Thesis (MScEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: First-order second-moment structural reliability methods are used to assess the reliability of
a prestressed concrete beam. This beam was designed for imposed office floor loads and
partitions following the limit states design method as provided for by the applicable South
African structural codes, viz SABS 0100-1:1992 and SABS 0160:1989.
The reliability is examined at two limit states. At the ultimate limit state of flexure the
ultimate moment of resistance must exceed the applied external moment at the critical section,
while at the serviceability limit state of deflection the deflection must satisfy the codespecified
deflection criteria. Realistic theoretical models are selected to express the flexural
strength and deflection of the prestressed concrete member, while appropriate probabilistic
models are gathered from the literature for loading, resistance and modelling uncertainties.
The calculated reliability index at the ultimate limit state of flexure (3.10) is lower than
expected in view of the fact that this represents a non-critical limit state in the case of a Class
2 prestressed concrete member. This condition can be explained with reference to the
relatively high uncertainty associated with the modelling error for flexural strength. The
calculated reliability index at the serviceability limit state of deflection (l.67) compares well
with acceptable practice.
The study further focuses on the sensitivity of the reliability at the two limit states of interest
to uncertainty in the various design parameters. The ultimate limit state of flexure is
dominated by the uncertainty associated with the modelling error for flexural strength, while
the contribution to the overall uncertainty of the ultimate strength and area of the prestressing
steel and the effective depth is less significant. In comparison the reliability at the
serviceability limit state of deflection is not dominated by the uncertainty associated with a
single basic variable. Instead, the uncertainty associated with the modelling error, creep factor
and prestress loss factor are all significant. It was also demonstrated that the variability in
beam stiffness is not a major source of uncertainty in the case of a Class 2 prestressed
concrete member.
It is recommended that the present code provisions for ultimate strength and deflection should
be reviewed to formulate theoretical models with reduced systematic and random errors. The
effect of the uncertainty associated with the creep and prestressed loss factors should also be
adressed by adjustment of the partial material factor for concrete at the serviceability limit
state of deflection. Furthermore, research must be directed towards formulating an objective
failure criterion for deflection. The uncertainty in the deflection limit must therefore be
quantified with a probability distribution. / AFRIKAANSE OPSOMMING: Eerste-orde tweede-moment struktuur betroubaarheid metodes word ingespan om die
betroubaarheid van 'n voorspanbeton balk te bereken. Hierdie balk is ontwerp vir opgelegte
kantoor vloerbelasting en partisies volgens die grenstoestand ontwerp metode soos beskryf in
die toepaslike Suid-Afrikaanse boukodes, naamlik SABS 0100-1: 1992 en SABS 0160: 1989.
Die betroubaarheid word ondersoek by twee grenstoestande. By die swiglimiet van buiging
moet die weerstandsmoment die eksterne aangewende moment oorskrei by die kritieke
balksnit, terwyl die defleksie die kriteria soos voorgeskryf deur die kode moet bevredig by
die dienslimiet van defleksie. Realistiese teoretiese modelle word gebruik om die buigsterkte
en defleksie van die voorspanbeton balk te bereken. Verder is geskikte waarskynlikheid
modelle uit die literatuur versamelom die belasting, weerstand en modelonsekerhede te
karakteriseer.
Die betroubaarheid indeks soos bereken vir die swiglimiet van buiging (3.10) is laer as wat
verwag sou word in die lig van die feit dat hierdie nie 'n kritieke grenstoestand
verteenwoordig in die geval van 'n Klas 2 voorspan element nie. Dit kan verklaar word met
verwysing na die relatiewe groot onsekerheid wat geassosieer word met die modellering fout
vir buigsterkte. Die berekende betroubaarheid indeks vir die dienslimiet van defleksie (1.67)
vergelyk goed met aanvaarde praktyk.
Die studie fokus verder op die sensitiwiteit van die betroubaarheid by die twee grenstoestande
onder beskouing ten opsigte van die onsekerheid in die verskillende ontwerp parameters. By
die swiglimiet van buiging word die onsekerheid oorheers deur die bydrae van die modelering
fout vir buigsterkte. Die bydraes tot die totale onsekerheid deur die swigsterkte en area van
die voorspanstaal sowel as die effektiewe diepte is minder belangrik. By die dienslimiet van
defleksie word die betroubaarheid nie oorheers deur die onsekerheid van 'n enkele basiese
veranderlike nie. In stede hiervan is die onsekerheid van die modellerings fout, kruipfaktor
en voorspan verliesfaktor almal noemenswaardig. Daar word verder aangetoon dat die
veranderlikheid in balkstyfheid nie 'n belangrike bron van onsekerheid in die geval van 'n
Klas 2 voorspan element is nie.
Daar word aanbeveel dat die bestaande voorskrifte in die kode vir buigsterkte en defleksie
aangespreek moet word deur teoretiese modelle met klein modelonsekerhede te formuleer.
Die uitwerking van die onsekerheid van die kruip- en voorspan verliesfaktore kan aangespreek
word deur 'n aanpassing te maak in die parsiële materiaalfaktor vir beton in die geval van die
dienslimiet van defleksie. Navorsing moet verder daarop gemik wees om 'n objektiewe
falingskriterium vir defleksie te formuleer. Die onsekerheid van die toelaatbare defleksie moet
dus gekwatifiseer word deur 'n waarskynlikheidsverdeling.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52430 |
| Date | 12 1900 |
| Creators | Brand, W. W. (Willem Wouter) |
| Contributors | Retief, J. V., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various foliations) ; ill. |
| Rights | Stellenbosch University |
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