# Slab Thickness



## MOOK (Feb 21, 2009)

To determine slab thickness for deflection criteria, ACI code use equations such as

ln/30 and ln/33

where

ln = clear span of slab

Is ln the short dimension of the slab or the larger dimension?? and where in the code it is mentioned?


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## kevo_55 (Feb 21, 2009)

Mook,

My guess is that you're finding out the minimum thickness of a slab such that you don't need to calculate the deflection, right?

I seem to remember from that question that it is a one-way slab. For a one way slab you'll be looking at the closest supports and place your steel in the proper direction accordingly.

I hope this helps!


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## MOOK (Feb 22, 2009)

Thanks Kevo for your answer. It seems that I did not make the question clear enough.

We agree that in one-way slab, we use the short distance.

My question concerns about 2-way slab. Some references use the long distance while others use the short distance.


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## MA_PE (Feb 22, 2009)

obviously the question depends on the aspect ratio of the 2-way slab. The load will tend to go to the short span first. As the aspect gets closer to 1:1 the spans become equal. I don't have the ACI code in front of me but I would not expect the definition of the term to be ambiguous.

Off the cuff I'd say check the short span.


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## teda (Feb 22, 2009)

For sure, it shall be length of clear span in long direction of two way construction. Note table is using ln, which is explained in notation at the beginning of the chapter.


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## MOOK (Feb 22, 2009)

I found the answer in ACI, it is the long span.

Thanks guys


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## McEngr (Feb 23, 2009)

MOOK said:


> I found the answer in ACI, it is the long span. Thanks guys


Be sure to note that the aspect ratio of long to short span shall not exceed 2. Otherwise, the 1-way slab deflection table shall control (if beams are between supports). If it exceeds 2 and you have to go with one-way slabs, then you can use equation 9-13 from ACI 318-05. Nonetheless, it sounds like you have figured it out.


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