Page:America's Highways 1776–1976.djvu/440

 Computing Moments for Continuous Concrete Bridges, including design procedures and moment charts, which was published in the January–February–March 1944 issue of Public Roads.

I-480 exit ramp in Omaha, Nebr. This was one of the first major interchanges where curved composite steel box girders were used.

While the use of AASHO H20 loading was quite general, the new and heavier H20-S16 loading was adopted for bridges on routes that would probably be included in the Interstate System. There was a general trend toward the use of heavier loading design of bridges on all highway systems.

With the toll turnpike era starting again after the war, good architectural treatment of structures was encouraged, especially where improvement could be made without substantial increase in cost. Consulting engineers and architects were retained for design of these toll road structures and for other major bridges on the Federal-aid system.

In the 10 years following World War II, the march of new developments in the bridge field resumed. One of the most widespread was the use of composite steel beam bridges, which enabled the deck slab to work with the steel beams as a main load-carrying member.

Another development was the use of welded bridges. Welded steel bridges, except for minor details, had not been permitted on the Federal-aid system until after World War II because of the lack of toughness (and resultant welding inadequacy) of most structural steel used for bridges. At first, welding on bridge members was limited to welding flange cover plates to beams and web to flange connections on plate girders. At BPR’s urging, steel producers developed a weldable steel for bridges which had sufficient chemical controls to produce a tougher steel. The American Society of Testing and Materials (ASTM) adopted the Specifcation for Structural Steel for Welding (A373) in 1954. ASTM A373 steel was supplanted in 1960 by ASTM A36 steel which had the same toughness with a higher yield point. The availability of these steels rapidly expanded the scope and volume of welded structures and eliminated the riveted plate girder save for very large bridges and railroad structures. More versatile designs and economy of weight were obtained with welded designs.

At about the same time, high strength bolts were developed for use in place of rivets in the connection of structural members. These bolts, tightened to their proof load, clamp the plates tightly together and transfer the stress in joints by friction rather than by bearing and shear on the bolt.

However, the most significant development of this period was the use of prestressed concrete in which highly stressed steel wires introduced compressive forces into the concrete to offset the tensile stresses 434