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The subject of this thesis is the static and dynamic analysis of the sacred Byzantine church of Panagia Paregoritissa, which is located in Arta. The church of Panagia Paregoritissa was built in two stages and the final stage dates back to the 13th century a.C.; at the time Arta was the capital of the Despotato of Epirus. The church is a masonry construction, it has large cubic shape and at the roof it has six domes, one central, four perimetric and a dome which is supported by marble columns.
The support system of the central dome is unique and particularly audacious, since part of it are 43 marble columns. The dome is carried by four arches, which are supported by pairs of marble columns, which are placed onto horizontal marble cantilevers. These cantilevers are placed onto another pairs of marble columns, which are carried by a second row of cantilevers. The aim of this thesis is to perform the static and seismic analysis of the whole model and more specifically the support system of the central dome.
The first important part of this work is the graphical 3D representation of the church with the use of the software AutoCAD. The geometry is based on the drawings of A. Orlandos, who had documented and measured the building in 1963. Next part is the development of the numerical model of the church with the use of the finite element analysis software ABAQUS. For the simulation 3D quadratic tetrahedral elements were used. Every part of the model was meshed independently with the free mesh generator of the program. In general, the mesh of the more massive parts was not dense, to control the size of the model, except for the parts which develop the highest stress and have a significant stress variations. The final model, which was created, consists of 685888 3D finite elements and has 3421020 degrees of freedom. The characteristic values of the mechanical properties of the different materials were selected from bibliography and relevant references. The foundation level was considered 1m below the ground level.
The final model was analyzed considering an elastic behavior of the materials. A static analysis was performed to determine the limit state corresponding to self-weight, the load of the cladding at the roof and the load due to snow. After that the eigenfrequencies of the model were computed. In addition, modal analyses were conducted without some parts of the model, in order to examine the influence of these parts to the stiffness of the whole model and the support system of the central dome.
Also, a dynamic analysis was performed, following the mode superposition method to determine the seismic excitation response. More specifically, five response spectrum analyses were conducted for different cases of seismic loading. Through the response spectrum analyses the influence of the vertical component of the earthquake to the support system of the central dome was investigated. Finally, two dynamic time history analyses were conducted for the earthquake that occurred in Aigio in 1995 of seismic magnitude Mw=6.4 and for the earthquake that occurred in Leukada in 2003 of seismic magnitude Mw=6.2.
The main results of every analysis are presented and compared. More specifically, deformed geometry of the model is presented, the Von Mises stresses that give an overall measure of stressing as well as compressive and tensile stresses at some critical regions. The stresses which develop in elements that are part of the support system of the central dome are emphasized. In the case of time history analyses the max displacement and stresses for the crucial time step and the most significant time history diagrams are also presented. Through this analysis it is possible to identify the regions of the model which are vulnerable and can lead to some type of failure. These regions may also need repair and /or strengthening and further a more detailed analysis and redesign. Through all the analyses cases that have been carried out it is clear that the construction is very stiff, since it is built by firm masonry. The vertical loads alone do not over stress the construction, which is mainly vulnerable to lateral loads due to earthquakes.
The mode shapes and the effective masses, which were computed, indicate the uniqueness and complexity of the church of Panagia Paregoritissa. From the results of the modal analysis it is concluded that the construction is more flexible and therefore vulnerable in the X direction and that in the vertical direction it is quite stiff. During the response spectrum analyses high stresses are developed but with the exception of the dome shell, which is supported by marble columns, the elements are moderately stressed lower than their strength. On the contrary, due to the seismic loads of Aigio and Leukada several elements develop stresses higher than the strength of the materials. The crucial load case is the time history of the earthquake that occurred in Leukada.
With reference to the support system of the central dome, during all the analyses the marble columns, which support the central dome, do not develop significant Von Misses stresses, unlike the masonry of the ledger, which is vulnerable to failure. However, in the case of an earthquake it is possible the marble columns to develop significant cracks or may break. Also, the support system of the central dome is not affected particularly by the vertical component of the earthquake. Finally, the timbered beams are very important to restrain the arches that support the central dome.
It turns out that the weight of the central dome is carried primarily by the masonry of the ledge and the surrounding walls and partly by the marble columns besides the aesthetics that leave the opposite impression.
The main conclusion of the present study is that in case of a strong earthquake the structure will suffer significant stresses that may lead to local or more extended failure. This together with the already accumulated damage in certain part of the church point to further systematic experimental and analytical studies that will determine the adequate repair and strengthening measures, which will enhance the overall performance of this monumental church. |
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