Deflections of reinforced concrete flat slabs

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Deflections of reinforced concrete flat slabs

Author: Eigelaar, Estee M. | Size: 17 MB | Format: PDF | Quality: Original preprint | Publisher: University of Stellenbosch | Year: 2010-03

ENGLISH SUMMARY: It is found that the serviceability limit state often governs the design of slender reinforced concrete members. Slender flexural members often have a percentage tension reinforcement less than 1.0% and an applied bending moment just above the point of first cracking. For such members, the available methods to evaluate the serviceability conditions produce inadequate and unrealistic results. The evaluation of the serviceability of a slender member includes the calculation of the predicted deflection, either by empirical hand-calculation or analysing a finite element model, and the verification using the span-to-effective-depth ratio. The focus of the study is on flat slab structures. It investigates the different deflection prediction methods and the span-to-effective-depth ratio verifications from various design standards. These design standards include the ACI 318 (2002), the SABS 0100-1 (2000), the EC2 (2004) and the BS 8110 (1997). The background to the methods, as well as the parameters which influences the deflection development for lightly reinforced members, are investigated in order to define the limitations of the methods. As a result of the investigation of the deflection calculation methods, an Alternative Approach is suggested and included in the comparisons of the various methods. The deflection prediction methods and the span/effective depth verification procedures are accurately formulated to predict the serviceability behaviour of beams. Additional approaches had to be used to apply these methods to a two-dimensional plane such as that of a flat slab structure. The different deflection prediction methods and the span/effective depth verification methods are calculated and compared to the recorded data of seven experimental flat slab specimens as performed by others. A study by Gilbert and Guo (2005) accurately recorded the flexural behaviour of flat slab specimens under uniformly distributed loads for test periods up to 750 days. The methods to evaluate the serviceability of a slender member were also applied to slab examples designed using South African standards. The study concludes by suggesting a suitable deflection prediction method for different parameter (limitation) categories with which a slender member can comply to. The typical span/effective depth ratio trend is also presented as the percentage tension reinforcement for a slender member changes. It is observed that the empirical hand-calculation methods present more reliable results than those of the finite element models. The empirical hand-calculation methods are accurate depending on the precision to which the slab was constructed relative to the actual slab design. The comparison of the deflection methods with South African case studies identified the role played by construction procedures, material parameters and loading history on slab behaviour.

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