Which is more economical - Drop Caps or thicker slab

[DEL2000]

I have heard from some engineers that it is more economical to avoid drop caps for punching shear by adding up to some number of inches of concrete. The thinking is that a good majority of cost in concrete construction deals with the formwork materials and labor. (This seems to be similar to moment frame columns in steel design, where you can double the weight of your column to avoid doubler plates, and it would still be more economical).

Other people in our office would prefer to make the slab the absolute bear minimum it needs to be to make flexure work, and then add drop caps and shear steel as required.

Any opinions on which philosophy is correct? Is there a cutoff point for the thickness of concrete added for shear to make it more economical?

 

[Taro]

When I have done framing studies for contractors' price comparisons, they have always chosen to make the slab as thin as possible and add shear reinforcing (e.g. studrails) at the columns. Their second choice was a thicker slab and the last choice was drop caps. This might be reversed in areas where labor is cheap compared to material costs.

 

[dik]

Drop panels should be approximately the thickness of the slab (a little thinner, maybe) and can effectively reduce the amount of concrete and reinforcing... Their economy is determined on the forming type and reuse and as noted, the labour costs. The use of drops can reduce the overall building height by coordinating the mechanical, etc with the space between the drops. The Imperial Broadway building in Winnipeg saved about 4" per floor.

Dik

 

[csd72]

There is a reason why punching shear reinforcement was invented. A flat form is much cheaper to build than one with drop panels.

dik makes a very good point though, there are other things to consider. You also check floor penetrations and get an idea of the amount of embedded conduits needed as these can render a thin flat plate next to useless if not considered.

Be careful about cutting things too fine as this can be false economy. Reducing a slab from 9 to 8 inches saves you 1" of height, but reduces the gross concrete stiffnes by 30%.

 

[JAE]

Today it might be more economical for fully flat forms as csd72 and Taro suggest.

But I have to say, over the years I've seen many more drop panel slabs (i.e. flat slabs) than simple flat plate floors.

 

[271828]

I don't know how the numbers come out, but we've designed a monstrous amount of flat slabs with drop panels. Most of these were hospitals, so folks would be perpetually coring holes next to columns. We oversized the drop panels enough so that they could lose a side (or two?? forgot) and still be ok for punching shear.

For at least one of these, we had a prelim pricing package and a DD package, both with the contractor involved and they didn't recommend bumping the whole thing up a couple of inches and eliminating the drop panels. We didn't ask about it, but they never brought it up.

 

[ron9876]

It probably depends. If a residential building the architect probably won't go along with the drops. If you can use flying forms the drops may interfere with that system and have a significant impact on cost. But in a stick formed building where you have the option to use drops it almost has to be cheaper.

 

[dik]

With substantial drops (approaching the slab thickness, a lot of negative moment is drawn towards the support where you have the added depth... Also with the greater thickness the stiffness can be 6 or so times the stiffness of the slab proper, this greatly reduces deflections... Forming may be a matter of constructing 'flying forms to accommodate the drops... this was done on the Imperial Broadway building. The construction manager built a mockup to establish the floor to floor height... It was about a 20 storey building...

Dik

 

[hokie66]

On multi storey buildings, cost comparison of floor systems alone does not tell the whole story. Affect on columns and footings can be a big part of the evaluation. So which is the appropriate and economical solution depends on many things. In a given big city, these parameters have all been factored in and everyone who works there should know which way to go. Even then, times change.

 

[rapt]

I prefer drop panel slabs (not drop caps which only assist with shear).

Drop Panels
- attract more moment into the negative area and place the extra concrete where the higher moments are,
- get rid of punching shear problems and the congestion around the columns caused by reinforcing for punching shear (the usefullness of which I often question)
- reduce slab thickness by about 15% and overall concrete volume and flexural reinforcement requirements
- reduce deflections

All for the addition of a thickening for a Length of L/6 all the way around a column. Yes, the formwork cost goes up but flying forms can handle this. But the benifits far outweigh this in my opinion and if designed and costed properly the drop panel slab would win.