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CMU Lap Length 2

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SteelPE

SteelPE

Structural
Mar 9, 2006
2,747
I have a 12" CMU wall that I am looking to calculate the required lap length for the reinforcing. The wall is somewhat heavily reinforced as it is a shear wall in a long skinny building. Currently I am using 2-#6 bars per cell with a cover of 2 1/2" to the outside face of the masonry wall. f'm = 2,000 psi, building code is IBC 2015.

I am calculating a required lap length of 90.7 (db)^2.... or 51 inches using equation 8-12 form ACI 530. I have run the walls design through some software I have from NCMA and they are giving me some obtuse lap length of 84 inches.

Am I calculating this lap length correctly?

I understand that we can switch materials out and such... but at this point this is where we are, so I am just looking for an answer to my question, not responses asking why I don't use material XXX vs CMU. It's a non starter at this point.
 
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Shouldn't the lap length be in accordance with ACI 318? The bars are lapped within concrete fill, not mortar fill.

BA
 
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Bars are lapped in a grouted space not concreted space.
 
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I got 51 inches as well. Is there some language about the 2 bars in one cell.
image_ethiiz.png
 
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The software gives lots of references but not for this specific calculation .

I did design the wall to be a special reinforces shear wall, but I don't think that matters.

Of note in the table provided by GC Hopi... I have a set of drawings for a similar set of walls done by a large well known engineering company. Their lap splice chart lists the same 54" for #6 bars with 2" of cover.

Also of note.... I calculate 64" (5'-4")of cover given the same conditions. So I don't know if there is an error in my calculation or an error in the chart provided above..... but it's reassuring that others are getting similar numbers.
 
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There's a clause in 2107 of IBC that requires a minimum 50% lap length increase for bars stressed greater than 80% of the allowable tensile stress. Maybe it is applying that clause somehow?

Also, try switching to a different code year in the software and see if the lap length changes at all.
 
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IBC 2015 has its own equation for development length (Section 2107.2.1) that you can choose to use instead of ACI 530's equation. That's where the 50% lap length increase clause gets used, but it still doesn't get you up to 84".

ld = 0.002*db*fs*1.5 = 0.002*0.75*32,000*1.5 = 72"

Structural Central
 
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GC Hopi said:
To get 54" the table use a K value of ~2.4 inches which is the dimension to the center of the bar (K = 2"+0.5*0.75")
Click to expand...

K is supposed to be the clear distance not the center to outside face distance........ my 2 1/2" is from edge of bar to face of CMU.

I think ProgrammingPE has it correct with regards to where the 54" comes from
 
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K is the masonry cover, not the distance to the center of the bar, so using K = 2" is correct.

We can validate the table for #5 bars instead since the 72*db upper limit does affect the result for them.

For #5's, ld = 0.13*db^2*fy*gamma/(K*√f'm) = 0.13*0.625^2*60,000*1.0/(2*√2000) = 34"

Structural Central
 
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Some codes have a factor for increasing the length if you have a certain amount of fresh grout below the bar (similar to ACI concrete code), unless you're using an expansive admixture. Not too familiar with American stds (may already be including this in your calculation if appropriate).

 
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Agent666, that isn't included in the US codes for masonry. Masonry grout and concrete are different enough that not everything is parallel between the two materials.

SteelPE also consider bumping up the f'm to 2500. You may likely be getting that already since most block produced are above the 2000 psi minimum. And if you can reduce the bar size, that has a bigger impsct. For masonry, smaller bars more frequently spaced is preferred over larger bars at greater distances. Although with 2-#6 you may be at your limit.
 
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