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104 CONDITION OF BRIDGES

Design loading.—On the 29,450 miles of rural roads approximating the location of routes of the recommended interregional system, there are 8,435 bridges. Seven hundred and twenty of these bridges are designed for loading inferior to the H15 standard loading of the American Association of State Highway Officials. The greatest number, 7,040, are designed for loadings at least equivalent to the H15 standard loading but inferior to the H20 standard loading. Only 675 are designed for the H20 standard loading, and of these 151, the spans of which are 26 feet or less in length, may be considered as adequate for the support of the H20-S16 standard loading recommended for bridges to be constructed on the interregional system. This classification, however, is based upon the design of the existing structures and takes no account of deficiencies resulting from depreciation. Although the available data do not permit a definite statement, it is probable that a substantial number of the bridges originally designed for H15 loading, by reason of deterioration, are not now safe for the support of vehicular loads contemplated by that standard.

Horizontal clearance.—In table 18 the numbers of existing bridges having horizontal clearances of various dimensions are given in relation to the surfaced widths of the roads on which the bridges are located. This table shows that of the 8,435 existing bridges, 1,261 or nearly 15 percent have horizontal clearance of less than 20 feet, and more than half of these are located on roads surfaced 20 feet or more in width.

Of the widest bridges—those with horizontal clearance of 60 feet and more—there are 110; and 33, or 30 percent, are located on roads which in their existing state were surfaced with only two lanes of more or less adequate width.