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Bridges Failure in Kobe Earthquake -1995

 
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B.V.Harsoda
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PostPosted: Mon Jul 11, 2016 4:38 pm    Post subject: Bridges Failure in Kobe Earthquake -1995 Reply with quote

Bridges Failure in Kobe Earthquake -1995:



On 17th January, 1995, the Hanshin/Awaji Earthquake struck the densely populated Kobe, Japan, area with a Richter magnitude 7.2. In terms of magnitude, this earthquake was not as large as some previous earthquakes in Japan. However, the close proximity of its epicenter to an urban area, along with its severe ground motion and large ground displacement, resulted in more than 5,500 fatalities and extensive damage to lifeline systems i.e., highways, bridges ,railways, buried pipelinees, etc. The depth of the hypocenter was 14 kilometers, which is considered to be a relatively shallow earthquake

The Japanese began designing bridges for seismic loads after the great 1923 Kanto Earthquake. At first, the only seismic design provision for steel and concrete bridges was the application of a lateral design force equal to 20 percent of the weight of the structure. The latest seismic design specifications were issued in 1990. A major change was a required check for the ultimate capacity of reinforced concrete piers using a ductility approach under an assumed magnitude 8 earthquake, where force levels of up to 1.0 g are applied to piers.


In this KOBE Earthquake At least 60 percent of all bridge structures in the Kobe area were damaged. The damage to the majority of reinforced concrete columns was no surprise once the column reinforcement details were examined. The predominate failure mechanism of these columns was either shear failure or flexural shear degradation failure. These two types of failure are related to a nonductile design that is the result of inadequate shear or confinement reinforcement. These bridges were designed and constructed in the 1960s before the introduction of modern seismic codes.


In Kone Earthquake The failure was triggered at the point where some of the longitudinal reinforcement was terminated. The 180 longitudinal bars at the base of the 3.1-meter (m)-diameter columns were reduced to 120 bars 2.5 m above ground level. These columns were designed according to pre-1980 design specifications and were scheduled for retrofitting. The 1995 tentative design specification no longer permits the termination of longitudinal reinforcement in columns at mid-height. It also calls for reduction of shear reinforcement spacing from a maximum of 30 cm to 15 cm.



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