Axial Shortening Result from Etabs

[KC__]

Hi everyone,

I am facing a problem in extracting total axial shortening results from Etabs. I have plotted the total shortening for one of the vertical members by extracting the Uz value from joint displacement table in Etabs (Graph on the right).

The graph on the left is plotted by a previous consultant. As you may see, my total shortening graph looks more similar to the 'sub to' shortening graph on the left. Anyone could advice on how to get to the 'total' shortening graph which should be increasing all the way up the stories?

 

[centondollar]

One would expect the shortening to decrease linearly along building height if the controlling load is gravity and the analysis is linear static.

From strength of materials, stress = strain * elastic modulus --> strain = F / (E*A) and shortening = FL/EA, i.e., the strain is largest at the base and the gradient of strain is the rate of change of total vertical loads. With a completely symmetric building with identical floor plans and floor loads, the story versus total shortening plot would be linear. Adding lateral loads changes things slightly, of course, since load due to wind and earthquakes are non-linear functions of height.

Asking the consultant what "up to" and "sub to" refer to might be a first step to answer your specific question.

 

[milkshakelake]

I agree that knowing what "sub to" and "up to" would help. Without that context, the graph doesn't make sense. If you take the joint displacement, it should always be increasing as you go up. There should never be a case where the total displacement has an inflection point and starts decreasing.

In terms of ETABS, auto construction sequence could be used. I'd first check it with D+L load combinations to see if it's on the right path.

Edit: Removed my math, units weren't working out so it's not relevant

 

[chris3eb]

I'm guessing that the "total" is how much the overall column has shortened (ie (Uz_L30_Loaded - Uz_B5_Loaded) / (Uz_L30_Unloaded - Uz_B5_Unloaded)) and the "sub to" is how much a single story has shortened (ie (Uz_L30_Loaded - Uz_L29_Loaded) / (Uz_L30_Unloaded - Uz_L29_Unloaded)). And then "up to" is "total" - "sub to", but I don't exactly understand why that one is useful

 

[milkshakelake]

@chris3eb That can't be it because the units are mm, and yours is a unitless strain type measurement.

 

[chris3eb]

@milshakelake, you're right. I meant to put a minus sign between the terms rather than division so:
(Uz_L30_Loaded - Uz_B5_Loaded) - (Uz_L30_Unloaded - Uz_B5_Unloaded)
(Uz_L30_Loaded - Uz_L29_Loaded) - (Uz_L30_Unloaded - Uz_L29_Unloaded)

Or in other words:
Length_B5toL30_Loaded - Length_B5toL30_Unloaded
Length_L29toL30_Loaded - Length_L29toL30_Unloaded