Page:America's Highways 1776–1976.djvu/438

 The AASHO Bridge Committee’s continuing function was to develop policies and specifications for design and construction of highway bridges. Public Roads bridge engineers served as chairman of the AASHO Bridge Committee from 1921 until 1953 and as secretary from then until the present time.

The AASHO Bridge Committee gradually developed the bridge specifications and issued the first printed edition of the AASHO Standard Specifications for Highway Bridges in 1931. These “Standard Specifications” were not just specifications, but served as a standard or guide for the preparation of State specifications and for reference by bridge engineers. They combined design criteria and policies with detailed specification guidelines. Primarily, “Standard Specifications” set forth minimum requirements which were consistent with the current practices. The same objectives prevail in the current edition.

A notable innovation in the specifications was the use of a truck system of live load instead of road rollers. The loadings, designated as H-20, H-15, and H-10, specified basic two-axle design trucks of 20, 15, and 10 tons, respectively. The H-truck loading was a basic truck in each lane of the bridge, preceded and followed by a train of trucks each weighing ¾ as much as the basic truck. An equivalent “lane loading,” consisting of a concentrated load plus a uniformly distributed load for each lane of loaded structure, was provided.

During the “great bridge” era between 1900 and World War II, there was a phenomenal increase in the number and size of lesser bridges as well as a growth in technical knowledge. The biggest new development was reinforced concrete. The short-span timber bridges were being displaced by concrete slab or concrete I-beam bridges. The reinforced concrete box culvert also became common. But aside from this, most of the growth consisted of further developments and refinements of existing bridge types. Riveted joints in steel trusses replaced the earlier pin connected joints, and stiffer and more substantial truss members developed. Steel rolled beam and built-up “plate girder” structures became common. The small arch brigebridge [sic] changed from stone to concrete and began to grow in span length. Wider bridge roadways were being used.

There were three new, or at least different, developments that were significant beginning in the 1920’s. The first was the construction of a “parkway system” in Westchester County. This progressive county began building roads with some control of access and separation of cross traffic. This kind of road meant bridges wherever traffic was to be separated, even where there was no river to be crossed. Usually, these grade separation structures were not very large but did need eye appeal. The result was the rebirth of the stone arch (or stone-faced at least) and its variation, the rigid frame. The Westchester Parkway used many stone-faced reinforced concrete arches and frames for the Bronx River, Cross County, Saw Mill, and Taconic Parkways. Arthur Hayden was the bridge engineer at Westchester, and his book on the Rigid Frame Bridge has become a classic. The parkway and freeway concept spread to Connecticut where the Merritt and Wilbur Cross Parkways were built, again with emphasis on esthetics. Every bridge on the Merritt Parkway is different. This road is still one of the most delightful and scenic in the United States.

The second development was the almost exponential increase in engineering theory and application. While there were many whose studies made these new frames and arches possible, there was one outstanding contribution. In 1932 Professor Hardy Cross of the University of Illinois presented the method of moment distribution for determining the moments and shears in continuous beams, arches, and frames in a paper to the American Society of Civil Engineers. Prior to this time, the design of continuous frames was a tedious, intricate and time consuming process which discouraged the use of indeterminate structures. Moment distribution came at a very favorable time—at the beginning of the widespread use of highway grade separation structures for which rigid frames were especially appropriate.

The third significant factor was the growth of Federal interest and activity in the highway field. Public Roads was still a relatively small agency, but it helped the States through the transition to heavier automobile loadings. While the designated Federal-aid system at the start of the Federal-aid program was in most States essentially a system of county roads located and designed for preautomobile traffic, the quality of bridges increased dramatically during the 1920’s because of the cooperation of the States and Public Roads, increased experience, and improved criteria, specifications and guides. The example of the improved highways and bridges created a demand for similar improvements on other State highways. In general, bridges on the Federal-aid system in this period were of short to medium spans and of moderate cost. The Federal -aid allocation and the State highway funds were not sufficient to finance high cost structures and at the same time to construct other highway facilities in the State. Consequently, high cost bridges and tunnels were frequently built by bridge or tunnel authorities or private interests as toll facilities.

On U.S. route 5 in Vermont, large vehicles were forced to straddle the pavement centerline in crossing this narrow bridge with its sharp-curved approaches.

During these years, the AASHO Bridge Committee continued to develop the bridge design specifications. Basic two- and three-axle truck design loads were substituted for the truck train loading in 1941, 432