IntroductionThe three moderate-sized shallow-focus earthquakes namely, the 1986 Dharamsala earthquake (M w = 5.5), the 1991 Uttarkashi earthquake (M w = 6.8), and the 1999 Chamoli earthquake (M w = 6.5) occurred in the Himalayan region. The Dharamsala earthquake occurred in the Kangra region of Himachal Lesser Himalaya, at the north-western edge of the rupture zone of great Kangra earthquake of 1905. This earthquake was recorded at nine stations of the Kangra strong motion array, deployed in the epicentral track of the Kangra earthquake in Himachal Pradesh [1]. The two moderate earthquakes, viz., the Uttarkashi earthquake and the Chamoli earthquake, occurred in the Garhwal Himalaya that form part of the Western Himalaya. These earthquakes caused severe damage in and around the Uttarkashi and Chamoli regions. The epicenters of these earthquakes were located in the seismic gap postulated between the rupture zones of the 1905 Kangra earthquake (M w~8 .0) and the 1934 Bihar-Nepal earthquake (M w~8 .4) [2]. The Uttarkashi earthquake was recorded at thirteen stations while the Chamoli earthquake was recorded at eleven stations of the strong motion array, deployed in the Garhwal and Kumaun Himalaya [3]. The purpose of this array was to measure the strong ground motion due to gap-filling earthquakes that were expected in the seismic gap. The strong motion recordings of these three moderate earthquakes have been analyzed for the purpose of identifying near-fault pulses. To allow for pulse detection, standard methodology proposed by [4] has been adopted. Near-fault pulses are attributed to forward directivity, fling, hanging wall and vertical effects [5]. Forward directivity occurs when the fault rupture propagates towards the site with a velocity close to the shear wave velocity. Further, sudden tectonic motions due to an earthquake can produce impulsive motions leading to permanent displacements at the site. Such motions have one sided velocity pulses [6,7]. Pulse-type ground motions possess immense damage potential, specifically for intermediate and longperiod structures [8]. Out of 33 strong ground motion recordings, only three recordings showed pulse-type characteristics. A brief description of the methodology employed for pulse detection, followed by comparison of spectral characteristics of near-
Although the application of the Alternate Path method has well proven its efficiency towards increasing the structural robustness of bare frame structures against progressive collapse, it should be further developed for bridge structures, and especially cable stayed bridges, as they are the only type routinely designed for cable loss phenomenon. The main objective of this paper is to analyse a cable stayed bridge for multiple types of cable loss phenomenon to develop a profound knowledge regarding the overall structural response to local failure and the possibility of a failure progression throughout the structure. This paper demonstrates numerical modelling and analysis of a typical cable stayed bridge through a nonlinear static procedure using SAP 2000. The response of the structural model is discussed for multiple types of cable loss cases to identify a definite progressive collapse pattern. Furthermore, a categorical investigation of the impact ratios was also done for different structural properties through a dynamic analysis to recognize the lack of robustness in the structure. The results indicate different design considerations for different elements of the structure and also a variation in the possibility of failure progression through the cable stayed model depending upon the location of the failed cables. The prevailing collapse type is discussed with some suggestions for a more robust design.
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