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