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Title: Deflection Control of Concrete Beams Accounting for Shear Deformations

Author(s): Adam S. Lubell

Publication: Symposium Paper

Volume: 328


Appears on pages(s): 7.1-7.18

Keywords: deflection, diagonal cracking, MCFT, reinforcement ratio, serviceability, shear deformation, beams, slabs, design

DOI: 10.14359/51711151

Date: 9/12/2018

The cross-section and reinforcement in a concrete beam must be selected to provide sufficient strength at the ultimate limit state while limiting the service deflection to an acceptable magnitude. ACI 318 analytical models for flexural capacity and deflection of slender beams assume that plane sections remain plane after bending and perpendicular to the longitudinal axis, but this hypothesis ignores the presence of diagonal cracking and related deformations associated with the imposed shear. This paper reports on an analytical deflection model developed using simplifications to the Modified Compression Field Theory that superimposes contributions from the flexural deformations arising from member curvatures and the shear deformations arising from diagonal cracking. The model is shown to be in better agreement with test data than the ACI 318 deflection model that only accounts for curvatures. A parametric study was completed using the model to gain insight into the influence of beam span-to-height ratio and the longitudinal and transverse reinforcement ratios on beam deflection. Recommendations are made on using a holistic design approach to satisfy both strength and serviceability requirements for a given span-to-height ratio.