Externally prestressed members: Evaluation of second-order effects

This paper presents the results of analytical investigation of the strength and structural behavior of concrete members, prestressed with external ten dons. The behavior at both the serviceability and ultimate limit states is evaluated. A nonlinear analysis model, based on the incremental deformation method, is developed to predict the entire response of concrete members or iginally designed with or strengthened by external prestressing. The propos ed analysis reproduced experimental results of deflection and tendon stress responses with remarkable accuracy. A parametric study was undertaken to e valuate the behavior of concrete beams either designed using external prest ressing or strengthened by it. The analytical results demonstrated that the second-order effect, associated with the progressive change in eccentricit y of the tendons with increasing member deformation, is the main factor tha t distinguishes the behavior of external tendon from an internal unbonded t endon system. Because of this effect, undeviated external tendons mobilized lower nominal flexural resistance and inelastic deflections than did devia ted tendons. When used for strengthening concrete flexural members, providi ng a moderate amount of external prestressing steel leads to significant de flection recovery, some reduction in the live load deflection, and a substa ntial increase in the yield load and ultimate flexural capacity of the memb ers.