Horizontal navigation clearances have increased in recent years to accommodate the increasing size and volume of marine traffic. The intense competition among port cities to attract ocean shipping has led to replacement of existing older bridges with those providing wider and taller navigation clearances. However, there are a number of other reasons for increased spans over that required for strictly navigation clearance requirements. One of the most obvious is the economic trade off of shorter spans requiring deep water foundations, as opposed to longer spans requiring shallow water foundations or foundations completely out of the water and on land.
Another reason is the concern for ship collision with piers. Besides the safety issue and potential loss of life resulting from a ship collision, there are a number of associated economic impacts: the closing or impairment of port and highway traffic resulting from a collapsed superstructure in the navigation channel, the repair or replacement of the bridge, the damage or loss of the vessel, and the potential for a hazardous materials spill from the damaged vessel. A risk analysis will generally reveal that consideration of a span longer than strictly required by navigation channel requirements is well warranted.
An emerging concern is where hazardous materials have settled to the bottom of the river or bay. Foundation excavation under this condition requires costly containment of this material and its relocation. This then requires minimization or elimination of piers in the waterway leading to longer spans. For the reasons given above, there appears to be a trend toward longer and longer spans.