Which is more economical - Drop Caps or thicker slab
Which is more economical - Drop Caps or thicker slab
(OP)
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?
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?






RE: Which is more economical - Drop Caps or thicker slab
RE: Which is more economical - Drop Caps or thicker slab
Dik
RE: Which is more economical - Drop Caps or thicker slab
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%.
RE: Which is more economical - Drop Caps or thicker slab
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.
RE: Which is more economical - Drop Caps or thicker slab
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.
RE: Which is more economical - Drop Caps or thicker slab
RE: Which is more economical - Drop Caps or thicker slab
Dik
RE: Which is more economical - Drop Caps or thicker slab
RE: Which is more economical - Drop Caps or thicker slab
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.