Reinforced Masonry Wall - Calculation of Lap Splice for dowels subjected to shear 1

[oengineer]

I know that Equation 8-12 in ACI 530 can be used to determine the lap splice length for rebar in tension & compression.

Would it be acceptable to apply equation 8-11 for rebar subjected to shear?

I do not see anything in the code specifically about lap splices for rebar in shear.

 

[jayrod12]

What is the application?

 

[oengineer]

What is the application?

Well, I am building a CMU wall inside of an existing box culvert (RCB) to act as a restricitor for the flow of water. The Existing RCB is 10' wide x 10' tall, but the proposed CMU wall is only 4' wide x 10' tall. I am doweling rebar into the top & bottom slabs to connect the reinforcing rebar to the existing RCB. The CMU wall has been designed as a simply supported member, so the dowels in the TOP & BOTTOM slab are only receiving shear loads. My goal is to calculate the required lap splice length for the dowels into the CMU wall.

 

[phamENG]

I'd look at these options:

1) Does the shear demand exceed the shear capacity of the mortar in the bed joints? If no, then you don't need rebar passing through that interface. Top of wall may be a challenge, though, since the top of the culvert is already in place.

2) Determine the shear friction required, and fully develop the bar size required to get the shear capacity you need.

3) Try to design the rebar like an adhesive anchor.

4) Use stainless steel or aluminum angles anchored in the culvert and the wall. Be careful about corrosion potentials and this is probably your best bet.

 

[oengineer]

I'd look at these options:

1) Does the shear demand exceed the shear capacity of the mortar in the bed joints? If no, then you don't need rebar passing through that interface. Top of wall may be a challenge, though, since the top of the culvert is already in place.

2) Determine the shear friction required, and fully develop the bar size required to get the shear capacity you need.

3) Try to design the rebar like an adhesive anchor.

4) Use stainless steel or aluminum angles anchored in the culvert and the wall. Be careful about corrosion potentials and this is probably your best bet.

So the rebar were designed using an adhesive anchorage system. We will be using rebar and an adhesive anchor to dowel into the existing top & bottom slabs. The calculations have been done and the dowel/anchorage system works for the applied shear loads. I am just trying to determine/verify the required lap splice length need to for the dowels into the proposed CMU wall based on shear. Equ. 8-11 in ACI 530 mentions primarily compression & tension.

 

[phamENG]

If you have room to develop tension, that's your best bet for the sake of simplicity. If you really want to dig into it, look at shear loading of anchorages in the top of a CMU wall. That's probably the closest approximation of the behavior.

 

[oengineer]

If you have room to develop tension, that's your best bet for the sake of simplicity. If you really want to dig into it, look at shear loading of anchorages in the top of a CMU wall. That's probably the closest approximation of the behavior.

Since the dowel rebars are just resisting shear, would it be acceptable to use Eq. 8-11 from ACI 530 for this situation? I would like to confirm that the equation is applicable for my situation.

 

[phamENG]

No. That's for wires. Wires and bars are not the same thing. 8-12 should work well enough, though.

 

[oengineer]

No. That's for wires. Wires and bars are not the same thing. 8-12 should work well enough, though.

I am sorry, I meant 8-12.

Thank you.

 

[oengineer]

I am curious. I have seen a minimum splice length for #6 bars as 4'-10". It appears, based on using Eq. 8-12, you can get away with a foot less for the splice length. Whwere is the 4'-10" splice length for #6 rebar coming from?

 

[phamENG]

That would depend. Where did you see that number?

 

[oengineer]

That would depend. Where did you see that number?

I saw them on 2 structural plan notes for 2 different schools, by 2 different engineers @ 2 different engineering companies. So I figured it must be right. But if you do the calculation for the lap splice length for #6 rebar into an 8" wall, using f'm = 1500 psi with Eq. 8-12, you can get a lap splice length of 3'-8".

Instead of 4' -10" lap splice length.

Is it common to use 4'-10" lap splice for #6 rebar in CMU?

 

[Celt83]

the formula for lap length has changed a bit in recent code updates TEK 12-061A1 gives a good summary: Link
edit: I guess that was 8 years ago so recent may not be the best word choice...still tend to see a lot of lap schedules based on the 2009 IBC.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Not that I'm aware of. They may some other concern specific to the application. Is this a seismic area? I believe there are certain ductility requirements that can be met by increasing lap lengths, but I'm not certain. If that's the case, it would make sense in a school.

One thing to consider is that, even though they're different engineers at different companies, they may have worked together at some time or the companies may share some historical link that created a mixing of "typical" details and "general" notes. There are plenty of rules of thumb for lap lengths, and I've seen more than a few drawings with "Lap all bars 62 bar diameters."

 

[oengineer]

Thank you all for your help

 

[oengineer]

the formula for lap length has changed a bit in recent code updates TEK 12-061A1 gives a good summary: Link
edit: I guess that was 8 years ago so recent may not be the best word choice...still tend to see a lot of lap schedules based on the 2009 IBC.

Based on TEK 12-061A, it makes it seems as if Eq. 8-11 should be used for reinforcing bars, not just wire.

But the ACI 530 states that Eq. 8-12 is for reinforcing rebar & Eq. 8-11 is for wire.

Why is there a contradiction?

 

[oengineer]

Based on TEK 12-061A, it makes it seems as if Eq. 8-11 should be used for reinforcing bars, not just wire.

But the ACI 530 states that Eq. 8-12 is for reinforcing rebar & Eq. 8-11 is for wire.

Why is there a contradiction?

If anyone knows the reason for this, please let me know.

 

[phamENG]

Can you explain your reasoning a little more? The only thing that I see that might give that impression would be equation 1 under 2009 allowable stress design. That equation is not the same as 8-11 (0.15 does not equal 0.2). You'd also be mismatching codes. The 2009 IBC was not based on ACI 530-13, it was based on -08. It was revised in 2011 as well (and included in the 2012 IBC). In 2013 the entire code when through a big shakeup/reorganization (though not as big as 318). I don't have a copy of -08, so I can't say for sure what equations were used, but I would suggest you stick to whatever code is applicable to your project.

 

[r13]

Without arguing the equations in code, think that shear friction works only under the premise that the bar is to be stretched by the shear force to yield, so the splice need a full tension splice length to develop/continue the required strength.

 

[bones206]

I don't think it matters to the wall if the dowels are cast into the concrete or post-installed with adhesive. As long as the dowel is anchored to the support, it shouldn't make any difference to the length of the lap splice.