CMU & Concrete 'Shear Friction', Development Lengths

[cpiedra23]

hey guys.

please see attached detail.

http://files.engineering.com/getfil...11-b64d-6f0798f8a746&file=20160503_095833.jpg

i'm checking the CMU wall anchorage for out of plan loads per ASCE 7-10, Sect. 12.11.2.1

i thought i should check the hook development length "ldh" into the concrete per ACI 318-11, Sect. 12.5.2, and the #5 dowel for shear friction per ACI 318-11, Sect. 11.6.4. they both seem fine.

for the CMU, i was going to check the development length "ld" in the CMU per ACI 530-11, Sect. 2.1.7.3.


some questions:

1) the conc hook development length is mentioned for hooks in tension. similarly, the cmu development length is mentioned for tension. i don't have tension in these dowels but only shear. are these sections still applicable? do i get a reduction for shear only?

2) is there a similar 'shear friction' section for cmu? is this a necessary check?


any thoughts would be greatly appreciated. thanks!

 

[KootK]

1) You're misinterpreting how shear friction works. It is clamped friction between the mated surfaces that resists the shear. The rebar provides that clamping force and is therefore only in tension. It's a bit murkier when the surfaces are smooth but let's not get into that just yet.

2) To my knowledge, shear friction has not yet made it into MJSC code but I believe it to be on the ballot for future incorporation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cpiedra23]

1) oh wow i didn't really think about that (the clamping together) - thanks for that insight. however, in my case the surfaces are smooth and nor roughened. there is a key present but my supervisor said to ignore the the key in resisting lateral movement of the wall. therefore, it'd be only the dowel itself resisting the movement in our analysis. thought?

2) thanks for the input on this as well.

another thought;

in the wall moving laterally, i wonder if i should check the break out of the CMU, with the small edge distance (7.625/2)? ACI 530-11 Sect. 2.1.4.3.2 has shear designs for headed and bent-bars, but i'm not sure if it necessarily applies to reinforcing dowels?

thanks in advance for any advice.

 

[KootK]

I'm going with the assumption that your CMU is a bearing wall that will be installed before the slab is poured. Let me know if that's not the case. With regard to the smooth surfaces, I'll say this:

1) You could direct the masons to overfill the top bond beam a bit and leave it rough. Whether or not they'll actually follow that instruction is questionable of course. This would give you both a bit of a rough surface and some keying of the grout into the slab. Obviously, you wouldn't want the keying to go too deep.

2) If the surfaces are smooth then, yes, I would expect the dowels to see some direct shear. That said, this condition happens all the time in concrete and ACI does not require us to design for dowel shear. Nor does it require us to design for breakout. So... I don't.

3) The weight of the slab itself will provide some clamping action independent of the rebar. You might be able to make use of that as well.

but i'm not sure if it necessarily applies to reinforcing dowels

Yeah, that's a problem. They do have provisions for post installed rebar used as anchors which would be similar. European codes have some provisions for true "dowel action" connections but that's another cup of tea. It would be questionable whether or not you could apply the ACI breakout provisions to CMU anyhow in my opinion.

there is a key present but my supervisor said to ignore the the key in resisting lateral movement of the wall.

What's the nature of this key? Parallel or perpendicular to the wall? Key down into CMU grout? Or does the top of the wall project into the slab a bit? Do tell.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cpiedra23]

the wall is indeed bearing, and this is an existing condition.

1) it is an existing condition

2) a) doesn't the shear friction of ACI 318-11, Sect. 11.6.4 address the shear dowel design? b) laterally (out of plane of the wall) i would think some consideration would be required especially with a small edge distance. in the plane of the wall (for diaphragm shear), i don't think this would be necessary?

3) that makes sense, for tension. how would i consider it for shear? a friction coefficient using the weight?


"Yeah, that's a problem. They do have provisions for post installed rebar used as anchors which would be similar. European codes have some provisions for true "dowel action" connections but that's another cup of tea. It would be questionable whether or not you could apply the ACI breakout provisions to CMU anyhow in my opinion." --thanks for the input!

"What's the nature of this key? Parallel or perpendicular to the wall? Key down into CMU grout? Or does the top of the wall project into the slab a bit? Do tell." --it's parallel to the wall keyed down into the CMU, centered on the 7.625" width. my supervisor feels it's a proper detail for construction but should not necessarily be used theoretically to restrain the wall laterally. behavior wise i believe it would, but the positive connection would be at the dowel so i see his point.

 

[KootK]

2) a) doesn't the shear friction of ACI 318-11, Sect. 11.6.4 address the shear dowel design?

ACI gives you the shear friction method which has the mating surface resist the shear with the help of the tension supplied by the rebar for the clamping. With smooth surfaces, there will certainly be some shear in the rebar dowels but the method supplied by ACI does not consider that explicitly.

i would think some consideration would be required especially with a small edge distance. in the plane of the wall (for diaphragm shear), i don't think this would be necessary?

I agree with your instincts here but that simply is not how the ACI shear friction method works. When you have a non-smooth surface, it's important to realize that there is very little shear in the dowel so it makes sense not to be concerned with edge distances etc.

a friction coefficient using the weight?

Precisely. You basically use the reliable, permanent dead load on the connection to replace some of the clamping tension that would otherwise be provided by the shear friction dowels.

my supervisor feels it's a proper detail for construction but should not necessarily be used theoretically to restrain the wall laterally. behavior wise i believe it would, but the positive connection would be at the dowel so i see his point.

I believe that, in most cases, shear keys should not be used. Unless you have significant axial force in play, they don't work worth a damn. And they're a pain in butt to install. That said, if you're confident that you have such a key, I see no problem with using it. How is it constructed? Leave the top course of CMU ungrouted and allow the concrete to flow into it? I'd love to see a sketch of that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cpiedra23]

the key is constructed supposedly using a shaped 2x4 centered on the wall and along its plane (in and out of the detail), with the top of the 2x4 flush with the top course of the masonry. the 'shaped' sides are beveled outwards. sorry i don't have a second to sketch but hope that helps.

anyway thanks for all your input and help on this! much appreciated.

 

[sandman21]

ACI 318-08 11.6.8, requires that shear-friction reinforcement shall be anchored to develop fy on goth sides of the joint.

Kootk is right that shear-friction is not in ACI 530. I suggest you get your hands on Reinforced Masonry Engineering Handbook 7th edition, section 5.10.2.3.2 goes over this topic, you can conservatively check the reinforcing as an anchor bolt. Or you can use the allowable shear strength from ACI 2.3.6 and compare it to the perpendicular shear area using a reduced d, from the outside face to the centerline of bar.

 

[KootK]

Or you can use the allowable shear strength from ACI 2.3.6 and compare it to the perpendicular shear area using a reduced d, from the outside face to the centerline of bar.

This is the diagonal tension check right? If so, I don't believe that would be appropriate as an interface check.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

IT's the section for typical shear design, ACI 530 uses the in-plane strength equations for out of plate, so I am not sure what you mean. When selecting the area, Anv, the effective depth should be used with the area of the grouted cell.

 

[KootK]

IT's the section for typical shear design, ACI 530 uses the in-plane strength equations for out of plate, so I am not sure what you mean.

I'll try to clarify. At any section where shear exists, two criteria (at least) must be satisfied:

1) Shear friction along the same plane as the applied shear.
2) Diagonal tension across a plane at an angle to the applied shear.

Both criteria must be satisfied. It's insufficient to satisfy only one or the other. ACI530 2.3.6 Checks #2. However, that should be trivial at the joint since, presumably, the wall proper has already been designed to satisfy diagonal tension. So the critical thing would remain the shear friction check.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.