Uplift in shear wall is too high

[Jeepitou]

Hello everyone,

One of my colleague is designing a 4 story wood building and having some trouble with the design of the hold down. The shear wall are parallel to the joist, so they don't support any dead load. The uplift at the base of the building for the smaller wall is 45000 lbs, we can't seem to find any hold down with a good enough capacity to resist that kind of uplift.

So... Does my load make no sense? Is there something I am missing possibly?

(Sorry for my bad english, it's not my first language).

Thanks for helping,

 

[KootK]

That's a big hold down force. You may need to rework your building lateral system. What are you using for your compression chord stud pack? 45K is about equivalent to stacking 18 Toyota Corollas on top of your stud pack.

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.

 

[Jeepitou]

I don't know yet, still trying to figure this out. I have shear wall of 30' long with uplift of 33.7kips and shear wall of 6' long with 45 kips.

Can you tell me if my seismic load is out of proportion? I got 461 kips for a 4 stories building of 12000 ft^2.

Thanks for the answer

 

[msquared48]

Roof = 120k. (10 psf dl)
3 floors @ 180K ea = 540k (15 psf dl)
Total DL = 660k
Base shear = About 0.15 X 660 = 99 K

Your base shear is way too high.


Mike McCann, PE, SE (WA)

 

[Jeepitou]

I have a 1 1/2" slab on each floor. I used 20 psf for the roof and 50 psf for the floors.

The national building code won't allow me to use such smalls loads.

I also have to consider 25% of the snow load and 50% of the live load.

We also have the worst soil category for this, obviously.

Anyway, we decided to use steel brace in the worst direction, since we already had some steel columns.

Thanks for you answers.

 

[jdgengineer]

With interior partition weight, exterior walls, nonstructural topping slab (common for 4 floor wood buildings) you are probably closer to average of 15-20psf roof and 30-40 psf floor. I might come up with double Mike's base shear but yours still sounds pretty high. With redundancy factor i would be getting a little closer to yours but still off 30% or so. Looks like Mike assumed SDS of 1.

I'd review the loading you used again, and shearwal layout. You likely want to use the shrinkage compebsating devices for your holddowbs. These can take some nasty uplift loads but Anchorage of the device will be problematic if your loads are too high.

Side note SDS is nearly 2 in some areas by me!

 

[msquared48]

OP:

Based on what you said, I would still use 15 psf DL for the roof unless you are using concrete tile, where I would use 25 psf.

However, I would use 15 (wood framing) + 20 (conc)= 35 psf for the floors.

You should probably add 5 psf for the interior and exterior walls too to be safe.

Are you in Seismic ... C, D, E...? which one?

Even though there is little load to the walls, there is still some dead load at 10 psf minimum to lower the overturning a little, plus probably at least a couple of feet at each floor.

Sounds like your code is far more restrictive with having to throw in 50% of the floor live load. We do not have to do that unless it exceeds 100 psf.

I think you will need more walls as Kootk said, or some transfer beams either above or below the shear walls to lower the holddown forces.



Mike McCann, PE, SE (WA)

 

[Jeepitou]

I'm in seismic E.

Also, after studying the code, I don't think I need to consider 50% of the live load, it is only for the load combination.

 

[msquared48]

Are you in California?

Mike McCann, PE, SE (WA)

 

[RFreund]

Multiple hold downs with multiple posts at the end of the wall.

EIT

http://www.HowToEngineer.com

 

[Jeepitou]

No I am in Quebec.

 

[Jeepitou]

Any of you have a rule of thumb of about how long of shear wall you need in a building according to it's area/number of floors? I feel like I have a lot of shear walls, but the force is still huge...

 

[sandman21]

You have a mid-rise wood building, you should not be designing this like a house. An important consideration when selecting a hold down device is to consider shrinkage of the structure. Consider using a shrinkage compensating device such as Simpson ATS. The added beneficent is that 45k uplift is small for these system and rods and posts can be provided to handle these loads.

 

[jayrod12]

Finding a tie down system for that load is easy. Finding something to anchor it into is the hard part.

 

[msquared48]

I've heard that China is taking Building Applications for our tie-downs.

Mike McCann, PE, SE (WA)

 

[RFreund]

Whats wrong with multiple hold downs?

EIT

http://www.HowToEngineer.com

 

[jdgengineer]

Nothing is "wrong" with multiple holdowns. Although if you have several you'd have to justify that the outer ones don't fail before engaging the capacity of the inner ones (unzipping)

The bigger issue is with taller wood structures shrinkage becomes a more critical component and the shrinkage of the wood can cause the drift to be excessive if a shrinkage compensating device is not used.