Shear in walls aci318

[hangingchad]

I have a concrete box type structure that is approximately 16' inside from wall to wall and 40' tall and subjected to earth and water pressure on the outside. The ACI code special provisions for walls for design of shear perpendicular to the face of the wall references the engineer to the section of the code for "Special Provisions for Slabs and Footings." For the condition where the wall is governed by beam action and each critical section to be investigated extends in a plane across the entire width, one is referred back to the code provisions for beams. These provisions require a minimum area of shear reinforcing with exceptions, one of which is slabs and footings. Does this mean that minimum shear reinforcing is not required in a wall even though each critical section is designed as a beam? Even if the wall is spanning primarily one way? The commentary says the reason that minimum reinforcing is not needed in slabs is because of the ability of redistribution of stress, etc but I don’t see that possibility in a one way slab.

 

[breaks]

Yes, you're correct in saying that minimum shear reinforcing is not required in a wall even though each critical section is designed as a beam. Even though you may be DESIGNING the wall as a beam, the wall doesn't act like a true beam. A point load on one portion of wall can be distributed, to some extent, away from the actual section of the wall loaded, through the concrete and reinforcing steel provided. Minimum reinforcing (most of the time 200/fy for flexure) is adequate to prevent the cracking associated with shear problems in concrete.

Basically, you are only designing the wall as a beam in flexure and not in shear.

Matt

 

[JAE]

I agree with breaks, but you still have to check shear. The code requires that the maximum factored shear, Vu, at any section be less than phiVc (this is ACI Equation 11-1 which must always be met). The provisions in the code for minimum shear reinforcing, spacing, etc. do not apply to slabs/walls, nor does the provision for the phiVc / 2 limit prescribed in section 11.5.5.1.

 

[breaks]

Thanks JAE, I forgot to mention the most obvious part! You still need to check for shear strength adequacy, just not as a beam.

 

[breaks]

Another thing: in my post above, "minimal reinforcing" should actually read "provided reinforcement (which is usually at least b*d*200/fy sq.in./ft. for flexure)". Obviously, some walls will need more than the minimum reinforcment required by ACI 318.