SYMPOSIA PAPER Published: 01 January 1976
STP33373S

Stress Spectrums for Short-Span Steel Bridges

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U. S. Steel's Research Laboratory is currently completing research on Project 12-12, sponsored by the National Cooperative Highway Research Program of the National Academy of Sciences, and entitled “Welded Steel Bridge Members Under Variable-Cycle Fatigue Loadings.” The main objectives of the program are to develop fatigue data on welded bridge members under variable-amplitude random-sequence stress cycles representative of stress spectrums such as occur in actual bridges and to develop an analytical method of predicting the fatigue behavior under variable-amplitude stress spectrums from constant-amplitude fatigue data.

This paper summarizes available field measurements of stresses in short-span bridges under traffic, and describes the stress spectrums that were developed from these measurements for use in the testing program. The available field data show that the passage of a vehicle over a bridge produces a single major stress cycle and superimposed vibrational stress cycles. In most types of bridges, the vibrational stresses are small enough to be neglected; however, the described method of deriving a simple equation for the stress spectrum is applicable to field data with or without vibrational stresses. The frequency of occurrence of stress cycles can be defined by a family of skewed Rayleigh probability-density curves; a particular curve from the family is defined by the modal value, Srm, and the dispersion, Srd. The major stress cycles (or stress ranges) are added to the dead-load stress, Smin, which remains essentially constant during the life of the bridge. In general, the stress cycles are arranged in random sequence. The stress spectrums used in the main test program, therefore, are defined by three parameters: Srm, Srd, and Smin. The fatigue-test control tapes corresponding to these spectrums record 500 individual loads that satisfy one of the family of Rayleigh probability-density curves and are arranged in a random sequence. The tape is continuously cycled throughout a fatigue test.

Author Information

Klippstein, KH
Research Laboratory, United States Steel Corporation, Monroeville, Pa.
Schilling, CG
Research Laboratory, United States Steel Corporation, Monroeville, Pa.
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Details
Developed by Committee: E08
Pages: 203–216
DOI: 10.1520/STP33373S
ISBN-EB: 978-0-8031-4672-3
ISBN-13: 978-0-8031-0344-3