Pratt
truss bridge 
. 
One
widely used design was the Pratt truss, created by Thomas and Caleb Pratt
in 1844. 
In this
design the diagonal struts point toward the midpoint of the bridge, in the
opposite 
direction of the Howe truss. 
. 

Fig. 168  Pratt truss bridge 

. 
◄ 13 = 2 ( 8 )  3 
therefore stable 
(static demonstration models) 
click image to enlarge 

. 
As shown
below this arrangement subjects these struts to tensile stresses (since
the blue 
highlighted rubber bands are stretched). This permits the diagonal
struts to be made 
thinner
and lighter from iron or steel resulting in a more efficient structure. 
. 


Fig. 169  Load induced 
stresses in the Pratt truss 
(training aid model) 
click image to enlarge 


. 
Although
Pratt trusses could be built with wooden upper chords, vertical posts, and
end 
posts,
which are subjected to compressive stresses, they were mainly alliron or
allsteel 
structures. One advantage of an all metal truss bridge design is
that the internal forces 
its
structural members are exposed to can be statically determined because the
struts can 
be
pinned together to form flexible joints. This allowed engineers of
the day to more 
reliably
predict how their designs would perform under load than earlier designs,
which 
had
fixed joints and therefore were indeterminate. 

Recall that when a
structure is in equilibrium it is stable and will not move or deform 
significantly when
subjected to an outside force as long as the sum of the forces acting on 
and in it are equal to
zero. This requires that the sum of all of the stressing and
reacting 
forces acting on each of
the joints of the structure are equal to zero. Thus three basic 
structural conditions must
be present for a static analysis to be done on a truss design: 
1) the structure must be inherently stable, that is, its stability
is due to the triangular 
arrangement of its members; 
2) the joints must be flexible; and 
3) the structure must be in a state of equilibrium and the sum of
all forces must be zero. 
This
allows the external load to be dissipated throughout the structure so that
the internal 
stresses
and reactionary forces within each member are aligned parallel with the
member's 
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Page 106
 Building stability  Pratt truss bridge 

