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PRYING FORCE IN ANCHOR DESIGN

 
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vnacharya
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PostPosted: Mon May 18, 2020 12:00 pm    Post subject: PRYING FORCE IN ANCHOR DESIGN Reply with quote

Hi,


During Bolted/Anchor Connection design, it is obvious to consider Prying action in to account. However, inclusion/exclusion of prying force for Concrete break out failure is the topic under discussion now.
In the present case, I am having a 16mm thk. Anchor plate on top of the RCC slab. On top of it, a central Fin plate is connected which is subjected to Tension force (Due to wind) under critical load combination. Factored Tension force (2Te) from the analysis is 58kN and Total prying force (2Q) is 89kN!! Thus, 2Te+2Q=147kN.
Under Tension load case, for post-installed  Anchor (Say HILTI) following checks are done:
1. Steel Strength Check
2. Pull out check
3. Concrete Break out strength check

As per my understanding, Prying force is an induced force which will exert an additional force on the bolts. However, if we treat Anchor plate assembly as a whole, prying is not an additive force. Thus, "I presume, Prying force is to be considered for Anchor Steel Strength Check, Pull out check. However, prying force need not be considered for Concrete Break out check". The reason for above said statement is, in the present case, as the breakout cone is larger than "Prying force region i.e. Le", within the concrete break out failure cone prying force is getting balanced or nullified.

Even though, I could have made prying force as negligible by increasing the anchor plate thickness to 25mm or through stiffeners, I would like explore the concept in detail. Hence, I would request your valuable insight regarding this. If you can suggest any specific code reference, it would be more helpful.



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mdshameer
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Joined: 30 Jun 2014
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PostPosted: Tue Jun 16, 2020 6:17 pm    Post subject: Reply with quote

Dear Acharya,

For the record, in my experience, prying forces is neglected altogether in case of anchor bolts (steel-to-concrete connections). The reason for this, as per AISC Steel design Guide - Base Plate & Anchor Rod design, when the base plate thickness is calculated assuming cantilever bending about the web and/or flange of the column section, and because the length of the rods result in larger deflections than for steel to steel connections. Consistently, Embedment Design examples, ACI 349.2 also neglects prying forces by preventing yielding of tension side of base plate. The same approach is also used by Indian Codes in solved examples of SP-38 & SP-40.
However, in case of plastic design, I suppose, AISC limits minimum thickness for base plate. For earthquake resilient design, IS: 800 - 2007 also explicitly asks to consider prying force.

Your observations are different in that the prying force is large. Load case is predominantly tension. BM is minimal - so entire connection is in tension. I think your observation that for the assembly as a whole, the prying force in the bolt may be balanced by plate bearing force on the concrete surface looks reasonable. Two additional considerations from my side:
One: For single anchor, concrete breakout governed by 1.5 x embedment, what happens if tributary line of bolt for prying reaction is outside the breakout cone? Equilibrium may not hold good then. Does such case arise?
Two: If I roughly estimate entire 300mm as tributary line and say about 6mm (1/4 of 25mm) as width of bearing, a lower limit estimate of bearing stress due to the reported prying force of 89kN is  12.36 MPa. Is there a possibility of concrete bearing failure during prying.

From R&D point of view, may be a FEA analysis should be done. From a design office POV, you should make the base plate stiff enough not to pry.

Thanks
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