Flexural ductility design of high-strength concrete columns

In the seismic design of a structure, it is necessary to provide not only sufficient strength, but also a minimum level of flexural ductility for reinforced concrete (RC) columns. Eurocode EN1998-1 directly specifies such minimum flexural ductility, while Chinese code GB50011 limits the normalized design axial force to achieve a nominal minimum flexural ductility. American code ACI 318-08 uses the tension steel strain at peak resisting moment to control the failure mode. To provide the required flexural ductility, a much lower axial strength reduction factor is assigned to compression-controlled failure than to tension-controlled failure. To develop an effective strategy for flexural ductility design of RC columns, it is necessary to identify the essential parameters and control them properly. This is particularly important to those cast of high-strength concrete that is inherently more brittle. The essential parameters identified include the maximum normalized axial force and maximum normalized neutral axis depth at peak resisting moment, as they help to guarantee various flexural ductility requirements. Their relationship with the flexural ductility is studied using a rigorous full-range moment-curvature analysis procedure. Empirical formulae and tables are also developed to facilitate flexural ductility design of RC columns. Copyright (c) 2010 John Wiley & Sons, Ltd.