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Pry out of headed anchors from concrete foundation

 
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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PostPosted: Thu Jun 25, 2020 3:48 pm    Post subject: Pry out of headed anchors from concrete foundation Reply with quote

Dear All,


Er Mandeep Singh Kohli raised the following question in ASCE Forum. As it may be interesting to many of you I am posting it here along with the nice answer by Er Er Neeraj Agrawal

I am facing a challenge in the current project i am working on. We are designing the Pedestal and the Anchor bolt for the Steel Column to Pedestal Connection and we are using Anchor rods with tack welded Heavy Hex Nut at the bottom with washer Plate. Since the forces are quite Huge we have used Anchor Reinforcement for the Tension and Shear Breakouts. The problem we are facing is with regards to "Pry Out" failure under shear loads. We are not able to satisfy the Pry Out Capacity.
Now reading a little literature and the code commentary i understand that PRY OUT failure mainly happens for  Short and Stiff anchors around 4 to 6 times diameter. The code treats the failure as a pseudo Tension failure and uses the same capacity as that of tension cone break out, since the failure cone for pry out and tension breakout are both very similar and start from the head of the anchor bolt.
The anchor bolts that we typically use have a minimum 12d embedment and mostly much more than this to provide sufficient development length to rebar above and below the failure plane. My Question is :

1.) Is the same clause applicable for such long anchors, since pryout seams improbable in such a circumstance. Should we rally check Pry out for anchor bolts around 12 times diameter.
2.) Since the capacity for the Pry out is a step function based on the Tension cone breakout capacity varying from 1 times to 2 times the Breakout capacity, can we use the capacity of  Anchor reinforcement instead of concrete, since the same reinforcement which prevents  the tension cone breakout will also tie the pry out failure crack.

Will really appreciate your feedback.

Er Neeraj Agrawal of Gurgaon Replied:

I think you are referring to very old version of ACI 318. ACI 318-2019 version gives following information which may be useful to you:
Pryout failure of cast in anchors longer than 65mm is double of tension breakout failure (Cl 17.7.3). Tension breakout failure capacity of anchor group Ncbg may be found by clause 17.6.2.1 (b).
If the strength is still less than what you need then code allows you to provide reinforcement in lieu of this check. Please see footnote 3 under table 17.5.2 and explanations given in commentary clause R17.5.2.1(b). Code has also given figures R17.5.2.1b(i) and (ii) explaining how to place the reinforcement. From these figures you will also notice the shear cone actually starts from the underside of base plate and not from bolt head that is why reinforcement is shown close to base plate and code has also written a remark "as small as possible observing cover requirements" for the location of reinforcement from top of pedestal.
Please check above  and it should solve your problem. Alternatively, I suggest you to use shear key for shear forces and design bolts only for tensile forces . 2019 version of ACI has also included an exclusive new section on shear key related checks for concrete design. See section 17.11 for details.

Best wishes
Subramanian

P.S. All of you stay safe and healthy
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