Near-Source Ground Motion Studies for Northridge and Hanshin Earthquakes

Strong near-source ground motions contain large, rapid displacement pulses that can have severe effects on structures. In the U.S. it is generally agreed that our design codes do not adequately address this kind of ground motion, and changes are being made to increase the design force levels, which will result in stronger buildings. In Japan, the design force levels for buildings have for some time been considerably greater than in the U.S. Even so, Japanese engineers are also concerned about
the adequacy of their design provisions for strong near-source ground motions.

The issue of building strength appropriate for near-source ground motions is investigated in this report. Two different heights of buildings are selected, 6 stories and 20 stories, and for each height, designs of three different strengths are produced. The first design is according to the 1994 Uniform Building Code (1), and the second design follows current Japanese provisions (2). The other 6-story and 20-story buildings were designed by Kajima corporation and they exceed the Japanese strength requirement. All six buildings are steel moment frames.

One important part of assessing near-source ground motions is to quantify the size of the region that is affected. An aspect of this is that near-source effects are directional and so are confined to only a portion of the near-fault zone. In this study, ground motions at a grid of sites sufficient to cover the region of near-source effects are employed. This is accomplished by using simulated ground motions. Two earthquake simulations are run, one based on the 1994 Northridge earthquake (magnitude 6.7) and the other for the 1995 Kobe earthquake (magnitude 6.9). In addition, various actual
records from these and other earthquakes are employed.

The near-source displacement pulses from the ground motions used here are from moderate size earthquakes, and they will not be as large as those in large earthquakes. Neither will be the duration of strong shaking. Effects from large earthquakes are topics for future research.