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.

 

[JedClampett]

In general, CMU reinforcing is designed for working stress level forces and stresses (not always) and concrete is designed for strength level (full yield of reinforcing). So it makes sense that the laps are shorter.

 

[hokie66]

It doesn't make sense to me. Knowing the quality of grouting, I want more lap length in CMU.

 

[MrHershey]

Agree with Jed. Typically CMU reinforcing is limited to 24 ksi stress. Concrete lets you go to 60 ksi, but then goes Phi factor of 0.9, plus LRFD load factors. Equivalent to CMU would be to use Omega of 1.67 with ASD combinations, which reduces your 60 ksi to 36 ksi. So you're using ~50% more of the rebar strength in concrete design than in masonry design.

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.

 

[MrHershey]

Obviously breaks down when you go to strength design of masonry. I'm not sure why the lap length for ASD masonry and strength design of masonry are the same when you're allowed a higher stress in your bars. Perhaps someone else can enlighten on that aspect.

 

[wannabeSE]

Mark,
I don't see Ψs in your eq 12-1 calculation. This will take 20% off the development length for #6 and smaller bars

 

[MrHershey]

Correct. Omitted by mistake. Though perhaps still illuminating given that there's no small bar factor for masonry.

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.

 

[hokie66]

I think this is a case of practicality superseded by equations. There is no way that a bar, of any size, is developed or lapped in masonry grout as well as in concrete. When you try to pull out the bar, it doesn't know what code provisions dictate, but rather how tightly it is held. Skimping on laps in masonry (and then in common practice not making sure that grouting is complete), doesn't sit well with me.

 

[wannabeSE]

Our typical detail specifies 72 x bar diameter for lap splices in masonry. This is consistent with the IBC amendments to the masonry code. Reference section 2107.2.1, and 2108.2.

 

[mike20793]

Hey guys, most of y'all are referencing the older version of TMS 402 (2008 and older). I'm referring to the procedures for IBC 2012 and TMS 402-11. There were significant changes to the code between the 2008 and 2011 versions. The 2012 IBC combines allowable stress and strength design lap splices. The equations are no longer separate.

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.

 

[mike20793]

Correction to my post above... For reinforcing centered in the CMU, the 72 x bar diameter doesn't control until you are splicing #8 bars. If you have bars at the face of the CMU, it will control for a #5 bar and above.

 

[KootK]

Do you foresee any issues with getting your upper and lower bars aligned in the field Mike? There was a thread here recently about how footing dowels are even tough to get lined up with CMU cores. If there may be a lateral offset in the bars, that's even more of a justification for conservatism in your laps.

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

 

[mike20793]

Good point KootK. We haven't had problems with this similar detail in the past, but it is something that should be considered. I will check out that thread to gather some more information.

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.

 

[dcarr82775]

Mark,

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.

 

[KootK]

There's a fellow on here that goes by TXStructural. He's quite involved in ACI committee work. I believe that he has mentioned that updated lap testing is working its way through the system. Hopefully he can jump in to confirm that.

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.

 

[mike20793]

I'm wondering if the cover provided is having a large impact on the lengths. The ACI equation is based off minimum cover criteria, which is usually less than the cover of a bar centered in an 8" block. I'm also wondering if the variability in concrete mixes vs the variability in coarse grout mixes has an impact. A concrete mix with very large aggregate would have a harder time developing a bond to the reinforcement, but this is just a thought.

 

[hokie66]

As design engineers, as opposed to those who do the research, we must always consider what happens in the field. There is very little economy to be gained in decreasing lap lengths, and a lot of comfort to be gained by remaining conservative.

 

[stevenspm]

What I am do for lap spices for any CMU is to follow IBC Sec 2107.2 for lap splice length of 0.002 * bar diameter * fs, which for a #5 results in a lap splice of 30" which I then I increase by 50% for bars stressed greater than 80% of the allowable stress. This does result in lap splices greater than those for concrete.

 

[mike20793]

That equation actually results in a lap splice of 40 inches because of the 33% increase in allowable stresses and is conservative because it does not consider cover (the lap splice length at the face of a cell is the same as centered in the cell for that equation). Check out NCMA TEK 12-6A. They lay out the provisions of IBC 2009 and IBC 2012 pretty well with good comparisons. NCMA recommends not using it because it is extremely conservative based on their recent testing. The IBC 2012 does allow either procedure to be used.

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.

 

[concretemasonry]

Mike 20793 -

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.