Time-Dependent Deflections of Prestressed Members: Rational and Approximate Methods


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Title: Time-Dependent Deflections of Prestressed Members: Rational and Approximate Methods

Author(s): Alex Aswad

Publication: Special Publication

Volume: 129


Appears on pages(s): 101-114

Keywords: camber; creep properties; deflection; prestress loss; shrinkage; prestressed concrete; pretensioning; viscoelasticity; Design

Date: 12/1/1991

The long-term camber prediction under sustained loads is an important part of the design of pretensioned flexural members. Critical members are noncomposite roofs and bridge members which have medium or large span-depth ratios or elements made of lightweight concrete. Currently the most common approximate design method is one that relies on empirical multipliers applied to initial cambers and deflections. When compared to a rational approach, substantial differences in prediction of long-term camber or deflection are noticed. The approximate method appears to overestimate the permanent sag or underestimate the camber. It also does not consider certain creep, shrinkage, and relaxation properties. It is concluded that such methods may be unreliable for critical members and that the rational method is preferred. For preliminary design of longer spans, revised multipliers are suggested for use with the approximate method.