Dynamics of Highway Bridges, HR-67, 1961

(1961) Dynamics of Highway Bridges, HR-67, 1961. Transportation, Department of

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Abstract

The AASHO specifications for highway bridges require that in designing a bridge, the live load must be multiplied by an impact factor for which a formula is given, dependent only upon the length of the bridge. This formula is a result of August Wohler's tests on fatigue in metals, in which he determined that metals which are subjected to large alternating loads will ultimately fail at lower stresses than those which are subjected only to continuous static loads. It is felt by some investigators that this present impact factor is not realistic, and it is suggested that a consideration of the increased stress due to vibrations caused by vehicles traversing the span would result in a more realistic impact factor than now exists. Since the current highway program requires a large number of bridges to be built, the need for data on dynamic behavior of bridges is apparent. Much excellent material has already been gathered on the subject, but many questions remain unanswered. This work is designed to investigate further a specific corner of that subject, and it is hoped that some useful light may be shed on the subject. Specifically this study hopes to correlate, by experiment on a small scale test bridge, the upper limits of impact utilizing a stationary, oscillating load to represent axle loads moving past a given point. The experiments were performed on a small scale bridge which is located in the basement of the Iowa Engineering Experiment Station. The bridge is a 25 foot simply supported span, 10 feet wide, supported by four beams with a composite concrete slab. It is assumed that the magnitude of the predominant forcing function is the same as the magnitude of the dynamic force produced by a smoothly rolling load, which has a frequency determined by the passage of axles. The frequency of passage of axles is defined as the speed of the vehicle divided by the axle spacing. Factors affecting the response of the bridge to this forcing function are the bridge stiffness and mass, which determine the natural frequency, and the effects of solid damping due to internal structural energy dissipation.

Item Type: Departmental Report
Keywords: Dynamics, Experiments, Fatigue tests, Highway bridges, Impact loads, Live loads, Static loads, Structural analysis, Vibration tests
Subjects: Transportation
Transportation > Bridges and tunnels
Transportation > Roads and highways
Transportation > Research
Transportation > Design and Construction
Transportation > Data and Information Technology
ID Code: 17266
Deposited By: Iowa DOT Library
Deposited On: 30 Jun 2014 12:36
Last Modified: 18 Sep 2014 15:33
URI: http://publications.iowa.gov/id/eprint/17266