cmu wall to concrete slab connection

[smvk3]

How would one go about designing a reinforced concrete slab-to-top of cmu wall connection in the attached sketch? What provisions in the masonry code would you use in designing the 90 degree rebar dowel attachment for the forces shown? This connection is for a storm shelter. I only have experience with this connection when using a cast-in-place or precast concrete wall in which I would use the shear friction provisions to calculate the required rebar dowel area (using the resultant shear V = sqrt(V1^2 + V2^2) and then add to that required area the area required to resist the tension load T. I would then make sure I properly lap the dowel with the wall reinforcement and make sure my slab edge is thick enough to have enough hooked development length for the dowel into the concrete slab. However, I was unable to find anything in the masonry code to design these dowels.

 

[KootK]

My current understanding is that the CMU folks don't have their own version of a shear friction provision. Clearly, though, they need one. For lack of better alternatives, I'd be inclined to just use the same method that you described above for concrete for the CMU. Obviously, one would expect the mechanism of resistance to perform similarly. That said, there's always a little danger in extending something that was based on testing beyond the range of that testing. Do grout or block generate the same amount shear plane interlock as concrete? Dunno. On average, I'd actually expect more roughness out of the top of the bond beam than your average precast element.

Not a big deal but I personally prefer this version of your detail if you can get Ldh up top. I feel that it's a bit easier to build and should perform at least as well.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[smvk3]

That is what I am thinking about doing (using the provisions for concrete) however I am unsure if I should do this as my calcs will have a 3rd party reviewer and I would hate for this to be an issue during that stage of the project.

 

[KootK]

...I am unsure if I should do this as my calcs will have a 3rd party reviewer and I would hate for this to be an issue during that stage of the project.

I hear 'ya but, realistically, what are your other options? Bolted clip angles at the soffit? I do a bit of precast hollow core engineering work and run into similar problems there often with plank to block connections. EOR's will specify wacky things with supper long z-bars broken into plank cores etc that erectors won't do. When I suggest simpler details and the shear friction thing and get challenged on it, I respond "got a better idea?". I rarely get challenged on it and, so far, "got a better idea" has a 100% success rate.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[bones206]

I recall seeing somewhere that shear friction for masonry was being worked into upcoming versions of TMS 402.

 

[EZBuilding]

In the past I have considered the rebar dowel as if it was a post-installed dowel into the concrete and used appendix D limit states as the controlling criteria for the load.

There really isn't a perfect solution out there. If you rely on shear friction alone, you would never be able to get the connection to work if you didn't have a wall above the slab.

 

[masonrygeek]

The 2016 version of TMS 402 has shear friction requirements.

 

[smvk3]

EZbuilding, could you elaborate on this? I use shear friction all the time for these connx for cast-in-place walls for storm shelters.

 

[CURVEB]

If I understand EZbuilding correctly, I have often struggled with this as well. The part of shear friction that I get hung up on is the requirement that the reinforcement has to be fully developed on both sides of the friction plane. Typically this is very difficult to do in a slab of any usual thickness (8" or so). A detail like Koots is one possible solution. Another is to use very small bars and a thicker slab.

 

[EZBuilding]

I didn't mean to imply that you couldn't use shear friction for this application, just wanted to provide an alternative option for consideration.

CURVEB mentioned the concerns related to using shear friction for these types of connections to thin slabs. I have seen other engineers proportion shear friction capacity based upon proportion of development length reached, but that is swimming in muddy waters at best.

Based upon my experience with out-of plane design of masonry walls due to high wind pressures I often find that the controlling element in the design is based upon the bending of the wall not on it's attachment to it's supporting structure. The attachment loads are generally small due to the frequency of reinforcement required.

Example: 10'-0" wall with 100 psf (LRFD) out of plane pressure. Wall is reinforced with #5 @ 32" o.c. The reaction at the slab support due to the out of plane pressure is 1.33 kips. I would check this reaction against the appendix D provisions of ACI. Or throw it into PROFIS software with the balance of demands you are considering.