Why does column buckling occur when the load is parallel to the column?

Question

I'm studying Euler's work on structural engineering from a book out of curiosity and it is mentioned that he developed a mathematical theory describing the buckling of columns under a parallel load (the weight-force of the load is directed down along the column). The theory is covered quickly without much motivation.

But this got me thinking; why does a column "buckle" in the first place? If the load presses the column down, why does the column even start deflecting sideways? I know this happens in real life since this fact is easily confirmeable with household objects, but theoretically, why do objects start deflecting sideways instead of just compressing under loads? This might be something obvious and maybe I'm just overthinking but I find this curious nonetheless.

Answer 1

Euler buckling occurs because the world isn't perfect. So that theory assumes that there is an initial infinitesimal deviation along the column (assuming the column is in fact not perfectly vertical*). This deviation causes a bending moment along the beam, which increases the deviation, which increases the bending moment, which increases the deviation...

For loads lower than the Euler load, this vicious cycle eventually stabilizes and the beam doesn't buckle. For the Euler load and above, the cycle never stabilizes and the deflection goes to infinity.

Obviously the real world has initial deviations and other problems which are much higher than "infinitesimal". So in the real world, columns buckle with loads far lower than the theoretical Euler load.

_{* This is the assumption for Euler buckling, but another possible deviation is that the load is actually not perfectly centered on the column. In the real world, both cases probably happen simultaneously}

Answer 2

Think about a "thin" beam, for example a strip of springy steel. It is very easy to bend the strip into a curve, compared with stretching or compressing it along its length.

When it is bent into a curve, the length of the strip measured around the curve does not change significantly, and that means the straight-line distance between the two ends becomes smaller.

If you try this experimentally with something you can bend easily with your hands, you will find that a graph of the force against distance between the two ends is not a straight line - the effective stiffness reduces as the load increases and the beam curves more.

On the other hand, the stiffness when compressing the beam along its length without bending it is constant (and equal to $EA/L$, as shown in any strength of materials textbook).

Since it is impossible to make a perfectly straight beam in the real world, the beam will buckle when the end load reaches the point where the stiffness in "bending sideways" becomes less than the stiffness in "perfect compression".

Euler's formula gives a fairly good approximation to that load, though it makes a few more assumptions (for example, about the shape of the beam when it bends sideways) which are not completely accurate. But since the tolerances in the beam geometry are also unknown, Euler's formula is good enough to be useful in practice, even though it usually over-estimates the actual buckling load by a factor of a few times (say between 2 and 5 times) compared with real life.

Because the beam becomes more flexible after it buckles, if you apply a constant end load (e.g. the weight of something pressing on the end of the column) the buckling results in catastrophic failure, as the beam curves more and more until it breaks. On the other hand, if you apply a controlled displacement to the end, the process is reversible and when the load is removed the beam will return to its (nominally) straight shape, with no permanent damage.

Answer 3

Not all the columns fail under compression by buckling. In steel columns shorter than slenderness ration of 50 they fail by direct compression.

It is the principal of stability bifurcation and it appears not only in columns but in failure mode of many other shapes, such as beams, trusses, vessels, and the buckling pattern could be quite complex. Fro example if you cut the cap and bottom of a can of coke and put it under a micro control press, it will buckle along diamond pattern on its wall, twisting around the vertical axis.

In columns it happens because of the elastic behavior of the material leading to bifurcation, be it steel or aluminum, wood, etc.

It is not due to residual imperfection in manufacturing of the column, neither due to load not applied at perfect centre, although those conditions will affect the reaction of column but that belongs to another topic.

As you increase the load applied to the column compression stress develops on the area of the cross section. This stress is applied evenly over the surface of the section, $$ \sigma = P/A$$ But this stress is constantly seeking out ways to force the column to curve so as to release the stress by creating small variation in intensity distribution over the surface area while the total stress is constant, hence creating a lateral momentum, but up to buckling force this virtual stress is just not enough to force the buckling. When the load reaches the buckling level the column will fail by randomly bending on either of two sides which have bigger slenderness ratio.

Answer 4

Another way to look at is that Euler buckling is sometimes referred to as Euler instability. Basically its a stability problem.

Consider the following:

Take a bottle. Start tipping it. Up to a point the forces and moments are such that the bottle returns to its original position.

Should you tip it over a certain point the forces and moments are such that it accelerates away from the vertical position.

Euler Buckling is essentially the same.

PS: even if you consider tension the above holds true. Euler buckling never happens during tension (the system is stable). In the example with the bottle, if you hang the bottle from the neck and disturb the base, no matter what the bottle always returns to the resting position.

Answer 5

In my opinion, i think that all previous answers describe well the mechanism of buckling but didn't mention exactly the reason of buckling event. The fact that volume of member before loading will, some how, remain constant after loading, so the vertical strain that occur due to compression load is decreasing the member volume unless there is a horizontal strain occur. Such horizontal strain at critical load produce a horizontal force that cause buckling. Even the member is perfectly vertical and the load is perfectly centred, the buckling of member will occur at the critical load due to horizontal strain

Answer 6

It buckles because its simply easier for it to buckle then to get crushed, take example of a thin guitar string, lets say you try to apply compressive force on it from ends, it will obviously buckle and not compress. This is the most basic nature's law, nature will always take the easiest path to do things, for example a lightning will strike through air which offers it the least resistance to come down. Similarly the load required to buckle a ling column is less than the load required to crush it (in other words a column will offer less resistance to buckling then to crushing) and hence the buckling takes place.

Answer 7

If the load is applied through the centre-line of the column then there is no side force, but if the load is offset, but parallel, then there is a sideforce which leads to buckling.