The city of Vienna is located in a weak earthquake zone with a seismic period of 475 years. The strongest earthquake in this period happened in the year 1590. Another strong earthquake with the intensity of 60% of the 1590-earthquake occurred in the year 1972. It just caused small damages to existing buildings. Although there were many wars but fortunately just few natural disasters, the city of Vienna still has a well preserved city-core inside the 1. district. This area was inside the city defence wall, the 'Ring'. This wall has been removed between 1858 and 1875 and it is now the 'Ring Road' (Ringstraße). Outside this area there are newer buildings, called -Gründerzeit--buildings. They have been built between 1850 and 1910 in the age of promoterism. These buildings still dominate the townscape of Vienna. They are taller and more slender than the older mainly baroque buildings. 'Gründerzeit'-buildings are clay-brick masonry buildings with wooden joist floors. The roof floor is mostly a timber floor, the cellar floor a brick arch. There are about 30.000 such buildings in Vienna. They are beloved because of their architectural style and high rooms. Since the 1970s the attic conversions 'Dachgeschoßausbauten' are very popular. These additional new apartments, instead the old roof, were built without special static rules except the control of the masonry and soil pressure. Since June 2009, when Austria as a member of the EU harmonized and implemented the Eurocodes, new rules have to be applied when changing the structure of old buildings. The problem for structural engineers is that there are no generally accepted static models for traditional masonry buildings. Using simple models as cantilever models, only very low capacities <50% can be calculated for Viennese 'Gründerzeit'-buildings. At the search for a realistic model, the author found rules for Truss and Frame models in the Eurocodes EC 8 and EC6. Such models are frequently used in engineering practice, even for masonry buildings mainly outside of Austria. Following the rules of the Eurocodes, the engineering practice and of course existing research works, a 3D-truss-modell was developed within a working circle of the Austrian Engineering Association (ArchIng). The author implemented traditional wooden joists and their connections in the model. A further developed model was presented by the author in the D-A-CH 2013 congress in Vienna. A realistic model is essential for the correct distribution of the lateral forces to the shear walls and calculation of the earthquake capacity of the building. The Model named CATS (Computer Aided Truss & Shell Model) is a linear model for static and dynamic analysis of structures built in accordance with the rules in the Eurocodes. The bracing ability of traditional wooden joist floors in 'Gründerzeit'-buildings were tested in laboratory and field tests. The results have been verified by 'hand', using simple mechanical models. The calibrated results were then implemented in the CATS-Model. The comparative calculations have shown that traditional wooden joist floors have a significant influence to the bracing ability of the traditional 'Gründerzeit'-buildings in the earthquake load case.