Improving design limits of strength and ductility of NSC beam by considering strain gradient effect

In flexural strength design of normal-strength concrete (NSC) beams, it is commonly accepted that the distribution of concrete stress within the compression zone can be reasonably represented by an equivalent by a and B to stipulate the width and depth of the stress block. Currently in most of the reinforced concrete (RC) design codes, a and B are usually taken as 0.85 and 0.80 respectively for NSC. Nonetheless, in an experimental study conducted earlier by the authors on NSC columns, it was found that a increases significantly with strain gradient, which means that larger concrete stress can be developed in flexure. Consequently, less tension steel will be required for a given design flexural strength, which improves the ductility performance. In this study, the authors\' previously proposed strain-gradient-dependent concrete stress block will be adopted to produce a series of design charts showing the maximum design limits of flexural strength and ductility of singly-and boubly- NSC beams. Through the design charts, it can be verified that the consideration of strain gradient effect can improve significantly the flexural strength and ductility design limits of NSC beams.