Shearwall height

[Cap07]

This is a pretty basic question about shearwall height. Let's say I have a 1-story house with 8' stud walls. The framed wall rests on the floor system, which consists of 2x12 joists (see attached .jpg file for a cross section).

For the shearwall height, do I use 8' or 8'+ depth of the floor?

 

[BAretired]

For the shearwall height, do I use 8' or 8'+ depth of the floor?

In what context? What are you attempting to do? Depending on what you are doing, it could be either.

BA

 

[Cap07]

Hi BA,

For example:
1) Checking the maximum height to width ratio for shearwalls (IBC Table 2305.3.4)

2) Calculating the load/ft to the roof diaphram (do I use 8'/2*wind pressure, or 9'/2*wind pressure).

3) Calculating the holddown loads: (unit shear) x (wall height)

Thanks for your help.

 

[sandman21]

It's to the top of the foundation, 8' + floor, Section 2305.3.5. Why is the wall resting on the 2x12? Seems to me to be a waste.

 

[Cap07]

Hi sandman,

I found the section in the IBC that you referred to - the shear wall height is defined as:
1. The maximum clear height from the top of the foundation to the bottom of the diaphragm framing above.

For reference, I've attached the figure from the IBC.

Maybe I'm mistaken, but "clear" height would seem to mean from the top of the floor sheathing to the bottom of the diaphragm framing above. If you look at the 2nd story in the figure, the clear height goes from the floor diaphragm below to the bottom of the framing above. I guess my confusion is this:

For the shearwall height of the 1st story - do you measure the height from the mudsill that rests on the foundation, or from the 2x plate that the wall studs rest on?

Also, I don't understand your comment about the wall resting on the 2x12 being a waste.

 

[msquared48]

1: Per the diagram to the underside of the floor/roof diaphragm framing... i.e., to the top of the associated double wall plate.

2. 9'/2, but this would be for a single story structure. For a multi-story structure, it is the distance between the floor/roof diaphragm plywood above and the floor diaphragm plywood below divided by 2.

3. Diaphragm plywood to top of concrete foundation wall or footing.



Mike McCann
MMC Engineering

 

[Cap07]

Hi Mike,

Thanks for your answer... I'm not quite clear on number 1: for checking the maximum height to width ratio, would I use 9' as well? [9'=from the top of the stem wall to the top double plate, or 8'=from the top of the floor joists (diaphragm) to the top double plate]. Please see the picture I attached in my first post - do I measure the height for the ratio from the top of the foundation stem wall, or from the top side of the joists (where the wall studs rest)?

Thanks for your help.

 

[Teguci]

The code seems to be addressing how the sheathing will react to lateral loads. It requires that your shearwall segments act like deep beams with, what I would think, is a very stiff wall section. This exercise is intended for determining the maximum lateral shear your wall will take. This section should not be used for addressing how your loads (wind and seismic) are distributed to the diaphragms or how you will need to anchor your shear wall.

Ex. Using this calculation I can determine that wall A can resist ## Kips of lateral shear. I only need it to resist # Kips. Ok, I do not need another shear wall in this direction, now I will design the connections.

 

[Cap07]

I have a wall that has a lot of windows in it. I'd like to use as many of the full height wall segments as possible for shear walls, but I need to check that they meet the height to width requirement. So my question is: when checking this ratio, do I use 9' (from the top of the stem wall to the top double plate), or 8' (from the top of the floor joists and sheathing where the the studs rest, to the top double plate).

My thought is that the latter height (8') is actually the "clear height."

 

[Teguci]

Short answer - 8'

For all intents and purposes, the top of the floor acts as a fixed base and the bottom of the next floor's joists act as a fixed top. The shearwall is then checked for empirical stiffness.

 

[BAretired]

1) Checking the maximum height to width ratio for shearwalls (IBC Table 2305.3.4)

2) Calculating the load/ft to the roof diaphram (do I use 8'/2*wind pressure, or 9'/2*wind pressure).

3) Calculating the holddown loads: (unit shear) x (wall height)

I am not familiar with the named code, but I believe:

1) Use clear height for h/w ratio.

2) Use clear height/2 plus roof thickness plus parapert height.

3) Use total height above concrete foundation for holddown loads.

At least, that is the way which makes sense to me.

BA

 

[mudflaps]

First rule of house framing - Design the way the framers will try to build it, if they don't look at the details.

Yeah, I know you're the engineer and they SHOULD build it like your details, BUT you'll just wind up having to justify what got built instead of insisting it be torn out and done "right". Course the owner will be wondering why you didn't do it the regular way in the first place. The framer will be pretty uncooperative if you give him much grief too. So, now for some engineering theory.

The top plate of the wall is assumed to be a drag collector so that's the top of the shear wall, not the roof sheathing. A similar rational exists for the bottom plate, it re-distributes the load along the floor even though you normally should specify enough sill plate nailing (per your detail) to transfer the panel shear directly into the floor sheathing/rim joist within the length of the shear panel. This is in case the framers cut the bottom plate at one end (a door)or both ends (two doors) of the shear panel.

As BA said, "it depends on what your are trying to do". In the case where the wall bottom plate gets cut, you'll need to transfer all the shear to the rim joist, then into the mud sill, within the length of the shear wall if the 'rim joist' is just a series of blocks. In this case the wall height for calculating overturning is 9'-0". If the wall sill plate is continuous then you could use a wall height of 8'-0".

Now for how the framers want to install plywood. Your detail indicates the wall plywood starts at the top of the foundation. This means there will be a horizontal plywood splice 7'-0" up the wall measured from the floor (unless the framers order 10'-0" sheets - yeah like that's ever gonna happen). If this is a tract house, all plywood will be installed by a "plywood crew" who will "correct" all the spacing and alignment errors in the stud work. The plywood crew will be planning to start the plywood at the top of the floor and that's the way it will be when you go to inspect the framing before the city inspector will comes out.

Bottom line - Design like the framers usually work and you'll delay the onset of stomach ulcers (for a little while anyway). However, if you really want to re-invent the wheel, be prepared for battles on the construction site, and ulcers. If you really, really want a non-standard detail, put a big cloud around it and a big "NOTICE" arrow pointing to it. Then maybe, but just maybe, the framers will see what you want to do, and do it.

So, 8'-0" of 9'-0"? Use 9'-0" just to make sure you've got a little "fat" in you design. Oh, and watch out for that electrician with the 2" diameter hole drill. ;-)

Old CA SE

 

[mudflaps]

Sorry for the ramblings. Sometimes I take my eye off the goal line.

Old CA SE

 

[sandman21]

If you strap across the windows you can define the height per 2305.3.5.2 as being from the foundation to the strap. However, it still needs to be taken from the top of foundation. In addition, the 2x12 creates a hinge in the wall any horizontal load will cause a moment on the 2x12 that has to be resisted by the sill plate. I would just run the concrete up and use a ledger to attach the floor to the foundation.

 

[StructuralCA]

I will second BAretired
For checking h/w we use clear height (sole plate to top plate). Floor thickness is considered very stiff and will not deform under lateral load. h/w limitations are based on expected wall deformation.

For uplift calculations, we take full height from diaphragm sheathing to diaphragm sheathing below. That is because lateral forces are assumed to be concentrated at diaphragm plywood. We normally add first floor thickness to full height in first floor since floor thickness (rim) will not be designed for uplift at ends. If there is high crawl space, then we design cripple walls as shear walls or simply extend first floor shear wall to foundation and just use total height measured from foundation to second floor. This is just simple statics.


M.S. Genidy, PE
Structural Soft, LLC

 

[Cap07]

Thanks StructuralCA - that really clears things up for me. And thanks to everyone for all of your input - you have been a lot of help and it is very much appreciated!

Cap