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RE: shear friction from horizontal top reinforcement, to resist holdown anchor pop out in A footing?

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I would also be interested in seeing your solution, if you would email it to me.  We tend to use the pullout cone method, but we generally use mass concrete bored piers (unreinf. circular pile) here. 
 
The notions you propose make sense.  I believe there are at least two mechanisms of failure
 
1) is debonding of the rod, of which we here very little these days and
2) shear cone failure of the concrete.
 
To overcome debonding sufficient anchorage length is required. Where this is not available in a straight length we would put a 90deg bend on the end of the bar.  However for large diameter bars developing large tension forces a local compression failure can occur at the concrete interface on the inside of the bend. This is a function of the bend radius and I have an old copy of a C&CA (UK) handbooks that has a design approach for this problem, where the radius of the bend is modified from the standard radius to minimise the compression. An alternative was to go with the standard bend but put a large dia. pin bar inside the radius to distribute the bearing and provide a dowel action.
 
With regard to the shear cone failure, the SAA concrete code has an approach for flat slabs where a steel shear head is cast into the slab.  (I think this was also in the ACI, I've not seen one for some years). To me the basic notion of the shear head is to actually increase the shear perimeter, ie enlarge the pull-out cone surface area, for shear.
 
But as suggested earlier in the list "there is not a conspicuous history of failures" from this mechanism, to paraphrase a local document.
 
 
Roy Harrison
 
Roy Harrison & Associates
Structural Design Engineers
Para Hills  - South Australia
 
-----Original Message-----
 
-----Original Message-----
From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com]
Sent: Wednesday, 4 October 2006 11:02 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: shear friction from horizontal top reinforcement, to resist holdown anchor pop out in A footing?

I’d like to see your PDF.  I don’t know if I can be of any assistance, but I have considered the top steel in resisting anchor rod pullout.  I don’t have a complete answer, or even a good partial answer, but if you have a plain concrete footing and a footing with top steel, it seems pretty obvious that the footing with top steel with have greater pullout resistance.  I’ve considered shear friction and simply the rebar steel shear strength (0.4*Fy).  I think there is a factor to account for the rebar in the ACI design method, but I don’t recall right now.

 

 

BDH

 


From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil]
Sent: Tuesday, October 03, 2006 4:28 PM
To: seaint(--nospam--at)seaint.org
Subject: shear friction from horizontal top reinforcement, to resist holdown anchor pop out in A footing?

 

Can you use horizontal reinforcement in the top of a footing to resist the pop out of a concrete tiedown anchor rod?  I  talked about this idea  with an intelligent engineer and he said it can't be done. 

 

I drew a free body diagram of a pop out cone with the reinforcement developed in the cone and with the shear friction parallel to the cracks with the vertical of shear friction component cancelling out the upward force and the horizontal component cancelling out.  I'll e-mail a .pdf of the solution if someone wants to look at it and tell me where it goes South.

 

He said the tension is acting vertically across the inclined crack failure plane, and you've got the shear component parallel to the crack and tension component perpendicular to the crack, and that the shear friction  formula in ACI 318 for reinforcement at an angle to the crack only applies to force component parallel to the crack. 


This is where my head starts hurting assuming the free body diagram is inappropriate, couldn't you still increase the amount of horizontal reinforcement across the crack to cancel the horizontal component of tension perpendicular to the crack and add more shear friction for the component parallel to the crack ect...etc.... 

 

I don't believe horizontal reinforcement couldn't clamp concrete from popping out with enough horizontal reinforcement clamping the pieces together. 

 


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