Optimization of unbonded post-tensioned concrete ribberd floors

Abstract

Floors represent a great portion of the total cost of multistory reinforced concrete structures. The ribbed slabs, benefited by the reduction of concrete in the tension region, become a good alternative when it is necessary to overcome large spans. Solutions more efficient with respect to structural behavior and economy can be obtained combining the ribbed slabs with post-tensioning unbonded tendons. This way, it is important to find the optimum section and vertical tendons profile which improve its capabilities. The traditional design process can be aided by the use of optimization techniques. Hence, several works with cost optimization of isolated ribbed slabs of reinforced and prestressed concrete have indicated new design trends of formwork. In this research, it is performed the optimization of a typical one-way multiple spans ribbed floor. Formulations to minimize the cost of the slab are studied, considering the cost of material, formwork and labor. The design variables are the dimensions of formwork, the vertical profile of unbonded post-tensioned tendons and the number of tendons, considering all variables as discrete ones. The analysis is performed by grid analogy considering the entire floor, considering the beams as non-deformable elements. A Finite Element Model is used to stress analyses. The load balancing method is used to simulate the effect of prestressed unbonded tendons. Constraints on Serviceability and Ultimate Limit States are verified. A Genetic Algorithm of an open-source program, that has presented robustness for structural optimization, is used to solve the discrete optimization problem. In order to investigate the effects of optimization process against the traditional modeling, literature examples have been chosen trying to represent the widest range of cases faced in practice.