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."

 

[dik]

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]

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]

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]

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]

I have attached the detail in question for review.

Rotate pdf counterclockwise to get correct orientation

 

[maramos]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

nutte

I have the 6th edition what section is it under?

 

[nutte]

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]

The Chapter in mine is called "Design of Connections." It's in the "Connection Angles" section.