Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 48

Chapter 3
Simplified Design for Beams
and One-Way Slabs

3.1 INTRODUCTION

The simplified design approach for proportioning beams and slabs (floor and roof members) is based in part
on published articles, 3.1-3.6 and in part on simplified design aid material published by CRSI. 3.7,3.10 Additional data
for design simplification are added where necessary to provide the designer with a total simplified design
approach for beam and slab members. The design conditions that need to be considered for proportioning the
beams and slabs are presented in the order generally used in the design process.

The simplified design procedures comply with the ACI 318-11 code requirements for both member strength and
member serviceability. The simplified methods will produce slightly more conservative designs within the limitations
noted. All coefficients are based on the Strength Design Method, using appropriate load factors and strength reduction
factors specified in ACI 318. Where simplified design requires consideration of material strengths, 4000 psi concrete
and Grade 60 reinforcement are used. The designer can easily modify the data for other material strengths.

The following data are valid for reinforced concrete flexural members with › = 4000 psi and f y = 60,000 psi:

modulus of elasticity for concrete E c = 3,600,000 psi (ACI 8.5.1)

modulus of elasticity for rebars E s = 29,000,000 psi (ACI 8.5.2)

minimum reinforcement ratio (beams, joists) ρ min = 0.0033 (ACI 10.5.1)

minimum reinforcement ratio (slabs) ρ min = 0.0018 (ACI 7.12.2) and (ACI 10.5.4)

maximum reinforcement ratio* ρ max = 0.0206 (ACI 10.3.5)

maximun useful reinforcement ratio** ρ t = 0.0181 (ACI 10.3.4)

3.2 DEPTH SELECTION FOR CONTROL OF DEFLECTIONS

Deflection of beams and one-way slabs need not be computed if the overall member thickness meets the
minimum specified in ACI Table 9.5(a). Table 9.5(a) may be simplified to six values as shown in Table 3-1. The
quantity n is the clear span length for cast-in-place beam and slab construction. For design convenience, min-
imum thicknesses for the six conditions are plotted in Fig. 3-1.

Deflections are not likely to cause problems when overall member thickness meets or exceeds these values for
uniform loads commonly used in the design of buildings. The values are based primarily on experience and are
not intended to apply in special cases where beam or slab spans may be subject to heavily distributed loads or

* The ACI 318-11 does not provide direct maximum reinforcement ratio for beams and slabs. The maximum reinforcement
ratio was derived from the strain profile as limited in ACI Section 10.3.5.
** For tension controlled sections with φ = 0.9