CMU Lap Splice vs ACI 318 Lap Splice 1

[mike20793]

I recently designed a retaining wall that had a CMU screen wall above it for IBC 2012. I had #5's at 24 inches in the CMU and #5's at 12 inches centered in the stem of the retaining wall, so I extended alternating bars from the stem to lap splice the CMU vertical reinforcement. When I detailed it, then double checked it, I noticed that for a #5 bar, the tension lap splice length for CMU is less than the Class B tension lap splice length for the concrete. For CMU (f'm=2000 psi) the lap splice comes out to about 20 inches but for concrete (f'c=4000 psi) it comes out to 31 inches using the provisions of 12.2.2. This seems counter intuitive to me. I understand 12.2.3 can decrease the lap splice length in concrete based on confinement of transverse reinforcement, but TMS 402 also has a provision to reduce the lap splice length. Has anyone else noticed this before? Does anyone know why this is the case? I'm having a hard time convincing myself that coarse grout and medium weight CMU can develop and splice a bar in a shorter distance than concrete.

 

[MrHershey]

dcarr-

You most certainly do use higher rebar stresses in strength design for masonry. LRFD vs ASD in other materials, I typically agree. For whatever reason masonry is different.

ACI 530-08 Chapter 2 limited this stress to 24 ksi with a 1/3rd increase allowed for wind or seismic. ACI 530-11 removed the 1/3rd increase, but increased the allowable stress to 32 ksi (1/3rd higher than 24 ksi).

ACI 530-08 and 530-11 have you use full stress for reinforcing steel, design equations are based on fy instead of Fs, with a phi factor of 0.9 typical for tension-controlled applications where the steel is providing the primary resistance.

60 ksi x 0.9 = 54 ksi

LRFD factored loads are typically around 1.5 that of ASD before the application of phi (or omega) factors.

54 ksi is 50% more than 24 ksi x 1.5 (36 ksi) and 12.5% more than 32 ksi x 1.5 (48 ksi). In other words, even after accounting for the different load combinations, you're still using lower steel stresses in chapter 2 than you are in chapter 3. Less so now than we were previously for non-wind or seismic design. But still less.

Why the lap splice requirements are identical is a question I'd love to hear answered.

 

[KootK]

I believe that there is something important missing from your analysis Mark: consideration of the design method. With allowable stress, we use an elastic design method (linear stress variation); with strength design, we use a plastic method (Whitney rectangular stress block). For the same applied load and reinforcing, allowable stress analysis will yield higher rebar tension. If you factor that into your numerical comparisons, I think that you`ll find more parity in the rebar utilizations.

While the global safety factor presents itself differently in the two methods, both allowable stress design and strength design methods intend full utilization of reinforcement. At least, that`s the outcome when the amount of steel provided is in close agreement with the amount of steel required by calculation. That`s why the lap splice lengths are identical for the two methods.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MrHershey]

Still, disagree. Even when accounting for that, you get different stresses.

Away from my desk and don't feel like doing it by hand, but this link has an example done in ASD.Link

Results in an allowable strength of 270 k-in (using the older 24 ksi limit).

Run the same design in strength design and you'd expect to get a nominal strength 50% higher (405 k-in), which would be in line with LRFD loads coming out 50% higher.

Instead...

a=As*fy/(0.8*f'm*b)=0.44*60000/(0.8*1500*8)=2.75 inches
PhiMn=Phi*As*fy(d-a/2)=0.9*0.44*60(28-2.75/2)=633 k-in

633 k-in is 56% higher than you'd expect, close to the 50% stress difference I noted above.

Unless I'm missing something serious here.

 

[KootK]

I'll buy that. On slide ten of the document that you linked, there's actually a graph that pretty much settles it. Now I'm even more confused. I thought that the two methods were more closely calibrated than that.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.