ACI 318-11 Special Reinforced Concrete Shear wall minimum reinforcing

[jeffhed]

I have a question on the minimum reinforcing for a special reinforced concrete shear wall. This is a small building (40'-0"x40'-0") so the loads are not very high at all. If I turn to section 21.9 - Special structural walls and coupling beams. I see table R21.9.1 - Governing Design provisions for vertical wall segments. The table specifies when to design the element as a wall or as a wall pier. So for my particular wall in question it is 10'-0" tall and 3'-8" long which gives a hw/lw ratio of 2.72. Walls are 8" thick so my lw/bw ratio is 3.67x12/8 = 5.51. So according to table R21.9.1 my wall should be designed as a wall pier per 21.9.8.1. 21.9.8.1 refers me to sections 21.6.3 - 21.6.5. Section 21.6.3 states that longitudinal reinforcing must not be less than 0.01 Ag which would require 0.01*8*12 = 0.96 sq. in. Definitely does not make sense. Section 21.9.2 is for reinforcing, so does table R21.9.1 not govern for minimum reinforcing? Section 21.9.2.1 states that minimum ratios must not be less than 0.0025 except if Vu is less than Acv*Sqrt(f'c). This is true in my case so I can refer back to section 14.3. I guess it's just confusing that table R21.9.1 seems to require much more steel instead of being listed after section 21.9.2 as an exception. Furthermore, section 21.9.4.3states that walls shall have distributed shear reinforcing providing resistance in two orthogonal direction in the plane of the wall. if hw/lw does not exceed 2, reinforcment ratio pl shall not be less than pt. So this looks like if I use #4 bars then my minimum ratio cannot be less than 0.0020 in each direction? Even though my loads are way below the requirement in section 21.9.2.1. I haven't done concrete shear walls in a while, I have been doing mostly basement walls and grade beam. Just wanted to be sure I was reading all of this correctly.

 

[PSEPK]

IMHO, ACI section 21.6.3 is applicable in your case. This section requires longitudinal reinforcement to be in between .01 Ag and .06 Ag, treating the wall similar to a column. Required minimum longitudinal reinforcement, therefore, should be .01*8*(3.67'*12) = 3.52 sq. in. (NOT "0.01*8*12 = 0.96 sq. in.")

On, "I haven't done concrete shear walls in a while, I have been doing mostly basement walls and grade beam."

Reviewing the relevant portions in PCA Notes (sections 21 on 'Walls' and 29 on 'earthquake engineering'), might help you a lot.

 

[jeffhed]

PSEPK,
My calculation of 0.01x8x12 was for required steel per foot of wall. I did not mean that to be the total steel in the wall, just trying to show why I wasn't sure that 21.6.3 would govern because 0.96 sq. in. per foot of wall is more than (4) #4 bars and more than (3) #5 bars. But I if thats what governs then I'll do it. Just wanted to be sure I was reading it correctly.