CMU Lap Splice vs ACI 318 Lap Splice
CMU Lap Splice vs ACI 318 Lap Splice
(OP)
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.






RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
A little example as a proof:
If you plug db=0.625 inches, fy=60 ksi, f'c=f'm=4 ksi, and the maximum spacing/cover amount allowed by each code (cb+Ktr/db)=2.5 for ACI 318, K=5db for TMS 402) into the equations for both concrete and CMU, you'll get the following:
Eq. (2-9) of TMS 402-05:
0.13*db^2*fy/K/root(f'm)
=0.13*0.625^2*60000/(5*0.625)/root(4000)
=15.4 inches
Eq. (12-1) of ACI 318-05 multiplied by the 1.3 splice factor:
3/40*fy/root(f'c)/((cb+Ktr)/db)*db x 1.3
=3/40*60000/root(4000)/2.5*0.625 x 1.3
=23.1 inches
23.1 inches = 1.5 x 15.4 inches
So there you go. You're using 50% more of the steel strength, so you need 50% more lap.
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
I don't see Ψs in your eq 12-1 calculation. This will take 20% off the development length for #6 and smaller bars
RE: CMU Lap Splice vs ACI 318 Lap Splice
Include that factor in there and you've got 18.5 inches for the #5 instead of 23.1 inches.
If you run again with #7 bars instead of #5s where the the small bar factor would not apply, you still get the 50% increase in length for concrete over masonry.
32.4 inches vs. 21.6 inches.
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
Mark, yes I don't understand why they combined strength and allowable stress for the 2011 code cycle. After talking with TMS a bit, I'm thinking its because allowable stress was recalibrated, based on testing, to closer match the results of stregnth design. Testing proved that the allowable stresses were a bit conservative, hence the increase in steel stress.
Wannabe, the 72 x bar diameter doesn't control until your splicing #8 bars. I'm leaning with hokie; I just couldn't have a lap splice length in masonry shorter than concrete, so I used the concrete lap splice length.
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: CMU Lap Splice vs ACI 318 Lap Splice
I spoke with some people on various TMS committees this morning. While they agreed, it doesn't feel right that the CMU lap splice length is shorter, they are very confident in the new equations present in the 2012 IBC and TMS 402-11/13. They are based off very recent testing. One member mentioned that the ACI 318 lap splice equations have been around for a 'while' and may be based on old testing and be conservative, similar to the older provisions of TMS 402; but this is merely speculation.
RE: CMU Lap Splice vs ACI 318 Lap Splice
You are not using more steel strength in LRFD v ASD, can't disagree with that thought process more.
I have always wondered why the lap splices are so short in masonry, feels so very wrong given I don't think there are more than 2-3 decent masons practicing in the US anymore.
RE: CMU Lap Splice vs ACI 318 Lap Splice
Is there a document somewhere that describes lap testing in CMU? It might be interesting to compare that to concrete testing. I assume that CMU testing would have been done on bars embedded in actual block rather than just mortar?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
RE: CMU Lap Splice vs ACI 318 Lap Splice
After speaking with a few people from ACI, I'm realizing the major difference is cover requirements and mix variability specifically with aggregate size (the testing was done in the 90's). They did mention they are looking at reducing the lap splice length based on research being conducted right now. It won't be in the ACI 318-14 but maybe the 2017 version.
RE: CMU Lap Splice vs ACI 318 Lap Splice
There is a subsrantial difference between actual bonding between CMUs and concrete.
In a CMU unit grout meeting ASTM requirements must be 8"to 11"slump with fine aggregate. The grouting rate is specified and final consolidation is required. During this period and the final curing the CMUs absorb the excess water and provide a moist curing environment. The main difference is the high slump for 100% contact and suction of the water out of the mix as necessary. The ACI 530 "Code and Specifications" and most masonry texts explain the process in detail. In high rise buildings, many engineers place a maximum strength limit of the grout supplied to maintain compatibility and reaction of all materials. That is the reason for historic testing records to develop the design standards since masonry has always been a wall-based strength of actual samples instead of individual materials.
The ACI code is published and distributed by ACI. The writing of the code is done by members of various association (ACI, NCMA, BIA, TMS, etc,) and professionals that write numerous. The testing is done the labs that have the physical ability to do full size testing with a high - It is fun seeing a 22'high 8"wall tested in flexure with high degree if instrumentation, or a vertical/horizontal joint torn apart.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: CMU Lap Splice vs ACI 318 Lap Splice
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.
RE: CMU Lap Splice vs ACI 318 Lap Splice
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.
RE: CMU Lap Splice vs ACI 318 Lap Splice
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.
RE: CMU Lap Splice vs ACI 318 Lap Splice
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.