pony wall shear wall (seismic retrofit)

[mikeCTE]

greetings,

i'm working on seismic retrofitting my basement (seattle area). originally, i thought my pony walls would be 48" or less, allowing me to use 4'x8' sheets of plywood horizontally. however, after opening up the walls, i now find that i'm at 52".

my question pertains to orientation of plywood panels. if i orient vertically, i would add a vertical stud as required for edge nailing. if i orient horizontally, where would the ideal location for the horizontal blocking be located? 1/2 of the height? at the very top? etc? or, would it be better for me to just add 6" of blocking in the stud bays at top and bot plates so i could use a 48" wide sheet?

also, to confirm my methodology: i am adding holddowns at every discontinuity in my walls. so, for example, at a window, i will add a holddown near the window and then at the opposite end of the continuous pony wall (plus shear anchors too). then, after plywooding, i will add some CS16 straps around the windows to tie everything together.

thanks

 

[KootK]

if i orient horizontally, where would the ideal location for the horizontal blocking be located? 1/2 of the height?

Technically, I don't think that it matters. You just need the blocking at the joint. Personally, I like the vertical sheathing idea if that doesn't cause you any contructability heartache.

i am adding holddowns at every discontinuity in my walls. so, for example, at a window, i will add a holddown near the window and then at the opposite end of the continuous pony wall (plus shear anchors too)

The location of the hold downs, and the demand on them, should follow from the assumptions made in the engineered design. Maybe FTAO, maybe segmental. Certainly, placing hold downs at all discontinuities would be conservative in terms of the number of hold downs required if not, perhaps, the demand on each hold down.

i will add some CS16 straps around the windows to tie everything together

1) Those straps will need to engage blocking running beyond the opening to be effective.

2) I'm not 100% sure but I suspect that the strapping may only be effective when installed before the sheathing.

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.

 

[mikeCTE]

thanks for the response.

question on the strapping? it seems to me that it would be better to get your plywood to fully bear on your studs so that the nails can shear properly. if you put strapping in first, this will create a gap. if you put strapping in on top of the plywood, the nails seem to shear better and it seems like you would get the same shear value regardless.

​

 

[KootK]

I disagree regarding the strapping.

Firstly, the strapping should be of such a gauge that it does not interfere with the fastening of the sheathing to the studs and blocking. There is a gap but it's of no practical significance.

Secondly, I believe that the load path in these situations goes:

[sill/head plates] --> [strap} --> [blocking beyond the opening] --> [sheathing]

As such, by putting the straps outside the sheathing, you're creating a 1/2+ gap in the portion of the load path represented by the connection between the strapping and the blocking.

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.

 

[mikeCTE]

sure, the straps are 16 gauge (or 11 gauge for some of the ones i plan to use). that's enough of a gap to me. any gap between the plywood and the studs would seem to be a weak point in the shear mechanism in the nails.

if you put the strap on the outside of the plywood, yes, there's a 1/2" of material between the strap and the stud, but there's at least continuous bearing of shear material, right?

 

[KootK]

sure, the straps are 16 gauge (or 11 gauge for some of the ones i plan to use). that's enough of a gap to me.

If demand were that high, I'd probably either alter the design or go with two straps side by side so as to facilitate the use of a more manageable gauge. A three millimeter thick strap represents a TON of tension force. You must be running that strap practically to your neighbor's house in order to develop that much force into your sheathing.

if you put the strap on the outside of the plywood, yes, there's a 1/2" of material between the strap and the stud, but there's at least continuous bearing of shear material, right?

I'm not sure what "continuous bearing" gets you with respect to shear resistance? Bearing doesn't help resist shear forces except, perhaps, in the friction sense which isn't what you're doing here. The strap's job is to transfer axial force from the sill plate to the blocking beyond the opening. That 1/2" gap weakens the capacity of the nails to perform that function.

It's not as though you can't put the straps outside the sheathing if that's really what you want to do. The fasteners would just need to be adjusted to reflect there reduced effectiveness with that 1/2" gap in there. See the notes below from Simpson's literature. You basically have to use longer / larger shank diameter nails to compensate.

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.

 

[mikeCTE]

thanks for that post (and this discussion). i'm already using 10d common nails (3"x.148") so it seems that nailing the straps to the top of the sheathing will work for me.

one of my walls will indeed need a strap running to my neighbor's property. i only have room for one 7ft long shear wall on a 45' long wall that's a soft story filled with windows and garage doors...

 

[KootK]

You're most welcome mike. I learned a couple of things here myself. Best of luck with your basement retrofit. I suspect that you'll have one of the sturdier homes in the area.

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.

 

[mikeCTE]

one more question if you'll humor me.

i'm planning to use an interior wall in the basement as a shear wall (plywood and all). i'm not sure of my slab thickness yet, but assume it's 4" max - so not really enough to do shear anchors and certainly not enough for holddowns.

on one side of the wall, it will butt into a 4' tall foundation wall, so lots of places to anchor into. on the other end of the wall, it will tie into a column supporting a floor beam (8x12). the tributary area the column supports is about 120SF (kitchen space, and some interior walls, but no roof). i'm tempted to just tie the shear wall into the column and use it as an alternative holddown. do you see any problems with this (aside from not being part of a prescriptive code)? my worry is that i could actually create uplift in the column and damage the floor in a seismic event. unlikely, but still...

i really don't want to have to saw out a chunk of floor, pour a footing, and connect my wall to that...

 

[msquared48]

I see no problem with that at all. Done it many times in the past.

Mike McCann, PE, SE (WA)

 

[mikeCTE]

any special considerations i should take?

 

[msquared48]

Just make sure the dead load seen by the column is greater than the uplift you generate from the shear wall.

If you know the connection of the column to the footing and the footing size, you can also include the dead load of the footing, and probably some of the 4" slab too, within reason...

Mike McCann, PE, SE (WA)

 

[KootK]

my worry is that i could actually create uplift in the column and damage the floor in a seismic event. unlikely, but still...

Given the level of the forces that you seem to be tossing around, I would definitely perform at least a cursory numerical check to make sure that both the column and the foundation wall can deal with the uplift that you'll be asking them to resist. Some other things to consider with regard to the hold down aspect of things:

1) Assuming that you can tie it down with existing dead load, you won't be adding any new compression load to your column with your uplift case. You will, however be introducing a new eccentricity to the compression load on the column (1/2 column width).

2) You may be introducing significant new compression loads into your columns, walls, and and footings at the non-uplift end of the shear wall. Perhaps load casing can take care of that for you, I'm not sure.

4" max - so not really enough to do shear anchors

I think that you have to find a way to do shear anchors to connect to the slab on grade. If you don't, you'll have the following problems to deal with:

1) Your sill plate will be come one rather hard working collector element dragging loads over to the ends of the wall where it will be resisted through bearing.

2) Where your sill plate abuts the column, you'll be delivering a large concentrated load to that column. You may fail the column in compression perpendicular to grain and potentially shear the column from its base support if it's not wrapped in concrete.

3) You'll need to run some calcs to check that the soil behind the concrete wall can resist the concentrated load that you'll be delivering to it without to much movement.

I'm sure that you can find some kind of shallow embed Hilti anchor to fasten to the slab on grade however.

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.

 

[mikeCTE]

sorry, i should have been clearer.

i am planning to use anchors in the sill. i will be test drilling some holes to see what kind of depth i can expect and will likely toss in anchors at 16" OC.

the top of this shear wall would be continuously fastened to the beam above, so i suspect that even if i technically overload the column in compression as part of the shear wall element, the shear wall itself would have vertical load bearing capacity, albeit on slab-on-grade.

 

[msquared48]

You should still be able to find some chemical or epoxy anchors that will work for transferring the shear to the slab. If you are in a high seismic region, just make sure they are rated for that. Some are not.

Mike McCann, PE, SE (WA)

 

[mikeCTE]

i am planning to use simpson titen screw anchors. these are typical for seismic retrofit projects in the seattle area and recommended in the prescriptive retrofit material provided by the city of seattle, portland, and other similar areas.

 

[msquared48]

Used them too.

From your logo, do you work at CT Engineers in the Interbay area?

Mike McCann, PE, SE (WA)

 

[mikeCTE]

nope. CivilTech Engineering in bellevue. unfortunately, we never work on residential projects - just heavy structural stuff. otherwise, i'm sure this wimpy seismic retrofit would be second nature to me.

 

[msquared48]

I work in Kirkland - close.

Mike McCann, PE, SE (WA)

 

[mikeCTE]

dibble?