Assessment of Ductility Measures of Prestressed Concrete Flexural Members: Application to Hybrid Steel–FRP Systems

Abstract

Ductility is an important structural design characteristic for ensuring that any overloaded structural member or structure shows visible warnings of an incipient or impeding failure through large deformations and/or excessive cracking. At present, there is a knowledge disparity in quantifying the flexural ductility of prestressed concrete (PC) systems, particularly with the evolution of prestressing reinforcement to incorporate nonmetallic fiber-reinforced polymer (FRP) tendons. There is a need to adopt innovative prestressing technologies and new prestressing systems to facilitate the use of partially bonded, unbonded (externally and internally), and hybrid tendons in which the prestressing reinforcement may consist of bonded–unbonded steel or bonded–unbonded FRP tendons or a combination of both. With the development of new prestressing systems incorporating FRP tendons, accurate evaluation of the ductility of these systems has become a serious concern. In an attempt to converge on a rational approach for evaluating the ductility of hybrid PC members, a review of the evolution of ductility measures proposed in the technical literature is carried out, covering conventional methods and ductility measures specified in design codes. Based on a critical assessment of these measures, it was found that the concept of net tensile strain offers, at present, a most rational approach for evaluating the ductility of prestressed concrete flexural members, which can be extended for hybrid PC members. Accordingly, a simple yet accurate design-oriented method is developed for quantifying the ductility of hybrid PC members using conventional measures such as curvature ductility ratio or, more importantly, using the net tensile strain concept. In this study, different methods proposed in the technical literature for evaluating the flexural ductility of prestressed concrete (PC) members were reviewed and assessed, with particular focus on the applicability of these methods to hybrid PC members—those utilizing a combination of different prestressing systems and/or prestressing materials. Based on the results of this assessment, it is concluded that the net tensile strain concept used in design codes for designing ductile structural members can be extended easily and rationally to any type of hybrid PC systems. These include systems in which the prestressed tendons consist of a combination of bonded steel–unbonded fiber–reinforced polymer (FRP), bonded steel–unbonded steel, bonded FRP–unbonded steel, and bonded FRP–unbonded FRP. In analogy with the flexural strength and analysis approach adopted in design codes, a direct design-oriented method is developed, which would allow design engineers to effectively and efficiently estimate the nominal moment capacity of any type of hybrid PC systems, quantify their ductility by means of the curvature ductility index (or ratio), and establish their maximum reinforcement limits utilizing the net tensile strain concept.