Why is a neutral axis necessary in beam theory?

[chandran]

in beam theory it is mentioned that there should be a neutral axis which remains
constant in length before and after bending. Why should there be a neutral axis
like this?

 

[Astronuc]

In a beam, the neutral axis is the transition between compressive and tensile forces.

Push down on a beam and the bottom of the beam (e.g. flange) is 'stretched'. The top of the beam is constrained by the material below (the height or cross-sectional thickness), and so the top of the beam is in compression.

Somewhere between the tensile force and the compressive force, continuity requires the force to be zero, or neutral, neither compressive nor tensile.

 

[Copperstar]

Hi,

Does this mean that the neutral axis stays horizontal?

I understand that there must be a fiber somewhere in between that is neither subject to compression nor tension, but I cannot visualize it. I do not visualize a fiber, somewhere in the middle, staying horizontal when the beam is bent.

It might be a dome question, but could someone explain me more about this or suggest any material that I could review to understand this.

Thank you,

 

[Andy Resnick]

No, the neutral surface is constant with respect to the geometry of the beam- imagine there's a grid painted on the beam that deforms with the beam.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/10/Beam_bending.png/280px-Beam_bending.png

Not a dumb qurestion!

 

[jayman16]

if you divide the beam into fibers then the top fiber becomes shorter with compression and the bottom most fiber elongates with tension(this is for the case where the beam bends such that is smiling at you). Neutral axis/surface is a just a fiber that lies between these the top and bottom and does not change in length although it is curved