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Neutral Axis outside the section in case of columns

 
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naman247
SEFI Member
SEFI Member


Joined: 27 Oct 2010
Posts: 15

PostPosted: Wed Dec 07, 2011 1:58 pm    Post subject: Neutral Axis outside the section in case of columns Reply with quote

Hello sefians....

I dont know whether the doubt what I am asking is legitimate or not, But this thing is really making me unconscious.

How can a neutral axis in case of columns can be outside the section?

Add to that our code says that maximum distnce which you can take for the neutral axis being outside the sction is 4D (4 Times the depth of the section).

I really dont know how it is possible.

Can anybody tell me how this is possible?

How can Neutral axis be outside the section in case of columns?

Please if anybody knows kindly reply to it..
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bijay sarkar
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Joined: 14 Dec 2009
Posts: 314

PostPosted: Sun Feb 12, 2012 5:46 am    Post subject: Reply with quote

Dear Naman,

A "Neutral Axis" is a line where there is no stress, neither the compressive nor the tensile. As you can see, this is also expressed in the wordings of "Neutral Axis".

Say, an axial compressive load is acting on a column. In this case, whole column section will be subjected to equal compressive stress throughout the sectional area. In such cases, neutral axis will be formed at INFINITY. This is PURE AXIAL case.

Now think, as a small quantity of moment is applied along with the above axial load, the neutral axis becomes a finite one, but at far away from the section. With gradual increase in the moment quantity, neutral axial comes nearer and nearer to column section. At some value of this moment neutral axis will enter into the section.

From above, we can see that NA can be formed not only at 4D, but also upto infinity.

Now question comes how much deep, a neutral axis of column can enter into the section ?
You may imagine that when a neutral axis enters into the section, neutral axis divides the sectional area into two parts : Tensile part and Compressive part. If neutral axis further and further enters into the section, there will be some point of time when tensile force in the tensile zone becomes equal to the compressive force in the compressive zone. This is the PURE BENDING case and vertical compressive load carrying capacity becomes zero.

Thanking you,

Bijay Sarkar
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