3x Sill Plate in 2-story wood shear wall

[StructTaco]

I have a two story shear wall in seismic category D per the 2006 IBC. It has 665 plf of shear. Per Table 2306.4.1, I can use the 3/8" struct-1 sheathing with 15/32" values per footnote d. Footnote i is also referenced and states that 3x sills are required for shears over 350 plf, and refers to 2305.3.11. 2305.3.11 says that all sill plates and members received edge nailing at abutting panels must have 3x sills.

Question: is this specific to foundation sill plates, i.e. first floor only, or any shearwall that is to transfer she greater than 350 plf, i.e. first and second floor walls?

Thanks as always

 

[msquared48]

Foundation sill plates only.

There are other locations where 3X minimum members are required, but they are studs at splice joints between plywood panels.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[StructTaco]

Thanks Mike,

is there a code section that says foundation sill plates only, it only says 'sill' in the code...?

 

[shaneelliss]

@Mike: Why would this apply to foundation sill plates only? What is different about the mechanics of this versus the mechanics of the foundation sill? I would think that because the attachment is made with nails to the floor below that a 3x member would even be more important on an upper floor than when the connection is made with bolts and large washers. Isn't pull through and sill splitting the main concern? What am I missing here?

 

[woodman88]

It applies to sill plates connected to the foundation because wood is a flexible material where concrete/masonry is not.

Garth Dreger PE
AZ Phoenix area

 

[dhengr]

The issue is that at the edge of a sheet of osb or plywd. you need to rows of nails, at fairly tight spacing, to achieve those allowable shear values for the sheathing. So, you need a thicker stud or plate, so that the nails from adjacent sheets don’t start splitting the stud or plate, and still meet edge distance and spacing criterial for the stud and the sheathing. “At abutting panels” is the operative phrase. Once the req’rd. shear level is low enough, you shouldn’t need the 3x edge backing members any more. You shouldn’t get bolt pull through in the plates because you are req’rd. to use larger washers under the nuts on the A.B’s.

 

[shaneelliss]

In section 2305.3.11 there is no mention of foundations or abutting panels. All it says is that 3x sill plates are required for any shear wall that exceeds 350 plf.

woodman: my first thought was that this might be a stiffness issue, but if you have an upper floor wall on top of a lower floor wall tied together through the floor with blocking, there won't be a lot of flexing going on will there?

dhengr: I understand that with the larger washers in a foudation sill tied with anchor bolts the pull through is limited. My argument was that with an upper floor shear wall, the sill plate is tied to the floor below with nails and those nails could pull through a 2x sill plate much easier than a 3x sill plate.

 

[dhengr]

shaneelliss:
I believe the primary intent of the 3x edge backing (blocking) members is as I described above, and to protect against panel edge buckling. At high panel shear levels the edge nailing req’mts. get much tighter also. Regarding upper level plates, think this way: you will have the nail heads in bearing, plus 1.5 or 2.5" nail pen. through the plate; and then how much actual nail tip pen. into the holding member? Is that joint going to fail in pull through, or pull out at the tip, from the holding member? If you have uplift at that joint you should not be counting on nail pull out (basically not a good way to load nails structurally). The sheathing should span across that joint, with the nails now in shear, and pick up that uplift loading, or straps or hardware can be used to span that joint for tension loading. Ultimately, full story, full height threaded rods can be used at critical locations, from the found. to the upper plates to take these uplift loads to the found. There is usually special shrinkage compensating hardware on these rods to keep them engaged as the bldg. shrinks and settles.

 

[shaneelliss]

Ok I agree that the nail is more likely to pull out than to pull through in tension, but I still don't see why a 3x would be required at the connection to the foundation and not at the connection on an upper floor.

So maybe this is just a definition question. Is a "sill" plate only the member attached to the foundation? I have seen "sill" and "sole" plate used for the bottom member of a wall. If "sill" plate is only for foundation attachment then the requirement of 2305.3.11 would only apply to that case. But if "sill" plate is also used to describe a second story bottom plate on a wall, I still don't see how you can get around the requirement in the code for a 3x bottom plate.

 

[woodman88]

As sole and sill are not defined in the IBC, the definition of them falls per section 201.4. My Webster's Dicitionary defines sill as "A horizontal member or beam forming the foundation...of a structure..." and sole as "3. The lower part or bottom of anything."
I have been looking at the ANSI/AF&PA SPDWS-2008 (as in the IBC 2009) and it looks like the requirement for 3x sill plates may be gone and replaced with large square washers extending to within 1/2" of the plate edge. But I have not read it completely yet. If I find referance to a 3x plate later I will post it here.
You can download a copy of the ANSI/AF&PA SPDWS-2008 at the below link.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

Woodman88: Technically, a "sill" plate is "defined" in the IBC in Figure 2308.11.3.2.

Shaneelliss: Check out notes c and d for table 2306.3.1. Abutting panels are mentioned, with 3X members needed where the nailing is tight and staggered.

Also, "Sole" plates are referred to in section 2308.9.3, in the 4th paragraph after section 8.

FYI all here, check out the Architectural Graphics Standards book if you have one. In my fourth edition, page 214, there is a good residential framing sketch showing the difference between sole and sill plates.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[StructTaco]

so what's the consensus? The way I read the code, all edges of plywood for shear over 350 plf in SDC=D should be 3x, which means that if the plywood stops at the sill plate, 1st or 2nd story, then the sill must be 3x.

 

[msquared48]

No.

The 3X member rule is applied to studs that see two adjoining 4X8 ply panels connecting to them. On a 2X member, this nailing requirement would split the H*^^ out of the stud. Consequently the 3X minimum requirement. The same does not apply to the double top plate or the sole plate as there is no plywood panel splice there, usually. If the situation at an exterior wall is such that a horizontal splice occurs on a sole plate, then it should be a 3X minimum, but this is usually not the case.

For a sill plate, no such splice ever occurs, but the high bolting requirements that led to longitudinal splitting through the bolts in the Northridge event led to the 3X plate requirement with washers. Note the exception though in 2305.3.11 allowing 2X plates for loads higher than 350 plf if the number of bolts required by analysis is doubled. Staggering the bolts here would be a good idea.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[StructTaco]

OK, but if I have a 2nd story wall with 665 plf of shear, the plate is required to be 3x because it exceeds the upper limit of the exception in 2305.3.11, and because I am using 10d @ 3"o.c. which also requires 3x???

Sorry, the light bulb is not quite on yet, flickering, but not bright yet...

 

[woodman88]

Msquared48: Technically, Section 201.1 “Scope. Unless otherwise expressly stated, the following words and terms shall, for the purposes of this code, have the meanings shown in this chapter.” and Section 201.4 “Terms not defined. Where terms are not defined through the methods authorized by this section, such terms shall have ordinarily accepted meanings such as the context implies.” defines the words used in the code. With the definitions section of each section defining words for that section only.
Can you please point out the section that states Figures can be used to define words in the code. As I read it, the Figure you are using only uses the ordinarily accepted meaning of the word “sill” and does not define it.
Sorry about my ravings, I have dealt with too many building plan/inspectors who have used a figure or statement in a unrelated section to mandate unneeded changes to my plans.

StructTaco: It is my opinion that sill plates are only used at the foundations.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

Woodman88:

I agree that neither is defined in words in the code. However we also have the time tested axiom that "a picture is worth a thousand words." Some things are just better defined in a picture.

Hey... There's the solution for cutting the code down to size!

Cheers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Another minor point to consider here. You can have a sole plate sitting on top of a sill plate in direct contact - a double bottom plate situation - where the sill plate is wet set into the top of the concrete wall, and the sole plate, which is part of a preframed pony wall, set on top of the sill plate. Happens all the time.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

StructTaco: If you are transferring the second floor shearwall shear (of 665plf) with plywood abutted at the second floor wall sole plate, then yes, you need a 3x sole plate at the second floor wall. Not because of section 2305.3.11. Otherwise the required edge nailing of the shearwall can be done on a 1.5” (2x) member or on half of a 2.5” member (1.25” at abutted 3x).

Garth Dreger PE
AZ Phoenix area

 

[StructTaco]

thanks everyone, much appreciated