Moment Redistribution to Wall 2-Way Slab

[BadgerPE]

I am back to working on the analysis of an existing 2-way slab that we have previously discussed. I have a question regarding how much moment is distributed to the middle strip when there is an exterior bearing wall present. See attached (3) pages for details.

I am analyzing this slab (circa 1968) using the equivalent frame method (EFM) because there are multiple violations of the direct design method (DDM) and I would like to be consistent for the entire analysis. When a flat slab has exterior columns at framelines and a wall which the slab is cast monolithically with (upper 3' of wall) how much moment is carried by the column strip and the middle strip? The existing column strip and middle strip have the same negative reinforcing (15) #6 bars over 13.5' so my thought is initially that 50% goes to each strip. However, that seems a little odd since the column strip is significantly stiffer than the wall. Another reason that I am questioning the 50% distribution is because the critical location of the column strip occurs at the face of the drop panel, which significantly reduces the moment found at the column CL. The critical location for the middle strip would be at the face of the wall and therefore, the moment would be much higher if it was evenly distributed. Maybe the critical location is considered to be at the same location for both the middle and column strips due to the inherent stiffness of the drop slab and capital??

I found some distribution factors in the 1967 UBC, but I assume they are for use with the DDM.

Any help would be great!

 

[KootK]

Any chance it would work if you assumed that the walls were not there at all? That would be my first choice if it's possible as I'm always a little skeptical of the capacity and stiffness of the wall/slab connection. If that doesn't pan out, I might be tempted to FEM a couple bays with the wall modeled pinned to get a better sense of what's going where. With column capitals and drop panels I agree, the middle strip ought to draw more than 50%.

A couple of questions for you:

1) Do you know the wall/slab connection detail? Can you describe it?

2) Whats the goal here? New, heavier loads to be supported? Will cracking/serviceability be a big deal?

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.

 

[BadgerPE]

I don't think it will because I get significantly reduced allowable loads when I assume 100% is carried by the column strip. Since there is the same amount of reinforcing in the middle strip, I assumed it was design to transfer "some" moment and I should take advantage of that if I can. 2-way slab analysis is new to me, so I'm trying to stick with hand methods so I can understand the underlying theory without getting into FEM.

To answer your questions:

1) See the detail on the 3rd page of the original attachment. The slab had a turn-down "beam" that was about 3'-5" deep. I assume the wall reinforcing crosses the joint to make the wall/slab continuous.

2) I am establishing what the capacity of the slab is as-designed. There has been significant degradation/water infiltration because this slab is exposed to the elements. I am attempting to spreadsheet everything so we can make modifications as needed once the field analysis is carried out. That won't happen until Phase 2 begins because the CIP slab has a drainage mat/wearing surface installed above. Ultimately, the loads may be reduced (gasp) because the proposal is to install a lightweight green roof system (archy's idea) to eliminate some DL and reduce the walking surface to reduce LL demands.

 

[KootK]

Since there is the same amount of reinforcing in the middle strip, I assumed it was design to transfer "some" moment

I'm sure that it was. For the sake of precision, when it comes to the lateral distribution of slab moments in the DDM, there's really no transfer or redistribution involved. The coefficients are intended to reflect elastic stress distributions straight up. That's in contrast to the longitudinal moment distribution which, I believe, does incorporate some redistribution.

See the detail on the 3rd page of the original attachment.

I saw that but that detail didn't say anything about the disposition of the rebar in the joint. Obviously, standard hooked top bars alone do not make for a moment connection with the wall. You'd need L-bar dowels etc that lap with both the wall and the slab reinforcing. Perhaps the rebar schedule clarifies that part of things, I'm not sure.

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.

 

[BadgerPE]

Hmmm....after consulting the rebar schedule, they are in fact standard 6" hooks on the negative moment reinforcing. Still seems that there should be "some" capacity there. What that is though, I don't know.

 

[KootK]

Still seems that there should be "some" capacity there.

If those hooks don't extend into the wall and aren't lapping with some corner bars, the moment capacity at the joint is zilch. It seems to me that, with part of the wall being cast with the slab, there must be some corner bars in there. Probably just matter of figuring out how to prove that.

You've got those parapet bars crossing the joint. Too bad those parapet bars aren't a little longer. You could have perhaps used the parapet to clamp the joint.

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.

 

[BadgerPE]

Right....

I just reread section 13.6.4.3 in ACI which says if the slab is supported for at least 0.75L2 then it can be assumed that the moment is distributed evenly across the L2 dimension.

 

[BadgerPE]

However, I will probably use a more conservative number. Maybe 80/20 if that works out.

 

[rapt]

The transverse distribution of moments/shears in a flat slab is basically caused by the deflection of the slab at the support as you move away from the columns across the panel. If there is a continuous wall, there is no deflection, so in general the distribution would be 50/50, or more correctly constant moment (widths of strips may be different).

With a column in the wall in the column strip, you could calculate relative stiffness of the column+ wall in the column strip and wall only in the middle strip distribute to each based on the relative stiffness.

This obviously assumes that the slab can develop the moments in the different areas.