In the restoration or repair of old buildings, the correct estimation of the stress-strain-relationship of the existing structure is an important task. Masonry stiffness is crucial to the load bearing behaviour of a structure. When detailed investigations are lacking, it is common practise to evaluate the masonry stiffness from the compressive strength by standardised empirical relations established for various types of modern masonry. This paper illustrates the danger of this approach in two case studies. The first example is a stone bridge upon the Main at Ochsenfurt. The thickness of the mortar joints in this construction allowed the sampling of mortars suitable for the direct testing of the stress-strain relationship under uniaxial compression. Thus it was possible to calculate the masonry stiffness from the measured elastic modulus of stone and mortar. It became evident that a first rough estimation according to the building standard would not have been safe. In a second case study dealing with masonry pillars it is shown that the quality of mortar samples are influenced by the position of mortar in the masonry section.
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