Page:America's Highways 1776–1976.djvu/445

 were developed for the longer spans. Reinforced concrete box girders and composite structural steel box girders were constructed to conform to the sinuous and undulating lines of the ramps and viaducts. Without the computer many of these structures could hardly have been analyzed and designed realistically or economically.

As happens so often, a breakthrough in one area seems to lead to one in another area. Prestressed concrete has already been mentioned. New structural steels were also developed to permit larger, more heavily stressed bridges. In 1959 and 1960, a new high-strength low-alloy structural steel for riveted construction, ASTM A440, and a companion steel for welded construction, ASTM A441, were produced to supersede the then-standard silicon steel.

In 1961, a temporary specification for a 90,000–100,000 pounds per square inch yield strength steel for long-span bridges was approved by Public Roads for use on Federal-aid projects. After 3 years, an ASTM specification for this steel, High-Yield-Strength, Quenched and Tempered Alloy Steel Plate, Suitable for Welding, ASTM A514, was issued, and the temporary specification was discontinued. This same procedure was followed in 1966 with the high-strength columbium and vanadium steels. ASTM issued a specification for the steel, A572, without notch toughness (impact testing to insure steel will resist fatigue cracking) requirements. BPR accepted the ASTM specification for use on Federal-aid projects with welded steel only if a special provision covering notch toughness was included in the project specification. This was done, but the use of the special provisions was discontinued in 1974 when ASTM included notch toughness tests for all structural steels.

The use of curved bridges produced other problems besides design, e.g., the great amount of scrap metal resulting from cutting the flanges to the curve. This required development of new fabrication methods whereby members were fabricated straight and then heated and bent to the desired curvatures, thereby eliminating the waste.

To achieve better quality control in welding bridge members, nondestructive testing by radiography, magnetic particle and ultrasonic testing were developed by Public Roads bridge engineers. Ultrasonic testing of welded groove joints was found to be a faster, more sensitive and cheaper method of locating weld defects than the methods previously used.

Among the many long-span bridges built on the Interstate System to cross wide waterways, the Poplar Street Bridge over the Mississippi River at St. Louis is one of the most notable structures. A truss or arch span was not acceptable since it was considered that its bulk would detract from the nearby Gateway Arch and the Eads Bridge. The 8-lane bridge, constructed in the mid-1960’s with a 600-foot center span, is the Nation’s longest box girder bridge and the first large structure in the United States to use orthotropic design. Orthotropic design, developed in Europe, consists of main girders and a stiffened steel plate deck welded together so as to act jointly in supporting the structure. The stiffened plate deck serves a fourfold purpose as bridge deck, stringers, and an upper flange for both the floor beams and the main girders, reducing the dead load of the structure.

Pedestrian crossings of freeways became necessary in some locations to avoid undue division of established neighborhoods. Topography and personal safety of pedestrians generally dictate the design of overpasses. Protection for the traffic below from objects dropped or thrown from pedestrian crossings led to the use of enclosures or high fences of closely meshed wires on the overpasses. Many unique and attractive structures have been developed for these overpasses.

The AASHO Road Test near Ottawa, Illinois, discussed in Chapters 4 and 6, is a good example of Federal-State cooperation on research. Public Roads bridge engineers designed and prepared the plans for the 18 bridges in the program. The bridges were designed with high working loads so that fatigue failures could be expected. There were 18 steel and/or concrete beam spans representing contemporary practices.

At the conclusion of the regular test traffic in December 1960, 7 of the 11 surviving bridges were subjected to accelerated fatigue tests, and two were tested to failure by increased vehicle loads. The mass of test-to-failure data on the behavior of the bridges under repeated loading has proved to be of major assistance in subsequent studies for developing specifications and design revisions.

Seven years later, full size welded plate girders were fatigue tested at Lehigh University under the sponsorship of the Welding Research Council. Analyses of the results of these tests resulted in design and specification changes for welded and riveted plate girders and confirmed the integrity of properly designed and constructed welded girders.

Since the late 1920’s, research and development has been greatly broadened in the nonstructural areas, such as hydraulics. While bridges and culverts have been used for centuries to cross streams and rivers, the structural elements of these crossings have attracted most of the attention of engineers with less attention given to the bridge’s capacity to accommodate floods, except for very large bridges over major rivers. Progress in highway hydraulics and drainage was slow, and in the early days, designs were frequently based on judgment without well developed engineering technology or adequate rainfall data. Drainage structures, including bridges, were commonly sized by using empirical formulas developed in the 19th century.

Progressive engineers had long recognized the shortcomings of drainage design and had stressed the importance of estimating the magnitude and frequency of flood flows and the risk of damage. The lack of hydrologic data and the dearth of information on hydraulics of highway structures made it difficult to develop policy and design procedures.

Research on hydraulics and hydrology started in the 1920–1930 era, mainly encouraged by Public Roads and other agencies of the Department of Agriculture, and after World War II, extensive development in the application of hydraulic engineering principles to highway design began in Public Roads. During the 439