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.


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Page 110 - Building stability - Comparing truss bridge models

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