Angle Bearing on concrete - Prying force

[maramos]

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."

 

[maramos]

nutte,
any chance you can scan the sheet for me?

 

[dik]

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]

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]

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]

Thanks for all your help.

 

[KootK]

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]

ADeRaj, that is exactly what is in my book. Thanks for posting it.

 

[BAretired]

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]

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]

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]

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]

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]

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]

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]

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]

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.