INFLUENCE OF SEMIRIGID JOINTS ON STEEL RAILWAY BRIDGES

Abstract

The dynamic response of a bridge structure depends on various parameters such as action of repetitive loads in terms of cycles of operations, resonant vibration, vehicular system (train composition, axle spacing and speed) and the bridge system itself. All these parameters attribute to the failure of a bridge in high cycle fatigue and thus should be considered in fatigue life assessment. In conventional methods of analysis of Steel Open Web Truss Railway Bridge and Plate Girder Bridge. [as per RDSO], under both static and dynamic loading, the members are assumed to be connected at rigid or hinged joints. However, in reality steel trusses are reinforced at their joints by Gusset plates, which possess rotational flexibility. Similarly the. intermediate joints in the Plate Girders also have some flexibility. These joints lose their rigidity due to repetitive passages of train loads and hence, loss of rotational rigidity is to be taken into account in the fatigue life assessment of bridges, as it tends to alter the response of the structure under dynamic loading that comes from moving trains. In this type of flexible system with rapidly varying loads, the components of the bridge respond dynamically to the loading environment, thus the time history obtained is a function of both load variation and dynamics of the structure which consequently affects the fatigue life assessment. The present article attempts to study the influence of Semirigid joints on steel railway bridges considering its rotational flexibility for different train composition, axle spacing and speed. A comparative study is also performed considering three different spans each for both open web truss girder and plate girder bridges. These bridges have been designed as per codes of Indian Railway standards for Steel Bridge and Bridge rules. Semirigid connection is modeled with rotational and linear springs about an axis .perpendicular to the plane of the rivets. The linear direct integration time history analysis has been performed, using SAP2000 and fatigue life is evaluated using both Rainflow Counting Method [Matsuishi and Endo, 1968; ASTM1049E-2011] and Palmgren-Miner Linear Damage Rule [Palmgren, 1924; Miner, 1945]. Allowable S-N (stress life) diagram has been used as per BS: 5400 Part 10:1980.Shorter spans for both the types of bridges are found to be more susceptible to fatigue damage considering joint flexibility. The study shows that Steel Railway bridges in use today have substantial fatigue life.