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A single span-to-depth ratio function is proposed for control of deflections in one- and two-way concrete construction. The equation can be applied to one-way slabs, beams, flat plates, flat slabs, and edge-supported two-way slabs. Effects of cracking, time-dependent deformation, boundary conditions, applied loading, and target deflection-to-span ratio are taken into account. Design examples are provided to demonstrate the application of the proposed equation for proportioning members for deflection control.
Keywords: beam; concrete; deflection; serviceability; slab; span-depth ratio.
(ProQuest Information and Learning: ... denotes formulae omitted.)
INTRODUCTION
The ACI Building Code (ACI Committee 318 2005) provides two approaches for control of deflections of concrete members. In the first approach, minimum thickness requirements are provided for both one- and two-way construction. The minimum thickness is specified as a fraction of span length. In the second approach, deflections are calculated and compared with code-specified limits. The minimum thickness provisions are attractive due to their simplicity. Limitations of the current minimum thickness provisions, however, have been discussed by a number of authors (Grossman 1981, Rangan 1982, Gilbert 1985, Hwang and Chang 1996, Scanlon and Choi 1999, Scanlon et al. 2001, and Bondy 2005). The main concerns can be summarized as follows:
1. The amount of deflection that can be expected for a selected thickness is unknown;
2. The minimum thickness is independent of applied load;
3. The range of span lengths for which the provisions can be used is not specified;
4. The minimum thickness values for one-way construction are for members not supporting or attached to partitions or other construction likely to be damaged by large deflections; and
5. There is no limitation given in the code for application of the provisions for two-way construction other than a statement in the Commentary of ACI 318-05 that "These limits apply to only the domain of previous experience in loads, environment, materials, boundary conditions, and spans."
In this paper, a generalized minimum thickness equation is developed for one- and two-way nonprestressed construction in terms of span-to-depth ratio incorporating applied loads, long-time multiplier, allowance for cracking, and target deflection-to-span limitation. The formulation is based on the approach suggested by Rangan (1982) for one-way construction and extended to two-way construction by Gilbert (1985). The proposed equation extends the formulation presented by...