change in shear strength along wood shear wall

[radiocontrolhead]

I have an existing wood framed shear wall and want to add a bit more capacity to it. I want to strengthen say the last few feet with a higher strength panel, nailing,etc. If I can resolve the additional overturning at the new wall chords, could this be an acceptable approach?

 

[KootK]

I don't see it. Your unit shear in the sheathing should be constant between your chords. As such, you'd still be limited by the capacity of the weaker part of the wall.

 

[jayrod12]

Koot, do you think that a stiffer wall would attract more load? In this instance since the chord would be deflecting the same amount for each wall segment, the stronger segment would take more of the load?

 

[radiocontrolhead]

the way I'm picturing it is that the weaker wall will carry it's own uniform unit shear up until the new wall where a chord for both the weaker wall and stronger wall are now shared. The resultant chord reaction is then summed appropriately.

I am likely approaching this from a relative stiffness relationship at this point where sheathing is sharing common stud framing.

 

[RPMG]

If there are two walls in line, of stiffness k1 = 1 and k2 = 2, the total wall stiffness is 3, where k1 takes 1/3 and k2 takes 2/3 of the load. Theoretically, that concept applies to wood shear walls, but stiffness has to be added to the walls with the unit shear forces resolved, accordingly.

 

[KootK]

Koot, do you think that a stiffer wall would attract more load? In this instance since the chord would be deflecting the same amount for each wall segment, the stronger segment would take more of the load?

Nope. If we're still using the same two, original boundary chords, I stand by this:

Your unit shear in the sheathing should be constant between your chords. As such, you'd still be limited by the capacity of the weaker part of the wall.

Think of it in terms of vertical shear in the wall rather than horizontal. They're complementary of course.

If we're claiming a third, intermediate chord at the boundary between the reinforced and un-reinforced parts of the wall, then we might be able to work something out.

 

[radiocontrolhead]

Koot,

Yes, I am definitely looking at in terms of vertical shear.

The idea is to in fact use an intermediate chord between the weaker and stronger walls.

 

[KootK]

All good by me then. Aside from the sharing of the intermediate chord, it's basically just two separate walls in line at this point. I'm curious, what prevents you from just doing the whole wall? Part of the length in a different room that can't be touched?

 

[radiocontrolhead]

Koot,

Yes that's correct. I am introducing just enough strengthening to meet the existing building codes "10% rule"

 

[KootK]

So how long's the original wall and over what length does the reinforcing extend?

 

[radiocontrolhead]

Koot,

two segments of wall, 22'-6" long each for a total original wall length of 45'-0". Both sides are sheathing with a capacity of 145 plf.

removing a 5 foot portion of 1 segment leaves me with a demand/capacity ratio exceeding 10% and making up for it by strengthening one side w/ 6 feet of new sheathing.

calcs attached, need to clean them up though

 

[KootK]

I see. For that tiny improvement are you sure that you can't just fudge your calcs and leave it alone? Some creative rounding or something? 12.5% not so far from 10%.

 

[radiocontrolhead]

I tried to find another way to justify it but couldn't think of anything else. I'm afraid the 5 feet removal may have been wishful thinking and likely need another 18" of removal. I'll know once I start detailing the opening against existing framing.

Have any suggestions?

 

[BridgeSmith]

Is the shear capacity of the wall governed by the shear capacity of the panels, or the number of nails/screws? If my memory serves me, for typical sheathing, the fastener spacing has to be really tight before shear capacity is actually limited by the panel itself.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[radiocontrolhead]

BridgeSmith,

I am in agreement, however, these are existing gypsum board walls so i'm not quite sure that necessarily applies to these as well?

 

[KootK]

Ideas...

1) gypsum shear wall capacities do indeed increase with closer fastener spacing.

2) you've taken your demand as the existing wall capacity, right? Is it within your purview to calculate the demand for real with the hope of shaving it down some? You can do a fair bit with 40' of shear wall on a single line.

 

[JAE]

I think that if you imagine your dual shear wall condition in a ridiculous extreme it helps you understand it.

Assume your original shear wall is a giant wet noodle....low low very low stiffness.
Your new shear wall is a typical nailed plywood shear wall....much higher stiffness.

With a constant top of wall chord delivering the lateral wind/seismic load, you have a constant delta - lateral deflection a the top.
Both segments of walls have moved X inches laterally.

How much force flows through the wet noodle compared with the plywood shear wall? (Hooke's Law says there's a LOT more load in the new wall and if the noodle is very wet, probably no load in the original wall).

So if you create a shear wall extension with higher stiffness, the extension will take a greater percentage of LOAD than the wet noodle will....even though they are deflecting the exact same amount.
Unit shears in each wall will correspond with that load difference.

If both unit shears for each wall fall below the maximum allowable unit shear capacities for each wall, you should be good.

 

[KootK]

If we'll be combining wood panel shear wall segments with gypsum panel shear wall segments, we may need to consider the limitations below.

 

[radiocontrolhead]

Okay, I'm getting cornered...Next thought: What is meant by "the same materials and construction? Here is my interpretation...

I think I will take JAE's recommendation and base it off of relative rigidities and avoid using plywood, and strengthen with a higher strength gypsum board sheathing (considered the same material and construction).

 

[phamENG]

radio - yes, as long as you're using GWB on all walls in the line you're ok.

You are using the SDPWS (source of Koot's last post), correct? It's important to remember that there are three distinct factors the effect the deflection (equation 4.3-1): deflection in the underlying framing, deflection of the shear panel (GWB in your case), and slip in the hold downs at the ends of the wall. The first two will vary linearly with the unit shear, but the hold down slip may not - you have to refer to manufacturer data to determine it. Then, you have to iterate as you redistribute based on elastic deflection, verify the deflection, and redistribute the loading, etc.