huangyhg

suspended slab supported on three sides
hi,
can anyone give me some assistance in designing a slab on three sides?
i have a 7'-0"x15'-0" balcony and the architect does not want to have a front edge beam. i can either design as a cantilevered slab however i would like to keep the slab thickness to 6". i was considering cantilevering two beams but i'm not sure this will make much of a difference because of the aspect ratio. can anyone give some advice on designing slabs supported on three sides and if this would make any difference.
suggestions are also welcome.
regards,
adrian
the architect also doesn't want any columns at the front of the balcony.
i am assuming the 7'0" is cantilevering out the 15'0" is the width of the balcony.
i would just cantilever the slab for the 7'. that is not that long of a cantilever and you shouldn't have any trouble keeping the slab at 6" thick.
doing a very rough check of 100 psf ll, i am getting somewhere in the neighborhood of a #5 @ 12".
thanks myerges ,
your assumption is right however, what about deflection? isn't that an issue, esp. since aci recommends an minimum of l/10 for slab thickness in lieu of more detailed assessment.
regards,
adrian
ataman-
yes, you will have to do a quick deflection check (probably by hand). i just did it with #6 @ 12" and the tip is under l/360 for total load.
the tip deflection will depend on the backspan, eg. you could get upward deflection with live load inside only.
you could look at a tapered slab thickness; 7" at the building tapering to 5" at the tip results in better performance for the same concrete volume.
why don't you try a one way spanning slab?
clefcon
what about the influence of the boundary conditions along the sides that are 7 feet long?
clefcon
one way spanning slab is actually worse.
henri2
i'm not sure what you mean exactly
myerges
how do you do a quick calculation for deflection of concrete? is there some sort of rule of thumb. the procedure i know is pretty invloved and includes calculating effective moments of inertia which is based on the dead and live load stresses. how do you account for long term deflection.
regards,
adrian
ataman-
you are correct. the procedure is in section 9.5.2.3 of aci 318-05. the long term deflection multiplier in is 9.5.2.5. calculating ie is not overly time consuming.
apsix-
you can get upward deflection with live load on backspan only, depending on magnitude of ll and length of backspan; however, the greater concern (i believe) is downward deflection. in addition, a tapered slab will perform better for deflection, but 7" to 5" is still not complying with the l/10 rule and deflection will still have to be checked. with the tapered slab, it is a much more time consuming process.

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