Seismic response of steel frame buildings to near-source ground motions

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 two 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). All four 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. Three earthquake simulations are run: one based on the 1994 Northridge earthquake (Mw 6.7), another for the 1995 Kobe earthquake (Mw 6.9), and the third of a hypothetical Mw 7.0 earthquake on the Elysian Park fault in Los Angeles. In addition, various recorded motions from actual earthquakes, including Northridge and Kobe, are employed. Previous studies of strong near-source ground motions have shown the potential for large story drifts in buildings and even collapse (3-7). Future design measures for near-source ground motions will likely have to consider all contributions to the strength of a building in order to be feasible. It follows from this that since future design methods will be based on assessment studies like the present one, the assessments should also be done as realistically as possible and include all important contributions to the strength of a structure. Such an approach is followed here. In addition, since strong near-source ground motions will likely produce severely nonlinear responses, it becomes necessary to include structural degradation effects. In this study, due to the common occurrence of welded connection failure in the Northridge and Kobe earthquakes and because this behavior would seem to be an important degradation mode, treatment of connection fracture is included. As there is now wide-spread interest in fracture of welded connections, this is an important part of the present investigation. The Appendix contains a description of the computer program used for the analyses. Some of the notation used in this report is defined in this Appendix