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maramos (Civil/Environmental) (OP)
12 Aug 09 16:08
I recently got a comment back from a reviewing engineer with the following comment regarding one of my analysis.
In the analysis i was calculating the prying (tensile) force on a bolt.  I was asked to multiply my moment arm
by a factor of 0.85 to account for the difference in strain rates between concrete and steel.  Exact comment is below.

Does anyone have an idea of how the 0.85 is calculated?

Detail description:
Angle anchored to side of concrete curb under eccentric gravity load.  Angle is bearing on concrete.

"That is your elastic return coefficient.  It is similar to the Whitney stress block that is assumed in concrete design.  The concrete is not elastically equivalent to the steel, so to account for the difference in strain rates one multiplies the "arm" in the concrete by 0.85."



 
dik (Structural)
12 Aug 09 17:08
It might be a measure of the depth of the compression block. For small amounts of rfg, I used to use a factor of 0.9.

Dik
rowingengineer (Structural)
12 Aug 09 17:18
I would query why you are calculating prying force for a bolt for a plate connected to concrete, generally I have always considered concrete to be 8 times softer than steel such that prying force from steel to concrete connection is not likely to develop.  

When in doubt, just take the next small step.
 

Ron (Structural)
12 Aug 09 17:24
I would not consider that to be a valid request, and here's why...

The elastic extension of this application would be measured in thousandths of an inch.  That doesn't change the impingement angle, therefore the load on the bolt doesn't change appreciably by the angle difference.  Given that, even when the angle deflects and the bolt extends under stress, the entire load is ultimately transferred to the concrete...so why decrease that by a factor?

 
Tomfh (Structural)
12 Aug 09 22:34

Quote:

I have always considered concrete to be 8 times softer than steel such that prying force from steel to concrete connection is not likely to develop.  

I have heard that argument before, but I'm not convinced of it. Consider a claw hammer pulling out a nail. The timber quite easily provides the necessary prying force to remove (or snap off) the nail, despite timber being much softer than steel.
maramos (Civil/Environmental) (OP)
12 Aug 09 23:12
I have attached the detail in question for review.

Rotate pdf counterclockwise to get correct orientation
maramos (Civil/Environmental) (OP)
12 Aug 09 23:33
in my origional post i stated that the moment arm was to be multiplied by 0.85 which is incorrect.  its the distance from bolt to pivot point (as shown in sketch in my previous post) that the reviewing engineer is asking to be multiplied by 0.85.  

 
rowingengineer (Structural)
12 Aug 09 23:38
I will take some honey with my words as so I can eat them. Looks like I was talking through my hat, for your situation you must include prying as you have defined it. You definitely have a "claw hammer pulling out a nail".  Wasn't how I saw the connection in my head.

 

When in doubt, just take the next small step.
 

racookpe1978 (Nuclear)
12 Aug 09 23:57
aren't there two  parts of the "prying" problem ?

One is the movement (compression) of the concrete under the (assumed perfectly rigid!) steel baseplate => The pivot point of the column + baseplate is therefore somewhere between the downwind edge of the baseplate and the center of the plate.  If the "perfectly rigid plate" deflects as well as the concrete compressing under the plate, then the pivot point is further moved from the edge.

Second is the bending force on the bolts.    
demayeng (Structural)
13 Aug 09 0:02
The 0.85 increases the bolt load and is an approximate distance to the centre of a 'compression block' (as dik said).

The compression block could be a result of the steel being unable to pivot perfectly around the corner point because of the difference in material properties (the steel will push into the concrete)
 
Tomfh (Structural)
13 Aug 09 1:14
Ok guys, I have a puzzle for you. Assume dim "X" is 1000 times the plate thickness "t", i.e. the leg is very long.

What is the approximate lever arm in terms of t?
 
maramos (Civil/Environmental) (OP)
13 Aug 09 8:31
dik,
anymore insight or text book reference to the 0.85 or the 0.9 that you use?  also what does rtg refer to?

sorry i'm new at this.

Thanks for all your help guys.  
JLNJ (Structural)
13 Aug 09 11:52
These can be maddeningly difficult little problems!

If you can, move the fastener up the leg of the angle to increase the "x" dimension. This will give you a larger moment arm and a smaller force in the bolt (assuming the plate is stiff enough to activate it). Don't forget to combine the shear with the prying tension.
 
Lion06 (Structural)
13 Aug 09 12:02
I don't know if I consider this "prying" as much as a tension load from the eccentric shear.  Either way, you can't use Dim "X" to get the tension.  That assumes that the concrete bears against the angle only at the very tip.  The 0.9 that dik references is a typical "jd" value, or "d-a/2" value.  

You could actually set up the equilibrium equations which will factor in the length of the angle (out of the plane of the page), Dim. "X", f'c, and the eccentric shear.  I've tried this before, and am always surprised to find that either the tension force is ridiculously high or that the anchor actually ends up in the compression block.  Somehow they're not falling down all over the place, so maybe I'm missing something.
nutte (Structural)
13 Aug 09 12:08
The PCI Design Handbook shows this exact scenario.  In the 4th edition, it's on page 6-22.  They use the moment arm equal to 5/6*X, or 0.83.  This comes from an elastic stress distribution of the angle bearing against the concrete.  Your reviewer is right on target.
Lion06 (Structural)
13 Aug 09 12:13
nutte-

The only issue I have with that is that all those "rules of thumb" for estimating that distance don't take into account the actual load.  It changes depending on the load (because the moment changes).  If you work it out, sometimes it just doesn't work on paper.  Am I missing something obvious?
mudflaps (Structural)
13 Aug 09 12:40
0.85 - - sounds like the "j" part of jd when designing in WSD concrete. For balanced steel ratios "j" is 0.85. I think it's as simple as that. Of course if you recalculated using the actual bolt tension you would get a "j" value much closer to 1.00. But it is a plan checker, give him what he wants and move on.


Old CA SE
maramos (Civil/Environmental) (OP)
13 Aug 09 12:41
nutte

I have the 6th edition what section is it under?
nutte (Structural)
13 Aug 09 12:42
Not if you're using an elastic stress distribution against the concrete.  If you're taking the concrete as plastic, with a rectangular stress distribution over some small depth, then yes, you'd need to do a more rigorous analysis.  This will ive you a larger moment arm, and less bolt tension.  Thus, the elastic method would be conservative.
nutte (Structural)
13 Aug 09 12:44
The Chapter in mine is called "Design of Connections."  It's in the "Connection Angles" section.
maramos (Civil/Environmental) (OP)
13 Aug 09 13:05
nutte,
any chance you can scan the sheet for me?

 
dik (Structural)
13 Aug 09 13:06
maramos:
The factor is to account for ju or (d-a/2) as a fraction of d. The rfg stands for reinforcing.  For small percentages of steel, the concrete strength has little effect on the flexural strength.

Dik
Ipetu (Structural)
14 Aug 09 1:41
maramos:

Attached pages were taken from the Canadian Prestressed Concrete Institute's Precast and Prestressed Concrete Design Manual (third edition). Section 4.14 is on connection angles. Hope this is helpful to all following this thread.

ADeRaj
phuduhudu (Structural)
14 Aug 09 7:56
I agree with structuralEIT that this is not really prying. Prying is where you have direct pullout tension on a bolt that is magnified by the steel connection. This is just plain old bending.

Interestingly I am working on a direct pull out problem for a column base with large uplifts and I thought I had to consider prying. Interesting that some say you don't need to consider it because of the softness of the concrete. I imagine that a high strength grout bed is not going to give a lot to relieve your prying force.
maramos (Civil/Environmental) (OP)
14 Aug 09 10:50
Thanks for all your help.
KootK (Structural)
14 Aug 09 11:23
Hold on RowingEngineer.  I'm not so sure your assessment was out to lunch.

As illustrated by the claw hammer example, there will be some prying.  I believe that the disparity in material stiffnesses will tend to minimize the prying though.

I've often wondered this very thing with respect to column anchor bolts under tension.  We generally assume rigid behavior and rarely consider prying.  Why not?  Because we consider a typically sized steel base plate cantilever to be much stiffer than the grout / concrete in contact with it.
nutte (Structural)
14 Aug 09 11:46
ADeRaj, that is exactly what is in my book.  Thanks for posting it.
BAretired (Structural)
14 Aug 09 14:03
I don't think anyone responded to the puzzle proposed by Tomfh, i.e. what happens if X is increased without limit?  Is it still safe to say that the tension in the fastener is P*e/0.85X (or P*e/0.9X)?  

I believe the answer is no because the deflection of the vertical leg of the angle is restrained by the concrete.

BA

mudflaps (Structural)
14 Aug 09 17:31
BA - I think you're right, what with the deflection of the angle getting smaller as the thickness increases. You'd need to check the slope of the deflected angle leg against the slope of the compressed concrete to find out where the center of contact occurs. In the case of a really long angle leg of moderate thickness you might find the toe of the angle actually lifting off the concrete. You'll need a finite element program to see that happen.

As for the difference in "E" values between the angle and concrete - it doesn't matter. When we do a FBD at midspan of a concrete beam we assume the plane section remains plane during bending. At that point the other side of the FBD could be the other part of the concrete beam or a massive steel block. The only thing important is to know the "E" values of the concrete and rebar on our side of the FBD. In this case it's concrete and a bolt of some kind.

As I recall, prying in steel-to-steel connections, you don't adjust the "x" dimension if you keep the steel stresses below yield. But then again, I don't have the 13th edition to know if they've changed that. The plan checker is asking about of a 0.85 factor on the assumption that the stress in the angle is below yield. If it hasn't then the "X" adjustment is the "j" value from WSD concrete. For a lightly loaded bolt "j" could approach 1.0.

Now, what do you do if the angle is so thin it yields. I think that's what Tomfh was getting at, in a round about way. In a case like that the "X" adjustment could drop considerably.  

Just for fun, think about the stress distribution under the toe of the angle with the shear generated by the concrete stresses having to go around the bolt hole. Another fun activity for those with finite element programs. Food for thought.

The 'old' guys knew about this 'stuff' and made the problems go away by making the members thicker. There's very little new under the sun.

Old CA SE
KootK (Structural)
14 Aug 09 18:00
Sure the relative stiffness and crushing strength matter.  Suppose you replace the concrete with rigid insulation.  Now how much prying action do you get in the angle?
mudflaps (Structural)
14 Aug 09 19:19
KootenanyKid - The assumption is the other side of the FBD is at least as good as the side the side you are working on. If it's better, then the weakest side is what you should be calculating. Suppose you had a concrete beam cantilevered off the flange of a really heavy steel column. Would you really be concerned about about what's happening in the column when checking stress in the rebar and concrete?

Old CA SE
KootK (Structural)
14 Aug 09 20:32
Mudflaps,

That's precisely the point.  The assumption that the other side of the FBD is as good (E/fy) as the side that you're working on is false.

The concrete beam / steel column example doesn't resonate with me I'm afraid.  In fact, I think it supports the conclusion that stiffnes does matter.

Relative to the concrete beam, the steel colum is flexuarlly stiff and has a high crushing strength.  That's why you can ignore what's happening in the column when doing the flexural design of the beam.

The legs of the angles discussed here are very flexible (flexurally) compared to the concrete on which they're mounted.  That's why prying becomes an issue in the first place.  If the angle was a rigid as assumed in the suggested concrete stress distributions above, there would be no prying action to speak of.

Check out the sketch that I've linked.  It probably does a better job of conveying my ideas on this than my ramblings.

A while back, out of curiousity, I did prying action calcs for some typical base plates (uplift) using the AISC provisions.  The bolt force amplifications were ENORMOUS.  If the concrete doesn't move/crush out of the way to relieve the prying action on the base plates, we've been seriously underestimating our anchor bolt forces.

Kootenay


 
Tomfh (Structural)
14 Aug 09 20:44

Quote:

Now, what do you do if the angle is so thin it yields. I think that's what Tomfh was getting at, in a round about way.

I don't believe yielding matters. You can assume elastic behavior.

Perhaps you are right about FEA being the only way to properly investigate the issue.

 
Tomfh (Structural)
14 Aug 09 20:54

Quote:

If the concrete doesn't move/crush out of the way to relieve the prying action on the base plates, we've been seriously underestimating our anchor bolt forces.

I agree. Concrete in bearing is often a lot stronger and stiffer than a steel bolt and cantilevering steel plate.

You can't just ignore prying on the assumption that the steel will win the fight.
KootK (Structural)
14 Aug 09 21:13
This reminds me of a related issue:

Back in 2003, I worked for a company that purchased a software package called RISA-Base.  It's an FEM package for doing base plate design.  It's cool.

Anyhow, the program lets you chose whether or not you want to consider the base plate as rigid or modelled using FEM and the base plate's real calculated stiffness.

Naturally, I figured that the true FEM would be the way to go since the compuational effort on my part was the same either way.  Here's what happened...

If I modelled the base plate as rigid, I got normal results for base plate thicknesses (3/4" etc.). If I used FEM, however, there were huge stress spikes beneath the web and flanges of the column.  In order to iron out the stresses to something reasonable, the base plate thicknesses had to be on the order of 4".

After some thought, I came to the conclusion that this makes sense.  A base plate is essentially a cantilver.  And cantilevers aren't too stiff when it comes to resisting transverse loads out at their tips.  So, I wondered, why the heck do base plates work then?

I think that the concrete crushes locally -- and minutely -- immediately below steel column sections when they are loaded heavily.  Then the load spreads out to the cantilevered portions of the base plate until you develop enough resistance to match the load on the column.

If my assumption is correct, however, I find it odd that it's never stated explicitly anywhere (books etc).  Base plates are pretty darn ubiquitous.  If anybody has any thoughts on this, I'd love to hear 'em.

 
rowingengineer (Structural)
14 Aug 09 22:46
OK I'm finished eating and am back,
First of all can we get a few things sorted let's set two situations:
1. The OP question is in relation to a lever arm type "prying" where the length of steel after the bolt is high.
2. "Prying" on a bolt in a base plate or similar has a low distance from the bolt to the edge of plate completely different.

While I do believe KK is on the right in regards to prying for number 2. I think a new thread on this is required.  

As for number 1, a lever arm situation, I think KK sketch bottom right-hand corner is close to the real situation for ultimate failure assuming you have anchor that can continue to displace ie a ductile type failure, thus for this to be true the bolt must have the ability to strain, and the angle must be of a thickness that ensures a gap can open.

However my experience with this type of situation is that you can have failure by concrete pullout. This is why on precast clips from steel columns to precast walls they have found it necessary to do something like the attached.  This has mainly occurred in skinny panels with small embedment depth for large chemical anchors.
 

When in doubt, just take the next small step.
 

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