Deflection of Partially Prestressed Members


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Title: Deflection of Partially Prestressed Members

Author(s): Dan E. Branson and A. Fattah Shaikh

Publication: Special Publication

Volume: 86


Appears on pages(s): 323-364


Date: 8/1/1985

Unified procedures for computing the deflection (using Ie) and centroidal axis location (using a,) of nonprestressed and partially prestressed members under short-term loading are presented. These procedures include the average effects of tension stiffening and the random distribution of cracks along a member. Also included is the simplified Ie method adopted for the ACI Code (R/C) and PCI Handbook (P/C) since 1971, and AASHTO (R/C) since 1973; as well as the proposed Australian Concrete Code and the PCA Notes on the 1983 ACI Building Code for both nonprestressed (R/C)and partially prestressed (P/C) members. The unified I, method is based on the zero deflection point as the reference, whereas the simplified Ie, method for prestressed members is based on computing the live load deflection increment from the 'prestress camber' -minus - 'dead load deflection' point. Results by the two Ie methods are shown to be in relatively close agreement. A comparison is made between the centroidal axis location (ae d), based on an empirically determined average effective partially cracked section; and the theoretical centroidal axis location (c1), as determined by Nilson for the maximum tensile stress section. The results are thought to be reasonable. For computing long-term camber and deflection, a simple multiplier method used in the PCI Design Handbook, and a somewhat more detailed multiplier method used by ACI Committee 435 since 1963 and also used in the PCA Notes on the 1983 ACI Building Code, are applied to partially prestressed members. Results by the two methods are shown to be comparable.