Improving the Ductility of Concrete Beams Reinforced with Topologically Optimized Steel

To address the sustainability challenges faced by concrete structures, various attempts have been made to optimize the reinforcement layout with a topologically optimized strut-and-tie model (STM). However, most studies have focused on theoretical discussions and the few available experimental studies have only discussed the prepeak behavior of optimized beams. The postpeak behavior, especially the ductility of beams with optimized reinforcement, has not been addressed, although it is one critical criterion for ensuring structural safety. Moreover, current topology optimization methods mostly adopt linear elastic material constitutive behavior, which neglects the intrinsic strength difference between steel and concrete material and has been found to cause low ductility in concrete beams. To address these challenges and enhance ductility with optimized reinforcement, this study proposes new frameworks for designing concrete beams with optimized reinforcement. The first framework enhances the elastic-material model-based optimized reinforcement layout with a postprocessing scheme to enhance concrete compression strut ductility. The second framework develops a new optimization formulation by introducing an asymptotic nonlinear material model, which considers both the stiffness and strength difference between concrete and steel material. An experimental and numerical program was conducted to compare the structural performance of concrete beams with optimized reinforcement from different frameworks. Results show that the new frameworks have limited impact on the peak strength but increase ductility of the optimized beams. Compared with the design from the conventional bilinear model, the design from the nonlinear model reduces steel consumption by 8.2%.