ETABS Long Term Slab Deflection Load Combinations

[joberempt]

Hi everyone, I'm having some issues with an ETABS model. My load combinations are not behaving the way I would think they would. I have load patterns set up for long term deflection checks, and then combinations that add them together. However, the load combinations are not adding correctly.

I am following what is described here, as SAFE and ETABS Ultimate address floor cracking in a similar manner. I am including all slabs in floor cracking analysis. I have these load cases giving the shown deflections:

Immediate - All Loads = -1.152"
Immediate - Sustained = -0.695"
Long Term - Sustained = -1.678"
Long Term - Sustained (Dead Only) = -1.384"

And then my load combinations are as shown:

However, when I run these combinations, they both give me the same deflection value, which doesn't make sense:

Deflection = -2.818" minimum for both combinations.

Why would ETABS be giving the same deflections for a D+L combination and a L only combination? And why do the case deflections not linearly add up to the final combination deflection? If you've encountered this before or have any guidance, it would be much appreciated. Thanks!

 

[milkshakelake]

Might be something wrong in how you set up the cases? Long term and cracking stuff is handled by load cases, not load patterns. Also, it simply won't run if you don't have the license to run it in ETABS. Try a test in SAFE and see if it works; then it's a license issue. Also, CSI recommends 25% live load but I don't see 0.25 factor anywhere, unless that's in the cases. You could also be adding up combinations and cases that have the same name by accident; make sure they're all named differently so you don't pick the wrong one.

 

[joberempt]

Thanks for the response! I do have the load cases set up, thought it would be cluttered to add them to the original post. They're shown below if you want to take a look. The 0.25 factor is added there. The cracking is short term for the short term cases, and then long term (with the appropriate values for concrete strength/time/etc.) for the long term cases.

I definitely have the license to run it in ETABS, and quadruple checked all the naming.

Also, since creating this post I created a dummy model with 4 columns and one slab, and the same issue occurs. I checked with a coworker with one of his models, and his behaves the same way. I would really love to get to the bottom of this issue. I have contacted CSI support, so hopefully they will have an answer. If they do I will post it here for anyone with the same question in the future.

 

[joberempt]

Hi all,

This hasn't gained much traction but in case someone out there comes across it and finds this thread, here's CSI's response:

It looks like it's a known issue. Which is a bit concerning, but as long as you add the load combinations separately it should be the same. I'm not sure I trust that completely though.

 

[jayrod12]

I believe, and I say believe because I don't generally do the modelling for slabs at our office, we post process for the deflections. This is probably the reason we do so.