to "prevent" splitting in a single 2x is 3in...and that's the spacing for all of the prescriptive portal frames. So it doesn't need to be doubled for that (though it is a good idea).

And you don't need the extra meat for the shear load - a single sole plate will give you a little over 1k per anchor, and you need at least two. I think it was APA who published a white paper on them and they can only take about 1800lbs for the shortest and widest version. Now if you start looking at external wind pressures and using those anchors in your uplift load path, you may need a little extra...

 

[phamENG]

JungleJoe - that Simpson TB is the only place I've seen that. Easiest thing to do is to reach out and ask their engineers where it came from. They're very responsive and helpful.

 

[JungleJoe]

I'd think you need the extra meat to get the shear load into the sill plate. The anchor bolts with a single plate work fine, but getting the shear into the sill from the wall itself is the challenge. If the shear load is 1800 lb. then you'd have 900 plf into a 2' wide panel which I don't think a single row of nails at 3" would work for that.

 

[dhengr]

What do “2015 IRC (R602.10.6.2) and 2018 IBC (2308.6.5.2).” actually say and pertain to? Somebody post them for our/my reading. What did they read into those code sections? I no longer go broke buying code eds. which I seldom use. Much of that stuff (tabulations, frame cap’ys., etc.), are in good part, based on their testing and approach to the analysis of the particular problem (not some code para.), so call Simpson’s engineering dept. and ask them where it came from, or where their citations say that. If they intend these to be used under the IRC, without any engineering guidance or actual design, they will intentionally be conservative where there is any doubt.

 

[JungleJoe]

Hopefully you can read these or at least zoom in.

 

[ChorasDen]

Code is available online at iccsafe.org, links below.

https://codes.iccsafe.org/content/IBC2018/chapter-23-wood

https://codes.iccsafe.org/content/IRC2015/chapter-6-wall-construction

 

[dauwerda]

The only thing I see indicating this is the image itself. The 2012 IRC does in deed show a triple plate where as it is shown as a double plate in the newer versions. It is not specifically called out in any of them that I can find.

 

[dhengr]

JungleJoe: Thanks for the info. and your efforts, but when I enlarge the figures, the print gets too fuzzy to read very well. Copy quality is lost in file transfer or something, if you can read your copy.
ChorasDen: Thanks for the links, those I can actually enlarge a bit, and read. The IBC section presentation looks much like what you might expect to see in the IRC, maybe almost lifted from it, very prescriptive in nature, maybe from some of Simpson’s early work and testing.

I don’t see anything in those exact code sections that really gives me the answer or direction for explaining the conundrum. I’d want to talk with some Simpson engineers and the code people who might remember how that all developed. These days, codes are changing so fast and the want to bring new products to market are happening so fast, and no real history record of the development is being kept, to help guide us with these types of questions or discrepancies. I’m sorry I can’t be more helpful.