Howe
truss bridge |
. |
The Howe
truss bridge, patented by William Howe in 1840, used diagonal wooden
struts to |
stabilize the Queenpost design in a pattern that could be repeated for any
length bridge. |
Note
that the red colored diagonal struts in the following images point away
from the mid- |
point of
the bridge's span. |
. |
 |
Fig. 166 - Howe truss bridge |
 |
. |
◄ 13 = 2 ( 8 ) - 3 |
therefore stable |
(static demonstration models) |
click image to enlarge |
|
. |
This is
done so that when the bridge is loaded these struts experience compressive
stresses |
induced
by the force of the load and the shear forces it generates between the top
and |
bottom
chords of the bridge. As demonstrated in Fig. 167 a) below, the diagonal rubber |
bands
highlighted in red collapse when a vertical load is applied to an
idealized model of |
. |
 |
|
click image to enlarge |
a) Compressive loading of truss diagonals |
b) Shearing of top and bottom chords |
. |
Fig. 167 - Load induced stresses in the Howe truss
(training aid model) |
. |
|
the Howe truss. This indicates they are reacting to compressive
stresses. The shearing |
action caused by the top chord, which is in compression, attempting to
slide past the |
bottom chord
which is in tension,
is detailed in b). |
. |
In the
early nineteenth century, when wood was plentiful and iron was relatively
expensive, |
the Howe
truss design was popular because iron was used only for the vertical
struts which |
are
subjected to tensile stresses mainly. Heavy wooden timbers were used
for the diagonal |
struts.
However, because most of the dead load on a bridge is concentrated towards
the |
mid-span,
as the span increases the weight of the heavy wooden beams becomes more |
costly
to support due to the large bending moment exerted by their weight.
As iron became |
less
expensive and more readily available, truss bridge designs that made
greater use of its |
unique
properties became more popular. |
. |
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to Knowhere |
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Page 105
- Building stability - Howe truss bridge |
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