Keywords: finite element method, transient heat transfer, solar radiation, geodetic monitoring, structure displacements, cathedral.

This paper reports on the geodetic monitoring of Saint Vitus' Cathedral and St. Georges' Basilica in the Prague Castle and determines whether the measured displacements were caused by temperature extensibility or by other influences. By the geodetic monitoring vertical and horizontal distances between the monitoring points were measured several times a year. These measured displacements were compared with the displacements aimed from the theoretical analysis.

The theoretical thermo-mechanical analysis of the part of the structure was provided using a three-dimensional numerical model. Analysis was divided into two steps. The first one is a numerical simulation of the distribution of temperatures in the structure. The second one is the stress and displacement analysis which has as the input the distribution of temperatures. To find out the temperature field in the structure the numerical solution of the transient heat transfer equation was used. On the surface of the structure two effects were simulated: heat transfer on the interface sandstone-air and the intensity of the absorbed solar radiation [1].

The next step is the comparison of the theoretical and measured deformations for the periods: the time of geodetic measurements. The analysis of St. Vitus Cathedral shows that displacements obtained from the model correspond well with displacements obtained from geodetic measurement during the periods "1" - "4". But the difference between theoretical and measured displacements is in the fifth period more than 3 times higher than in previous periods. This difference could be caused by moisture changes in the structure, but this impact should not be radical if the weather conditions before the measurement were similar. The next possible effect is the gradual increase in the irreversible deformations which can be caused by cycling thermal and moisture strain. A significantly higher difference between theoretical and measured displacements in this case could also be caused by the horizontal movement of the soil. This movement was recently detected in a geologist drill hole which is in the foundations of the cathedral.

The method shown in this paper could help to detect the beginning of an irreversible degradation process in the structure by comparing the theoretical displacements and the measured displacements. The method is at the beginning but in the future it could differentiate between the displacement caused by the temperature from those caused by another impacts and it could provide an alert to possible problems with buildings.

1

H. Hens, "Building Physics - Heat, Air and Moisture", Ernst & Sohn Verlag, Berlin, 2007.

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