Application of the I-Effective Method in Calculating Deflections of Partially Prestressed Members

F or computing curvatures and deflections of partially cracked members, the effective moment of intertia (Ie , I e) method l,1,3 provides a transition value between well-defined limits in the uncracked or I,) and fully cracked (I, r ) states. The method is applicable to shorttime deflections of non-prestressed and prestressed members alike, and empirically accounts for the effect of tension stiffening (increased stiffness due to concrete in tension, including between cracks). It is also applicable to individual sections, to simple beams, and between inflection points of continuous beams; so that a practical solution that takes into account the random distribution of cracks for all cases, including non-uniform members, is available. The method is similar to the approach+ of determining the partially cracked deflection as an intermediate value between the uncracked (State 1) and fully cracked (State II) deflections. The I-effective method has been adopted for the 1971 and 1977 ACI Building Codes, 6 the 1971 and 1978 PCl Design Handbooks,? the 1973 and 1977 AASHTO Highway Bridge Specifications,8 and the 1977 Canadian Building Code .9 However, its application to non-prestressed and prestressed members has been somewhat different in the past;$ with the live load IQ for prestressed members determined from the prestress plus dead load deflection point, as shown in Fig. 1, and not from the zero deflection point, as in the non-prestressed case using the dead