Captive Columns

[Fatima Khalid]

Asalamualaikum

It is said that “ There should be no columns with height to depth ratio less than 75% of the nominal height to depth ratio of typical column at that level”

for example I have a building having column clear height 10feet and its cross section is 18”x 18”, next to column there is an opening of 3ft wide by 6 feet high till the soffit of the beam. How to check the above provision? what height to depth ratio is need to be considered. I have studied that depth of the column should be parallel to wall? why it needs to be parallel to the wall? 

Nominal height to depth ratio = 10feet/18inch

short column height to depth ratio (unsupported) = 6feet/3feet

Is the above calculation correct?

Thanks

[Saifuddin18]

Dear Fatima, I am unable to understand your query, can you please elaborate?

[Fatima Khalid]

While doing seismic assesment using FEMA357, there is a provision for captive columns whether they are present in a structure or not? So the documents states that “There should be no column with height to depth ratio less than 75% of the nominal height to depth ratio of column at that level”

If its more than 75%, it means a captive column is present in the structure

Now I am confused what is height to depth ratio of column? and what are we comparing this column with?

[Nustian371]

This is sort of creating a short column in the structure due to architectural features. 

On a certain storey level you will have your story height and most of the columns are of same length but if any column due to any addition to structure is reduced in size it will cause a short captive column which  attract more seismic shear due to stiffness. we try to avoid these elements or detail them adequately. See attached commentary it will explains you much better hopefully.  

[Saifuddin18]

17 hours ago, Fatima Khalid said:

While doing seismic assesment using FEMA357, there is a provision for captive columns whether they are present in a structure or not? So the documents states that “There should be no column with height to depth ratio less than 75% of the nominal height to depth ratio of column at that level”

If its more than 75%, it means a captive column is present in the structure

Now I am confused what is height to depth ratio of column? and what are we comparing this column with?

Got you now. I have not done any practical design or check for Captive Columns but as per my understanding, the height of columns refer to the unsupported length(say L₁) and depth means the dimension of column along the wall(Say D₁). So you do L₁/D₁ for captive column. Now take average depth and lengths of columns at that storey and let them be L₂ and D₂. Now ratio L₂/D₂ for normal column. You compare both these ratios for the given allowable percentage of 75% i.e., {0.75xL₂/D₂}>L₁/D₁.

Hope this answers your question.🙂

[Saifuddin18]

11 hours ago, Nustian371 said:

This is sort of creating a short column in the structure due to architectural features. 

On a certain storey level you will have your story height and most of the columns are of same length but if any column due to any addition to structure is reduced in size it will cause a short captive column which  attract more seismic shear due to stiffness. we try to avoid these elements or detail them adequately. See attached commentary it will explains you much better hopefully.  

Dear Nustian371, I think Fatima is not asking for the theory or concept behind the captive columns and how they behave. Instead she was asking about the explanation of the limit set by FEMA on such columns.

[Saifuddin18]

@Fatima Khalid But one thing to note here is that the unsupported length we are considering for our captive columns is the one supported with walls. In your case L₁=10', D₁=18". L₂=4', D₂=18".

0.75xL₁/D₁>L₂/D_2.

Which will be true in your case. That means that your columns will be considered captive columns only if the wall height exceeds 7.5'.

Would like to here from other group members too about this.

[Fatima Khalid]

@Saifuddin18 are you sure that unsupported length would be 4’ rather than 6’? 

[Saifuddin18]

@Fatima Khalid, as I said, I have not done such checks personally in a practical project so I can't guarantee what I said but as per my understanding, it should be 4'. If you consider your unsupported length as 6', you are not going to get a Captive column for lengths where your wall height increases which doesn't seem correct when you go through the captive column definition. As per my understanding, if your wall height increases, the column should go near the threshold of captive column.

Let me know your take after considering my views.

[Simple Structures]

On 2/22/2020 at 10:32 AM, Fatima Khalid said:

I have studied that depth of the column should be parallel to wall? why it needs to be parallel to the wall? 

1.     "Captive Columns" are in fact start of as unrestrained (long) columns which then become "laterally restrained", by walls coming into the side of them, normally in one direction. They are therefore captive in one direction.

2.     Under seismic loading, the column is subject to lateral - horizontal forces in BOTH direction - so for this directional force this column length & depth (in each direction) needs to be checked.

3.     For direction into the building, where the column is clear full height, no lateral restraint: 10'/1.5'

4.     For direction along the building perimeter frame: 6'/1.5'

5.     It important that structural engineers look at BOTH structural & non-structural architectural walls, beams, coming into the 'side' of the column, and thus making the column captive - and design accordingly.

6.     One way to overcome this is to have the architectural wall run past the face of the column on the outside. This will mean cantilevering the slab sufficiently to have the wall sitting on slab. Wall detailing and tying to column detail would be be considered carefully in this case.

7.     This Captive or Short column always attract much higher proportion of lateral earthquake loading due to their modified slenderness/stiffness, and will therefore need to be designed AND detailed accordingly to avoid its failure.