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 all-iron or
all-steel |
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 |
Back
to Knowhere |
 |
Page 106
- Building stability - Pratt truss bridge |
 |
|