Shear Stress in Two-Way Flat Slabs from Moment Transfer 4

[MegaStructures]

ACI code 318-14 section 8.4.4.2 gives provisions to transfer slab moments to columns through flexural action and eccentric shear as can be seen in the first two images below. My question applies to the shear requirement only. The provisions are clear for determining if shear reinforcement is needed. However, once it is determined that reinforcement is needed it is not clear whether the eccentric shear should be considered on the second critical section a distance d/2 away from the last reinforcement. Is eccentric shear only considered at the first critical section? Or should it be considered when finding the point that shear reinforcement can be stopped? Image of second critical section is shown in the third picture.

 

[KootK]

Is eccentric shear only considered at the first critical section? Or should it be considered when finding the point that shear reinforcement can be stopped?

I believe that the eccentric shear should be considered at all sections where punching shear is to be studied, not just the first one out from the column face. Otherwise, you'll have lost track of some of the moment that needs to get transferred from the slab to the column.

 

[r13]

This is my understanding. You have to make sure at the outer critical location (beyond shear reinforcement), Vu <= phi*Vc.

 

[MegaStructures]

KootK I don't suppose you could point me towards a reference that works through that calculation could you? I've read through several texts and they stop the shear calculation by stating "concrete shear is inadequate, thicken slab, or add shear reinforcement". The polar moment of inertia calculation for the second critical section is a bit intimidating.

 

[KootK]

I'd share such a reference happily if I knew where to locate it MegaStructures. Unfortunately, I'd have to go searching for it just as you may. You might try the verification manuals from the stud rail suppliers (Decon etc) or ACI's publication on monolithic slab to column joints.

 

[KootK]

You may be in luck after all. Check out Deker's contribution here: Link.

 

[MegaStructures]

Thanks KootK, it was worth a shot that you might have a reference from a previous project. It's interesting this topic seems to be almost avoided by all of the most recommended textbooks and the PCA design notes. I have not tried the publication for monolithic slab to column joints. I will give that a review and see if I can find anything useful. Thanks for your input.

 

[KootK]

We simulposted. Check out my last.

 

[Celt83]

ACI 421.1 has the info your after and a general form formula for the shear perimeter properties.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[r13]

IMO, moment itself does not add to additional shear in the slab. The additional shear at the edge column is due to the unbalanced moment causes a twisting stress, which is an addition to slab shear stress, at the critical section, thus shear reinforcement is required to counter/balance the moment and to resist the slab shear. Once the moment is balanced, the concern of torsional phenomenon ceases to exist, therefore, the shear check on the next critical section (at d/2 beyond the shear reinforcement) only concerns the regular slab shear. The question is, what is the required shear reinforcement length. The length can be found by simply equate the applied shear force and the available concrete shear resistance without reinforcement, that is Vu <= ∅Vc. The location can be easily identified on the shear diagram.

 

[r13]

This may help, though no design. Link

 

[r13]

This is also a very interesting read. Link