ACI 349-01 MULITIPLE SHEAR KEY / lugs

[msboobathi]

DEAR EXPERTS
I am designing multiple shear lugs to resists 2700 kN. is my unnderstanding is correct as per the attachment for concrete breakout .
any other checks to be prefromed for breakout.
if i have an example solution it will be much helpful.
if this check is not satisiyed shalll we go for additional reinforcement to resist the etc shear

Regards
M.S.Boobathi

 

[KootK]

I believe that you can only utilize the breeakout capacity associated with the projected area of the rearmost shear lug here. Reinforcing may well be required.

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.

 

[msboobathi]

Dear Kootk
But ACI 349 is allowing to sum the contribution of all lugs/fins. because of the limited area of concrete I would like to use
All allowances.

Regards
M.S.Boobathi

 

[structSU10]

If the failure plane develops on the back most lug, then the front lugs are pushing on an unanchored portion of concrete - the overall strength is limited by the back most lug.

I think the multiple lugs described in ACI would be multiple lugs where their common line is perpendicular to the applied shear.

 

[TehMightyEngineer]

Multiple lugs in the configuration shown will reduce the load on each lug, allowing redundancy and reduced size of the lugs. However, I agree with the above replies that your concrete breakout strength will be unchanged vs. one single lug.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WARose]

Agree with the others. By the way, I'd check to be sure that (punching shear) formula is ok for the lug. That appeared in a AISC design guide as well......and years later someone with AISC told me it was light for that application. (I.e. non-conservative.)

[red]EDIT[/red]: Elaborating a bit further on what I was talking about above:

Two methods are considered to estimate the strength corresponding to the observed
blowout failure mode of the concrete. One of these methods, commonly referred to as the
45 degree cone method, is prescribed by ACI-349 (2006) for the concrete shear capacity
of embedded shear lugs and is featured in the AISC Design Guide 1 (Fisher & Kloiber,
2006). A second estimate of strength is based on the concrete capacity design (CCD)
method, which has been adapted by ACI-349 (2006) for anchorages in concrete under
tension or shear loading. The main difference between the two strength prediction
methods is associated with the size-effect in concrete derived from fracture mechanics
theory (Bažant, 1984). An inspection of the test-to-predicted ratios indicates that the 45
degree cone method (currently proposed by the AISC Design Guide 1) may be
unconservative for large concrete edge distances (i.e. large concrete foundations) due to
this size effect. The 45 degree cone method has a mean test-to-predicted strength ratio of
0.51 (COV = 0.14). The test-to-predicted ratio determined with respect to the CCD
method indicates the inclusion of the size effect in the analysis provides a more accurate
estimate of strength. In this case, the mean test-to-predicted ratio is equal to 1.07 with a
COV equal to 0.19. The longer shear key is observed to be stronger on a unit basis, as
compared to the shorter shear key. The larger local bearing stresses of a shorter lug may
increase the likelihood of early crack initiation.

https://www.aisc.org/globalassets/a...ar-transfer-in-exposed-column-base-plates.pdf

 

[msboobathi]

Thanks for the replies
As the shear is more and it is embeded to limited concrete , is it the only way to provide shear reinforcement to avoid this failure
Can i provide in the form of stirrups around the shear key

Regards
M.S.Boobathi

 

[TehMightyEngineer]

Yes, ACI-318 appendix D (or chapter 17 in the latest edition) covers reinforcing concrete to preclude a shear and/or tension breakout failure. You could provide hoops that are in contact with the lugs and engage the entire concrete pier. This way the shear failure becomes not much different than a concrete column shear failure mode.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

For the scale of load that you’re trying to wrestle, I suspect that you’ll want to use methods like this: Link

I din’t Feel that it’s necessary —- or even advisable — to have your stirrups in direct contact with the lugs.

Given the degree to which your load exceeds plain concrete breakout capacity, something akin to a serviceability check might be prudent as well. All those stirrups won’t be engaged until after the plain concrete breakout cracking occurs.

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.

 

[TehMightyEngineer]

KootK, interesting that you don't think we should put the hoops against the lugs. My thought was exactly to avoid serviceability level cracking before engaging the hoops. Just curious for your thoughts on your recommendation.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

Just curious for your thoughts on your recommendation.

This is the picture of things rolling around in my head. I see lots of potential for slip and local stress concentration related failure modes that will either go unchecked or will, at the least, be rather difficult to check. And my sketch assumes that the bars are literally in contact with the lugs. The reality of field placement probably means that there would be small gapping instead. With that being the case, I'd think that you'd have even more potential for concrete crushing and slip but, conversely, you still wouldn't have enough concrete in the gap to improve bond on that side of the bar meaningfully.

In a frou-frou philosophical sense I feel that, fundamentally, the righteous path here goes LUG -> STRUTS -> STIRRUPS (similar to the Widianto stuff). I feel that it's a bit of a "cheat" to attempt to short circuit the concrete in that chain.

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.

 

[TehMightyEngineer]

Hmmm, good thoughts about the crushing of the concrete. Extending the hoop to the corners and using compression struts would be the easiest way to mitigate that. I agree with your points.

Regarding "cheating", I typically use similar reinforcement details for reinforcing lifting devices in large precast structures and it's pretty much required by ACI to have the reinforcement in contact with the lifting device (in my understanding) to be able to use the reinforcement for this purpose.

In the interest of academic discussion; what about a configuration such as this?

I agree it's not easier or cheaper (you either need to erect the lugs and baseplate prior to the pour or you need access to make the tight field welds). However, I believe it solves your comments.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

Regarding "cheating", I typically use similar reinforcement details for reinforcing lifting devices in large precast structures...

Currently, I recognize three kinds of valid concrete connections:

1) CCD
2) STM
3) Anything backed by, and used in accordance with, testing.

I've learned a LOT about connections following #3 based on my recent travels in the world of precast. So much so that I almost feel someone should put on a seminar: "Connection secrets that precasters know and EOR's don't". If I had the knowledge I possess now fifteen years ago, I could have exploited it to great advantage I think.

Anyhow, your revised detail does little to alleviate the concern. I think that you'll still be loading the lug oddly, although that can surely be designed for. I also feel that your sketch downplays the impact that bar bend radius may have coming off the lug. All that said, if you tell me that this is a #3 connection, that's good enough for me.

it's pretty much required by ACI to have the reinforcement in contact with the lifting device (in my understanding) to be able to use the reinforcement for this purpose.

Can you point me to this? Frankly, it sounds like something that I should already know.

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.

 

[TehMightyEngineer]

Great discussion and points. I would pay good money for a seminar for you to share your wisdom on precast connections. Even dealing with this stuff daily I often get backed into corners that I find really no good reference information on other than my own engineering judgement. Ungrouted shear keys likely being the most common grey area.

Can you point me to this? Frankly, it sounds like something that I should already know.

ACI 318-14 commentary R17.5.2.9 and associted fig. R17.5.2.9a and b.

Specific quote that addressed both the contact requirement and the impact of bend radius:

To ensure yielding of the anchor reinforcement, the enclosed anchor reinforcement in Fig. R17.5.2.9a should be in contact with the anchor reinforcement with a maximum diameter similar to a No. 5 bar. The larger bend radii associated with larger bar diameters may significantly reduce the effectiveness of the anchor reinforcement and, therefore, anchor reinforcement with a diameter larger than No. 6 is not recommended.

They also specifically note that a strut and tie model is an acceptable alternative to the two details they give.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

I would pay good money for a seminar for you to share your wisdom on precast connections

I was kinda thinking that you would do it and I could lurk via webinar.

Even dealing with this stuff daily I often get backed into corners that I find really no good reference information on other than my own engineering judgement.

I get a lot of mileage out of European publications. I wouldn't go so far to say that they're more advanced than we are in North America but they often approach things from a different perspective and sometimes that makes all the difference.

ACI 318-14 commentary R17.5.2.9 and associted fig. R17.5.2.9a and b.

Gotcha, thanks. So you're essentially still in CCD country.

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.

 

[TehMightyEngineer]

I was kinda thinking that you would do it and I could lurk via webinar.

Hah! Fine, we'll do it together.

European publications

I really should read more of those publications. I agree they tend to be just different enough that they get some unique rationalizations that the US codes don't have.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[msboobathi]

Thanks for All
If some experts shares his idea in a detailed book or report format it will be great .which includes FAQ
Once clear concepts are kept on the forum i will make spread sheets and share here.
Regards
M.S.Boobathi

 

[bones206]

Here is a good example you can follow, including reinforcing ties around the lug pocket:

Link

This study has some good background theory discussion as well as experimental results:

Link

The Widianto paper is really good too.

 

[msboobathi]

Dear All
caculation and paper is not taking the shear from back lug. once again going back ?

Regards
M.S.Boobathi

 

[msboobathi]

Dear All
1) I herewith attached calculation based on discussion
2) The shear key breakout strength is less than available .There is a need of reinforcent. is it ok to provide rebars for ( Required strength- available strength OR i need to provide Total shear
3) Whether shear reinforcement to be anchored please refer the detail and comment
Thanks in advance


Regards
M.S.Boobathi