Deflections of reinforced concrete flat slabs

Eigelaar, Estee M. (2010-03)

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010.

Thesis

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.

AFRIKAANSE OPSOMMING: Die diensbaarheidstoestand is in baie gevalle die bepalende faktor vir die ontwerp van slank gewapende beton elemente bepaal. Slank elemente, soos lig bewapende buigbare beton elemente, het gewoonlik ‘n persentasie trekbewapening van minder as 1.0% en ‘n aangewende buigmoment net wat net groter is as die punt waar kraking voorkom. Die metodes beskikbaar om die diensbaarheid van sulke elemente te evalueer gee onvoldoende en onrealistiese resultate. Die evaluering van die elemente in die diensbaarheidstoestand sluit in die bepaling van defleksies deur berekening of die analise van ‘n eindige element model, en die gebruik van die span/effektiewe diepte metode. Die fokus van die studie is platbladstrukture. Die doel van die studie is om die verskillende metodes vir die bereking van defleksie asook die verifikasie volgens span/effektiewe diepte metodes van die verskillende ontwerp standaarde te ondersoek. Die ontwerp standaarde sluit die ACI 318 (2002), SABS 0100-1 (2000), EC2 (2004) en die BS 8110 (1997) in. Die agtergrond van hierdie metodes is ondersoek asook die parameters wat ‘n rol speel, sodat die beperkings van die metodes geidentifiseer kan word. As ‘n gevolg van die ondersoek na die beperkings van die metodes, is ‘n Alternatiewe Benadering voorgestel. Die Alternatiewe Benadering is saam met die metodes van die ontwerpstandaarde gebruik om die verskille tussen die metodes te evalueer. Die defleksievoorspelling en die span/effektiewe diepte verifikasie metodes is korrek geformuleer om die diensbaarheid van balke te evalueer. Ander benaderings was nodig om die diensbaarheid van blad blaaie te toets. Die onderskeie defleksievoorspelling en span/effektiewe diepte metodes is bereken vir sewe eksperimentele plat blaaie soos uitgevoer deur ander navorsers. Gilbert and Guo (2005) het ‘n studie uitgevoer waar die buigingsgedrag van die sewe plat blaaie, met ‘n uniforme verspreide las vir ‘n toetsperiode van tot 750 dae, akkuraat genoteer is. Die metodes om die diensbaarheid van ‘n slank element te toets, was ook op Suid-Afrikaanse blad voorbeelde getoets. Dit was gedoen om die Suid- Afrikaanse ontwerp van ligte bewapende beton elemente te evalueer. Die gevolgetrekkings stel ‘n gepaste defleksie metode vir ‘n slank element vir verskillende beperking kategorië voor. Dit is ook verduidelik hoe die tipiese span/effektiewe diepte verhouding met die persentasie trek bewapening vir ‘n slank element verander. Dit is bevind dat die imperiese handmetodes om defleksies te bereken, meer betroubaar as die eindige element modelle se resultate is. Die imperiese handberekening metodes is akkuraat relatief tot hoe akkuraat die blad konstruksie tot die blad ontwerp voltooi is. ‘n Vergelyking van defleksieberekening met Suid-Afrikaanse gevallestudies het die belangrikheid van konstruksieprosedures, materiallparamteres and belastingsgeskiedenis geïdentifiseer.

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