Immediate Deflections of Simply Supported, Bonded, Class T and Class C Prestressed Concrete Beams

Abstract

Design practice for prestressed members has moved toward allowing partially prestressed members, which are allowed to crack under service loads. To define design requirements, the ACI 318 Building Code explicitly defines different classes of prestressed flexural members. A simple procedure for computing immediate service-load deflections that accurately incorporates the effects of cracking in Class T and Class C partially prestressed concrete members is not yet standardized. The primary focus of this study is to propose and evaluate the accuracy of a simple method for estimating the immediate deflections of cracked prestressed flexural members subjected to service loads. To achieve this a database of tests representing only beams expected to crack under service loads was created from published data and used to compare four methods of calculation of deflection. The outputs of the methods were compared to reported measured deflection values at key service-load stress levels. Conclusions were made about which of the methods provided accurate results. The methods differed in approach when calculating the effective moment of inertia, cracked moment of inertia, and identifying the load level at which the transition of uncracked behavior transitions into cracked behavior. Overall, of the four methods compared, the proposed method was found to be the most accurate method when predicting immediate deflections for cracked, prestressed flexural members. The proposed method uses a cracked-section moment of inertia calculated including prestressing effects, a decompression moment at which uncracked behavior transitions to cracked behavior, and an effective moment of inertia based on moment ratios relative to the decompression moment.