Author : Sushank Dani 1
Date of Publication :24th January 2018
Abstract: Fatigue is a localized and progressive cumulative damage accumulation due to continuous cyclic movement on a structure. The effects due to this can be dangerous as compared to conventional static load. On bridges, fatigue occurs due to the passage of vehicular load. In railway steel bridges, there are structural members with low dead load stresses but high live load stresses due to movement of wagons and locomotives. These high live load stresses cause a decrease in strength well below the design stresses. This, in turn, reflects the reduction in useful life of the bridge. Here, live load stresses are assessed for different members and details of bridges. Various analytical methods suggested such as nominal stress method, Hot spot stress method, Effective notch method, etc. The results of these methods are compared with the field measurement data obtained from strain gauges or structural health monitoring methods. The nominal stress method has been mentioned in various codes like BS 5400: 1980 (Part X), Steel Bridge Code, RDSO, etc. While the hot spot stress method depends on the finite element analysis of bridges. It finally concludes with the calculation of stress concentration factors for a particular detail. The failure due to fatigue does not depend upon the maximum stress, but on the stress range (absolute difference of maximum and minimum stresses) and the number of cycles corresponding to that stress range also called as stress history. Using the S-N curve and Palmgren-Miner’s cumulative damage rule, the damage assessment for each moving load is calculated followed by useful life estimation of a structure. This is a brief methodology for nominal stress method. The revised fatigue Appendix ‘G’ in Steel Bridge Code, RDSO has incorporated this method in addition to geometrical stress method (or hotspot stress). The nominal stress method is widely used for fatigue evaluation and design of steel bridges, but the hot spot stress method is more accurate and effective. Though the field measurement data presents most accurate information for determining the fatigue life of the structure, the above said two methods can be applied successfully to fatigue analysis and design. The main objective of the study is to find the difference in methodologies for assessment of fatigue and its application on railway steel bridges. Recently, a number of studies have been started on the use of probability concept in fatigue life determination of bridges. Though, these require combined basic of reliability and probability so they are more dependent on experimental and statistical data. Based on the above analysis, a predefined maintenance and inspection schedule can also be prepared. This schedule depends upon the service life of each structural member or details
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