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 Aframe 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. 
. 
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Page 110
 Building stability  Comparing truss bridge models 

