concrete design question

huangyhg

concrete design question
i have not been exposed to concrete design too much in the real world and i have general questions:
1. if i have a structural slab that is suppoted by 4 walls.  the slab sits on the top of the walls and dowelled into the wall with bent rebars (one leg into the slab and one leg into the wall).  so is this fixed or pinned?
2. do you guys have any pointers on how to design the slab?  i was going to design it as if it were 1 way slab and design it to span the short direction and then put the rebars the same way both ways.  shall i follow aci and make the slab thick enough so i dont have to worry about deflection? some of you may say to ask my supervisor to train me but i dont think he knows any better than i do and he has a pe stamp!.  
thanks!
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1) you have to consider the joints as pinned connected, because, the walls may not have been designed to carry fixed end moments and the required amount of reinforcement for considering the joint fixed will be too large and may not be economical.
2) for deflection control, you can use aci-318 suggested thickness.
for designing reinforcement, yield line theory is most applicable for this kind of situtation.
1. from what information you have provided, it seems that the slab would be considered simply supported (pinned) by the walls.
2. aci is likely the correct code document, but it would depend on where the structure is located and who has code juristiction. if you are in the us this should be the case.
you need to look at the ratio of the span lengths to determine if the slab is a one-way or a two-way slab. you'll need to evaluate how much deflection is acceptable and verify that you meet that requirement. aci gives some guidance on minimum slab thickness.
good luck!
well, do you think my approach of designing it as one way slab (pin pin) on the short direction will be a satisfactory design?  then ill just put the same rebar going the longer direction.  this project is not very big so i do not have the time right now to really learn it to save amount of steel and concrete.  i have a feeling the pe will love and trust whatever design i come up with (which is scary).  so i want to make sure i do it right in so little of time.
from what i re  
designing as a 1-way slab will be safe and conservative. the rebar spanning the short direction should be in the lower layer.
you should also have top steel at the supporting walls, especially at the corners, to avoid cracking due to negative bending moment. however, i would still design it for pinned supports.
yeah, thanks apsix.  that is what i was thinking too.  thank you everyone!
you could design as two way (put the short direction steel in bottom) by use of tables. in the british standard theres a simple table to use not sure if equivilent in us. should be considered simply supported. if you design as one way spanning design will be conservative so be ok, check deflection as one way spanning as well, (short span). effect of four support sides will mean deflection will be less than you allowed for. generally if the ratio of spans > 2 then one way spanning.
check out the aci website for an example calculation.
when dealing with a single slab bearing on 4 walls, you will also have to account for mxy and myx moments.  you can look at pca design of rectangular tanks for the design procedure.  i would definitely buy this book (it's only like $40) and use this design methodology.  they have an example in the book for a tank design with a roof slab, which is basically what you are describing.  is the wall you are supporting below ground?  if so, the walls will have to be designed like a concrete tank.  they will not be braced until the top slab is poured.  if you do not account for this in your design of the wall, you could have a failure or at least a wall with large deflections or large cracks.  also, be careful in your detailing of the wall.  you should look in the aci detailing manual for help in detailing a concrete wall or "tank".
you could certainly design it as a one way slab and it would be safe (i.e. it won't collapse), but depending on the aspect ratio (i didn't see it mentioned above, just that one side is longer than the other.  there is a difference between a ratio of long to short of 1.25 and 4) it really wants to behave as a two way slab.  it won't collapse designing it as a one way slab, but you may have some serviceability issues.