One Way Shear

[Transferbeam]

Hi Folks, I just have a quesiton about designing two-way slab without beam: according to the new Canadian standard CSA23.3-04, the one way shear needs to be distributed to column strip and middle strip in proportion to the moment distribution factor,by doing so apparently the one way shear rather than punching shear checking will become critical in the two way slab design which seems not correct,in my opinion the whole slab strip will evenly pick up the slab shear. could anyone lend me a hand on this, please?

 

[asixth]

I would not say that the slab strip will evenly pick up the slab shear.

Take an example of an internal span two-way flat slab design where the distribution of moments may be 65% negative moment and 35% positive moment. Of this, 75% of the negative moment at the supports is distributed to the column strip and 50% of the positive moment is distributed to the column strip at midspan.

So now looking at the column strip, the distribution of total panel moment is 48.75% as negative moment at the supports (0.65*0.75) in the column strip and 17.5% positive moment midspan (0.35*0.5).

The shear force that correlates to this moment distribution would be approximately 33.1% of the total panel load (0.331*w*L). Therefore based on this moment distribution, the shear force at the support in the column strip is approximately twice that of the shear force in the middle strip (33.1% c.f. 16.9%) so the shear force is not evenly distributed across the panel but is more concentrated in the column strip.

 

[BAretired]

I don't think I have ever seen an example of a two way slab where one way shear governed. Usually, shear around the periphery of the column governs.

A footing, on the other hand can be governed by one way shear and in that case, I would assume the shear uniformly distributed across the width of footing.

BA

 

[Transferbeam]

Thanks a lot to asixth and BAretired.

asixth, I dont understand how you get the 0.331WL shear force distributed to the column strip? if we distribute this shear force in proportion to the negative moment distribution factor, then 75% shear force will be distributed to column strip and 25% shear force goes to middle strip; therefore, it is more than likely that the slab will fail in one way shear prior to the punching shear because the column strip needs to take 75% of the total slab shear.Apparently it is not correct and we all know that the two way shear always govern the flat slab design.

BAretired,I absolutely agree with you, I have never seen the one way shear govern the flat slab design either because it is believed that the whole slab strip will uniformly pick up the shear force.
But if you distribute most shear force to the column strip, you will see that will happen.what is your thought about shear distribution?

Maybe my question should be what is the proper shear distribution factor between column strip and middle strip?

 

[BAretired]

I do not have a copy of CSA A23.3-04, but if the column strip is considered to carry more load than the middle strip, it seems logical to accept the fact that shear stress will be greater in column strips than middle strips. Any attempt at tying it down precisely is an exercise in futility.

If the one way shear stress is plotted across an entire span, I expect it would be a smooth curve undulating up and down from a maximum at the middle of the column strip to a minimum at the middle of the middle strip.

I am surprised to learn that one way shear becomes the controlling factor in a two way slab. Perhaps there has been some new research indicating that this is the dominant type of failure. I would have thought punching shear was dominant.

BA

 

[rowingengineer]

I have only heard of shear governing over punching shear when punching shear reo is used.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[asixth]

If the moment distribution in the column strip is -0.4875*Mo at the supports and 0.175*Mo midspan, then this relates to 0.6625 of the total load (0.4875+0.175). If the moment distribution in the middle strip is -0.1625*Mo (0.65*0.25) at the supports and 0.175*Mo midspan (0.35*0.5), then the total load carried by the middle strip is 0.3375 of the total load (0.1625+0.175).

After the moment diagram has been calculated, the shear force can be calculated by superimposing the equivalent udl over a simple span and relevant end moments (see attachment).

You cannot distribute the shear forces in the same proportion as the moments. You need to calculate the shear forces based on the equilibrium the structure.

Consider the end span of a two-way flat slab with no edge beam. The moment distribution between the column strip and middle strip at the exterior column may be 100% to the column strip and 0% to the middle strip. This is not to say that there will be no shear in the middle strip, based on the equilibrium of the structure there will still be one-way shear in the middle strip.

 

[DaveAtkins]

I have struggled with the same thing, because I analyze two way slabs on RISA-3D (as equivalent frames, and then distribute moments and shears by hand to the half middle strips and the column strip. I seem to recall the one way shear sometimes becomes a problem in the column strip.

BUT--I cannot see how a column strip could fail in one way shear, while the adjacent half middle strips remain intact. It just makes more sense that a two way slab fails in punching shear around the column.

DaveAtkins

 

[apsix]

DA
I think that the middle strip wouldn't remain intact but would follow the column strip and also fail.

I do agree that I would expect punching shear to be more critical.

 

[Transferbeam]

Thank you all very much for your inputs which are really helpful to me.