Deflections of reinforced concrete flat slabs

[摘要] ENGLISH SUMMARY: It is found that the serviceability limit state often governs the design of slender reinforced concretemembers. 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, theavailable methods to evaluate the serviceability conditions produce inadequate and unrealisticresults. The evaluation of the serviceability of a slender member includes the calculation of thepredicted deflection, either by empirical hand-calculation or analysing a finite element model, andthe verification using the span-to-effective-depth ratio.The focus of the study is on flat slab structures. It investigates the different deflection predictionmethods and the span-to-effective-depth ratio verifications from various design standards. Thesedesign standards include the ACI 318 (2002), the SABS 0100-1 (2000), the EC2 (2004) and the BS8110 (1997). The background to the methods, as well as the parameters which influences thedeflection development for lightly reinforced members, are investigated in order to define thelimitations of the methods. As a result of the investigation of the deflection calculation methods, anAlternative Approach is suggested and included in the comparisons of the various methods.The deflection prediction methods and the span/effective depth verification procedures areaccurately formulated to predict the serviceability behaviour of beams. Additional approaches hadto 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 arecalculated and compared to the recorded data of seven experimental flat slab specimens asperformed by others. A study by Gilbert and Guo (2005) accurately recorded the flexural behaviourof flat slab specimens under uniformly distributed loads for test periods up to 750 days. Themethods to evaluate the serviceability of a slender member were also applied to slab examplesdesigned 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 depthratio 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 resultsthan those of the finite element models. The empirical hand-calculation methods are accuratedepending 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 playedby construction procedures, material parameters and loading history on slab behaviour.

[发布机构] Stellenbosch University