The National System of Interstate and Defense Highways
The Federal-Aid Highway Act of 1956 provided for construction of the 41,000-mile Interstate System with an accompanying source of funding. This System, which was to be a modern, safe, limited access, divided highway, set highway and bridge design engineers to work developing ideas for facilities which would be adequate for estimated future needs. The tremendous number of structures required to maintain access control encouraged continuous study of structure types and construction methods. Coincident with this was the dawn of the computer age. The capability of these electronic machines to perform lengthy mathematical operations permitted more refined analysis of increasingly complex structures in less time. Just as moment distribution gave impetus to continuous frame analysis, the computer made indeterminate analysis commonplace. The elimination of the computational drudgery brought a return to more basic and theoretical solutions and away from the approximation methods of the second quarter of the century. The computer also created new approaches, such as the finite element method.
High strength steels were used for the welded arch truss members of the Sherman-Minton Bridge over the Ohio River on I-64 between Louisville, Ky., and New Albany, Ind.
The numerous large and complex grade separation structures and extensive urban viaducts led to many bridge developments in designing for improved architectural treatments and economy. Deck structures were generally used with girders curved to follow the alinement of the roadway. Box girder design tech-
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