Elevated slab at CMU wall - though or interupt?

[MrStohler]

There is a discussion in our office regarding the best detail to use at the junction of an elevated slab with a CMU wall that would continue from below to levels above.

The elevated slab will be a joist supported 4" slab on 1.5" form deck. The wall is an 8" CMU shear and bearing wall.

One option is to stop the wall at the bottom of the slab, place the deck and slab, then build the wall up from the slab. There would need to be consideration of re-bar detailing at this junction to ensure proper shear transfer both from the slab and from the wall above. This has been compared to platform framing in residential construction.

The second option is to build to wall to a as tall as practical, then anchor a sill angle to the wall to support the slab perimeter and transfer shear then place the slab. If needed the slab can serve as a staging platform for any remaining wall construction. This might be considered analogous to balloon framing.

Each option has advantages and disadvantages
Op 1, Requires more coordination of trades and can interrupt the mason. It does allow easier placement and finishing of the slab.
Op 2, Provides a continuous wall with uninterrupted reinforcing.. It does not require the use of partial height blocks to match coursing with the exterior walls.

The NCMA TEK 5-7A Fig 12. shows the second option but does not appear to indicate whether it is preferred over the second.

Are there design details or case studies that make a case for one option over the other. What have been the experiences of the engineers and designers in this forum?

Thank you.

 

[StructTaco]

I have designed CMU buildings with parapets (one story only with this method of framing/construction), essentially your second option where the angle ledger supports the deck/slab.

In multistory design, I have designed the first option where the slab stops below the slab and the slab is poured, and then the next story starts, like platform framing.

I have only seen the continuous wall in parking structures, which I equate to a "house of cards" and then I've only seen the concrete shearwalls being constructed continuous through floors, not the masonry shearwalls (not sure why).

I'm in California, and this is just my own experience. But my gut feeling says anything multistory wouldn't feel right with the cmu "balloon framed" option...but I don't have an engineering reason why you couldn't or shouldn't do it one way or another.

I'd be interested to see other posts on this subject as well.

 

[msquared48]

Is this a shear and/or bearing wall, or a partition wall?

Mike McCann
MMC Engineering

 

[MrStohler]

Response to masquared48:

It is a combined bearing/shear wall (resists both shear and gravity). The slab/wall interface described is intended to transfer both gravity and shear loads from the second floor to the wall. The wall carries both gravity and shear from the roof over the second floor.

More background: It is a two story institutional building, IBC 2006, SDC-A, 90 mph wind exposure C. There is sufficient wall area that bearing has a much greater effect on wall design requirements than shear.

Thank you for the quick reply.

 

[msquared48]

I would lean toward maintaining the integrity of the CMU wall and frame the steel to the wall with afterset bolts or inserts as need be.

Sorry that I did not pick up what you said earlier in the first post about this being a bearing/shear wall.

Mike McCann
MMC Engineering

 

[jike]

If it is only a 2 story building, I would go with option 2. Less coordination and interuption for the GC. Most elevators and stair towers on short buildings are built this way in the midwest. Taller buildings use option 1.

 

[concretemasonry]

The break at the floor level is not necessarily a detriment. Have you checked with a GC and a masonry contractor for opinions and practicality?

You are going to have normal breaks in the wall construction depending on the scaffolding system, for rebar splices, grouting and OSHA approved wall bracing. You may also have a bond beam near/under the elevated bearing or attachment

The partial height block to maintaining the exterior coursing is not a problem and there are many industry details that are used. This is not a unique structure. It could be a bit more difficult with an 8" wall than a 12" wall.

Both methods will work.

Dick

 

[MrStohler]

Thanks to all for the quick feedback.

We have chosen Option 1. One of the primary reasons for making the selection is that it directly transfers load from the slab to the CMU, rather than from the slab to an angle and bolts to the CMU.

Further, the architect preferred it since it is less likely to have a gap that would effect the floor to floor STC rating.