but
their heights remain the same. As a result the relative differences
in their heights |
become
less of a factor determining their strength. Concurrently, the
increased bending |
moment
the models experience due to their increased spans becomes more of a
factor. |
That is,
the longer their spans the more the truss bridge models act like simply
supported |
beams.
Indeed a truss can be imagined as being assembled from many short beams
that |
are cut
from one long beam just as the arch was previously. It is the
triangulated |
arrangement of the beams that enables the truss design to be structurally
efficient. |
|
As with
the A-frame structure, although increasing the H/S ratio of a truss bridge
makes it |
stronger, there is a limit to how tall it can be for maximum efficiency
and economy. A |
basic
measure of a bridge's structural efficiency is the maximum load it can
bear in |
comparison to its own dead weight, its L/W ratio. Common sense
dictates that the L/W |
ratio
decreases as a bridge's span increases since its load bearing capacity is
decreasing |
while
its dead weight is increasing. A graph of the L/W ratio versus span
for the |
Kingpost/Howe,
Pratt, and Warren models bears this out. |
. |
 |
Fig.
177 - Graph of L/W ratio vs. span for the Kingpost/Howe, Pratt, and Warren
models |
. |
In real
bridges a higher L/W ratio means a more efficient use of materials and
greater |
cost
savings. Therefore, in many instances, it is more economical to span
a distance with |
several
shorter, squatter bridges than with one longer, taller bridge unless
there are |
other
overriding factors to consider such as topography or ship passage. |
. |
Back to
Knowhere |
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Page 110
- Building stability - Comparing truss bridge models |
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