ACI Appendix D, shear breakout parallel to edge

[smvk3]

For an embed plate with headed studs, free edges on both sides and the top, with the vertical load away from the top edge, what does the failure plan look like for the shear parallel to edge? Will the failure plan start from the anchor nearest the edge (with half of the vertical load) and spread out? Or will it start from the anchors farthest the edge and spread in the same direction (with the full load)? See attached figure.

 

[Lion06]

If that's in a column that goes down to a supporting element then I, personally, do not consider breakout for that condition.

 

[smvk3]

Don't you have to consider breakout parallel to the edge per the code? The bottom of the column/pilaster is support on a footing, and the edge distances on the top, right, and left sides are around 5".

 

[Lion06]

If you have those edge distances, the anchors are within the column cage. Those equations are based on completely unreinforced concrete. I would throw some extra ties in the area of the embed plate and call it a day.

I don't believe the failure mode lines you've drawn are accurate. Possibly there could be a failure mode similar to side face blowout (which is what you've drawn), but that's not the intention of the shear parallel to the edge breakout.

 

[charliealphabravo]

I'm not sure I understand your configuration but I do know that the failure mode can depend on whether the anchor rods are welded or only bolted through the embed plate. You get more credit for welded embed plates under Appendix D since the anchor rods farthest from the loaded face will be mobilized.

 

[smvk3]

Lion6,

How do the column ties resist the breakout? Don't you have to develop the ties on both sides of the breakout surface per Appendix D to rely on them resisting breakout?

 

[smvk3]

charlie,

This is an embed. plate with headed studs. I just don't know what the failure surface looks like when you just have a vertical load like that with an edge distance on both sides.

 

[charliealphabravo]

Have you tried the free baseplate design tools from Simpson Strong-Tie or Hilti-Profis? They are pretty snazzy and take into account the free edges and applied loads. I believe the Simpson version is the best right now.

A sketch would help. It sounds like you are describing a vertical embed plate for a beam rather than a horizontal embed plate for a column.

 

[smvk3]

Charlie,

I have tried those tools. However, I was just interested in knowing what the failure plane looks like for this type of condition and how you can use the column reinforcement to resist this breakout.

 

[Lion06]

smvk-

I don't think that you have a breakout failure mode with your condition. The failure lines you've drawn aren't a complete failure cone and are more similar to a side face blowout (for tension) than a shear breakout parallel to the edge.

For a shear breakout parallel to the edge to manifest itself, you would need the entire embed plate to break out of one side, not just one side of the anchors. The entire plate is not going to break out of one side of the column for your condition.

I've attached a sketch of the way I understand the shear parallel to edge to fail.

 

[Lion06]

I was referencing using the column ties to resist what you've shown as as something similar to side face blowout for tension. That's not the failure mode you're asking about and probably just confused the matter.

 

[JLNJ]

Does ACI 318 D6.2.1(c) apply? It gives a value to use but doesn't seem to talk about the failure mode.

Last time I looked into this I was trying to post-install some large vertical loads into an existing column. I remember being very frustrated at the time with the then-new appendix D and its parallel-to-the-edge provisions.

 

[smvk3]

JLNJ,

I believe ACI D6.2.1 (c) does apply (shear force parallel to edge). ACI Appendix D states that to calculate the shear force capacity parallel to an edge, you apply a fictitious load perpendicular to the edge, calculate the capacity with an edge modification of 1.0, and then multiply that by 2. I understand this part, however, I do not know what the true failure plane looks like under the vertical load in my attached sketch. I am interested to know how one could use the column reinforcement to resist this force.

 

[smvk3]

Lion06,

Wouldn't the breakout start from the anchors nearest the free edge force?

 

[JLNJ]

If you are inside the last row of vertical rebar in the column, I don't have a good picture in my mind what the failure mode would look like.

It seems to me that ACI could do better with this fairly-common scenario.

 

[charliealphabravo]

When the bolts are welded to the embed plate you are permitted to use a failure surface passing through the most remote bolts for shear perpendicular to the face. This is because there is strain compatibility between the plate and the bolts which can't be relied upon if the bolts are loose in the plate. In that case you would use the closest bolts to the loaded edge.

As you probably know, appendix D has very little guidance for taking credit for pier/column reinforcement to resist anchor rod failure modes. All you get is a small flat credit for secondary reinforcement which is never enough. Appendix D started out as a design method for anchor rods in plain concrete. I have been told that a strut and tie method is used to take full credit for primary and secondary reinforcement but I haven't needed to work though this myself yet. It would clearly help in your case.

 

[smvk3]

charlie,

I originally assumed that that was the case. However, when I ran it with Hilti Profis, I checked there breakout calculation numbers, and they used a failure surface that starts from the anchors nearest the edge (the first case in the attached figure) and multiplied it by 2. This implies that failure surface is simply the embed plate blowing out the corners of the column on both sides.

If you are correct, I would think that one could make the argument that you could use the column ties to resist that type of breakout. Looking at the failure from the top, the failure surface would almost "slice" through the column diagonally corner-to-corner. However, if the breakout is per the first case in my attached figure, I don't think one could use that argument because I just don't see how you could engage any reinforcement.

 

[Lion06]

smvk-

The failure cone you've drawn in does not fail the embed plate. The plate has not broken out of the column.

I just don't see this failure mode for this condition. For a similar reason that I don't see a shear breakout perpendicular to the edge. The only way it fails for shear parallel to the edge is for the entire plate to break out one side of the column. I don't see that happening, especially with a tied column.

 

[smvk3]

Lion06,

What if your edge distance is very small, say 2" or 3"? Are you saying that the ties would never allow the embed plate to break off the corners of the column?

 

[charliealphabravo]

Appendix D breaks down pretty quickly in piers and columns where there are multiple near faces. Hence the synthetic loads and failure surfaces you noted. It sounds like Profis is taking a conservative approach. Either that or perhaps it is not taking credit for the rigidity of the welded rods acting as a unit. Actually I think I remember that this is the case since the program is set up for post-installed proprietary anchors. I don't think it properly models rods welded to the plate.