Ultimate Capacity of Wood Shear Walls

[JAE]

With a wood shear wall
(wood 2x studs with sheathing on one side - built-up end posts each end - hold downs at the end posts with bolts into the concrete foundation)

one has to use magnified seismic loads to design the hold down bolt to the concrete.
This usually requires either an Ω factor on the loads or "the maximum force that can be delivered to the system".

If the Ω results in too large forces to design to - and you want to somehow derive a weak link somewhere that would represent the maximum force delivered to the system, you might find one of these that would control:
1. Max. ultimate diaphragm shear into the shear wall.
2. Max. ultimate shear wall sheathing shear capacity
3. Max. ultimate end post tension capacity.
4. Max. ultimate end post compression capacity
5. Max. ultimate shear of anchor bolts along the base of the shear wall - either wood sill bearing failure or anchor failure
6. Max. overturning moment that the footings can take.

Most of these involve the "maximum ultimate" capacity of a wood entity. The NDS and other wood publications don't seem to provide that value. The safety factor or ultimate state of wood members is never directly given. We are given Fb, Ft, etc. but with no knowledge of the integrated variability in "failure" capacity.

Anyone have any thoughts on this?

With steel, we are required to use an Ry factor to account for the "true" yield level of the steel.
With wood, we don't know what sort of integrated increase value is applicable to wood.

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[msquared48]

Wood shear walls are constructed in the field, sometimes by those not familiar with the limitations incurred by faulty construction techniques, irrespective of any inspectors. Hence the larger safety factors...

With steel, there is more plant control in the making of the steel, and welding is very controlled. So the line can be toed more...

Not so with wood.

So, I don't think I'd go there JAE...

Just use more walls or transfer beams above the shear walls to lower the hold down forces to the concrete stem walls. I usually limit the uplift to 2000# for a 6" thick wall, more for an 8".

if wind controls, no worries about Omega...

Mike McCann, PE, SE (WA)

 

[JAE]

Mike
Thanks for your reply

But in a high seismic condition we either must use the omega or use the largest load that can be delivered to the system. And wind doesn’t come close to controlling.

Wit the omega, there’s no anchor that works. Thus the need and desire to derive the largest load that can be delivered.

I know wood construction is more variable and has higher safety factors. Just want to know what they might be.

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[msquared48]

I think at least part of the solution for the connection problems we see to the concrete for seismic loads is to rethink the Omega factor, as that factor is playing a very large part in driving the concrete failure.

If you are going to use the factor, allow short term stress increases in the concrete as was allowed in past codes. Otherwise, back it down.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

Isn't there an option to design for "ductile" anchorage instead? Where you make sure the anchor rods yield in shear tension prior to concrete pull out?

Granted, I almost always use Omega. But, I still this was just the brute force way when you couldn't ensure ductile anchorage behavior.

 

[JAE]

Josh - with the end-of-the-wall hold down bolts I'm not sure shear is the dominant applied force as we would have all sorts of spaced "shear" anchors along the wall length - not sure we could ensure a ductile shear failure - maybe a ductile tensile failure but with standard Simpson hold downs I'm not sure you can even know what that RyFy value is.



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[JoshPlumSE]

I was definitely only about the tension anchorage for the hold downs. For shear, I would just decrease the spacing of the shear anchors until I can handle the omega level forces. I thought it was the tension pull out issues that you were mostly concerned with.

 

[JAE]

Yes -hold downs. You said "anchor rods yield in shear..." so I thought you were talking about the shear anchors.

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[msquared48]

Ok.

The obvious issues to be looked at would be the shear nailing and the end bearing of the columns.

I believe the values were arrived at through testing to failure. Then the failure loads were reduced by a factor of safety to get the ubc/ibc table design values.

So you would have to find the governing study and the subsequent reasoning for the FS used.

I have usually seen a FS of 4 applied in similar situations, but do not know here.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

Yes -hold downs. You said "anchor rods yield in shear..." so I thought you were talking about the shear anchors.

Sorry about that. Total brain fart on my part. I was thinking "tension", but somehow ended up writing "shear". Corrected the post to prevent further confusion.

 

[msquared48]

JAE:

If the walls are in an existing building that is being retrofitted, the IEBC allows for a reduced seismic load.

Could work.into the mix too.

Mike McCann, PE, SE (WA)

 

[JAE]

These are all for new construction.

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[msquared48]

Darn... oh well.

Mike McCann, PE, SE (WA)

 

[KootK]

Yeah, coming up with a limiting capacity based on the wood elements seems fraught with uncertainty. I've never seen this done but the logical place to put a fuse would seem be the boundary post steel strapping at the level above anchorage. Maybe DIY some A36 instead of Simpson hardware. You won't win any popularity contests with that of course. And even this has it's uncertainties. Who's to say that some extra tension resistance doesn't come about through sheathing continuity etc? Still, one has to draw a reasonable line in the sand and move forward. Omega's not a perfect thing either but it's looking better and better the more that I type.