ACI 318-11 / ACI 318-14 Minimum Drop Width Dimensions

[bryangallagher]

I am a structural engineer from Canada working on my first ACI suspended slab.

The first hiccup that I have come across is the the requirement that drop panel shall extend one-sixth of the center to center span. This is not included in CSA 23.3. The grid spacing for the project is 10.2m x 10.2m and the drop dimensions are 3.2m x 3.2m. This just barely doesn't meet the one-sixth requirement.

ACI318-14 / CL 8.2.4 (ACI318-11 CL 13.2.5):
A drop panel in a nonprestressed slab, where used to reduce the minimum required thickness in accordance with 8.3.1.1 or the quantity of deformed negative moment reinforcement at a support in accordance with 8.5.2.2, shall satisfy (a) and (b):
(a) The drop panel shall project below the slab at least one-fourth of the adjacent slab thickness.
(b) The drop panel shall extend in each direction from the centerline of support a distance not less than one-sixth the span length measured from center-to-center of supports in that direction.

My question is: if I am using FEM analysis can this minimum drop dimension be ignored? It looks like this requirement should only apply when using approximation formulas. It seems difficult to get around the "when used to reduce ... the quantity of deformed negative moment reinforcement" portion of this clause.

Is there a section in ACI that explains what clauses do not apply if using FEM?

Thanks for your help.

B Gallagher

 

[rapt]

It is a fairly common rule in design codes. It is in ACI, BS8110, and was in earlier Australian codes but unfortunately was misplaced in one of the revisions.

It has nothing to do with analysis, the requirement is for reinforcement design.

When determining the flexural reinforcement required, is it being calculated for design strips or for each row of elements? Normally it would be done for strips and in the areas around the column the section shape is a T section of overall depth of the drop panel with the flange having the slab depth. For this logic to work , any reinforcement calculated on this section is assumed to have an effective depth equal to the drop panel depth. If the drop panel is too thin, the bars at the edge of the flange are too far away from the side of the drop panel and cannot be assumed to have the effective depth of the drop panel.

Also, except for very heavily loaded slabs, this drop dimension normally results in punching shear being adequate at the face of the drop panel. If you make it smaller, make sure you check punching shear at the face of the drop as well as the face of the column.

 

[KootK]

Is there a section in ACI that explains what clauses do not apply if using FEM?

Not that I am aware of.

My question is: if I am using FEM analysis can this minimum drop dimension be ignored? It looks like this requirement should only apply when using approximation formulas. It seems difficult to get around the "when used to reduce ... the quantity of deformed negative moment reinforcement" portion of this clause.

Like you, I've always believed (and still believe) that the L/6 requirement could be waved when more detailed analysis methods such as FEM were employed. The trick, as you've discovered, is proving that.

The deflection control part of the equation is easy. The span to depth ratios in table 8.3.1.1 assume that a certain portion of the slab area will be the thickness of a drop panel of a certain depth. If you plan to calculate deflections explicitly per 8.3.1.1 and satisfy the limits of 8.3.2, then you're off the hook.

I've never fully understood the relevance of L/6 to the negative flexure design. ACI doesn't do much of anything to explain the logic there. Since rapt's explanation is is quite sensible (and the only one that I've ever heard), I'm happy to run with that.

At all of the US and Canadian firms that I've worked at, the practice has been to essentially ignore T-beam action in middle strip design. Instead, both the drop and the flange areas were designed independently for flexure assuming that the their respective flexural lever arms were based on their respective physical depths. I believe this to be a conservative approach and, were it to be employed, I would personally be okay with waiving the L/6 requirement for negative flexural design.

I've always though that the minimum drop depth provisions (t/4, 100 mm) were odd. While a very small drop depth may be relatively ineffective and perhaps difficult to form, I see no reason why a small drop depth would cause analysis or design problems.

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.

 

[bryangallagher]

Thanks Rapt and KootK.

KootK I agree with your approach of ignoring T-beam action. For negative steel over the column we always calculate the number of bars (area of steel) over the drop and the number of bars (area of steel) that ends up in the slab depth and calculated moment depth accordingly.
We also use SAFE to draw a narrow strip directly over the column to ensure that we have enough concentrated steel where we need it most.
I also agree that if you cover all your bases that deviating slightly from the L/6 rule is okay. In this case my 3200mm drop panel is only 200mm short of the L/6 limit, so I know that I am not pushing the system too far.

Thanks for the help!
BG

 

[KootK]

You're most welcome Bryan. Your situation is pretty regular so it's not a big thing but, in modern, highly irregular slabs, trying to figure out what L/6 ought to be is also a royal pain.

We also use SAFE to draw a narrow strip directly over the column to ensure that we have enough concentrated steel where we need it most.

If you don't mind, I'd like to know more about this procedure. Can you describe it in more detail? Which code provisions are you targeting with it? Are you dealing with the general lateral distribution of rebar in the top mat or the bars that need to be withing 1.5d of the column etc? Use of FEM to satisfy non FEM developed code provisions is of great interest to me.

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.