What is so intriguing about Mono-pole Traffic Signboard? – Bi-Axial Bending…

When it comes to
structural behaviour, the technical terms might sound a bit 
but actual behaviour is 

I am moving forward
with the assumption that you know the basic concept of bending.

“Biaxial bending of a
member is nothing but the bending of a member in both the axis simultaneously
because of the applied load.”

In general, it is same
as a beam or column bending which takes place because of the applied transverse
load. The only difference is that it occurs in both the principal axis, hence
the name “Biaxial Bending”.

This can be easily
seen in columns. Especially in the monopole structures. (Monopole is a
structure which is composed of only one column. For example, traffic signboard,
transmission towers, street lights, etc.)

For our easy
understanding, let us discuss this particular behaviour with an example.


The above image shows
a traffic signboard with a monopole column. Moving little deeper into the
behaviour of the column, lets first fix the principal axis.

For the general
convention of the axis, let’s take the fog line on the road (continuous
white line on the side of the road)
 as global Z-axis, and the one
perpendicular to it is the global X-axis.

Now, let us see how
bending would arise in both these directions.


Firstly, bending about
global Z-axis would occur because of the cantilever beam which is rigidly
connected to the column end. This cantilever beam holds the signboard, that
means, those dead load of the signboard will act on the cantilever beam which
in turn gets transferred to the column top joint as “vertical load” and “equivalent
 along Z-axis. This moment induces a bending of the column
about that axis (Z-axis).


Moving to the X-axis,
as you can visually see, the surface area of the signboard which is likely to
subject to the wind load. Assume that, the heavy wind is blowing, the
widespread signboard is standing against that. What probably would happen?

The wind that hits the
signboard in the direction parallel to Z-axis would result as “horizontal load”
and “moment” along X-axis. This is how the bending about X-axis arises in the

As it is evident that
these two load cases will simultaneously occur in the signboard structure.

What would happen

It will bend in both
the directions (biaxial bending).

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