called
stresses, develop within the panel due to the external forces that are
imposed on it. |
Every
atom in the panel is subjected to a compressive stress that tries to
squish the atoms |
together
and shorten the length of the panel. In response repulsive forces
between the |
atoms
react against their being compressed together and they push back.
The strength |
with
which the atoms of a material resist being compressed is called the
compressive |
strength
of the material. In the forgoing example the compressive force
exerted by the |
panel's
material (upward pointing red arrow) is equal to the compressive stress
(downward |
pointing
red arrow) induced in it by the external load so it remains
stable and rigid. |
|
Now take a rubber band and stretch it between your fingers.
The |
 |
external force your fingers apply to the ends of the rubber band (orange |
arrows) subjects its atoms to another kind of internal stress called |
tension. Tension forces are stresses that try to pull the
atoms in the |
material apart and stretch its length. Attractive forces
between the |
atoms react against their being pulled apart (blue arrows).
The strength |
with which they resist being stretched
apart is the tensile strength of the |
material. |
Fig. 127 - Stretching a rubber band induces tensile stresses ► |
|
|
All
materials possess compressive strength and tensile strength and they are
sometimes |
not
equal. For example concrete has high compressive strength but weak
tensile strength. |
Rubber
has weak compressive strength but moderate tensile strength. Wood
has moderate |
compressive and tensile strength. And steel has high compressive and
tensile strength. |
|
Structures differ in the way they react to forces of tension and
compression depending on |
their
geometry. In the previous demonstration you saw that the triangle is
inherently |
stable.
Let's analyze why this is so. Construct a triangle structure whose
legs are two |
small
square (SS) Polymorf panels that are pinged together at one edge and whose
base is |
a rubber
band (loop the ends of the rubber band around pinges attached to the free
ends |
of the
panels). Now press down on the apex of the triangle. You
notice a small reaction |
|
force pushing back on your finger that increases as the |
 |
rubber band stretches and the legs of the triangle spread |
apart. The reaction force that you feel is the tensile force |
(blue arrows) exerted by the rubber band material. It is
|
resisting being stretched by the tensile stresses (green
|
arrows) being induced in it when you push down on the |
triangle causing the legs to spread apart. The compressive |
forces (upward red arrows) exerted by the atoms of the
|
|
panel equals the compressive stresses (downward red
|
Fig. 128 - Forces and |
arrows) induced in the material by the downward load. And |
reactions of a triangle |
the upward reaction forces (upward purple arrows) exerted |
|
. |
Back to
Knowhere |
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Page 82 -
Building stability - Forces and reactions |
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