Angle with load on leg - bending analysis 1

[wb.c]

So there is this problem in the PE structural practice exam (NCEES). And I did my calculations and I'm not getting the same result.

The "official" solution is this:

Now here is my approach.
This is a simple AISC 360-10 F11 Rectangular bar yielding check.
The leg that has the load will bend where the moment is maximum and the cross-section of the leg is smallest. This is at a distance k from the bottom corner.
So my calculation was identical to the "official" solution, except that when dividing the moment y the moment arm, I used 3.125 (4- 7/8) in stead of the 3.8125 (4- 3/16).
To me it just doesn't make any reasonable sense that the angle will bend at a thicker cross-section. I've seen the fabrication process for making 60-degree angles, and they always bend at k.
So I'm getting 2.19 for the LRFD result.

There is also this topic, where a user takes a totally wrong approach by assuming the moment is applied at the outer face of the corner.

https://www.eng-tips.com/viewthread.cfm?qid=501218

Am I missing something here?

 

[dik]

I disagree... I suspect there is little difference in the angle loaded in tension from a force or from a moment...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[wb.c]

yeah that's on me I use prying to describe the type of motion the short leg is experiencing, need to get out of the habit of calling it that since it does not align with what prying action actually is.

Its just called bending.

I'll get the right image, yet...

So you would really only need to check prying if your base plate was flexible enough.

If the base plate was flexible enough to cause prying effects due to deformations, then it would look like this.

The tension in the rod would actually be significantly bigger in this case.
This most people will design the base plate to be thick enough to avoid this. A thicker plate is much cheaper than bigger anchors.

so, have we explained why your Lb is incorrect ?

Yes, this has been answered. Thanks
To summarize, in my initial assumption, I overlooked the fact that the moment will carry though to the horizontal leg. This is why my Lb was incorrect.
With proper application of the moment to the horizontal leg of the angle, the correct answer (same as the official answer) is obtained.

Thanks for the help. Maybe we need a thread on prying effects...

 

[dik]

Not at all... I didn't mean to imply that. My program checks for prying action 'all the time'... I built it right in. It also includes prying action for the design of the baseplate, too. AISC has a calculation that if you run it, it stipulates the minimum thickness of the plate that can be used so prying action is not a condition.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[rb1957]

the sketches of prying are completely different to the initial one. In dik's sketch "q" is like the useful load "P". In the initial sketch the prying couple is between the bolt and the toe of the flange ... nothing on the load side. Prying doesn't change the maximum bending moment.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[ProgrammingPE]

They do a little bit at the hole location and increase the moment at 2 a little by increasing T slightly...

T continues to increase after prying action begins, but the moment at 2 does not change. It remains equal to the plastic moment capacity of the base plate, which is the moment that caused the hinge to form and prying to begin. The increased tension does not increase the moment at 2 since it gets offset by the negative moment that now occurs at 1. See the moment diagrams I added to the drawing below:

Structural Engineering Software:

http://www.structuralcentral.com/tools/steel-connection

Structural Engineering Videos:

http://www.youtube.com/structuralcentral

 

[wb.c]

Yes, we can all agree that prying action does not affect the magnitude of moment in the horizontal leg in the case I presented. Like I said before, your horizontal leg could be twice as long on either side and moment will still be the same. The only thing that changes is the reaction at the anchor. Prying action is considered only for the connecting elements like bolts and rods. There is no prying check for plates. Prying is a reaction, and in the end there must be equilibrium. Original question has been answered. Feel free to start a new thread called “Prying action discussion”.

 

[rb1957]

wb, you're new around here ? this will keep going ... on, and on, and on, and ...

smile, and appreciate the entropy you've added to the world !

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WesternJeb]

wb.c, you asked for FREE engineering consultation on an online forum of nerds who love this and it seems like you have consistently gotten frustrated with this thread and been rude. disclaimer: I might have misinterpreted the intent of your messages, so sorry if I have but I really don't think so.

Please lighten up and reevaluate your expectations for a free online forum of the people you are asking for help, or move on .

 

[wb.c]

As a fairly "new" contributor, the first thing I did was read the posting policies, and like most forums, there is a basic stay on topic rule.

I'm not being rude, maybe sarcastic with my "sherlock" response, but that was merited.

I thanked people for their input and recognized my mistakes, if people want to have a argument about a totally unrelated topic, just start a new thread, as per the posting policies.
Is it rude to expect that?

Then again, if this forum is full of people who need to give their two cents on every question posted, even if there input is totally unrelated, then you're right, I should definitely reevaluate my expectations.

 

[rb1957]

hey, "sherlock" was fine, I quipped back with "dr Watson" and you seem ok with that. just banter.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WesternJeb]

Well it appears I fell victim to a classic case of "if you go looking for a fight, you'll find one." Sorry for assuming the ill intent, wb.c! My goal was to just try and keep things friendly.