ACI 318 Appendix D Vcb 3

[henerythe8th]

I've had several discussions relative to calculating nominal concrete breakout strength in shear of a group of anchors using Eq (D-22) using the 2005 edition.
I have also read many threads here.

Is there a limit relative to edge distance after which there is no need to use the concrete breakout strength?

I would really like to get this settled so that I can get on with this (and my life).

The argument could be made that as ca1 increases that ha increases to some point that breakout is no longer an issue.

It has also been argued that if ca1 is equal to or greater than hef then there is no need to use Vcbg.

If someone can provide clarity to this situation, it would be greatly appreciated.

 

[StructuralEngGuy]

Appendix D has a tendancy to drive me up the wall and this particular issue definitely contributes to that. I'm also interested to see what people here will say since ACI doesn't explicitly allow you to "ignore" concrete breakout in shear if your force isn't acting towards any close edges. Leaves me to believe you have to check it no matter what even though your (Av/Avo) factor gets exponentially smaller the farther you get from an edge.

 

[ash060]

They added a new factor in 2008 that helps with the far edge distance problem. D.6.2.8 equation D-29

 

[henerythe8th]

That could prove useful, but the code of record that I am bound to work to is 2005.

 

[bones206]

Section 9.3.4 in "Design of Fastenings in Concrete" put out by Comite Euro-International Du Beton says:

"For fastenings...[with shear load] with an edge distance in all directions c>= 60*d, it may be assumed that no concrete edge failure will occur".

Where d is the diameter of the anchor.

This is what I go by. Keep in mind that the guys that put out this design guide are the same guys who brought us the CCD method in Appendix D.

"It has also been argued that if ca1 is equal to or greater than hef then there is no need to use Vcbg."

I don't believe this argument is correct, in general terms. You would be ignoring breakout failures of short anchors that are still relatively close to an edge.

 

[Lion06]

That is a good number to have in your back pocket. I've never had a hard number, I just use my judgment when to discount it.

ACI doesn't explicitly allow you to neglect this failure mechanism in any circumstance, but PCA Notes does show in an example that you can neglect shear breakout for shear that is "not toward a free edge". Every concrete member has a free edge, so what constitutes "not toward a free edge" is presumably left to engineering judgment.

I've never heard of ignoring shear breakout for ca1>hef. I don't believe this is accurate, either. Even aside from short anchors - for normal embedment depths, say 12", I would not discount shear breakout with an edge distance of 13".

 

[PackerFan]

The "out" that is addressed in App. D is the third paragraph of RD.4.2.1. ACI 318 App. D does allow you to neglect this failue mechanism.

"For anchors exceeding the limitations of D.4.2.2., or for situations where geometric restrictions limit breakout capacity or both, reinforcment oriented in the direction of load and proportioned to resist the total load within the breakout prism, and fully anchor on both side of the breakout planes, may be provided instead of calculating breakout capacity."

So, basically, if you have "enough" rebar in the slab and you are far enough away to have it anchored on both sides of the failure plane. Ignore breakout and just check steel capacity and pry-out capacity and you are done.

Loads that are acting away from an edge can be checked reasonably by calculating the shear force as if it was towards the edge in the oposite direction of the load (if there is and edge in that direction) and multiplying it by 2. This is kind of addressed in D.6.2.1 for loads acting parallel to an edge.

Certainly an anchor that is only located hef away from the edge is not great enough to preclude concrete breakout. However, you are corect that the 2008 edition corrects an issue with concrete breakout for thin slab conditions. Before this correction, the breakout capacity would get smaller and smaller once you got further and further away from the edge in thin slabs since Av/Avo got smaller way faster than Vb got bigger. If I remember correctly, it now uses an adjusted Ca1 just like you would use an adjusted hef for tension situations where three or more edges are in contact with the breakout plane (D.5.2.3).

 

[PEinVA]

If I have a plate on the face of a column, and my force is acting parallel to the face of the column, the first paragraph quoted by PackerFan would apply to this situation correct?

I'm trying to verify a spreadsheet and software and am confused on why there isn't option to turn off this check.

TIA.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[Lion06]

RC-

His first paragraph does address it, but that's only in ACI 318-08, it's not in the '05 version.
That being said, if you have an embed plate in the face of a column and the force is acting parallel to the longitudinal axis of the column, then you can't have a shear breakout failure. There is no free edge in the direction of the load. The column goes straight to a footing, then to soil without ever having a free edge.

 

[PEinVA]

SEIT,
I agree, but there doesn't seem to be anywhere that says this doesn't need to be checked, so I was just confirming.

BTW, I did read that in an '05 as well.

Thx.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[Lion06]

It is there. No kidding. I wonder why the actual code text doesn't explicitly state that?

 

[PEinVA]

Ok, another question. I have a steel beam spanning between 2 concrete columns. These plates are attaching to the face of the columns and I'm attaching the beams with a double angle connection.

I believe I wouldn't need to review any sort of pryout loads on these bolts. If the bolts were to be put in tension due to pryout, then the rotation and distortion of the connection and plate would have to distort my beam in the major axis.

This just couldn't happen. Any comments on my train of thought?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[Lion06]

The double clip connection will allow enough beam end rotation to design the embed plate for shear only, but make sure to provide the proper return at the top. If it gets welded all along the top then it will start attracting moment and put tension on the anchors. The proper return is 2 - 4 times the fillet leg - so a 1/4" fillet wants no more than a 1" return.

 

[PEinVA]

Exactly. I was specifying max 1" at the top of the fillet on the weld to the plate from the double angle.

So if I have gone through using engineering judgement to not look at pryout and/or breakout, is steel in shear the only thing I'm reviewing?

This is my first real foray into Appendix D as it wasn't part of our building code until this new cycle.

Thanks for all the help.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[Lion06]

Yes, but to be honest, I would personally put some nominal moment on the connection. We used pinned assumptions, which is reasonable when the failure mechanism is ductile (as is the case in steel structures). This is not always the case for embed plates. If you have heavy loads or an Load/Capacity ratio over 0.7 (or so), I would put a moment on the plate equal to 20% of what the fixed end moment would be (that is generally considered the limit for simple connections).

 

[PEinVA]

I never design post-installed anchors to 70% capacity (unless there is no choice) I like to keep ratios at 50%.

I agree there might be some fixity, and moment get into the bolts, but...I just can't see how the bolts will fail or the plates will distort in my situation.

Granted there are situations this can occur but I don't believe this is one of them.

Thanks for the help and sharing your knowledge.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[vandede427]

318 App D is evil. stay away from it.

 

[sturr]

I have been using a pretty good program called "Quick Anchor". I don't know if it is acceptable to push software in this forum, so I will refrain from saying where to get it. But if it is okay, let me know thru a response and I will post the info.