You should understand from these demonstrations that the geometry of the stable, hinged

triangle structure allows several essential things to happen.  The external forces and

internal reactions are intimately linked.  That is, increasing the external forces directly

increases the internal reactions. Also the internal compressive and tensile stresses induced

by the external load and the internal compressive and tensile reactive forces exerted by

the structure's material are in direct opposition to each other.  As a consequence these

internal induced and reactive forces are aligned axially, that is, parallel to each other along

the longitudinal axis of the members.  It is these properties that cause the hinged triangle

structure to be stable since they enable the external and internal forces to be balanced.

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Now that you have a basic understanding of what makes structures stable and how external

loads cause structural members to experience compressive and tensile stresses; let's see

how these factors affect the design and performance of real structures like columns, beams,

bridges, buildings, spaceframes, and towers.

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Columns and beams of some form are the fundamental building blocks of most man-made

structures.  They are complimentary structural elements that function together to enclose

space or support a framework.  Columns (colored green above) are upright members whose

function is to support an elevated framework and transfer its loads to the ground.  They are

subjected to compressive stresses mainly since they are being squeezed between the load

bearing down on the top of the column and the ground pushing back on the base of the

column. Beams (colored yellow above) are horizontal members that span between columns

or walls.  Their function is to collect the load of the elevated framework of which they are a

part and transfer it to the tops of the columns or walls.  They are subjected to bending

induced stresses of tension and compression mainly since they are supported on their ends

by columns or walls and loaded in the middle by the weight of the elevated framework.

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 Page 84 - Building stability - Columns and beams