Analysis of prestressed concrete structures exposed to fire

摘要

The thesis presents the mathematical model and numerical procedure for geometrically and materiallyudnon-linear static analysis and fire resistance of prestressed concrete structures. The mathematical modeludis limited to pretensioned prestressed concrete structure. In the derivation of matematical model theudslip effect between concrete part of the structure and prestressing steel is considered, while the concreteudspalling is not included into the matematical model. Because the problem was very complex we decidedudto divide the thesis into two parts. The first part presents the static analysis and the system of fundamentaludequations of prestressed concrete structures considering slip effect between concrete part of structureudand prestressing steel. Reissner’s planar beam theory is used to model the concrete part of the structure,udwhile the prestressing steel is modelled as a rope model. The system of the fundamental equations has toudbe solved numerically. Therefore, a modified principle of virtual work is used to derive the strain basedudfamily of finite elements for a non-linear analysis of prestressed concrete structures. The applicabilityudand the accuracy of the proposed mathematical model and numerical procedure of prestressed concreteudstructures subjected to static load are illustrated and verified by numerical examples avaliable in literature.udIt was found out that the slip effect between concrete part of the structure and prestressing steel hasuda significant influence on kinematic and static quantities in prestressed concrete structures. In the secondudpart, which is the main part of the thesis the mathematical model and numerical procedure is expandedudto the analysis of prestressed concrete structure exposed to fire. Because the problem is very complex,udthe problem was devided into two phases. In the first phase the temperature and moisture content inudconcrete section are analysed. The Luikov matematical model with two coupled non-linear partial differentialudequations is used to describe the simultaneous heat and moisture content. Material parametersudare dependent on temperature and moisture content. Generally, the exact solution of this problem is notudknown, therefore the simultaneous heat and moisture content in porous material has to be solved numerically.udIn our case the finite difference method is used. The temperature field of the structure representsudthe temperature load in the second phase of the analysis, where the mechanical response of prestressedudconcrete structures simultaneously exposed to static load and fire is presented. Concrete, reinforcing andudprestressing steel all depend on temperature. Also, the slip-shear stress relationship between concreteudpart of the structure and prestressing steel depend on temperature. The geometric strain increment isudassumed to be the sum of increments of elastic, plastic, temperature and creep strain in concrete, reinforcingudsteel and prestressing steel and the increment of trainsent strain in concrete. The applicability andudthe accuracy of prestressed concrete structures subjected to static load and fire is illustrated and verifiedudby numerical examples avaliable in literature. It was found out that simultaneous heat and moisture contentudhas an important influence on the temperature and moisture content analysis in concrete. It was alsoudfound out that the slip effect between concrete part of the structure and prestressing steel significantlyudaffects on the stress and strain state of prestressed concrete structures, when taken into account.