Wide, shallow beam limitations 1

Hello, and thank you.
I need to determine if an existing CIP slab section is adequate. I have a segment of a slab that is 8' wide x 12" thick, spanning 25'. There are uniformly spaced longitudinal bars in the bottom (#5 @ 12")along with 2-#11 along each side of this strip. There are #5 @ 12" bottom transverse reinforcement as well.
Is there a limit to the width to depth of a strip in a slab that I can consider as a beam? I know the transverse reinf. is adequate to distribute loads to the additional #11 edge bars,but I don't see a code limit that applies to this condition (considering this strip as a wide beam works for my loading condition).
I also have a similar condtion with a 14' wide strip that also calcs as working except fo deflection.

Thanks for the input!

 

[ishvaaag]

Normal assumption is linear member till 4 to 1 proportion, yours is 8 to 1.

However I think to have read recently somewhere (I am unable to remember) it is acceptable in your range to calculate just as a beam ... having bigger reinforcement at edges only reinforces that.

In doubt, a FEM analysis will allow you to see the degree of concentration of the stresses for an non uniform loading, then decide if other reinforcement scheme is convenient.

 

[TXStructural]

What is the boundary for these slabs? If the slab works as a slab with #5@12" (which is approximately As_min, per ACI 318-08 10.5), the #11 on the boundary is probably not functional for flexure. ACI 318-08 10.6.3 requires relatively even distribution of bars across one-way slabs. I don't think the #11's would qualify.

If this is part of a larger slab, it is customary to bound openings with bars which replace the bars which are discontinuous through openings, which could explain the #11's. (Specifically required for 2-way slabs in ACI 318-08 13.4.2.) As a reference regarding effective width of a slab acting as a beam, for 2-way the column strip width is 1/4 of the least span direction in a panel.
It would be likely that a designer might detail every opening the same, for simplicity in construction and detailing, thus the #11 along sides of openings up to about 8 feet across.

It is doubtful that one would design this as a true single wide beam, especially without some kind of shear reinforcement.

 

[woody1235]

Thank you for your input!

I agree that we don't have very uniformly distributed bottom reinforcement - have the #5s that are uniform across bottom, bu the #11 aren't.

There are some other factors. This slab strip that I'm calling a wide, shallow beam, is actually an edge strip of a mlti-span 2-way PT slab. However, the slab was constructed with no longitudindal PT along this 8'wide strip (12' - 14' wide at 2 bays) - the uniform tendons stop at he edge of this strip. There are banded tendons at the perpindicular column lines that cross this strip (as well as shallow beams in this bay).

Thus, there is transverse PT compression, at least near the perpindicular columns, and longitudinal compression and flexure capacity from the adjscent parallel uniform tendons. I think considering this strip as something of a stand-alone mild reinforced only shallow beam is conservative.

Right now, a sharper pencil indicates that cons1dering these 25' clear span stips as simply supported wide beams works OK, with fair deflection (can't consider as multi-span continuous since no mild top steel at the perp. beams). I'm thinking that the concern with non-uniform reinf is mitigated by the intergral adjacent PT spab.

Thanks again

 

[JedClampett]

Also, in addition to the excellent advice above, as you've found out in your other situation, you're susceptable to deflection issues, based on ACI 318-08, Section 9.5.
If this is a construction mistake, I'd make them redo it. If it's checking an existing building, I'd add whatever thickness you need (or steel members) to make it servicable. A bouncy section like this is asking for trouble.