The nature of the obstacle being crossed will drive most superstructure design
decisions with the ultimate goal to achieve the least overall lifecycle cost for the
structure. For short (height) structures, trestle construction is favored due to the
economies of pile bents. Conversely, taller structures over good footing are likely to be
viaducts with longer spans supported by towers. Where there is insufficient clearance
over navigable waterways, moveable spans may be necessary. The addition of longer
or moveable spans to clear main channels does not significantly affect the design of the
balance of the structure. However, as the structure becomes taller, the economies of
pile bents are diminished due to the need to strengthen the relatively slender
components.