The animation to the right shows how the tensile (blue) and |
 |
compressive (red) stresses are distributed through the struts of.... |
a
fixed cantilevered truss when a load is applied to its free |
end. The load is moving farther away from the root. This |
demonstrates that the members making up the upper chord of |
the truss experience tensile stresses, the bottom chord struts |
click image to enlarge |
experience compressive stresses, and
the stresses induced in |
Fig. 187 - Stresses induced |
the diagonal members depends on
whether they point toward |
in a cantilever truss |
or away from the support. |
(static demonstration model) |
|
. |
The most
famous centrally supported cantilever bridge is the Forth Railway Bridge
over the |
Firth of
Forth near Queensbury, Scotland, which was completed in 1890. The
picture on the |
. |
 |
|
 |
|
Fig. 188 - Forth |
railway bridge |
|
click image to enlarge |
|
. |
right
shows a demonstration of the cantilever principle put on by Sir Benjamin
Baker, the |
designer, to convince the public of the viability of the concept.
You can see that the men |
seated
to the right and left, who represent the center supports, are
experiencing tensile |
stresses
in their arms, which represent the upper chords of the structure.
The wooden poles, |
representing the lower chords, are being subjected to compressive
stresses. The man in the |
middle
represents the load of the center of the span, which is being
counterbalanced by the |
block
weights, which represent the resistance of the other end of the cantilever. |
. |
Suspension bridge - typical
span 70 to >100 m ( 210 to > 3000 ft. ) |
. |
Some of the longest bridge spans in the world are |
 |
achieved by suspending the deck from steel cables |
that stretch between massive towers. The ends of |
these cables are anchored in the abutments at both |
ends of the bridge. Smaller vertical cables extend |
down from the main suspension cables to the deck, |
which may be a
truss or girder. The deck must be |
sufficiently
flexible so that each hanger cable shares |
click image to enlarge |
|
some of
the load, yet sufficiently rigid to resist wind induced twisting, or
torsion loads. Thus |
box
girders are often used. Although modern in the types of materials
and techniques used, |
the
suspension bridge is one of the oldest types of man-made bridges, dating
back to when |
it was fashioned out of
vines and ropes, and much later out of iron chains. |
. |
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to Knowhere |
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Page 114
- Building stability - Suspension bridge |
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