Side face blowout 2

[braves25]

I have read several posts about this but have not seen anything to help with using reinforcing to resist this failure mode (or I just missed it). I have a circular pedestal with 1.75" dia anchor bolts. 40" embed. I do not meet the 6do to satisfy the side face blowout. I cannot make the pedestal larger. Can I use stirrups around the anchor bolts (in addition to the ties at the vertical reinforcing for the pedestal) for this? If so, any design guides that relate to this? Thanks and have a great day.

 

[KootK]

Link

Answer's in there. Local reinforcing to the tune of 1/4 te tensile demand.

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.

 

[cliff234]

I don't like relying on reinforcing to resist side-face blowout. When I look at the illustration of side-face blowout in ACI 318 (page 415 in ACI318-08), I can't figure out how I can develop reinforcing steel within the narrow piece of concrete that is spalling off the rest of the concrete. The only way reinforcing steel will work will be if it can develop on either side of the crack. How can it develop if the width of the spalling blocks of concrete is only several inches wide? Those 1.75" diameter bolts are big!

Can you use a larger number of smaller diameter bolts?

Or can you extend the vertical reinforcing steel out of your pedestal (I am assuming that your pedestal has substantial vertical reinforcing steel) and thread the ends of the bars? Use the vertical bars as your anchor rods. That will make the side-face blowout issue go away.

Or another solution is to leave your vertical reinforcing steel as is, and instead of using headed anchor rods, use anchor rods comprised of vertical reinforcing bars threaded the top and lap spliced with the vertical bars in the pedestal. You might want to re-position your pedestal bars so that they lap splice with the anchor rods. And the reinforcing steel anchor rods would need to extend into the pedestal for at least the lap splice length.

 

[KootK]

I don't like relying on reinforcing to resist side-face blowout. When I look at the illustration of side-face blowout in ACI 318 (page 415 in ACI318-08), I can't figure out how I can develop reinforcing steel within the narrow piece of concrete that is spalling off the rest of the concrete. The only way reinforcing steel will work will be if it can develop on either side of the crack. How can it develop if the width of the spalling blocks of concrete is only several inches wide? Those 1.75" diameter bolts are big!

I struggle with this too. I harbor a more optimistic view of it now however, although I can offer little hard evidence to support my view.

The reinforcement strategy always bothered me because I previously assumed that the side face blowout cracking would have to occur prior to the reinforcement being engaged. As such, while you might be safe in terms of load transfer, you'd have a bunch of unsightly cracks present inviting corrosion problems and compromising shear capacity etc. And, under this model, you would have a need to develop the reinforcement either side of the cracks. It sounds as though you're viewing things in a similar fashion.

I now believe that the bursting reinforcement actually prevents the concrete from cracking / bursting. My, admittedly soft, "evidence" for this is as follows:

1) The spiral, Tf/4 bursting reinforcement normally provided is based on an elastic, uncracked analysis that assigns all of the stress resisting responsibility to the reinforcing. And the reinforcing should be the stiffest load path for the bursting stresses, kind of like how the bursting forces in well confined columns tend to "find" the column ties and vertical bars. Although it should be noted that heavily taxed columns will spall en route to fully engaging the confinement reinforcing.

2) The most analogous situation that I can think of is bursting reinforcement for PT cable anchorages. They're detailed similarly, bursting would be predicted to happen without them, they see loads very near their design loads, and you never see any cracking around them.



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.

 

[RFreund]

@Kootk:
Doesn't the article say that the spiral reinforcement does not increase side-face blowout capacity it only increases the load maintained after side-face blowout occurs?

EIT

http://www.HowToEngineer.com

 

[KootK]

Doesn't the article say that the spiral reinforcement does not increase side-face blowout capacity it only increases the load maintained after side-face blowout occurs?

Yup, that's right and is part of the dogma that I now question. I feel like the PT anchorage analog is a very persuasive one. If there's anybody out there that can dispute the validity of that comparison, I'd love to hear from them.

I'd also like to get my hands on the research quoted by Widianto. The detailing of the reinforcement in the article leaves something to be desired in my opinion. I believe that it should be placed like this:

1) Pick a vertical distance over which to get it done and provide all required reinforcement within that distance.

2) Laterally, provide enough space between the anchors and the reinforcement such that you're grabbing all of the wedge defined by #1.

Often, I see folks wrap the reinforcing tight to the anchors as Widianto shows. In my opinion, that's incorrect. It's certainly not what is done with PT bursting reinforcement. It also wouldn't satisfy any STM model of the situation that I can envision. Lastly, if you adhere to #2, you'll find that still limits edge distance very much as you see without the reinforcement. No free lunch it would seem. Well, I guess cheap lunch would be more apt. It's not as though the spiral ties are free. In fact, residential and light commercial contractors bitch about them endlessly.

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.

 

[KootK]

In fact, residential and light commercial contractors bitch about them endlessly.

And I can't say that I blame them. The ties are crazy hard to locate when done my way.

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]

Did you just respond to yourself?

Per ACI additional reinforcement does not increase the capacity of the design. So I almost always increase the edge distance regardless of if the ties provide additional strength or not.