Transactions on the Built Environment vol 15, 1995 WIT Press, ISSN

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1 Development of a methodology for seismic strengthening of Byzantine churches P. Gavrilovic*, V. Zelenkovska*, W.S. GinelP, L. Sumanov' "Institute of Earthquake Engineering and Engineering Seismology (IZIIS), Skopje, Republic of Macedonia Getty Conservation Institute (GCI), Marina del Rey, 'Institute for Protection of Cultural Monuments (RZZSK), Skopje, Republic of Macedonia Abstract A methodology for the strengthening of Byzantine churches has been developed which is consistent with the main principles of conservation and protection of historic monuments. The methodology is in full compliance with traditional construction techniques and the specific characteristics of the churches and modern requirements for seismic stability. Surveys of 54 Byzantine churches were performed to document the present state of the churches and their main characteristics, structural systems, typology and classification as a basis for development of an appropriate methodology for repair and/or seismic strengthening. For the purpose of verification of the methodology, field experimental investigations and detailed analyses have been performed on four representative churches. To study the dynamic response of structures during expected earthquake ground motions, shaking table tests have been performed. The model for shaking table tests of one of the churches, which was selected as a prototype representative of the Byzantine churches ("St. Nikita" church), was constructed to a scale of 1:2.75 and was tested in both its original and strengthened states. In addition to static tests of material samples taken from St. Nikita, quasistatic tests of three wall elements were performed, under axial and shear pressure, in order to obtain experimental values of the strength and deformability characteristics of the brick and limestone masonry, as well as the type of failure mechanisms. The seismic safety criteria were defined on the basis of investigations of structural response to expected earthquakes and seismic stability analyses were performed that took into account the characteristics and the artistic-historic value of the structure. The procedure, retrofitting techniques, and the principal results of the project will be presented. The retrofitting methods are easily applied and fully satisfy the conservation principle of minimum intervention-maximum protection.

2 38 Dynamics, Repairs & Restoration 1 Introduction The territory of Macedonia is well known for its numerous historic monuments, among the most distinguished being the churches from the Byzantine period (9th - 14th century) that exemplify the highest architectural creations and contain an extraordinary collection of fresco-paintings of important artistic value. The oldest monuments with preserved fresco-paintings that are found in Macedonia originate from the XP century. The most important are the cathedral church of St. Sophia in Ohrid, the church of St Leontie within the monastery of Vodocha, and the Monastery of St Bogorodica Eleusa (St. Mary Eleusa). The XIP and XIII* century Byzantine understanding of stylistic-artistic values are reflected in many of the preserved monuments. The fresco-paintings in St. Sophia, a church that was one of the domiciles of the Ohrid archiepiscopacy, were done by the medieval fresco-painters Mihailo and Eutihie who transformed the way of thinking at the Byzantine Court in a most striking way, through the expressive sense of monumentally, strong strokes and a characteristic athletic shaping of the figures of the painted saints. The monastic life in these monasteries gave rise to the development of the transcribing activity and many XIV^ century artistic works are still preserved in some of the huge monasteries. Located in one of the most active seismic regions in Europe, these structures have been exposed-not only to climatic and human damaging factors but also to the effects of earthquakes The high seismicity of the territory of Macedonia contributed to the partial or complete destruction of these cultural and historic monuments in the past. Earthquakes inflicted not only structural damage but also damage to the mural paintings and other artistic features of these monuments. The buildings belonging to the cultural and historic heritage of Macedonia are constructed of traditional local materials such as stone, bricks and different types of mortars. The mechanical and strength characteristics of these materials make the structural systems of these buildings non-ductile and they are there considered to be very susceptable to seismic damage. 2 Selection of the representative monuments For the purpose of development of appropriate methods and techniques for repair and strengthening of Byzantine type monuments, in general, and particularly the Byzantine monuments located within Macedonia, the research project "Study for Seismic Strengthening, Conservation and Restoration of Churches from the Byzantine period (9th - 14th century) in Macedonia" sponsored by the Getty Conservation Institute - Marina del Rey was initiated by the Institute of Earthquake Engineering and Engineering Seismology (IZIIS) - Skopje and the Institute for Protection of Cultural Monuments (RZZSK) - Skopje. The first phase of the scientific-research project involved acquisition of data on the existing state of 54 cultural-historic monuments which provided a basis for the investigation of the architectural-structural characteristics, structural typology, natural climatic and local conditions, and the historic-artistic values of the churches as well as their general classification. From the entire group of buildings that were the subject of these studies, four buildings were selected for detailed investigation on the basis of the following criteria: Building typology (architecture - structure); Present state of the buildings, extent and causes of damage; Scope and applied methods for conservation, restoration, reconstruction, strengthening and presentation; Authenticity.

3 Dynamics, Repairs & Restoration 39 The four churches selected on the basis of the these criteria were: St Nikola-v. Psacha, St Nikita-v. Banjani, St Bogorodica (St Mary) Zahumska-v. Trpejca, and St Bogorodica (St Mary)-v. Matejche. These can be catagorized as single-dome structures with an inscribed cross or its quincunx modification. They date from the same period (XIY^ century) and there is a real possibility for extension of results from the same kind of experiments. The degree of authenticity, with respect to original design and materials, of these churches decreases starting with the church of St Bogorodica Zahumska through St Nikita and St Nikola. The lowest level of conservation was done on the church of St Bogorodica, whereas the most extensive work were done on the church of St Nikola in the village of Psacha. The church of St Bogorodica in Mateiche was selected because it is of the type of churches with a developed cross and five domes and mostly because it was thoroughly reconstructed in In the 1980's, it was strengthened by reinforced-concrete elements (columns, belt courses, vaults, domes) which were incorporated into the already reconstructed walls. 2.1 Determination of the dynamic characteristics and seismic hazard parameters for the sites of the selected monuments Field experimental investigations were performed on the four selected monuments within the framework of the scientific-research project. The seismic hazard parameters of the sites of these churches were determined to define the theoretical and experimental studies needed for development of a methodology for repair and strengthening. The seismic hazard parameters were established on the basis of spatial, time and energy- seismic characteristics as well as attenuation of seismic intensity according to macro seismic field of seismic degree, which are defined on the basis of earthquakes that occurred in the territory of Macedonia between 1900 and Considering that the churches are rigid and brittle masonry structures constructed of stone and bricks and are characterized by vulnerable structural elements in their upper parts (roof and domes), they behave differently during local and distant earthquakes. The seismic hazard parameters for these churches were therefore defined separately for both local and distant events. The experimental in-situ investigations of the four selected monuments included ambient vibration measurements that were used to define the main dynamic characteristics (frequencies and mode shapes) and the damping coefficients of the buildings. 3 Shaking table tests performed on the St Nikita church model The church of St Nikita-v. Banjani was selected as a prototype-representative of Byzantine churches in Macedonia for which a methodology for seismic strengthening had to be defined and experimentally verified. The investigations performed for this church included definition of the physical-mechanical characteristics of the construction materials, seismicity of the terrain, main dynamic characteristics of the building and analysis of its seismic stability. Apart from the static tests of samples of materials (tufa, bricks and mortar), quasi-static tests of wall elements were performed under axial and shear compression in order to obtain the experimental values of the strength and deformability characteristics of the masonry, as well as the type of failure mechanisms. These were necessary for construction of the model to be tested on a seismic shaking table, (Figure 1).

4 40 Dynamics, Repairs & Restoration PROTOTYPE MODEL [ TOTAL MASS mp.z,60 t TOTAL MASS mm=2l f ) Figure 1. Comparison between the prototype and the model The model was constructed in the Dynamic Testing Laboratory of IZIIS to a scale of 1:2.75 and according to the designed proportions. Original materials taken from areas surrounding the church were used for its construction. The walls of the model were constructed in a typical Byzantine style. The model was fixed on the seismic shaking table and exposed to different types of earthquakes, i.e., the Breginj earthquake (type of a local shock), the Petrovac earthquake and the El Centro earthquake (earthquakes from distant foci) that were selected from both seismic hazard and structural response aspects. These were properly scaled and applied with maximum amplitudes from 0.02 to 0.43g (Table 1). TABLE 1. Specification of tests performed for the original model Test Code PET012N ELC015N PET035N ELC045N BRE006N ELC085N PET075N BRE012N ELC130N Input ace. («) Ace (g) level Maximum Response Ace (g) Disp (mm) top^ level Disp (mm) top Cracks None 2.6 At the tambour Failure The elastic limit of the model structure was reached at a^ = 0.17 g under the El Centro earthquake spectrum excitation. At a^x= 0.43 g (El Centro earthquake), the model suffered severe nonlinear damage, close to the state of failure. The main failure mechanism in the initial stage was loss of structural integrity and development of cracks through the main vaults after which damage occurred in the bearing walls. This was confirmed also by the drop of the natural frequency of the model from 11.2 to 6.6 Hz.

5 Dynamics, Repairs & Restoration 41 4 Repair, Strengthening and Testing of the Strengthened Model Based on the criteria regarding the seismic stability of the monuments and the architectural-conservation principles and requirements, an original method foi strengthening of such structures was developed. The model was repaired and strengthened in accordance with the defined methodology that was applied and demonstrated in a laboratory by a realistic approach (Figure 2). A photograph of the strengthened model is shown in Figure 3. In a real application, the load-spreading end plates would be located behind the exterior stone blocks and would not be visible. P058 P05 7 Section at the timber ties level Details of strengthening Figure 2. Applied method of strengthening of the model

6 42 Dynamics, Repairs & Restoration Figure 3. Strengthened Model Before Retesting i) The strengthening method involved incorporation of horizontal steel ties at locations formerly occupied by timber belts in the bearing walls, at the base of the tambour and the base of the dome. ii) Repair of the model was done by injection of the larger cracks after surface treatment and cleaning. The injection of the specially prepared emulsion grout was performed through metal tubes placed at 20 cm intervals, under a pressure of 0.6 to 1.2 atmospheres. Hi) For the purpose of increasing the bending resistance and the capacity for ductile behavior, a concept of structural strengthening was adopted involving incorporation of vertical steel ties at the ends of the walls, around the openings and through the tambour columns. Prestressing was performed to increase the axial stresses at plan and hence the tensile bearing capacity. The strengthened model was subjected to the same series of dynamic tests as the original model in order to make a direct comparison of the values of interest and to check the effectiveness of the strengthening method. Because of the obviously higher resistance of the model, it was tested under higher input acceleration levels than previously (Table 2, Figure 4). It can be concluded that the nonlinear state of the repaired model started when applying the Petrovac earthquake with a^ = 0.40 g. In order to obtain the stages of nonlinear behavior and estimate the damage level under higher expected earthquake intensities, tests were carried out by applying the El Centro earthquake with a gradual amplitude increase up to 0.49 g. At this level, cracks in the load-bearing walls, the tambour arches and damage to the dome occurred. The performance of a series of tests under a higher intensity (up to a^* = 0.75 g), gave rise to severe damage to the nonstructural elements (arches, cornices, lintels) and occurrence of cracks that were mainly concentrated in the west wall. However, the overall stability of the model was not affected and the damage that did occur was repairable.

7 Dynamics, Repairs & Restoration 43 TABLE 2. Specification of Tests Performed for the Strengthened Model Test Code ELC130S PET100S BRE020S PET140S ELC175S BRE027S ELC220S ELC280S BRE020S ELC280S BRE040S BRE053S ELC320S Input ace. (g) Ace (g) level Maximum Response Ace (g) Disp (mm) top level Disp (mm) top Cracks None First Crack Development of cracks elclsos, top, elc 150s, level l,a,nax=0.41g elc 150s, input ace, 5 Conclusion Figure 4. Acceleration time histories (ELC150S) On the basis of the results obtained in this program, the following can be concluded: The proposed methodology for seismic strengthening complies with the basic principles for conservation and protection of historic monuments. It is easily applied and satisfies the criterion of "minimum intervention-maximum protection".

8 44 Dynamics, Repairs & Restoration The experimental results show conclusively that the applied methodology for repair and seismic strengthening increases the bearing capacity and deformability of the structure (Figure 5) up to the required design level of protection. Figure 5. P-8 diagrams for original and strengthened model (Petrovac, a*,., = 0.2 g) We feel that the results of these and future investigations will contribute to a better understanding of methods for the protection of monuments of similar types existing in Macedonia, the Balkan region and beyond into the eastern Mediterranean areas. References "Study for seismic strengthening, conservation and restoration of churches dating from Byzantine period (9th - 14th century) in Macedonia", joint research project, IZIIS-Skopje, RZZSK-Skopje, GCI-Marina del Rey; Report IZIIS (vol. 1-6) - first phase of investigations, Report IZIIS (vol.7-10) - second phase of investigations.