MEASURES TO COPE WITH IMPULSIVE EFFECT OF NEAR-SOURCE EARTHQUAKE IN STRUCTURAL TRANSIENT RESPONSE

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Structural damage suffered from the 1995 Hyogoken-nanbu Earthquake (Kobe Earthquake) motivated the authors to initiate investigation into the quasi-impulsive effects of near-source earthquakes and to establish structural measures to cope with them. A cooperative research has been carried out involving various academic fields for four years since 1999, funded from Grant-in-aid for Scientific Research. Among significant research results, abstracted are the following :(1) The field surveys and rupture process observations of recent earthquakes have revealed that in Quindio earthquake, a large rupture occurred after a small initiation, and the big shake caused by the second event struck Almenia city, killing over 1000 people ; in Kocaeli earthquake, which killed over 30000 people, an earthquake fault appeared over a length of 100km, causing a maximum horizontal acceleration of 407gal and maximum vertical acceleration of 260gal near the fault ; in western Tottori earthquake, two clear phase … More s were identified, suggesting that the first rupture did not grow up continuously, but another big rupture succeeded later, indicating that shear stress might be completely and spatially released by large complex rupture around the second event. An earthquake"bright spot", proposed by Umeda, must have formed in a small confined region.(2) 3D dynamic finite-element simulation analyses have shown that the transient phase of structural response plays an important role, governed by stress waves, affecting heavily the following vibratory phase response, and possibly causing subsequent significant damage, in the case of severe earthquakes such as a near-source earthquake. The degree of this importance depends on ground-motion profile, site conditions and structural properties, such as shape, size, boundary or support condition, etc.(3) Actual severe damages in the Kobe earthquake were located near structural portions where abrupt changes in energy transmission must have taken place, with main rupture observed in heavy steel skeletons coinciding with such a location, and is understood as caused by this effect.(4) A particular large scale rupture experienced near a brace-column joint was initiated by brace breakage, which was caused by the combination of a great tensile axial force and transverse shear, and was transmitted to the adjacent column cross-section, which resulted in complete rupture.(5) Concentration of yield-hinge occurrence in a coincident story can cause severe whole story collapse. This is particularly dangerous, when occurred in an early phase of structural response to an earthquake.(6) In order to restrengthening an earthquake-damaged wooden house, a device of installing out-door metal poles together with bolted connections on the second floor level has been introduced, and found to be effective, transmitting horizontal resisting forces between the wooden frame and the poles, through experiment and earthquake response analyses.(7) As a result of Sharpy tests with varying speeds on steel specimens, it has been found that absorbable energy, and hence, fracture toughness also increase, as impact speed increases.(8) Sharpy tests have revealed that prestraining plays an important role in causing brittle failure of steel, and that plastic prestrain causes significant drop in absorbable energy capability. Less