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reinforcement design in two way slab

reinforcement design in two way slab

reinforcement design in two way slab

(OP)
can top and bottom bars both be used when cecking for shear?

RE: reinforcement design in two way slab

I assume you are talking about beam shear in a two-way slab, and that you are using ACI318 Equation 11-5.  If my assumption is correct, you can only use the steel on the tension side, which would normally be the top steel.

RE: reinforcement design in two way slab

The question is not clear.  Please elucidate.

BA

RE: reinforcement design in two way slab

Amplify...elaborate...expound...clarify...Add to... hairpull3

 

Mike McCann
MMC Engineering

RE: reinforcement design in two way slab

Mike,  I agree...wish I had said it first.

BA

RE: reinforcement design in two way slab

Well, he did say it was a two way slab.  As far as I know, ACI318 only allows the reinforcement quantity to be considered for beam shear, not punching shear.  And again assuming this is not a post-tensioned slab.  I was just trying my hand at mind reading.

RE: reinforcement design in two way slab

Anyway I find the question interesting in one sense. For a single shot shear, I mean, to form just the first compressive strut, the tensile shear as hokie66 quotes (other than forming part of some equation) can be used to quantify such ultimate capacity in shear by some assumption. In the EHE code, the connection zones and alike may be analyzed designed by special means such strut and tie schemes that warrant that the structural intent is met.

Furthermore there is the "general" applicability of the shear-friction theory, that also involves (or solves) the case of shearing action along an interface, but has wide range application such support of slabs on walls or design of composite studs.

Not to name the shear action itself on the rebar, one can see it drawn in the Paulay book, main rebar distorted in shear in a cracked section. Our codes have taught us to remain far from such thoughts, but surely we are still applying some of this stuff in distilled form when designing anchors.

RE: reinforcement design in two way slab

Assuming that the two-way slab system is either a flat slab or flat slab with relatively light live loading (<100psf/5kPa) you are referring to punching shear around columns and not beam shear or one-way shear, typically beam shear will not be critical in flat slab design but needs to be checked never less.

I will have to check my copy of ACI tomorrow but Australian codes do not include any contribution from tensile reinforcement (even though it does help), it is a function of the concrete shear strength, shear area and any effective prestress.

RE: reinforcement design in two way slab

asixth,

I am sure they do include it.

abe123,

You only include the tension steel.

Since you are asking this question then I recommend that you do some reading on concrete shear in order to understand the subject. The ACI commentary as well as a good text book will help.

Here is a good start:
http://www.pdhonline.org/courses/s153/s153content.pdf

As engineers we not only have to know, we also have to understand.

RE: reinforcement design in two way slab

The Australian code only handles tension steel in for punching shear as a detailing requirement.


 

When in doubt, just take the next small step.
 

RE: reinforcement design in two way slab

Here comes a dumb question, but why would tension or compression reinforcement play into two-way or beam shear at all?  ACI doesn't (at least from anything I've read) allow for any increase in Vn from longitudinal reinforcement.  You get the Vs from transverse (or bars at some angle) steel, but not from longitudinal steel.  Is that an accurate statement or am I missing something?

RE: reinforcement design in two way slab

SEIT,

You are missing (a little) something.  For beam shear, see Paragraph 11.2.2.1, Equation 11-5.  This contains a factor based on the amount of tensile reinforcement.  It doesn't help much, but the provision is there.  This is in ACI318-08, not sure about earlier versions.

RE: reinforcement design in two way slab

Tensile reinforcement acts as dowels across the diagonal tension cracks, thus boosting capacity.

RE: reinforcement design in two way slab

Thanks!
After I read it I remembered about the "more detailed analysis", I just always use the basic 2rootf'c.

RE: reinforcement design in two way slab

(OP)
Thank you all
I am calculating equipment use on the two way slab, to check for shear on the point load city engineer had objection that I can not use the bottom bars in calculation;  

RE: reinforcement design in two way slab

For a point load on a slab, the bottom bars can be the tension bars.  Depends where the load is.  But do you really need to include this factor?  As SEIT says, just use the simpler equation.

RE: reinforcement design in two way slab

I think that punching shear eventually needs to become a strut & tie thing.  When you're dealing with the typical case of a hogging moment, the top steel is your tie for those struts.

RE: reinforcement design in two way slab

Agree with hokie, if you have a concentrated point load located in/causing a positive moment region than the bottom bars will be the tensile reinforcement and therefore they will be used in the calculation of effect depth in the punching shear calcs.

Beam shear has a component that includes the reinforcement ratio [Ast/(b*d)] in the shear capacity of the concrete alone, this must be tensile steel, so depending on whether the top or bottom bars are the tensile reinforcement than this reinforcement is used in the calcs but under no circumstances can you sum both top and bottom reinforcement

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