topic Re: Am I doing this correctly? Forces and reactions in columns. in Robot Structural Analysis Forum

Am I doing this correctly? Forces and reactions in columns.

Anonymous — Thu, 23 Feb 2017 20:36:54 GMT

Hi all,

I created a dummy model with very simple geometry to check my results and it seems correct. However, when I move on a to a more complex model, my results are way off.

Either, I'm doing something wrong in Robot, i.e., reading the wrong numbers or modeling errors, or my hand calculations are way off.

I'm attaching a picture of my model.

What I'm doing for hand calculations is this:

1) I apply snow load, dead load for slabs and live load. For all slabs at once.

2) Loading is distributed to columns and walls according to load area. I think my calculations here should be fairly accurate and total forces seems reasonable.

In Robot, I want to check the same calculation:

1) Define loads and create a manual combination with all vertical loads at once. Run calculation (no errors).

What's the best way from here to get the axial force in each column per story level as shown on the drawing that's attached?

Do I need to check the value at each node?

Or can I use View -> Tables -> Select: Bars; Reactions; Forces?

Then, from Bars, I can right click the the screen and select Table columns and select Fx in Forces tab?

For some reason, the numbers seemed to work out correctly this time. So, maybe it was right after all? Can anyone confirm or tell me if there's a smarter way to get these results?

Would it be possible to have this displayed on the columns on the actual model?

Thanks in advance for any help! 🙂

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Thu, 23 Feb 2017 20:38:47 GMT

CumulativeLoads - Model with hand calculated values annotated. Per story.

CumulativeLoads2 - Model with nodes/bar numbers

CumulativeLoadsNode - Result for node 55. Seems to align fairly well with Robot values.

Attaching a final file now showing my other sheet.

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Sat, 25 Feb 2017 11:18:44 GMT

Can anyone confirm if I'm going about doing this correctly? Or is there a smarter way to find axial force/reaction (Fx) in the columns/walls?

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Sat, 25 Feb 2017 15:08:31 GMT

Either way, graphic or tables you will get axial forces (Fx)on the bar's bottom and top

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Sun, 26 Feb 2017 20:49:30 GMT

Thanks!

I've checked some of my values and it seems like I'm indeed doing it correctly for the columns.

How can I do the same for the walls (panels) that support the slabs?

Is it possible to find total axial force Fx for each wall? If so, how? Or do I somehow need to summarize the values at each node?

Thanks very much in advance! 🙂

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Mon, 27 Feb 2017 02:04:50 GMT

You can set a panel cut at the botton of each wall panel ,choosing "local direction of the cut" and choosing to show Nxx. Then in the graphical window the integral value will be the total normal force at the botton of each wall

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Mon, 27 Feb 2017 11:18:10 GMT

Hi,

Thanks. I just tried with a very simple model and my numbers seems to be slightly OFF. Can you clarify or tell me if I'm doing something wrong?

I created a simple wall with width 1,0 m and a linear load of 10 kN/m. This should give total axial force of 10 kN in the entire wall.

1. Panel cut

I choose between 2 points and parallell to Y axis (would X axis give the same cut). Points at start and end of wall.

2. Choose detailed, local direction of cut and Membrance forces - N - xx.

Integral value in red text on the screen shows 10,30 kN/m.

Should this not be 10,00?

Does this indicate I'm doing something incorrectly? Mind you, I'm used to hand calculations.

Thanks in advance for any help on this! 🙂

PS: I had some instability issues and changed from pinned to fixed support. This gave me 10,13, but still not 10,00.

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Mon, 27 Feb 2017 11:59:14 GMT

This is a general method for any panel shape when you want to know the normal force across any panel section (cut). In your case try a more refined mesh. It will probably give you more accurate results.
For retangulars panels you can also try Tables/ Reduced results for panels

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Mon, 27 Feb 2017 12:21:01 GMT

Hi, Rafael,

Thank you. I tried a very, very fine mesh for this panel and now I get 10,02 kN. So, it's simply a matter of meshing settings? These meshing settings are however way too fine for my other model, I think.

The important thing for me to know is that I'm reading and getting the accurate results.

Reduced results for panels. Please see attached picture. Am I looking at the correct one?

I don't quite understand what the middle value Cut means.

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Mon, 27 Feb 2017 13:18:47 GMT

Doing some more googling on this...

Is it perhaps smarter and easier to group my walls as core walls and then get the results directly in tables for core walls?

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Mon, 27 Feb 2017 13:45:23 GMT

You can use core walls , reduced results , panel cuts but the results will come from the same source which is the integration along/across finite elements which is better explained on the help files.

For the reduced results in tables , the cut is designated by the nodes on the 4 corners of the examined panel.

And you get only cuts on the edges and in midpanel distance

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Mon, 27 Feb 2017 13:50:15 GMT

Appreciate all the help, Rafael.

Thanks!

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Tue, 28 Feb 2017 12:05:19 GMT

Does this appear to be correct? Please see first attached picture.

I have a system of core walls that's U-shaped.

If I want the reaction for each wall, I assume I need to select ONLY one row/columns of walls and create a new view that I make cuts from?

If not, my cut will include the other walls as well, right?

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Tue, 28 Feb 2017 12:33:22 GMT

Yes , that's correct. The integral value will be to the walls being cut on that particular view.
Another way to get the normal force is marking "reduced forces" on the "cuts" tab on panel cuts dialog window.
Then results are displayed on a legend on graphic window. Forces on XYZ are related to global coordinate system. The Z value is what you need

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Tue, 28 Feb 2017 12:42:18 GMT

Thanks, Rafael!

I tried what you proposed with reduced forces and got this (please see attached picture). Not sure if that's what you meant?

Another related small trouble: The data in red letters on bottom right of my screen goes further than my screen extends. Is there a way to move it leftwards? I seem to be able to read what I need, but would be better to get all the letters.

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Tue, 28 Feb 2017 13:21:21 GMT

Try to move the legends to another corner on View/legend parameters
Yes that's it. Values for "integral" and "reduced force" Z shall be the same

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Thu, 02 Mar 2017 14:39:56 GMT

Hi again,

I'm now interested in doing the same thing on panel cuts to get Moments and Shear force.

It seems like I can simply use reduced forces and reduced moments. My moments appear to be correct.

But why is the integral value not correct? I thought that Myy would be the correct choice, but the moment is close to zero. Mxx and Mxy is also very low. Do I need to define the cut elsewhere?

If reduced moments and forces is correct, I'm happy to use that. But would simply be curious to understand WHY this is so.

Thanks in advance!

Please see attached picture.

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Thu, 02 Mar 2017 19:46:24 GMT

Integral value of a cut will only work for out of the panel plane moments.

So it will be "local direction of the cut" direction and Mxx

Re: Am I doing this correctly? Forces and reactions in columns.

Anonymous — Fri, 03 Mar 2017 05:47:56 GMT

Rafael,

Thanks.

I figured out of the plane would be xx. Regardless, when using xx, I do not seem to get the correct moment values and reduced moments and integral value is not the same. And I don't quite understand why?

Thanks in advance.

EDIT: Since I had other questions, I also made a similar post yesterday.

Re: Am I doing this correctly? Forces and reactions in columns.

Rafacascudo — Fri, 03 Mar 2017 12:21:32 GMT

But it is correct!!!!!

You should be looking at the Reduced moment about the X axis (global coordinates) . The value shown is zero which is correct for the loads applied . And it agrees with the integral value of the cut which is also ZERO.

The large value 958,46 is for the moment about Y axis(global coordinates) ,which is the in-plane moment at the panel base.