Axial Shortening - Multi Storey building

[Seb_]

Hi All

in my professional life as an engineer, i have always worked with low storey structures, say max. 4 levels.

I have never been exposed to tall building, however i have always questioned myself how do engineer deal with the axial shortening of tall building.

As an engineer, what needs to be achieved?

thanks in advance!
Seb

 

[milkshakelake]

I don't work with tall buildings myself, but in the 4-14 story range, we proportion the columns so that they have similar axial stress. Same with footings.

 

[ETABSUSER1551]

In my experience, as mentioned by MilkshakeLake, we size the columns based on the stresses they experience so that they are similar in size. This ensures comparable levels of axial shortening and minimal differential shortening. Another consideration is columns located close to cores. Cores typically see much lower axial stress under gravity compared to columns. While we prefer to place columns away from cores, when this is not feasible due to architectural constraints, we can build the columns with a preset (constructing the columns higher so they settle flat at some point in the future). In saying that I would be very careful when doing this, over presetting just like over precambering if you do too much they may never come out!

From an analysis point of view, we do a staged construction analysis to determine the extent of shortening. Inducta has proposed a methodology for this, which you can read about here: Inducta Axial Shortening Methodology."

 

[BAretired]

Axial strain takes place every day when the sun moves around the building and heats some columns but not others.

 

[Seb_]

Thanks everyone for their inputs!
@ETABSUSER1551 even with columns away form the core , wouldn't you expect to have the columns highly stressed compared to the core walls as the core walls have greater contact area? How would this be addressed ?

Thanks again!

 

[Eng16080]

A similar phenomenon was discussed in a webinar that I recently watched. The Tennessee Performing Arts Center experienced problems due to axial elongation where upper level floors were suspended from steel trusses.

 

[milkshakelake]

Thanks everyone for their inputs!
@ETABSUSER1551 even with columns away form the core , wouldn't you expect to have the columns highly stressed compared to the core walls as the core walls have greater contact area? How would this be addressed ?

Thanks again!

I'll be the first to admit that I don't actually do this, because core walls are very long and are subject to much lower axial stress from non-transient loading. It's kind of a spray-and-pray situation. I focus on equalizing the columns. It is simply impossible to do the same for core walls due to much larger area for P/A, and core walls have moment in them anyway, so it becomes super complicated. However, I'm just reiterating that I don't do high rises. I'm sure high rises have some shrinkage compensation mechanisms, like they do for outriggers.

Edit: To show something interesting, this detail below from "Outrigger Design for High-Rise Buildings" (thanks @KootK ) shows a shrinkage compensating device known as "flat jacks" in an outrigger.

 

[ETABSUSER1551]

Thanks everyone for their inputs!
@ETABSUSER1551 even with columns away form the core , wouldn't you expect to have the columns highly stressed compared to the core walls as the core walls have greater contact area? How would this be addressed ?

Thanks again!

If the columns are sufficiently spaced so that the differential settling of the columns and cores results in a difference of say about span/500, then the impact is minimal. In such cases, depending on the specific situation, further analysis may not be necessary.

 

[HDStructural]

In tall buildings, they also will have a table showing how much longer to make the columns to account for their axial shortening, so that each story's elevation is where it should be after all dead loads are applied.

Haven't designed any myself, but took a class in grad school taught by someone who has designed many.