Reliability assessment of a prestressed concrete member

[摘要] ENGLISH ABSTRACT:First-order second-moment structural reliability methods are used to assess the reliability ofa prestressed concrete beam. This beam was designed for imposed office floor loads andpartitions following the limit states design method as provided for by the applicable SouthAfrican 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 theultimate 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 codespecifieddeflection criteria. Realistic theoretical models are selected to express the flexuralstrength and deflection of the prestressed concrete member, while appropriate probabilisticmodels 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 thanexpected in view of the fact that this represents a non-critical limit state in the case of a Class2 prestressed concrete member. This condition can be explained with reference to therelatively high uncertainty associated with the modelling error for flexural strength. Thecalculated reliability index at the serviceability limit state of deflection (l.67) compares wellwith acceptable practice.The study further focuses on the sensitivity of the reliability at the two limit states of interestto uncertainty in the various design parameters. The ultimate limit state of flexure isdominated by the uncertainty associated with the modelling error for flexural strength, whilethe contribution to the overall uncertainty of the ultimate strength and area of the prestressingsteel and the effective depth is less significant. In comparison the reliability at theserviceability limit state of deflection is not dominated by the uncertainty associated with asingle basic variable. Instead, the uncertainty associated with the modelling error, creep factorand prestress loss factor are all significant. It was also demonstrated that the variability inbeam stiffness is not a major source of uncertainty in the case of a Class 2 prestressedconcrete member.It is recommended that the present code provisions for ultimate strength and deflection shouldbe reviewed to formulate theoretical models with reduced systematic and random errors. Theeffect of the uncertainty associated with the creep and prestressed loss factors should also beadressed by adjustment of the partial material factor for concrete at the serviceability limitstate of deflection. Furthermore, research must be directed towards formulating an objectivefailure criterion for deflection. The uncertainty in the deflection limit must therefore bequantified with a probability distribution.

[发布机构] Stellenbosch University