Page:America's Highways 1776–1976.djvu/439

 the concentrated truck loading for short span designs and the conventionalized lane loading for longer span designs.

This continuous steel truss bridge over the Allegheny River on I-80 in Pennsylvania was completed in 1968. The 68-foot wide concrete deck easily accommodates four lanes of traffic and a concrete median barrier.

The design formulas for concrete floor slabs were revised in the 1941 and the 1961 editions of the AASHO Bridge Specifications to conform to the results of slab tests.

New revised editions of the specifications continued to be developed and issued at 4- or 5-year intervals up to the present day. The revisions keep the specifications up-to-date on new design concepts, safety policies, use and development of materials and construction practices. The AASHO Bridge Specifications have become a valuable addition to the bridge building profession, serving as guides to State highway departments, cities, counties and foreign countries in the design of adequate but economical structures of all types.

A new design concept evolved during the 1930’s—that bridge alinement should conform to the overall alinement of the highway. Previously, bridges, which cost much more per unit of length than roadways, were generally located for minimum cost reasons, with little regard to approach road curvature. The new concept has resulted in much safer approach alinements.

The Federal-Aid Highway Act of 1944 authorized the National System of Interstate Highways. Initial standards for construction of the Interstate System were developed jointly by Public Roads and AASHO in 1945. The high standards for the Interstate System to meet present and future traffic needs also necessitated changes in the character of highway bridges. Standards for traffic capacity, load capacity, safety and appearance of structures were given careful study in order to provide for the safe and free movement of vehicles over and through bridges.

The field of the bridge engineer was broadened because of the number of large bridges required over waterways and because of the greatly increased number of grade separations.

Bridge railings were studied and led to the use of rail curbs and streamlined railings. Full width roadways (surface and shoulders) were advocated for safety on short bridges, and deck type rather than through type bridges were emphasized. Particular attention was given to horizontal clearance at underpasses, with greater clearances to sidewalls and center piers than had been customary.

Design with continuous spans was advocated. In anticipation of the increased use of three-span continuous reinforced concrete structures for urban and Interstate bridges, Public Roads prepared a report, 433