ETABS - Odd load distribution to columns

[FFCrystal13]

Hi everyone,
This is my first time posting on the forums and I have a question regarding my ETABS model. I was trying to analyze my model for load combo “C2DLLL04” and was interested in the axial loads at column D5, and grid D4 (at the edge of the wall) on the ground floor. According to my hand calculations, locations D5 and D4 should have a much higher axial load than what ETABS is outputting. I am assuming that the wall stiffness has something to do with it but I am unable to confirm this reasoning. Is there a reason why the locations above carry a much lower loading than what I am supposed to be seeing like what my hand calculations are supposed to be giving me? Any help is appreciated and suggestions for a fix is very much welcomed!

 

[Celt83]

Don't have the ability to open your model but if you configuration is what I think it is, this is a common problem with FEM models with Columns in or near walls. Columns are generally treated as bar elements where as walls are usually solids, so even when pinned the meshed continuous wall line soaks up a lot of load. The other issue is a lot of models will apply a reaction from the column above to the top surface node of the slab mesh while the bottom column is attached to the bottom slab surface node the stiffness of the slab mesh combined with the wall mesh is often enough to have the column above reaction get redistributed out so you will actually see the column below have less load then the column above it.

In these instances I tend to manually track the column axial forces. You can try placing a section of thick slab area over the column that matches the column dimensions and F'c or higher, the thickness will vary but I've seen half the column height above and below used, the top surface of this slab section should be offset to be up into the column above. This gets you a section of very stiff slab within the column bounds to limit how much load leaks out of the joint.



Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[FFCrystal13]

Thank you for the reply Celt83. I also have a follow up question to your reply:

If that is the case with the load distribution to the column/wall, then how would I know that the load distribution to the shear walls will be accurate when I perform a lateral analysis? Will you be recommending a manual calculation for the shear walls as well and compare it to the ETABS results??

 

[Agent666]

One other aspect that can come into play is the fact that often you reduce the f11 or f22 stiffness of the walls (depending on the orientation of the shell elements) to model cracking and effective stiffness. In a shell element this directly affects the vertical axial stiffness of the element and you get additional axial shortening which plays havoc with your gravity load distribution as your columns have no axial stiffness reduction by comparison.

Often this will mean using some judgement and perhaps another model without the flexural stiffness reductions to the shells to see if the distribution of axial loads better matches results of tributary area/hand load takedowns.

 

[Trenno]

Good point, Agent666.

Compare the stress levels in the wall and the column. I think you'll then see what is working harder and/or stiffer. Not sure how much elastic shortening will come into play - don't know the height of the building.