OBSERVATIONS DURING STABILIZATION OF OLD BELL TOWERS DAMAGED BY CRACKS

The structure of rubble walls of old bell towers is comparable with the structure of concrete with large aggregates-of course the strength of the mortar of limestone masonry is much smaller than cement stone of lowest quality. Nevertheless it is possible to compare the results of dynamic measurements on towers built by the named kinds of masonry with empirical results of concrete technology and it is also possible to interpret and explain this by fracture mechanical analysis. In this connection the measured data of the dynamic response of a building under shock loadings and periodical loading by shakers before, during and after a stabilization are: the lowest natural frequency; the damping ratio of Lehr; and the horizontal displacements of the tower during bell ringing. The observed masonries showed clear perceptible systems of macrocracks before reconstruction; whose direction deviates only from the perpendicular line. The big pores and cracks in the masonry could clearly be recognized by TV-observation of the interior of boreholes of diameter 100 mm. The cause of the cracks and their growth namely by earthquakes and bell ringing can easily be analysed by traditional suppositions of the theory of technical strength of materials and models of fracture mechanics. The measured effects of different kinds of consolidation of crack damaged walls will be shown in two examples. © 1990.