# Explaining Structural Mechanics with simple analogies.

• To: seaint(--nospam--at)seaint.org
• Subject: Explaining Structural Mechanics with simple analogies.
• From: G Vishwanath <gvshwnth(--nospam--at)yahoo.com>
• Date: Sat, 22 Sep 2001 05:53:45 -0700 (PDT)
```Dennis and Syed Faiz Ahmed,

I remember seeing that book too. (Structures for
Architects.) A copy was available in the British
Library at Bangalore. I was able to enjoy this book.

It will be an excellent reference that Dennis can use
at the moment.

While on the subject, I had a teacher who explained
very convincingly and in an amazingly simple fashion
how a folded plate gets strength by virtue of its
shape.

He took two books of equal thickness and placed them
about 6 to 8 inches apart on the table.

Next he placed a plain sheet of paper 8"X11"
supporting one short edge on one book and the other
short edge on the other. He now had a "thin slab"
spanning between the books and  ready to be loaded. He
placed his wrist watch on the top of this slab. The
paper couldn't support the weight.

The teacher than boasted that he could make the paper
support this weight without reducing the span or
adding material to the paper.

He then proceeded to make alternate folds in the sheet
of paper into strips (like a paper fan) and made a
folded plate structure out of this sheet of paper. It
was now stiff enough to support the watch and a pen
and pencil in addition.

I still remember this analogy better than the
differential equations and the mathematical jugglery
he later proceeded to do to teach us formally and in
the orthodox way the theory of plates and shells way
back in the early seventies when I was a post graduate
student of Structural Engineering  at the University
of Roorkee in India.

Another teacher illustrated convincingly how the
moment of intertia is related to bending strength. A
simple 1 foot long plastic scale was all that was
needed. He showed how easily it could be bent about
the weaker axis and how difficult it was to bend it
about the stronger axis.

I also remember the humorous analogy that my teacher
of theory of structures gave us to explain how moments
at a rigid joint in a frame get distributed among the
members meeting at the joint in proportion to their
stiffnesses. If a member was overstressed, the
solution was not always to strengthen the member. This
would merely make the member take more force.

In real life situations, we see this happening. In a
team , the stronger, and more capable and sincere
always shoulder additional burdens. The weak  manage
to escape work by simply continuing to be weak.
Making the strong even stronger merely makes them
shoulder additional burdens. The weak still get away.
Not for nothing do they say that in many organizations
80% of the work is done by 20 percent of the people.

Do others have any similar examples to contribute ?

Regards and have a good weekend.

G Vishwanath
Bangalore, India

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