Immediate Deflection of Partially Prestressed Concrete Flexural Members


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Title: Immediate Deflection of Partially Prestressed Concrete Flexural Members

Author(s): Peter H. Bischoff, Clay J. Naito, and Joseph P. Ingaglio

Publication: Structural Journal

Volume: 115

Issue: 6

Appears on pages(s): 1683-1693

Keywords: cracked section; deflection; live load; partial prestressing; prestressed concrete

Date: 11/1/2018

The ACI 318 Building Code permits calculation of immediate deflections for cracked prestressed concrete slabs and beams based on either a bilinear moment deflection relationship or an effective moment of inertia. The effective moment of inertia Ie is taken as a weighted average of the uncracked moment of inertia Ig and cracked moment of inertia Icr. Accuracy of deflection calculations based on Ie depends on the value of cracking moment and requires an upwards shift in the Icr response. A new approach is proposed for computing service load deflections of cracked prestressed members. The proposed approach is compared with the existing ACI 318 approaches using the measured load-deflection response of prestressed beams and is shown to provide a more accurate and consistent prediction of deflections than the existing approaches. An alternative trilinear model that is much simpler but not as accurate is shown to provide reasonable results as well. It is recommended to compute deflection of cracked prestressed members using the trilinear approach except in those instances where higher accuracy is needed.