Page:America's Highways 1776–1976.djvu/337

 This machine was used in 1922 to test the impact of solid rubber tires on test pavement sections. By raising the wheel and suddenly dropping it at impact forces comparable to truck traffic on the roads, comparative pavement design strengths could be determined.

Research in the field began in 1935 when road life studies were incorporated as part of the statewide planning surveys. At one time or another nearly all States cooperated in this research by providing the basic data which were analyzed by Public Roads staff. Although the purpose of the surveys was to determine average service life of road surfaces, the data also provided the means for obtaining construction costs, salvage values of retired roadway elements, and service lives of structures, gradings, and rights-of-way.

Based on an analysis of the records of surfaces and roadway elements previously constructed and depreciated, it is now possible to estimate the amount and cost of replacement of these elements. For example, the results of a study completed in 1971 indicated that of the total miles of roads remaining in service in 1968, 60 percent will be retired in 10 years and 87 percent in 20 years. Such information is essential to determine construction and reconstruction programs and corresponding revenue needs of a future period.

Early highway bridge research was conducted in response to the need to shape the AASHO standard specifications for highway bridges, which were gradually developed between the formation of the AASHO Bridge Committee in 1921 and the first printing of the standards in 1931. Subjects of early bridge research studies included the expanded use of welding and high-strength bolting for connections in steel structures, the widespread acceptance of continuous composite bridge design and new techniques of construction, such as orthotropic deck design and cable-stayed girder bridges. Other important developments were specifications covering the fatigue life of steel and the widespread adoption of new high-strength structural steels for bridges.

A unique aspect of early structural research activity resulted from the catastrophic failure of the Tacoma Narrows Suspension Bridge in the State of Washington on November 7, 1940, due to flutter induced by high winds. Under the auspices of the Advisory Board on the Investigation of Suspension Bridges, formed in September 1942, a wind tunnel large enough to accommodate a scale model of the entire Narrows Bridge was designed and built at the University of Washington. This tunnel was used to make exhaustive studies of the causes and possible remedies for such a failure. After extensive redesign utilizing the findings of that study, the Narrows Bridge was rebuilt and has since served well without excessive vibration or other evidence of distress. One feature of the redesign was a slotted or grid type of deck which largely relieved the vertical component of wind streams impinging on the bridge.

The extensive investigations which followed the collapse of the Narrows Bridge led to the construction in 1950 at the Fairbank Highway Research Station of the George S. Vincent Memorial Wind Tunnel, where model studies on the effects of winds on highway structures were conducted. Models of many of the major suspension bridges throughout the United States and abroad have been investigated for aerodynamic stability in this wind tunnel.

One of the subjects of those studies was the Golden Gate Bridge. Excessive wind-induced vertical oscillations at a maximum amplitude of 13 feet had occurred while the Narrows Bridge investigation was still underway, causing great official and public concern. Consequently, section-model studies of the bridge were performed in the Vincent wind tunnel. Some structural stiffening of the suspended portions of the bridge, as well as the introduction of slots in the deck to reduce the vertical component of the wind force, proved effective, as in the case of the redesigned Narrows Bridge, in remedying the excessive oscillation.

A great deal of research was also initiated on all of the common bridge construction steels in order to learn more about their sensitivity to brittle fracture. Once this research began to produce results, tentative toughness specifications for bridge steels, using the Charpy V-Notch impact specimen as a control, called for tougher steel with increased steel strength and thickness. These requirements were adopted by the AASHTO Bridge and Materials Committees for inclusion in the 1974 interim specifications.

Aerodynamic studies of suspension bridges are conducted in the George S. Vincent wind tunnel. A precise model of a portion of the bridge deck is mounted on springs that match the stiffness of the actual cable system with instrumentation to measure the oscillations produced by the wind stream. 331