◄ Fig. 153 - Deck plate


girder bridge

click image to enlarge

Fig. 154 - Pony plate  

girder bridge ►

the plates are stabilized against these stresses by cross bracing.  In the pony plate design,
where traffic passes between the plate members, bracing is provided by knee braces that
are located where the plates meet the deck.
Truss bridges - typical span 40 to 500 m ( 120 to 1500 ft.)
Just a individual beams can take advantage of the inherent strength and stability afforded
by the triangulated truss design, so too whole bridges can be built from trusses.  Many
different types of truss bridge designs have been built in the past.  The following section
covers some of the more regular ones including the Kingpost, Queenpost, Howe, Pratt, and
Warren designs.
Kingpost truss bridge
An early solution to spanning longer distances with solid beam bridges was to support the
mid-points of the main beams located on either side of the bridge.  This is done by building
a triangle on each side of the bridge whose legs are joined to the ends of these outside
beams.  A vertical tie strut extends from the apex of the triangle to the mid-point of the
beam.  This design significantly reduces the bending moment experienced by the beam




(static demonstration model)


a) details of truss

b) stresses and reactions

(scale visualization model)


Fig. 155 - Kingpost truss bridge    click image to enlarge

since the strut bears some of the load.  As shown in Fig. 155 b), the tensile stresses induced
in the struts by the load on the bridge's deck are distributed to the angled beam legs of the
A-frame as compressive stresses, which are passed on to the bridge's support abutments.
As with all triangular structures loaded at their apex, the compressive stresses concentrated
at the base of the legs try to pull the legs apart.  The internal tensile forces of the base tie

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Page 100 - Building stability - Kingpost truss bridge

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