Axial Shortening in Concrete Columns 1

[Lion06]

I'm trying to determine the axial shortening in concrete columns for a tall building, and I'm really hitting a wall. I have a very good paper by Ghosh that deals with creep and shrinkage at different ages of concrete, but I haven't been able to find an example of a calculation that goes through a full height column stick from foundation to roof.

The variables are so many. If you have a 60-story column and identical load and column height at each floor, and let's say we're just going to look at the shortening of the lowest stick of one column. You still will get different 60 different contributions to the bottom stick of one column, because f'c is constantly changing which means Ec is always changing, and the shrinkage factors are always changing.

It seems like an incredibly daunting task to calculate, even with a spreadsheet.

Does anyone have any literature that has an example of calc'ing this out?

 

[csd72]

In this type of problem you really need to look at the reason why you are trying to determine the axial shortening and work from there.

If it is just purely to maintain building alignment then you can probably discount wind and live loads.

If it is for wind sway then you can probably discount dead and live loads in the calculation e.t.c.

Also, why are you looking at combinations, the combinations are usually for strength only and not generally for serviceability.

These are all questions that you should be asking the engineer of record.

 

[Lion06]

We are the engineer of record, and I'm only looking at gravity loads. I didn't mention anything about combinations.

The issue is that if 10 stories have the same load, they will all result in a different creep component, because the E and creep components vary with the age of the concrete. So on day 7 when the first floor is cast the concrete has different E and creep and shrinkage characteristics than on day 14 when the second floor is cast, which is different than on day 21 when the third floor is cast.

 

[csd72]

Sorry, I read contributions as combinations.

I have seen a few articles that outlined the method but none that show it in detail.

I dont think there is any simple shortcut, it has to be done the hard way, no-one said that highrise was easy! Welcome to spending days on a task that takes minutes on a smaller building.

 

[JAE]

Lion06 - see attached pdf - these are papers I gleened from the ACI website:

 

[Lion06]

JAE - Thanks! That Fintel and Khan paper was referenced in a Ghosh paper and a later paper. I'm going to check that one out right now.