Review of ACI 349-01 Provisions for Spacing of Shear Stirrups in Slabs
Portions of ACI 349-01 are shown in the attachment. Westinghouse is looking for help in the
interpretation of certain provisions as outlined by the following questions.
1. Does the limit in 11.5.4.3 on Vs > 4√fc’ bwd apply to Vs provided or Vs required? The definition is
“nominal shear strength provided by shear reinforcement”. There are cases in the Code where Vs is
limited; 11.5.6.8 limits Vs to 8√fc’ even if more is provided. Does a similar argument apply for
11.5.4.3 when excess reinforcement is provided? i.e. when spacing is d/2 per 11.5.4.2, the nominal
shear strength is limited to 4√fc’ bwd even when more reinforcement is provided.
My understanding is that 11.5.4.3 says that if the required Vs is greater than 4 √fc’ bwd, then
the spacing of the transvserse reinforcement needs to be reduced to d/4 or less. This has got to
do with the fact that the corresponding demand shear force (Vu = Vc+Vs > ~ 6 √fc’ bwd) is quite
large, and can cause significant shear cracking in the concrete. The closer spacing of ties /
stirrups is required to keep the cracks closed and ensure that the Vc contribution (~ 2 √fc’ bwd)
does not reduce due to excessive crack widths.
If the demand (Vu is low), and Vs provided is excessive (i.e., more than 4√fc’ bwd, and much
more than needed), then the spacing need not be reduced to d/4 or less. This is so because the
demand is low, and there will not be significant concrete shear cracking that needs to be
constrained with smaller spacing ties.
2. Paragraph 11.12.3.1 states that the required area of shear reinforcement Av and Vs shall be calculated
in accordance with 11.5. It does not specifically identify that spacing should meet the provisions of
11.5.4.
a. Are the provisions of 11.5.4 also applicable to two way action in slabs?
b. If so, when excess reinforcement is provided, does the spacing have to meet 11.5.4.3 (same as
question 1)?
It is my understanding that two-way action in slabs is a different failure mechanism than oneway shear in beams. The concrete shear cracking will be dominated by diagonal tension
cracking with 45o angle orientation or less. This is different from the flexural-shear cracking that
occurs in one-way beams. This is probably the reason why two-way shear strength has a
significant contribution from the concrete that can be as high as (Vc = 4√fc’ bwd for square
column supports).
The spacing requirements of ACI 318-02 and ACI 349-06 are appropriate for slabs and two-way
action failure modes. The spacing requirements of 11.5.4.3 are applicable for one-way shear
situations rather than two-way. This was corrected in ACI 318-02 onwards.
Note that this was replaced in ACI 318-02 and subsequently in ACI 349-06 as follows:
11.12.3.3 The distance between the column face and the first line of stirrup legs that surround the
column shall not exceed d/2. The spacing between adjacent stirrup legs in the first line of shear
reinforcement shall not exceed 2d measured in a direction parallel to the column face. The
spacing between successive lines of shear reinforcement that surround the column shall not
exceed d/2 measured in a direction perpendicular to the column face.
3. Figure 1 shows the critical section considered for the evaluation of the elevator pit floor slab. The
critical section is distance “d” from the edge. The demand on the long edge is taken as the bearing
pressure acting on a trapezoid as shown. ACI paragraph 11.12.1.1 requires evaluation as beam action
extending across the full width. The entire pit width is 105”. Is this the full width specified by the
code or can one only consider the width of 80” of the base of the trapezoid?
Did you mean d/2 = 12.5 in. from the edge. The full width for one-way action will be the smaller
value of 80 in. in my interpretation. The perimeter at distance d/2 from the boundary of the
column is used for two way action only.
Area 2 elevator pit boundary
Critical section
d = 12.5"
N
57.0"
d
80.0"
d
Figure 1 Elevator Pit
d
ACI 349 Requirements for Spacing of Shear Reinforcement
ACI 349-01
Vs = nominal shear strength provided by shear reinforcement
11.5.4—Spacing limits for shear reinforcement
11.5.4.1 Spacing of shear reinforcement placed perpendicular to axis of member shall not exceed
d/2 in nonprestressed members and (3/4)h in prestressed members, nor 24in.
11.5.4.2 Inclined stirrups and bent longitudinal reinforcement shall be so spaced that every 45
degree line, extending toward the reaction from mid-depth of member d/2 to longitudinal tension
reinforcement, shall be crossed by at least one line of shear reinforcement.
11.5.4.3 When Vs exceeds 4 √fc’ bwd, maximum spacings given in 11.5.4.1 and 11.5.4.2 shall be
reduced by one half.
11.5.6.8 Shear strength Vs shall not be taken greater than 8 bwd.
11.12—Special provisions for slabs and footings
11.12.1 The shear strength of slabs and footings in the vicinity of columns, concentrated loads or
reactions is governed by the more severe of two conditions:
11.12.1.1 Beam action where each critical section to be investigated extends in a plane across the
entire width. For beam action the slab or footing shall be designed in accordance with 11.1
through 11.5.
11.12.1.2 Two-way action where each of the critical sections to be investigated shall be located so
that its perimeter b0 is a minimum but need not approach closer than d/2 to
(a) edges or corners of columns, concentrated loads, or reaction areas; or
(b) changes in slab thickness such as edges of capitals or drop panels.
For two-way action the slab or footing shall be designed in accordance with 11.12.2 through
11.12.6.
11.12.3 Shear reinforcement consisting of bars or wires shall be permitted in slabs and footings in
accordance with the following:
11.12.3.1 Vn shall be computed by Eq. (11-2), where Vc shall not be taken greater than 2 √fc’ b0d,
and the required area of shear reinforcement Av and Vs shall be calculated in accordance with
11.5 and anchored in accordance with 12.13.
11.12.3.2 Vn shall not be taken greater than 6 √fc’ b0 d.