Simply supported beam |
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A beam
that is supported at both ends with moveable joints is said to be simply
supported. |
One end
of the beam can rotate slightly around a support with a hinge joint,
called the |
fixed
node. The other end rests on some sort of roller or bearing surface,
called the rolling |
node. Fig.
141 shows how it is
diagrammed. Although the fixed node allows that end of the |
. |
 |
 |
fixed node
rolling node |
( demonstration model) |
Fig.
141 - Simply supported beam |
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beam to rotate slightly it cannot move vertically or horizontally.
The rolling node allows the |
other end to move horizontally and rotate slightly but it
cannot move vertically. Examples |
of simply supported beams include beam bridges,
truss bridges, gangways, etc. |
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The
following shows an idealized model of a simply supported beam before and
after a |
load has
been applied to it. The load is seen to impart a bending moment to
the beam that |
|
subjects its upper edge to compressive stresses |
 |
that tend to shorten it and subjects its lower edge |
to
tensile stresses that tend to lengthen it. In |
between there is a gradual shift from compressive |
to
tensile stresses. Notice that the purple line |
passing through the middle of the beam, called |
the neutral axis, remains the same length, which |
indicates the absence of these stresses there. The |
shift in stresses from compression to tension |
(training aid) |
causes the upper section of the beam to tend to |
Fig. 142 - Idealized model of
|
slide past the lower section. This induces another |
a simply supported beam |
stress in the beam called shear. This horizontal |
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shear stress is
greatest along the neutral axis. |
|
Solid
beams have very high shear strengths. Therefore shear stress is not
a major factor in |
beam
design except for very tall, thin beams. |
|
The
importance of providing a rolling node for one end of the beam is
demonstrated by |
the
model as that end of the beam moves in and out when the load is applied
and removed. |
If that
end is not allowed to move additional stresses are placed on the beam and
it will |
no
longer behave as a simply supported beam. |
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Back to
Knowhere |
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Page 93 -
Building stability - Simply supported beam |
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