Angle Bearing on concrete - Prying force
Angle Bearing on concrete - Prying force
(OP)
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."
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."






RE: Angle Bearing on concrete - Prying force
Dik
RE: Angle Bearing on concrete - Prying force
When in doubt, just take the next small step.
RE: Angle Bearing on concrete - Prying force
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?
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
Rotate pdf counterclockwise to get correct orientation
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
When in doubt, just take the next small step.
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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)
RE: Angle Bearing on concrete - Prying force
What is the approximate lever arm in terms of t?
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
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?
RE: Angle Bearing on concrete - Prying force
Old CA SE
RE: Angle Bearing on concrete - Prying force
I have the 6th edition what section is it under?
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
any chance you can scan the sheet for me?
RE: Angle Bearing on concrete - Prying force
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
RE: Angle Bearing on concrete - Prying force
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
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
I believe the answer is no because the deflection of the vertical leg of the angle is restrained by the concrete.
BA
RE: Angle Bearing on concrete - Prying force
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
RE: Angle Bearing on concrete - Prying force
RE: Angle Bearing on concrete - Prying force
Old CA SE
RE: Angle Bearing on concrete - Prying force
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
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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.
RE: Angle Bearing on concrete - Prying force
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.