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

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


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Page 82 - Building stability - Forces and reactions

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