Pomposa Abbey: FEM Simulation of Some Structural Damages and Restoration Proposals

Transactions on the Built Environment vol 39 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 Pomposa Abbey: FEM simulation of some structural damages and restoration proposals E. Cosmi (2), G. Guerzoni (%), C. Di Francesco 0), C. Alessandri (%) W Soprintendenza Beni Ambientali Architettonici, 48100 Ravenna, Italy (2) Department of Architecture, University ofFerrara, Via Quartieri 8, 44100 Ferrara, Italy EMail: [email protected]. it Abstract A static analysis of the Pomposa Abbey is proposed with the aim of verifying the effects of the tower self weight on the present deformed configuration of the church and the structural efficiency of the internal buttresses, added as reinforcements to the whole construction. In this paper the Authors present a kind of reinforcement to be considered as alternative to the existing ones; the proposal is supported by a numerical simulation carried out on a 3-D model of the Abbey, discretised and analised with FEM techniques. The structural solution is statically reliable and it allows to recover the unity of the internal space and to preserve the integrity of the existing structures. 1 Introduction The Abbey of Pomposa (Fig. 2), located in the middle of the River Po Basin, between the ducal woods of Mesola and the Mezzano nature reserve, has its origin in the VI century, when a large part of Italy was under Longobard rule; it was a centre for culture, work and prayer for the Benedectine Monks who reclaimed vast areas of sorroundings marshlands and collected relics from the Classical world, which was then in the phase of full decline. Initially under the power of the church in Ravenna, later on Pomposa managed to gain its independence, and was especially favoured by Pontiffs and Emperors and, in this way, little by little, it was able to increase its power, and the Abbot, as well as having ecclesiastical power, began also to wield civil power over large areas of land. In the XI century the complex of Abbey buildings were extended and a library was founded. Pomposa reached the peak of Transactions on the Built Environment vol 39 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 140 Structural Studies, Repairs and Maintenance of Historical Buildings its power in the XIV century, when its jurisdiction spread to include lands in eighteen different Italian diocese. But, in the same period, as a result of the river Po breaking its banks in the XII century and changing its course, the land sorrounding the Abbey was gradually being invaded by marshlands, which was destroying the secular work of the Benedectines. The decline of Pomposa was imminent; only a few months remained at Pomposa which, in 1654, on the departure of the last monks, becomed a modest parish in the diocese of Comacchio. Only in the XIX century, when the Italian Government took over possession of the Abbey, did the slow work of restoration and renaissance begin. All that remained of the old complex of Abbey buildings were the church, the bell tower, part of the monastery and the palace of justice. After being founded, the Abbey continued to grow until the XII century by the addition of apses, spans and an atrium. It shows clear architectural influences of the near city of Ravenna, expecially in the internal subdivision (quite usual in the late Byzantine basilicas) into a nave and two side aisles, with columnades and arcades (Fig. 1). The nave, more developed than the others in hight and width, ends up with a triumphal arch, a crypt and a semicircular apse. Wooden trusses and beams form the roof structures of nave and aisles, respectively. External and internal walls are formed of brick and mortar masonries which show no appreciable sign of physical decay but visible out-of-plane deformations and localised cracks due to a subsidence started during the construction of the near bell tower in the early XI century and continued afterwards. As a matter of fact, the enormous self-weight of the whole masonry tower, nearly 1 800 tons, distributed on a relatively small square base, produced uniform vertical settlements of the most compressible soil layers below and, as a consequence, the rotations and the above mentioned out-of-plumbs of the Abbey walls. In order to prevent a possible, further growth of these structural damages, a set of transverse masonry walls (Fig. 3) was built in the aisles and in the atrium between the XVI and the XIX century, long time after the occurrence of the first macroscopic subsidence. Although they had to behave as internal buttresses, serious doubts still remain about their static efficiency; moreover they have altered completely the perception of the internal spaces. For more than one year the Authors have been investigating alternative solutions to such a kind of reinforcement on the basis of results obtained in previous analyses and by using modern software packages for numerical modelling and 3-D visualisation. The results of this research are partially presented in this paper, expecially those concerning the structural analysis of the Abbey and the technical solution to be proposed in place of the buttresses. In particular, a static analysis was carried out to verify the effects of the tower self weight on the present deformed configuration of the Abbey, the actual structural efficiency of the internal buttresses and the validity of the new proposal; a well- known FEM code, MARC K62, was used to solve the various linear and nonlinear structural problems encountered. Although further investigations are needed, the solution proposed is statically reliable and allows to recover the unity of the internal space. 2 Historical Analysis Still readable traces on the monument, as well as the testimony of some historians [1-3], allow to subdivide the whole construction process of the church into twelve phases. In each of them the building was subjected to morphologic transformations which gave rise to considerable changes in the mechanical behaviour of structures and subsoil. The oldest part of it, dating back to the half of the VIII century, was originally two span shorter than the present building, without minor apses and crypt Transactions on the Built Environment vol 39 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 Structural Studies, Repairs and Maintenance of Historical Buildings 141 but with the same three naves as nowadays. A two-level atrium, adorned with double lancet windows, was built before the end of the X century in correspondence of the first two present spans. The XI century sees the beginning of a great deal of works: frescoes, mosaic floorings and marble "tarsie". Probably, by that time, the church had been already enlarged by incorporating with other two spans the previous atrium, the facade of which, modified by the addition of a tympanum, the closure of the double lancet windows and the reduction of the entrance portal, became the new front of the church. The construction of the semicircular apses of the side aisles (the Southern apse was demolished in the XIV century to allow a direct access to the Monastery) and of the crypt under the main altar goes back to the same period. After a short time the present atrium and the whole monastery were added. Meanwhile, in 1063 the imposing bell tower began to be built; its construction spanned over some decades corresponding to the economic and cultural bloom of the Abbey. The alignment of bell tower and atrium facade allows to suppose that the latter was built previously or, at most, at the same time. About 1150 the Northern apse was re-built, together with a part of the otside wall, after a collapse occurred for still unknown reasons; meanwhile the decline of Pomposa begins: the break of the river Po in the nearby (1152) leads to a slow but inexorable worsening of the geo-climatic conditions of the area and, in particular, to its progressive swamping. By the end of the XIII century the roofs of the side aisles were lifted up to the present level and the central part of the atrium facade was re-made, probably in consequence of a collapse occurred some years earlier, as confirmed by an oral, although not documented, tradition. Probably in baroque time, when most of the churches were modified in accordance with the new liturgical rules issued by the Council of Trento, the original crypt was demolished and the whole apsidal zone was brought to the same level as the nave. The construction of the eleven transverse masonry walls, supposed to behave as buttresses, represents the final stage of the architectural and structural transformation process of the church. Such a cumbersome consolidation work, the motivations of which will be explained in the next Sections, was carried out in two stages: all the buttresses, with the exception of the first of the right aisle and of the ones in the atrium, were built between the XVI and XIX century; the remaining ones were added in 1858, as reported in some historical documents. The successive works carried out on the church were mainly maintenance works and preserved the structural aspects characterising the building in the second half of the XIX century; the only exception is the crypt which was re-built in 1927 in the same place as the previous one and with an imaginary typology. 3 The Present State Some investigations carried out between 1985 and 1992 allowed to evaluate the physical state of the whole building [4]. The masonries, for instance, are not deteriorated and visible cracks interest only small wall portions. Nevertheless, the costruction of the church by successive additions of different parts caused inhomogeneities and irregularities in the masonry and in the brick textures and, in particular, a remarkable weakening of the connections between not coeval masonry structures.

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