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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Page 114 - Building stability - Suspension bridge

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