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Structural Slab

Structural Slab

Structural Slab

I am designing a structural slab to support a self contained hot-tub.  The property has a significant slope and in order to have the tub at the same height as the deck, an "mini foundation" has to be poured.  The walls are 9 feet tall and the "foundation" is 15' square.  The home owner wants to be able to store garden equip. etc. under the tub in this mini-basement.  

Can I design the slab on the top of this 15' square foundation as a one-way slab and just spec. the reinforcement for both directions?  

Thanks for your help,

RE: Structural Slab

Yes, that would be conservative.

RE: Structural Slab

That would certainly be the conservative approach.  Or you can design it as one-way with temperature reinfrocement transverse.  You will need to be sure you have the proper amount of corner reinforcement.  Personally if you didn't want to design it as two-way then I would at least take the time to do the two-way strip method.

RE: Structural Slab


Be careful.  Designing a two way slab as one-way with only temperature reinforcement in the transverse direction can lead to significant cracking as the loads redistribute from the initial elastic 2-way distribution.  Probably not a safety concern, but a serviceability issue and could have your clients doubting your qualifications.

RE: Structural Slab

To all,

Thanks for your help.  I normally do log/wood design and am comfortable with standard concrete beams, wall stuff, this was just a little beyond my comfort level.  I have several wood engineering programs but nothing with the capability to design (or offer design suggestions) for two way slabs and I like to be able to check my work with a program or another engineer (not an option in this case).  The one way slab design is within my comfort level with hand calcs. and software capability.  Plus I couldn't find any reference in ACI or my CERM book on how to design a two way slab with the entire bottom part of the slab in tension.  It seems like every design method, DDM or Equiv. Frame, involved some sort of interior support.  This project is a square slab, supported on all sides...and that's it.  

So anyway, I designed it as a one way slab and specified the rebar both directions, but also making sure my rebar ratio, summing both directions, was less than the max.  Perhaps overkill...but if anybody thinks this is an unsafe approach I would like to hear your thoughts.


RE: Structural Slab

A one-way slab may even be the preferred structural arrangement to allow access underneath the slab (c.f. a two-way slab with an edge band or edge beam).  For such a small plan area, I doubt you would save much on materials and labour with a two-way slab anyway.

RE: Structural Slab

According to the original post the slab is based on top of walls on all sides in a square configuration.  The slab doesn't know if you designed it as one-way:  it's going to act as a two-way slab initially no matter what due to the width to length ratio of the span.  Generally one-way action will not predominate until at least 2:1.  Agree with Taro that if it was designed as a one-way, significant cracking could occur before redistribution takes place. The method of designing such a small slab as a one-way in both directions is conservative and ok in this situation.  If you wanted to get fancy there are typical solutions (such as in MacGregor) for yield line analysis of square flat plates that would get you an upper bound for reinforcing.

RE: Structural Slab

Is it supported on all four sides?  It sounds like at least one side is open to allow access underneath for storage.

Agree though that the proposed approach of having one-way reo in both directions in conservative and acceptable.

RE: Structural Slab

Your approach is conservative with the same reinforcing in both directions.  You might want to dig up some information on 2 way slabs supported on flexurally stiff elements.  This will give you the magnitude of the negative moments.  With a 2 way slab, cracking can/will develop at right angles to the diagonals of the slab; these are referred to as corner levers.  The negative moment will also require dwls into the wall below.

RE: Structural Slab

I agree with WillisV that the slab will behave as a 2-way slab because of its width to length ratio. A simple rational 2-way slab design can be done using the ACI Coefficient Method. A sample problem can be seen in a book authored by George Winter and Arthur Nilson "Design of Concrete Structures". You'll end up with a more conservative thicker slab if you design the slab as 1-way and using the same reinforcing in both directions.

RE: Structural Slab

I don't see any problem designing the slab as one-way and specifying that steel in both directions.  The OP has already noted that the ACI methods do not apply because the slab in question does not meet the necessary criteria, such as a minimum of 3 spans in both orthogonal directions.  Some diagonal bars in the corners would be a good idea, and make sure that the structural depth reflects the fact that bars in one direction are shallower than bars in the opposite direction.  The slab will end up a bit thicker than really necessary but this is a small slab anyway.  It won't be that different in the end.

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