Controlling Deflection Serviceability by Span/Depth limits and Long-Term Deflection Multipliers for Reinforced Concrete Beams

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Title: Controlling Deflection Serviceability by Span/Depth limits and Long-Term Deflection Multipliers for Reinforced Concrete Beams

Author(s): N. J. Gardner and J. Zhang

Publication: Special Publication

Volume: 161

Issue:

Appears on pages(s): 165-196

Keywords: beams (supports); compression; deflection; tension; slabs; reinforced concrete; serviceability; span-depth ratio; thickness; Structural Research

Date: 4/1/1996

Abstract:
North American design codes offer two methods to insure deflection serviceability. The design engineer can calculate the live load and sustained load deflections and check that they are less than code specified limits. Alternatively, the codes give maximum span/depth ratios for which serviceability can be assumed to be satisfied and deflections do not need to be calculated. However, the span/depth provisions of ACI 318-89 and CSA A23.3- M84 do not consider many of the factors which influence the deflection behavior of reinforced concrete beams and may not be consistent with the code specified deflection limits. The immediate and long term deflections of reinforced concrete beams were calculated using a layered, nonlinear finite element model. The long term deflections were calculated by a hybrid technique using an effective reduced modulus for concrete creep and a conventional finite element, time-dependent load vector for shrinkage and tensile cracking. The modelling technique was verified using the extensive experimental data of Christiansen. Span/depth ratios are proposed, which include the effects of concrete strength, tension steel ratio, and compression steel ratio, for incremental deflection criteria of span/500 and span/250. Long term deflection multipliers are given for sustained moments of 30, 50, and 70 percent of the design ultimate moment.