Shear Reinforcement for Walls vs Beams

[kajnpdls]

Does anyone know why ACI requires twice the shear reinforcement for walls, Sect 11.9.9, than it does for a beam, Sect 11.4.7?
Eqn's 11-29 and 11-15 appear to be the same but for walls the code continues on to require vertical reinforcement- Why?

 

[ishvaaag]

In a beam the side that has (may be both) longitudinal steel completes the strut and tie (Mörsch analogy) scheme by which beams are reinforced in shear. In 2D elements usually the strut and tie scheme is locally reduced to the exam of some rhomboidal deformation of some rectangular element, this also provedes completeness for the strut and tie scheme. May be this and of course rheological effects mitigation what causes to put rebar in both directions.

 

[abusementpark]

Doesn't it only require a minimum vertical reinforcement?

A lot of times what you need for flexure will exceed this.

 

[ishvaaag]

Again to the question. Section 11.9 starts with...

11.9 — Provisions for walls
11.9.1 — Design for shear forces perpendicular to face
of wall shall be in accordance with provisions for slabs
in 11.11. Design for horizontal in-plane shear forces in
a wall shall be in accordance with 11.9.2 through
11.9.9.

Minimum shear requirement for slabs is ruled by

11.4.6 — Minimum shear reinforcement
11.4.6.1 — A minimum area of shear reinforcement,
Av,min, shall be provided in all reinforced concrete
flexural members (prestressed and nonprestressed)
where Vu exceeds 0.5?Vc, except in members satisfying
one or more of (a) through (f):
(a) Footings and solid slabs;

so it is clear that walls with shear perpendicular to face less than 0.5?Vc can go without shear reinforcement.

Now, for the in-plane shear in walls, section 11.9.9 establishes the minimum amount of vertical and horizontal reinforcement, total, then half at each face when doubly reinforce one may assume.

I was more or less in my previous post answering something related to both shearwalls and walls. It is typical for walls to require more Minimum horizontal reinforcement tan vertical, out continuous walls will show hairline cracks so better do not forfeit this minimum requirement. Of my previous post the consideration of the longitudinal reinforcement being a requirement of completion of the morsch scheme of resisting shear stands true.

Respect the vertical reinforcement the rebar required to meet flexure should be enough, yet the code bothers to require some minimum reinforcement. Remember the old practice for shells of reinforcement not be more distant than twice the thickness, well, ACI has its own rule.

Other than that, requirement to have 2D reinforcement is common to most 2D elements, say deep beams as well. Unsightly cracks may so be avoided, and stresses can be shown by analysis that wouldn be met by reinforcement otherwise.