Axial Shortening in Walls and Columns 1

[plopez]

Hello, I created a model of a multi-storey building and I find that the results are not the expected of a traditional static analysis for the beams that are supported by columns and walls because the shortening of axial column invests efforts in the M-V diagrams.

This happens because the program apply all the loads instantly and does not consider shortening part of the structure as it is being built.

I used the construction sequence case analysis and the
results do not change much.

What should be the way to model the building so that the beams are not deformed more in supporting column?

Thank you for the help ...

 

[stressed]

Could you please elaborate further regarding what you mean by "traditional static analysis"? Because walls, columns, and beams really do deflect due to gravity loads.

 

[lisica]

He wants to say that columns are shortening more than walls.
When the building is being built, it is compensated, but what about calculation?
I'm student, so please can someone give the answer for this problem?

 

[lisica]

I guess the solution would be to scale an area of columns with property factor. Is it OK?

 

[plopez]

Hi Stressed,I refer to an analysis of gravitational loads without the seismic forces.

Hi lisica,you are right...I'm trying with the F22 factor for walls (F22=10) and the modulus E (E=E/10) and I get some good results.

The problem is when I run the seismic analysis...the factors used in the gravitational change the modal periods.

Maybe the solution is to make 2 analysis (gravitational and seismic.)

What do you think about this idea?

 

[lisica]

I think that just using scale factor for cross-section area of columns would do the job. It should not modify modal perods for lateral directions. It is not necessary to use scale factors for wall properties.
So, my opinion is to increase axial stiffness of columns (not walls). But the question is, how much?

 

[JohnB965]

Hi plopez

I had a similar problem recently. I modeled a multi storey building recently using etabs and then did a column rundown using hand calculations. I found that the columns near the walls were not taking as much load as they should. I think this is because the beams or slabs are trying to span from the wall on one side to the next column beyond .

I then used pinned connections for the beam so it would articulate and got good agreement for vertical loads.

A lot depends on how the structure is meshed so I have learned the hard way not to trust ETABS results 100%. Always check column loads by hand and take the worst result.

 

[derim01]

ETABS calculations are correct. But as engineer you have to understand the nature of problem. It persists in all programs.
Read in the papers about using construction cases in ETABS.