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Structural Engineering and Mechanics
  Volume 27, Number 4, November10 2007 , pages 425-438

Dependency of COD on ground motion intensity and stiffness distribution
Mark Aschheim, Edwin Maurer and JoAnn Browning

    Large changes in stiffness associated with cracking and yielding of reinforced concrete sections may be expected to occur during the dynamic response of reinforced concrete frames to earthquake ground shaking. These changes in stiffness in stories that experience cracking might be expected to cause relatively large peak interstory drift ratios. If so, accounting for such changes would add complexity to seismic design procedures. This study evaluates changes in an index parameter to establish whether this effect is significant. The index, known as the coefficient of distortion (COD), is defined as the ratio of peak interstory drift ratio and peak roof drift ratio. The sensitivity of the COD is evaluated statistically for five- and nine-story reinforced concrete frames having either uniform story heights or a tall first story. A suite of ten ground motion records was used; this suite was scaled to five intensity levels to cause varied degrees of damage to the concrete frame elements. Ground motion intensity was found to cause relatively small changes in mean CODs; the changes were most pronounced for changes in suite scale factor from 0.5 to 1 and from 1 to 4. While these changes were statistically significant in several cases, the magnitude of the change was sufficiently small that values of COD may be suggested for use in preliminary design that are independent of shaking intensity. Consequently, design limits on interstory drift ratio may be implemented by limiting the peak roof drift in preliminary design.
Key Words
    reinforced concrete frames; interstory drift ratio; seismic response; seismic design.
Mark Aschheim? and Edwin Maurer; Civil Engineering Department, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053, USA
JoAnn Browning; Civil, Enivironmental, and Architectural Engineering Department, University of Kansas, 2150 Learned Hall, 1530 W. 15th Street, Lawrence, Kansas 66045-7609, USA

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