Numerische Simulation von unbewehrten und textilverstärkten Mauerwerksscheiben unter zyklischer Belastung

Kalker, Ines; Meskouris, Konstantin (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2007)
Dissertation / PhD Thesis

Abstract

For thousands of years man kind puts up buildings to protect himself against exposition to nature. In this course the simplest way to construct has been developed very early: more or less shaped stones are layered on top of each other. Mortar, clay or other matrix materials are inserted between the stones to keep them in their position and assure the distribution of forces to the foundation. Worldwide almost all historical buildings have been constructed in this way. Doubtlessly masonry construction has developed over the millennia/ centuries so that nowadays high strengths can be reached by optimising form and type of the stones as well as the applied matrix and many other factors. As a result of high compression strength masonry is suitable for bearing vertical loads applied by dead and live load. Horizontal load transfer however is extremely limited due to the low tensile strength of the material. Earthquake loads are particularly problematical as masonry is a brittle material with only low ductility. In spite of these characteristics masonry was and still is the prevailing construction method in many seismic vulnerable countries because of local availability and excellent thermal insulation. One possibility to improve the seismic capacity of masonry is laminar textile strengthening, enlarging the masonry’s tensile strength and ductility without charging the masonry with additional weight. Another advantage is the flexible application on existing and designed masonry buildings. Although the effectiveness of textile strengthening is well known so far no suitable numerical models for calculating the capacity of textile strengthened masonry shear walls exist. In the present paper a smeared model is developed in order to simulate masonry strengthened with a bidirectional textile fabric with variable orientation in a matrix of cementitious mortar. The brittle, non-linear material behaviour is described by a cyclic two-dimensional non-linear macro model which is based on the principal of equivalent uniaxial strain according to DARWIN and PECKNOLD. The idea of this concept is the decoupled formulation of the biaxial stress-strain-relationship for each principle stress direction. Then the current stress state depends not only on the present strain state but also on the load history. The main advantage of this formulation is the applicability for cyclic loading as well as the application of uniaxial material parameters and stress-strain-relationships which can be determined by simple uniaxial tests. The failure criterion defines the biaxial tensile and compression strengths as functions of the principle stress ratio for different bed joint orientations. If the strengthened masonry fails in tension the tension stiffening effect due to participation of the masonry between the cracks is calculated using a modified textile characteristic. The non-linear characteristic is approximated as a polygon whose sampling points are defined by the average textile strain between the cracks and the corresponding textile stress within the crack for different degrees of crack initiation. The average crack distance is calculated depending on the transfer length regarding the distribution of the masonry’s tensile strength for a certain degree of crack initiation according to KRELLER. The resulting textile stress within the crack is determined by the equilibrium of forces of the crack element considering the angle between strengthening direction and crack orientation. The applicability of the model implemented into the FE software program ANSYS is shown by simulation of unreinforced and textile strengthened masonry shear walls. The calculations have been made for vertically and horizontally loaded walls with different geometries. The simulations show that the model is able to represent the characteristic failure modes of masonry shear walls. Also the dependency between failure mode and material ductility is demonstrated. All in all the validity of the model for different masonry types can be demonstrated.

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