ACI 318-11 App D Side Face Blowout in Tension 4

[SteveGregory]

Referring to D.5.4 calculations, I have a pier with 4 anchor bolts (rods). If the spacing between the anchor rods is large enough, then each rod works independently and equation D-16 controls. So if the tension is concentric, the total allowable blowout would be 4X the amount calculated by D-16.

If the the rod spacing is smaller, the group equation D-17 is used. It is unclear to me if "A" I should use the result from Equation D-17 for the entire group of anchor rods or "B" multiply the result of D-17 by 4. I think i know the answer, but I would like some of you to weigh in with your thoughts or interpretations.

 

[SteveGregory]

Or plan "C", multiply results by 2 to capture the the other 2 anchors assuming a symmetrical 4-rod group.

 

[Lomarandil]

Interesting, this one is unlike the other provisions in ACI App. D (where the A_N/A_No term accounts for the effect of multiple anchors in group action).

D-17 multiplies the single anchor capacity by a value greater than 1, so I'd take its result (or 4x D-16 modified for perpendicular edge distance, if less) as the capacity of the group.

 

[SteveGregory]

It looks like, Table D.4.1.1 means the group capacity number does not change based on the physical size of the group or the number of anchor rods.

I am wondering if I can disregard Side-Face Blowout in tension (D.5.4) if I am using reinforcement developed as shown in D.5.2.9 to resist tension loads in the anchor rods???

 

[KootK]

I don't believe that you can disregard it as I don't see how the tension reinforcing would prevent side face blowout.

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]

Kootk, is correct side face blow out is a local compression failure of the concrete. Tension reinforcing will not prevent this failure mode

 

[KootK]

There's a feature of side face blowout that has always struck me as odd. So what if the side face blows out? With tension rebar, one should still be able to transfer the tension post blowout. Perhaps the issue then would be corrosion on a potentially exposed length of anchor rod.

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 a failure cone does not occur in the same direction. Ties would be the only thing that could restrain the failure but the spacing would be very close.

 

[MacGruber22]

I provide 2#3 ties spaced at 2" centered at the elevation of the head or nut on the anchor, and that keeps me sleeping for side blowout.

"It is imperative Cunth doesn't get his hands on those codes."

 

[SteveGregory]

I guess the ties near the head would be a good way to prevent side-face blowout. Although, you are left without a good way to quantify the blowout force.

The best way to fix it would be to increase the cover so that you meet the "magic" number 0.4hef. This could be accomplished by making the pier larger or by reducing the anchor rod spacing. A third strategy would involve reducing the embedment depth.

 

[KootK]

From Widianto's paper, you can estimate the demand for side face blowout ties as 1/4 the anchor rod force. Restraining ties need to be placed very close to the anchor head in both plan and elevation. As such, I don't normally trust the correct installation of additional ties unless special inspection will be undertaken. I also prefer the use of dedicated spirals at the anchor head rather than using standard ties tightly spaced and carefully located.

A fourth strategy would be to use rebar, deformed bar anchors, or an all thread bar like Williams or Dywidag. No anchor head, no blow out. AppD is really meant for shallow anchorage anyhow.

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.