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As one of the most violented earthquake countrys, earthquake disaster is one of serious natural disasters in China. At 2:28 pm, May 12th, 2008, the WenChuan earthquake happended in SiChuan province which took a big challenge to the seismic engineering. Accoring to the earthquake observing station, the biggest acceleration was 957.7gal at that time, a large of houses and other building were collapse, bridges in disaster area was broken, and transportation was cut off, which made the seismic rescuing much more difficult. As we all know, bridge takes big effect in the lifeline of relief work, so the seismic design of bridgeis very important; as one major load bearing part of bridge, the pier seismic design should be pay more attention to.In the ductile seismic design analysis of highway bridge, flexural stiffness of ductile structure(pier) should consider as the effective cross-section stiffness. At present, it often compute the cracked stiffness according to test data, empirical formula and finite element software. The research on cracking stiffness of pier is hardly found especially in the theoretical way.In the elastic range, based on the concepts of the mechanics, this paper, researched on the bi-reinforced rectangular concrete column, discussd the depth of compression zone and properties of the stiffness distribution, compared and analysed the results with the help of software and test. The work of this paper are listed as follow:1. Using "three-phase" linear stiffness, a simplly division of the cross-section stiffness distribution has been presented; meanwhile, a flexural stiffness formula for three stiffness phase has been suggested;2. By way of neutral axis counting of cracking cross-section for reinforce concrete column, the paper researched additional moment due to neutral axis deviation, and summarized the moment of physical force acting on column.3. Using the basic mechanics method, with the help of programme software, this paper counted and summarized flexural displacement and stiffness of column base on a example, and compared with the analytic software.4. Compared and analyzed with the cracking stiffness test of column. the paper summarized the variation trend of cracked stiffness of reinforce concrete column to make a cross-reference for the whole stiffness formula of column.
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