Evaluation ways of deflections of beams and one-way slabs

Increase of concrete and reinforcing steel strengths over the years have led to increased spans and larger slenderness of structural elements with lower flexural stiffness. Thus, limiting the deflections and the ability to evaluate them under service loads, have become an important part of the design of reinforced concrete beams and slabs. Design codes allow approximate yet detailed methods for the calculations of deflections. They consider the main parameters that affect deflections (including influences of creep and shrinkage) and are mostly based on interpolation between the influences on the deflection of cracked and uncracked cross-sections properties. Alternatively, codes provide a simplified 'slenderness requirement' that exempts the designer from more detailed calculations, where 'slenderness' is defined as the ratio between the beam span (or equivalent span) and its cross-section's height (actual or effective). The European code, as well as the Model Code2010 slenderness provisions are based on calibrated formulation from extensive detailed calculations of various cases. Yet, they require the designer to assume the reinforcement ratios (needed for flexural ULS verification) and do not refer explicitly to the design loads (but only implicitly, through the reinforcement ratios). An alternative procedure for the slenderness requirement is provided in the Israeli design code (SII). It requires the designer to use the given loads and concrete modulus of elasticity. This paper presents the slenderness limit of the SII code and shows in a numerical example evaluation of the above detailed calculation procedures for a wide range of spans, for residential buildings. The results of this example show the suitability of the alternative procedure compared with the one provided in EC2.