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Re: Flat Slabs

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I would use ACI's Direct Design method as a starting point and then make
modifications to account for skip live loads and that concentrated wheel
load. I believe the DD method is limited to a certain maximum permitted
ratio between DL and LL so you will HAVE to make some adjustments (I am
writing this email at home and do not have my ACI-318 handy).

You can get an idea of what slab thickness you will need for punching
shear by checking the total load on the pile with your DL(slab weight) +
800 psf live load. If it looks like your slab thickness as dictated by
bending will be much less that what would be needed for punching shear,
then you can stop your piles short and pour a blob of concrete at each
pile to create a column capital. Studrails are great for thin
post-tensioned slabs, but for your case a more economical solution will
probably be to use column capitals. The cost of studrails is often
justified by a savings in not having to form a drop cap, but in your
case there is no forming cost,... just a little more digging at each
pile (for the column cap).

Drop PANELS (used to increase "d" for getting more negative bending
moment capacity as well as increase punching shear capacity) will
probably be more effort than they are worth with your short slab spans
(12'-6") - just stick to a uniform slab thickness with column CAPS if
needed.

The biggest thing to watch for will be to make sure that all the piles
are cut-off at the correct elevation. I doesn't matter if they are low
(just pour more concrete at the pile), but if they are too high (and
poke into your slab thickness) you will get a big reduction in punching
shear capacity.

Hope this helps.

Cliff Schwinger



> looking at a 12" slab as a starting point.  Having only done one flat
> slab project (actually checking an existing building to insure it was
> designed for vertical expansion) I am trying to find out about any
> pitfalls I may need to look out for.  Based on limited investigation
> there may be a need for drop panels (don't think so for sure but is a
> possibility).  Also would love to get some ideas for references on the
> design of flat slabs.

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