CFS / Wood Structure Interior Bearing Walls on Thickened Slab 2

[RFreund]

In regards to say 3-5 story cold formed steel or wood framed structures for condo/hotel/multi-family type structures. Does anyone use thickened slabs for interior bearing walls? How about if they also act as shear walls?

In my particular case all the foundations will be on vibratory stone columns.

My concerns are:
Uplift resistance at shear walls.
Differential settlement where the thickened slab meets the exterior wall.

We are working with a contractor who has stated that they are using thickened slabs for a couple 5 story structures currently. I suppose the differential settlement may not be an issue because of the vibratory stone columns. Also I could size the thickened slab for bearing pressure and possible check to see what is required for uplift. Although uplift seems like a long shot...

Thanks

EIT

http://www.HowToEngineer.com

 

[KootK]

It's been a while but I've done a lot of this. Usually for heavily loaded demising walls where uplift wasn't an issue. I hate to say it, but we never worried about differential settlement at the perimeter walls. It's a point of some debate but I believe that your boundary element T/C forces need to be handled with localized foundations beneath the boundary elements. If you detail appropriately, you can sometimes use a length of exterior wall for tension anchorage.

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.

 

[RFreund]

KootK -> thanks for the response.

So you are saying that for shear walls you would have foundation and footing below the entire wall or have a thickened slab and then you would have a pier and footing below the boudry element?
I may be worrying too much about the differential settlement situation...

Thanks again

EIT

http://www.HowToEngineer.com

 

[KootK]

A thickened slab and a pad footing at the boundary elements. Not necessarily a pier.

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.

 

[haynewp]

If it's all on similar foundations (aggregate piers) I don't think I would be concerned with differential unless the geotech gave a high potential settlement difference for the site. Adding more piers in some areas can fix that, I would run the loads by your geotech and Geopier (if that is who you are using) if you have a concern.

Aggregate piers can also be designed to resist uplift.

 

[KootK]

Aggregate piers can also be designed to resist uplift.

Neat. What does that look like?

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.

 

[haynewp]

A steel rod with a plate on the bottom of it underneath the base of the pier.

 

[RFreund]

@Kootk -> thanks for clarifying

@haynewp -> Interesting and good to know. Thank you. I will ask the aggregate pier contractor more about this.

EIT

http://www.HowToEngineer.com

 

[manstrom]

I've been doing it for 20 years. Wouldn't think twice about it

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[RFreund]

Sounds like I am out to lunch on this one.

@Manstrom - how do you typically handle the T/C boundary elements. Also which reference so you prefer for SOG design?

Thanks again

EIT

http://www.HowToEngineer.com

 

[manstrom]

For uplift, I'll spread the load over a large area of thickened slab and regular slab. If there is uplift due to a worst case wind speed, it will have to take the whole slab with it. In a sense, it is more resilient than a wall footing since it is monolithic. Typically, I'm only seeing 3k uplift (with DL) or so at interior bearing walls, it's not a lot. On my 4 story buildings, it zeroes out when I include dead load.

If you have bigger uplifts, add a thickened slab footing (bigger blob of concrete)

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[RFreund]

@Manstrom -> Got it, thanks.

EIT

http://www.HowToEngineer.com

 

[KootK]

You guys ever see restraint cracking around thickened slab pad footings? I don't even though I would expect to as a result of the footing keying into the soil. I provide control joints at thickened slab strip footings but not thickened slab pad footings.

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.

 

[manstrom]

I do see shrinkage cracks in the slabs, but not concentrated around thickened slab footings. I detail control joints in my slabs, but most of the time, they don't cut them or they half ass it. If the slab gets carpet or hardwood, they don't really care. It is an acceptable crack to the client. If the owner gets picky, they fill the joint.

The savings from a stem wall footing to a thickened slab is huge.

My experience is in residential. Single family and large multifamily.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[RFreund]

I would imagine though that you would eventually see cracking in wall finishes at the intersection of the exterior wall / interior wall joint. I mean the slab is floating so that it can move independently of the footing. I suppose those these cracks are probably minor and acceptable. Eventually the room is repainted and there is no crack for another X years or settlement 'stops' and there are no more cracks.

EIT

http://www.HowToEngineer.com

 

[RFreund]

Couple more questions ->
This is probably debatable, but... When calculating dead load tributary to resist uplift, what is typically used for CFS walls which utilize X-strapping? Half the wall length? Or only the load tributary the stud (ie. 16" or whatever the stud spacing)?

I usually see X-strapping for CFS walls (exterior and interior), does anyone sheath these instead?

With X-strapping it would see your tributary dead load would be smaller than if you used sheathing, which would seem to offer more rigidity to the wall.

EIT

http://www.HowToEngineer.com

 

[KootK]

You can use the entire DL of the wall as resisting so long as you put the mass at it's rightful centroid in your calcs. I've sheathed CFM walls for bracing. Either way, the dead load treatment should be the same.

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.

 

[manstrom]

I haven't thought too much about DL contribution on X bracing, but at first glance, I would use the full tributary DL. The panels are fairly rigid.

If you need some belt and suspenders considerations, all of the walls that T into the shear wall will have some contribution as well in the event of a crazy wind event.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[RFreund]

So even with strap x-bracing we assume the wall is rigid in regards to uplift (i.e. any dead load applied to the wall can be used to resist tension uplift)? Having a difficult time convincing myself of this due to the fact that we are using discrete member as opposed to a sheathed surface. However I've seen it in a few places now...

EIT

http://www.HowToEngineer.com

 

[KootK]

I see your point and agree. With discrete bracing, I don't think that you can use mid-wall Gravity loads. It would put the bracing in bending if the bracing even grabbed the desired stud posts.

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.