Vertical Rebar in CMU Walls 1

[SteveGregory]

I typically place a single rebar in a cell and then only in the center and hope it gets placed correctly. I usually stick with smaller bars like #5 or #6 to keep the lap length down.

Does anyone ever use 2 layers (interior and exterior) of vertical rebar in a cell? How much clearance do you spec between the rebar and the inside face of the face shell?

What kind of rebar positioning devices (if any) do you specify to make sure the rebar is in the right place?

What is the smallest size block (U.S.) that you can place two layers?

 

[UcfSE]

I use two bars per cell for 12" cmu when necessary. I typically place it 8.5"-9" from each face. I keep the bars sizes down also, but will go to a #7 if I am using 12" cmu. Durowal makes rebar positioners that are often used. I have seen two bars per cell specified in 8" block, but coming from a masonry background, I have never seen this installed as intended. I believe it's one of those things that works on paper but not in the field. For a minimum, personally, I would recommend 10" block if you're going two bars per cell.

 

[concretemasonry]

Steve -

What is the height, thickness and f'm of the walls required for the walls to resist the loads? Typical and "I hope it gets placed properly" does not provide much information. The best place to start is the ACI/TMS 530 document that is the basis for most international masonry design.

The ASTM C90 requirements for CMUs that are 12" thickness have a minimum face shell of 1-1/2" although 1-1/4" is permissible with a load reduction for unreinforced walls. Unfortunately, that really applies to the vertical compressive strength of the units specified (usually, the minimums and not the actual strength of the units used) and the critia used for behavior of the structure, so the stress distribution can be significantly be affected. That is the problem with the traditional "cook-book" solutions for structural design.

Often, in some areas specially designed units with many times higher strengths and dimensions are used to minimize the wall thickness and reinforcement requirements.

As an example, many 20 story load-bearing masonry buildings have been constructed in the last 20 years using 6" partially reinforced walls with specially units because of the cost savings (no clean-outs, outstanding engineering details and minimal inspection). It is important to understand the benefits of masonry design if the units and details are specified from a professional standpoint.

If it is just a typical industrial/warehouse/big-box application the rule of thumb will work, but in different applications, there are other options possible using ACI 530 and ASTM C90 and appropriate specifications available to provide benefits to the owner/customer.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[SteveGregory]

For standard C-90 plain vanilla CMU, let's see...

11.625" - face shell(1.5") - d(8.5") - 1/2 #6(0.375) - 530.1 tolerance(1") = 0.25" clear

or 11.625" - face shell(1.5") - d(8") - 1/2 #6(0.375) - 530.1 tolerance(0.5") = 1.25" clear

 

[concretemasonry]

Steve -

If you want to get real specific, forget about the ASTM C90 minimums and look a the units actually being used/available and not the "plain vanilla" units that are rarely available when it comes to actual construction. I have been on the ASTM masonry committees (C12 and C15) for years that write the specifications. There are significant changes in the C90 specifications that were just balloted on, but the effects will take years to have an actual effect because of the minimal effects we engineers have on the specifying of the minimum properties of a unit, and most units are beyond the minimums (strength, minimum units dimensions and configurations). The days of the heavy, old configurations and low strength units have been long ago passed by since it costs next to nothing to make higher strength units and more to make the old heavy, thick units.

There is a wide difference in the actual dimensions that affect the location of the rebar due to local preferences they have far more influence on the units actually used than on the units supplied.

Take a look at what is available in the location of the project. Just because there is a minimum face shell dimension, does not meant that it can be assumed to be usable since a CMU has a top and bottom and the minimums are just the minimum and there may be different for construction reasons. As an example a 12" CMU may have a minimum face shell dimension of 1 1/4", but the "top" as installed may have a "flair" of as much as 2" to provide a wide mortar bed at the top to reduce weight and handling costs. On top of that, consider the compressive strength of the assembly that can have an f'm of 4500 psi for a hollow, ungrouted 2 block high prism, compared to the "canned" ASTM minimums for "normal" units.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[SteveGregory]

Dick and Ucf,
Thanks for responding to my post. As a design engineer, I have to use the minimum section properties and specify a minimum strength for the block and the mortar type. I need to look at the possible deflection of the wall and the building drift.

Also, I have to look at the constructibility of what I am designing. In this case, I need to consider what the "practical" maximum flexural 'd' distance may be used in my design. Most of our work is bidded by a general contractor. I have no control over which block company gets the work or even which general wins the contract. So, I need to keep it simple!

Steve

 

[steellion]

Specify non-metallic or stainless steel bar positioners to get the bar where it needs to be. Otherwise, "hoping" they end up in the right place is wishful thinking. If bar positioners aren't specified, they probably will not be used and the bar will be pushed over to the face shell by the grout. I would limit 8" and 10" CMU cores to one bar.