Title:
Proposed ACI 318 Equations for Immediate Deflection of Prestressed Concrete Members (Prepublished)
Author(s):
Peter H. Bischoff, Wassim Nasreddine, Hani Nassif
Publication:
Structural Journal
Volume:
Issue:
Appears on pages(s):
Keywords:
cracking; effective eccentricity; effective moment of inertia; immediate deflection; prestressed concrete; serviceability; structural design
DOI:
10.14359/51746721
Date:
3/25/2025
Abstract:
Design recommendations are presented for calculating the immediate deflection of cracked prestressed concrete members under service load. Inconsistency and sometimes confusion regarding the calculation of immediate deflection for the different approaches presently available highlight the need for a rational approach to computing deflection. The ACI 318-19 approach for reinforced (nonprestressed) concrete is broadened to include prestressed concrete. This involves the implementation of an effective moment of inertia taken together with an effective eccentricity of the prestressing steel used to define the effective curvature and/or camber from the prestressing force. Proposed revisions to ACI 318 are presented for prestressed Class T and Class C flexural members and clear steps are provided for calculating immediate deflection. The effectiveness of the new approach is validated against an extensive database of test results, showing reasonable accuracy and reliability in predicting deflections. The paper concludes with practical recommendations for implementation and a worked-out example to illustrate the proposed methodology. These findings aim to enhance the accuracy and consistency of deflection predictions in prestressed concrete design, contributing to better serviceability and performance of concrete structures.