Failure in Columns

[saadpervez1]

It is understood that there are different failure modes for columns depending upon the relative magnitudes of the applied bending moment and axial load. I am listing them down:

1) Large Axial load, negligible bending moment: failure by crushing of concrete, rft yields in compression

2) Large axial load, small moment, section in compression: failure by crushing of concrete with rft in compression

3) Moment larger than (2) so that tension develops on one side: rft on one side in compression, the other side in tension, concrete crushes on compression side.

4) Balanced Loading condition: self explanatory

5) Large Moment, small axial load: rft yields on the tensile side of the column

6) Large moment, negligible load: pure bending failure, like a beam.


My main concern is with condition no. 6. If we want to analyze an existing column, and we have the forces acting on it. How can we determine whether a pure bending failure is now possible, what are the numerical values, is it only dependant upon the experience of the designer to determine the failure mode or is there a mathematical way to approach it, a ratio or a range of values that governs this. So far my research has yielded nothing.

 

[IDS]

You don't need to select a failure mode before checking the column capacity. Just find the section design load capacity for design load eccentricity and compare with the design ultimate loads.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[saadpervez1]

@IDS

The problem is that it is an existing planted column and I have to check whether the connection detail is adequate or not (the rebar was drilled 250 mm into the existing slab). The development length of this connection will be determined by the mode of failure.

 

[IDS]

But what is the difference between 5 and 6? They are both controlled by tensile failure of the steel.

Unless there are some specific requirements in the code you are working to I don't see why the procedure for calculating the development length would be different.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JAE]

saadpervez1,
You started out with a general column behavior question and now you are referring to drilling in rebar into an existing slab. Very confusing.
Perhaps take some time to explain what your situation is and perhaps also include a drawing or sketch of the conditions. Might help clarify what you are asking and get better responses.

Check out Eng-Tips Forum's Policies here:
faq731-376