Flexural ductility design of high-strength concrete beams

Abstract

In the seismic design of a reinforced concrete (RC) structure, it is necessary to provide not only sufficient strength, but also adequate flexural ductility. This is particularly important to the design of RC beams cast of high-strength concrete that is inherently more brittle. Eurocode EN1998-1 directly specifies such minimum flexural ductility. To provide adequate flexural ductility to RC beams, Chinese code GB50011 limits the normalised depth of simplified rectangular stress block at peak resisting moment, whereas American code ACI 318-08 requires that the tension steel strain at peak resisting moment shall not be smaller than 0.004. The essential parameters identified for effective flexural ductility design of RC beams include the maximum difference of tension and compression reinforcement ratios and maximum normalised 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 beams. A comparison shows that the allowable differences of tension and compression ratios may be smaller than those specified in Eurocode 8 particularly for those cast of high-strength concrete. © 2011 John Wiley & Sons, Ltd.