No. 3 or No. 4 Horizontal Rebar Minimum

[Trillers]

Many of the designs I have seen use #3 at 16" OC horizontal for both CMU and CIP shear walls. However ACI 530 and ACI 318 both list minimum horizontal reinforcement that seem to require a minimum of #4 rebar, even when shear wall capacity exceeds shear requirements.

Is the minimum horizontal requirement strictly for longitudinal reinforcement or does this apply to minimum shear reinforcement as well?

Our colleagues and peers are split in their opinions. Can anyone provide clarity on ACI 530 and ACI 318 for minimum horizontal reinforcement?

Thanks!!!

 

[steellion]

I'm not aware of any code-specified minimums for horizontal reinforcement for masonry walls (unless you're in a high-seismic region). The code includes provisions to design these walls as "unreinforced". Typically, I will try to get the wall to work as unreinforced then include joint reinforcement at 16" OC.

ACI 318 specifies minimum horizontal reinforcement for walls, regardless of whether they are part of the lateral system. I think this reinforcement ratio allows less than #4 at 12" OC, but as a personal preference, that is what I start with for each mat of steel.

 

[msquared48]

.0025bt OS what I remember for shear walls.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Trillers]

Hi Steellion and msquared 48 - That's exactly what I'm referring to...so using rho as .002(ACI 318) or .003(ACI 530) would require greater than #3 bars at 16" OC horizontal for minimum reinforcement (rho x b x t or rho x Ag) of the wall section.

In the case we are looking at the shear wall capacity is greater than shear so we will need only minimum reinforcement. ACI says we can use #3 as a tie but then turns around and says min reinforcement is the rhoxbxt which in the case of 8" walls results in #4 bars.

Still wondering!!!

 

[TXStructural]

The overall project cost difference between #3 and #4, owing to the limited size of most projects, is very small, and most fabricators and placers prefer #4 and larger because it holds its shape much better (#3 typically warps too easily, even in the mills). Also, placers frequently need to be able to climb walls during placement, and small diameter bars spaced widely do not provide sufficient strength to do this safely.

Chapter 14 of ACI 318-11 does not specify a minimum bar diameter, so I'm not sure where you are getting such a requirement for CIP. The limitation for horizontal reinforcement is usually a function of maximum bar spacing (18" max.) and minimum reinforcement ratio (0.0020 or 0.0025, effectively acting as T&S in the case.) Reinforcement ratios below this are usually considered unreinforced.

 

[BruceSPT]

ACI 318-11
11.9.9 Design of Shear reinforcement for walls
Ratio of horizontal shear reinforcement area to gross concrete area of vertical section shall not be less than 0.0025.

14.3 Minimum reinforcement
14.3.3 Minimum rato of horizontal reinforcement to gross area shall be:
(a) 0.0020 for deformed bars not larger than No. 5 with fy less than 60,000 psi or
(b) 0.0025 for other deformed bars

21.9.2 Reinforcement
21.9.2.1 The distributed reifnrocment ratios for strutcural walls shall not be less than 0.0025, except for ........

The only place I know of that specifes bar sizes for walls is in 14.3.7 that requries not less than No. 5 bars around window, door or similar sized openings.

Bruce

 

[Trillers]

Thanks TXSTructural and Bruce SPT -

That was the crux of the problem - does or does not the following end up being #4 minimum because of the reinforcement ratio? For instance - if we want to design horiz reinforcement at 16" OC, then we would have 8 x 16 x .0025 = .32 Sq In. This means that #3 at 16" would be insufficient - or does it?

And one other designer insists that only the grouted cells of a Concrete Masonry Unit (Hollow Block)count for Ag because the CMU's are only 1350 psi strength. Using only the grouted cells for Ag would then make #3 @ 16" well within the reinforcement ratio requirement.

Like to hear your opinions on these as well - and thanks!!

 

[hokie66]

Ag is Ag, the gross area. That designer is trying to read something into a code section that is not there.

#3 bars could be used...you would just need more of them.