Column Reinforcement From Summary In Etabs

[EngrJunaid]

AOA Respected Seniors !

 

At station location 0.00 In column reinforcement summary the longitudinal steel is very high as compare to other location in the same column. My friend told me to ignore the 0 location reinforcement and provide the rebar in the column according to other station locations.

here is a snapshot of the column summary, so should i ignore the location 0 reinforcement ????

 

THANKS in advance.

[sameersaleem]

kindly do not ignore this loaction as it is default that software check the column at three location. kinldy check the moment and axial load at that location and check that there is no jerk in axial load and moment at that location.

[WR1]

First thing is why etabs is not taking end offsets automatically?

 

end offset should be at the soffit of beam for the column design.

 

if etabs is not taking it automatically you can assign it manually. No need for moment design at 0 location.

[EngrUzair]

Junaid,

 

As sameer has said, by default ETABS calculates reinforcement value at three locations along column height - at location 0.00 (start of column at floor level), at mid-height , and at top face of column (i.e., at base level or soffit of upper floor beam). According to my understanding, column reinforcement is designed for the largest of load effects at these locations. 

 

IMO, you should check your model carefully.  If you have applied 'frame end offsets' properly, and these are actually being applied by the software to the model (as may be verified from the moment diagrams for the load combinations involving earthquake loading for concrete frame structures, and wind load for steel frame structures), your reinforcement location values should be as given in above (with column height taken upto soffit of upper level beam). If OK, you SHOULD NOT ignore the reinforcement value at column location 0.00 in such a case. It will, in most of the cases, be the controlling value for the column.

 

If in doubt, check the column reinforcement design manually for the suspected locations, using applicable load combinations, to see whether the reinforcement provided by the software is OK or not. You should ignore the program provided reinforcement, only when you are sure that there is really some error in program output.

 

 

Regards.

[WR1]

Uzair but if the end offsets are correct (either put manually or automatically at the soffit of slab) then there should be no reason to design column at 0.0 location.

[EngrJunaid]

AOA !

Thanks for your kind responses.

I rechecked my Model, the end offset length have already AUTOMATIC FROM CONNECTIVITY option ON (screenshot is attached here).

I am uploading my model. I will really appreciate your time for checking my mistakes in the model in you convenient time.

 

 

3 FATA.EDB

[sameersaleem]

Dear Rana there is no end length off set issue here at location 0. or you are not familiar with ETABS. ETABS always calculate the reinforcement at 0 loaction. and as far as end lenght off set is concern for this then it will be below slab not above slab.

[EngrUzair]

@Rana,

 

My point of view, is as under:

 

1.   When we select & adopt a structural software, we believe that it works correctly at least for basic design-related calculations;  both the structural analysis i.e., computation of member forces and moments etc, and proportioning of members i.e., sizing of members and determination of reinforcement. 

 

2.   Now, if we believe that the software is performing correctly for the analysis part (which is not only most important and a harder calculation), will it be logical to say that design results of software (Which are a lot simpler and easier to calculate) are incorrect, WITHOUT verifying it manually?  In simple words, if analysis results of ETABS (or any other structural software) are acceptable to me, I will not object to its design output merely because I 'feel' that it is giving excessive reinforcement or erroneous results, UNLESS I check them manually and is prove them wrong.

 

3.   In the present case, if we are sure that all other calculations done by ETABS (including all the analysis and other design output) are OK, I see no reason for believing that the software is giving incorrect reinforcement (both in display as well as design summary) for the columns - the most critical members of the structure. 

 

4.   As such, IMHO column reinforcement (or other design results), displayed in graphic output or shown in design tables, SHOULD NOT be ignored ordinarily, without any soild reason.

 

 

Regards.

[UmarMakhzumi]

From an analysis point the 0 location shall be significant as the joint can be treated as rigid. I am not in to softwares so can't comment what SAP or ETABS would do, but if I am asked to do an analysis, I would make the above assumption and do the job.  Also, when in doubt please be conservative. I guess there is no right or wrong here. Engineering is all about judgement and this situation is one good way to exercise it.  

 

Thanks.

[WR1]

1. ETABS will never design a column or beam inside end-offsets (NEVER!) You can do joint design manually by taking moment inside the joint but I am talking here about the column design specifically.

 

2. ETABS will design a column at 0 location in some cases. For example at foundation level. There is no framing so end-offset will be zero and design will be reported at 0.0

 

3. It is important to check the local axis 1-1 direction (red) in column to see which one is I end and which one is J end.

 

 

As EngrUzair said,

we should be sure if ETABS or any other software taking the input and output correctly or not.

 

 

And Junaid, If you right click on C15 in your model, you will find out that end offset i = 0 (at floor level) and end offset j = 36inch (at soffit of beam). This is what sameersaleem was suggesting actually. 

 

so basically what ETABS is doing in your case is correct. It is reporting values at 0.00 that is your slab level.

 

If you want to go a step ahead you can change the i end offset value = half of slab thickness because it is center to center modelling.

[sameersaleem]

In my openion we should not change any end length off set to pass some member or reduce the steel %age. suppose if  we agree with rana that we can provide end length off set as slab half thickness then what for all other columns that the software repoting at 0 loaction and further thing is that if you can go to elevation 3 and exturude it shows that slab will be inside the beam by default and beam is exdtuded bleow column so how we can not provide end length off set as slab half thickness.

[WR1]

Sameer yes you are right for slab extrusion below beam. In this case you will not apply end offset as = half slab thickness. But I was talking about a general concept. Anyway, sometimes it is necessary to change end offsets manually. But for most of the part in ETABS end offsets are okay. For example when a beam is rested on another beam end-offset is not properly calculated. I am not sure if it is the same in ETABS 2013 or not.

 

I was talking in general for end-offsets. As I thought Junaid was talking about j end. But in this case Junaid, you dont need to change anything. You have to provide reinforcement at 0 location at i end.

 

Thanks and it was a nice discussion btw. Hope this clarifies your concern Junaid?

[EngrJunaid]

@sameer, Uzair, Rana & Umar...... THAAANK you very much for such a nice and fruitful discussion.

 

But Sir rana and sameer i am confuse that why it is giving 16 in2 (about 4.5%) reinforcement on the C17 & C18 (elevation 4) due to combo 3 and 4.

Is there any mistake in my model because for a 4 story structure its very heavy reinforcemnt...which i think is not logical...

[WR1]

Without checking your model I would suggest that you check on which combination you are getting 4.5% reinforcement.

 

Then check the moment diagram on that combination and make sure its okay!

 

You can always make a simple 2d model of a frame to compare the results.

 

It the column is at upper story, you will get more moment than axial force so e is more and so is the reinforcement.

[Muhammad Imran Zafar]

In ETABS frame section properties are assigned to line objects. However, actual structural members have finite cross-sectional dimensions. When two members such as a beam and column are connected at a point, there is some overlap of the cross-sections. In many structures, the dimensions of the members are large, and the length of the overlap can be a significant fraction of the total length of the frame element. ETABS provides the capability of defining end length offsets along the length of frame elements to account for these finite dimensions of structural elements.

• If the Automatic from Connectivity check box is checked, ETABS will automatically calculate the offset length. ETABS bases the end offset length at the end of the beam on the maximum section dimensions of all columns that connect to that end of the beam. This is the default.

  ETABS automatically calculates offset lengths for beam and column-type frame elements. It assumes the offset length for all brace-type frame elements to be zero. (You can define your own non-zero offset lengths for brace elements if necessary.) Also, the dimensions of brace elements that frame into the ends of column and beam elements are not considered when calculating the end offset dimension for a column or a beam.

  • Beams. When ETABS automatically calculates the end offsets along the length of a beam, it bases the end offset length at an end of the beam on the maximum section dimensions of all columns that connect to that end of the beam.

  • Columns. When ETABS automatically calculates the end offsets along the length of a column, it bases the end offset length at an end of the column on the maximum section dimensions of all beams that connect to that end of the column.

  • If the Define Lengths check box is checked, enter values for the length of the offset at the start (End I) and end (End J) of the line object.

                      (From ETABS)

 

SO, In my opinion it is better to define the end length offset manually depending on size of connected beams keeping rigid zone factor as o.

 

Thanks