Analysis problem on ETABS

[faisal45]

Dear All,
i use ETABS to do analysis of RCC building. sometimes i get that rebar percentage of column bottom portion is lower than the top portion of that column. for example today i have got at bottom portion (Ground floor) rebar percentage is 1.2% but in the top portion (10th floor) rebar percentage is 2.28%.

my questions are why these things are happening? and what is the solution to make design okay.

there is no error message in that analysis.

Thank you.

Engr. Faisal

 

[hokie66]

I don't think that is a function of ETABS. The bending in relation to axial load in the upper floors is greater, requiring more reinforcement. You should do some hand calculations to verify the program results, but there have always been some structures where columns above required more reinforcement than columns below.

 

[faisal45]

Dear hokie,
what is not the function of ETABS? its not clear.i took the value of rebar percentage of cross sectional area of column from ETABS and then provide column reinforcement on that basis.
and what is the solution when upper portion of column needed more reinforcement than the lower portion?

Thank you.

Faisal

 

[hokie66]

I meant that the ETABS results are not necessarily incorrect, and you could get the same answer with other software or with hand calculations. I just don't know in the case of your building. But if you need more reinforcement at the top than at the bottom, then use it. That is the solution...if you have verified it. If you are totally dependent on ETABS (or whatever program) to know how your structure works, you should not be designing structures without a mentor.

 

[faisal45]

Well i always do hand calculation for cross check. sometimes i increase cross sectional area of the column and sometimes i provide wall to reduce extra reinforcement at the top.it works most of the time.

Thank you.

Faisal

 

[asixth]

Is Etabs the best program to be using for column design? If built up correctly it can but for the most part I only find it using for lateral analysis and the distribution of lateral forces to the columns. For a building incorporating 10 levels, I think the floors would be meshed too coarse to give meaningful results from framing action and gravity loads.

Too answer your specific question, I reckon it would have to do with moments and effective lengths. The columns will attract a higher percentage of the lateral forces towards the top of the building thru frame action and the core will take over as you run down the building.

Does etabs calculate effective lengths?

 

[bookowski]

I don't recommend taking gravity load distribution from Etabs. The meshing and load distribution is not great. Since it sounds like you're using CSI products I would say that you should be using SAFE for your slabs and to determine or at least verify your column moments and axial loads. Etabs doesn't like fine meshing - and even if you get that working it doesn't account for slab cracking, SAFE is much better suited and you may find pretty significant differences in results.

 

[faisal45]

@bookowski i agree with you. safe is good for slab design. i use it for slab design.
Thank you.

 

[bookowski]

Yes, but you should also be using it to determine your column axial loads and end moments. Etabs is not trustworthy for this unless you get into really refining your mesh and it's not that user friendly for this. I would stay away from designing columns in Etabs unless you spend a lot of time on getting the model right with respect to meshing and load distribution.