Two-Way Slab-Order or Rebar Placement 1

[doka1]

I am working on small footprint 2 way slab 30" thick, surrounded by walls around the entire perimeter, essentially making a concrete vault. The spans are 29'7" and 24'....The plan dictates #9 at 12 o.c., top and bottom, both directions. The section illustrates the rebar in the 29'7" direction to be installed first...in the field the bars in the 24' direction were placed first.

Considering the bars are of the same size, does it really matter which is installed first?

 

[SlideRuleEra]

It should not matter which bars were placed first. If it does (theoretically), the design is too precise to allow for normal field variations.

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[theonlynamenottaken]

First off I'm assuming that you meant to say 30 inches thick and not 30 feet thick for the slab!

Technically it does affect the flexural capacity if the shorter span bars are placed closer to the top and bottom surfaces. However, the loss of moment capacity for the longer span would only be ~4% and at a slab thickness-to-span ratio of 1:10 I seriously doubt whether flexural capacity would be the governing failure mode anyway.

Shear capacity should not be affected. Negative moment capacity only comes into play if this slab is continuous beyond the stated perimeter to more distant supports. The only other reason the top bars could matter would be for positive moment ductility or for ridiculously high uplift forces like bouyancy or blast.

 

[doka1]

That was my thought... my logic was if the bars were different sizes then it would matter.
See Section picture below

 

[hokie66]

It is probably not a big issue with a thick slab, but I think the field got it right. The bending moment in the shorter direction is greater. Think about it...the deflection is the same for both directions, so the deflection/span ratio is greater for the shorter direction, therefore the moment is greater.

 

[KootK]

my logic was if the bars were different sizes then it would matter.

I don't agree with this logic. It doesn't matter much here because the thickness of the slab is large relative to the diameter of the bars. The potential change to the effective flexural depth is minimal. If this were a 10" slab, we'd be having a very different conversation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Archie264]

In many of the texts on two-way slab design the depth of steel used, d, is the average of the d in each direction. In other words, it's measured from the point of contact between the two perpendicular rows of steel and that depth is used in each direction. So, based on that (and everything written above) I think you'd be ok.

 

[hokie66]

I think we are all agreed that in the OP's case, the layer order would make little difference. But I think it important to know which way is correct, and in a slab supported on 4 sides, the shorter bottom bars should be placed first.

 

[dik]

... and the shorter top bars should be placed last...

Dik

 

[hokie66]

Well, dik, in the case of a slab simply supported on 4 sides, the top bars do little, except at the corners. And they are normally not full length. It is standard practice to provide nominal top bars around the perimeter, with support bars to suit.

That said, in the OP's case as shown in his section, with very thick wall supports, the designer may count on developing negative moments.

 

[dik]

Agreed, but, they marginally help as compression steel in the middle... (needed or not)