Stepped Foundations and Shearwalls 1

[medeek]

I'm looking at this residence right now and I have two questions or problems:

1.) The SDPWS (2012) has restrictions on perforated shearwalls such that the top and bottom plate are uniform (4.3.5.3, Item 6). To my knowledge no such limitation exist for segmented shearwalls. Can a segmented shearwall have steps in it (ie. one end is taller than the other and the bottom plate is discontinuous but the top plate is continuous)?

2.) On the east elevation in the picture below you can see that I have two stories with a daylight basement on the right side of the residence. Essentially three stories. I've got a couple shearwall segments drawn in but how to get much else out of this wall is causing me to scratch my head. Any thoughts on the best way to get more shear panels on this wall?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

 

[KootK]

1) I don't know about any code limitations but, rationally, the stepped wall should be possible, if a bit cumbersome. You'd need a vertical chord member at each step which you've probably got anyhow. You'd probably want strapped horizonral blocking at each corner as well which won't be popular.

2) could you collect your shear load at the main floor level and drag it back to the garage slab on grade rather than taking it down to the basement level foundation?

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.

 

[MikeHalloran]

All you need is a couple of picture windows that are stiff enough and strong enough to qualify as 'fixed structural glazing'. I wonder if polycarbonate can do that? ;-)


Mike Halloran
Pembroke Pines, FL, USA

 

[medeek]

In the south elevation you can see that the Main Floor is sitting directly on the stem wall for about 3 ft of the wall, the rest of the wall is supported by the stepped daylight basement wall. If enough anchor bolts could connect the sill plate in this 3 ft section of the wall then yes I would assume that the shear load could be successfully dragged into the stemwall foundation and not worry about the stepped wall below.

It's not apparent from the south elevation but the garage wall is 4 ft out of plane from the main residence wall.

I'm more worried about the East Elevation wall, he's got so many windows on the right side of this wall its hard to make anything work, but maybe I'm just not seeing it. I've never yet had to put a shearwall chord above a window header or beam, I suppose it could work but then you've got a point load on the header both in tension and compression and there will be more strapping and sizing to account for this, it just seems messy.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[moments]

Keep in mind also, with discontinuous shear wall chords you are required to apply the overstrength factor to the overturning loads.

 

[XR250]

Why not have the contractor run the basement walls up to the floor system?

 

[medeek]

At this point I think I should probably sit down with the designer and discuss what a reasonable amount of windows would be to make this work. In situations like this I've used specialized Simpson strongwalls before but that has also got me into trouble with the contractors and homeowners.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

I agree with your last statement. Get together with the designer. It's amazing how many don't understand the importance of keeping windows to a reasonable amount, AND keeping your windows in line from floor to floor.

 

[medeek]

I really dislike having to reconfigure or dumb down a design, somewhat of a cop out in my opinion. The whole reason this designer calls me is because I can usually make his designs work when they wouldn't otherwise using the IRC braced wall method and other prescriptive methods.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[SJBombero]

One thing to consider with a stepped shearwall is the increasing stiffness as the height decreases. Your shortest segment is going to end up attracting the majority of the force. Even increasing the nailing pattern on the taller segment, has minimal effect when the shortest segment height is 1/3 the tallest. I did a study on this for an apartment raised floor that was founded on a slope. In order for the shortest segment to not attract more load than its capacity, I had to eliminate the shortest segment and play with the nailing patterns of the taller segments to balance the loads.

 

[CBSE]

Remove the steps and concrete to the top of where you currently have it. End the concrete wall at your last step. It won't change any interior finish transitions. Besides, it will cost more to build with all of those steps.

 

[KootK]

The whole reason this designer calls me is because I can usually make his designs work when they wouldn't otherwise using the IRC braced wall method and other prescriptive methods.

If that's your competitive edge with this client then we'll have to dig a little deeper. See the sketches below. With some minor adjustment to the window locations, you should be able to slip in posts to take your chord forces to ground.

If enough anchor bolts could connect the sill plate in this 3 ft section of the wall then yes I would assume that the shear load could be successfully dragged into the stemwall foundation and not worry about the stepped wall below.

Expanding the concrete a bit on the south elevation makes sense to me. Another option to improve matters is to use a steel strap of some kind as your collector at the main floor level.

It's not apparent from the south elevation but the garage wall is 4 ft out of plane from the main residence wall.

This complicates matters but is not necessarily a deal breaker. You may be able to transfer the shear down to the main floor level and then across the main floor diaphragm laterally to get the load over to where it might transfer to the garage floor system.

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.

 

[KootK]

More gooder...

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.

 

[medeek]

I like what I am seeing KootK, however with a perforated shear wall doesn't a full height wall segment need to be at each end per Item 1 of 4.3.5.3. I think I could remove the left most basement window of the East Elevation then bring the concrete up to the edge of the perf shear wall you have drawn in for the first and second floor. If I use a drag strut back to the top of the foundation I probably can eliminate the 3 ft shear wall panel in the basement wall.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Someething along these lines:

The south elevation I did not redraw since I will go with what you have drawn above (KootK), I think that is probably as good as it gets.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

however with a perforated shear wall doesn't a full height wall segment need to be at each end per Item 1 of 4.3.5.3.

I would argue that, in all of the ways that matter, the perforated shear wall is done being a shear wall once it hits that main floor drag strut. After all, theoretically, there would be no shear required in the panels below the main floor. As such, I'd be willing to leave that one basement floor window alone.

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.

 

[medeek]

The shearwalls are only one of the issues with this particular design, I've also got an interesting wall of windows on the north wall as well as some interesting roof lines intermixed with a stairwell and beams. In hopes of better sorting this all out I'm going to meet with the designer tomorrow and see if I can gain more clarity and see how he feels about removing some of the steps from the foundation at critical locations.

A really rough mockup in SketchUp is here for those interested:

https://3dwarehouse.sketchup.com/model.html?id=b2204b12-d2c8-4821-831b-749d5e30c07a

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[msdmoney]

I would argue that, in all of the ways that matter, the perforated shear wall is done being a shear wall once it hits that main floor drag strut. After all, theoretically, there would be no shear required in the panels below the main floor. As such, I'd be willing to leave that one basement floor window alone.

I agree about leaving the window since everything is transferring in shear to the concrete through the drag, but the vertical chord holdown above this header (from the left side of the perforated) would need to transfer to the header and then have a holdown on either side of the header that goes to the ground.