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The Conservation and Strengthening Intervention on the Wooden Floors of the Ducal Palace in Sabbioneta (MN-Italy)

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The paper aims to inform on the executive phases and on the problems faced during the restoration works on some wooden floors of the sixteenth century Ducal Palace in Sabbioneta near Mantua in Italy, site in the World Heritage list since 2008. The particular historical, artistic and architectural interest of the Palace-and of the floors- required the involvement of expert restorers and a continous monitoring during the works by the Director of works, by the Manager of procedure and by the responsibles of Superintendence for Architectural Heritage and Landscape of Mantua. The paper describes the works performed mainly on wooden structures postponing in another place those relating to the restoration of the decorative elements.
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The conservation and strengthening intervention on the wooden floors
of the Ducal Palace in Sabbioneta (MN-Italy)
Francesco Augelli 1
1 Politecnico di Milano, Observatory for conservation of wood works, Via Durando 10, Milano, Italy
francesco.augelli@polimi.it
Keywords: Conservation, strengthening, intervention, floors
Abstract. The paper aims to inform on the executive phases and on the problems faced during the
restoration work on some wooden floors of the sixteenth century Ducal Palace in Sabbioneta near
Mantua in Italy, site in the World Heritage list since 2008.
The particular historical, artistic and architectural interest of the Palace-and of the floors- required
the involvement of expert restorers and a continous monitoring during the work by the Director of
works, by the Manager of procedure and by the responsibles of Superintendence for Architectural
Heritage and Landscape of Mantua.
The paper describes the work performed mainly on wooden structures postponing in another place
those relating to the restoration of the decorative elements.
Introduction
The Ducal Palace of Sabbioneta was built on the orders of the Duke Vespasiano Gonzaga
Colonna, from 1560 to 1561 on a preexisting building destroyed in a disastrous fire.
The building consist of four levels: basement, ground floor, first floor, mezzanine floor. To
these are added the attic partially feasible.
The floors subject of intervention are called, in order, by the name of the followings rooms:
Leoni (27.70 sm).; Angelo (4.60 sm.), Ottagoni (40.29 sm.) and Grappoli d’uva (45.96 sm.).
It‘s commonly thought that the construction of the floors is attributable to a period between
1577 and 1590.
The neglect and lack of maintenance of the building, for many years after the death of
Vespasiano and after the extinction of the Gonzaga family, had determined the need, in 1932, of
important restoration and safety interventions.
These interventions, although well executed, after almost 80 years, needed a check and further
safety measures. [1]
The interventions design was preceded by a careful analysis of the construction system and of
problems faced by. In particular on the structural ones. After the inspection stage we have been
made, in the critical points, instrumental checks by the use of a wood penetrometer in order to
verify the state of conservation structures.
The diagnosis phase in the yard has been made difficult by the conditions of the place because
parts of the wooden structures were submerged by dirt and debris.
The design and the interventions have been preceded by a detailed history research,
measurement works and surveys. All the phases of the work are described in the contributions of
the following authors: Augelli, Nicola, Petracco, Ronchi [2], Augelli, Bordina, Migliavacca [3],
Augelli and Amigoni [4].
The structural problems
The problems encountered had two main causes: the constructive system characterized by the
use of short pieces joint each other (the Serio’s floor system) [2] with the precarious state of safety
of joints between the various components and of these with the walls; the material decay
determined by old rain infiltrations from the roof. [3] & [4]
Advanced Materials Research Vol. 778 (2013) pp 865-871
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Many joints by wood were found fissured and cracked. Other wood and wood joints were only
partially cooperating. The ends of the beams frequently were leaning to the wall for extremely
limited portions. The metal systems of connection, both ancient (bands, brackets and nails) and
more recently introduced as a result of the restoration of the thirties of the twentieth century, did not
ensure the necessary security. All these causes had, as consequence, floors really inflected.
Another element found during the inspections was the presence of wood boring insects attacks
also widespread on the components of cedar wood that, despite of the lack of scientific data, has
always been considered a durable species.
Other problems widely present on all the floors were: instability of the decorative elements,
stains and surface deposits extended over all the surfaces.
The situation of structural components was recognized as extremely precarious in particular on
the floor of the Leoni’s room that in 1932, due to structural failure. In consequence of the thirties
restoration they made an intervention with the addition of buttresses and hangs anchored to new
timbers. The same applies to the Ottagoni’s floor that resulted, in some places, precarious. Such
intervention of structural support with metal elements were extended to all the floors. The
appreciable intervention of almost 80 years ago was no longer safe and therefore it was necessary to
provide for safeguarding both the extrados and the intrados.
First we wanted to solve the structural problems reiterating that the choices of action meet the
needs of conservation and structural material authenticity through specific interventions.
The operations performed
The criteria for intervention were the maximizing of the permanence of both decorative than
structural elements, of the material data and of any historical compatible stratifications.
We follow the iter suggested by standard UNI 11138:2004 [5]
Preliminary surveys and diagnostic tests [3 & 4] allowed us to obtain useful data for the purpose
of determining the specific problems of degradation and instability and then to determine, for each
problem, appropriate minimal intervention. [3]
A first important step in the work involved the providing to access to the attic. At the beginning
of the work the only accessible floor was that of Leoni's attic. Taking advantage of old buffered
openings, that have been reopened, it was possible to have a direct access to other floors, thus
avoiding the formation of expensive external scaffolding and the uncovering of parts of the roof
above the floors.
Was not possible to work going up the precarious floors, therefore wooden walkways have been
created leaning on beams in good conditions. Below, in the halls have been prepared the internal
scaffolding that allowed working safely on the structures and also carrying out the restoration of the
decorative elements. On these worktops was possible to proceed with the cleaning of the structures,
the seams of the joints and fissures with special screws and stainless steels, with the inclusion of
support elements in pultruded fibreglass, and with the hanging strands of stainless steel.
To ensure the maintenance of the interventions was realized a lighting system under roof.
The preliminary work carried out to improve access to attic areas were:
1. Partial demolition of two buffered openings that give direct access to the Angelo, Ottagoni
and Grappoli’s floors;
2. Provision of aluminium ladders to overcome some different level between the various floors
of the attic;
3. Provision of wooden walkways;
4. Realization of electrical lighting extended to the whole attic.
These preliminary intervention were essential for the future maintenance of the works.
866 Structural Health Assessment of Timber Structures
The general interventions performed for all the extrados of the floors were as follows:
1. Removal of debris, of rubble, of consistent and inconsistent deposits (guano) with vacuum
cleaner and brushes; (Fig.1)
2. Preservative treatment performed with biocide products dissolved in organic solvent applied
in two layers: one by brush and the other one by spray; (Fig.2, 3)
3. Insertion of ligatures and hanging strands, in correspondence of the only structural unstable
joints, composed by strands and turnbuckles stainless steel sleeve [4]; (Fig.4, 5)
4. Micro seams of elements severely cracked with self-tapping screws stainless steel of
particular conception (variable pitch) [4]; (Fig.6, 7)
5. Insertion of support elements of a high strength material, high durability and light in
pultruded fibreglass. [4]. (Fig. 8)
Fig. 1-2 – Cleaning with vacuum cleaner (left) and treatments with biocide of wood by brush (right)
Fig. 3-4 –Treatments with biocide of wood by spray (left) and insertion of ligature strands (right)
added to the old ones.
Fig. 5-6 –Insertion of ligature strands (left) and micro seams with self-tapping screws (right)
Advanced Materials Research Vol. 778 867
Fig. 7-8 –Particular of micro seams with self-tapping screws (left) and particular of pultruded
fibreglass beam.
General interventions for all lower surfaces of the floors were as follows:
1. Re-adhesion of unstable decorative elements; (Fig.9)
2. Dry-cleaning through the use of soft brushes, toothbrushes and vacuum cleaner/insufflator.
Cleanings with special tires and rubber micromole; (Fig.10, 11)
3. Homogenisation intervention of the discoloured surfaces with a mix of coloured chalk.
(Fig.12)
Fig. 9-10 –Shut down of instable decorative elements (left) and dry cleaning of surface by nylon
brush (right)
Fig. 11-12 – Dry cleaning of surface by special sponges (left) and homogenisation of colour surface
(right) with coloured chalk.
The intervention were carried out with the utmost respect, technically possible for the material
data but also of respect regarding the compatibility and reversibility of the materials and techniques.
It was decided to clean with dry methods and not wet methods to either to avoid any possibility,
by highly hygroscopic materials (first of all the wood) to absorb even small parts of any solvent or
surfactant, and to avoid the use of toxic chemicals always moreover, nowadays, eliminated by
restoration works.
868 Structural Health Assessment of Timber Structures
In agreement with the Superintendence it was also agreed to don't reconstruct any missing
decorative elements limiting the eventual reconstruction of the elements or parts of these necessary
for the structural purposes.
Conclusions
The structures safety intervention has partially confirmed the ancient technique on the use of
metal bands and buttresses and reinterpreting them with the most reliable and durable metals that
are in addition to old systems which were treated with a metal protective product. (Fig.13, 14)
In some cases it was necessary to insert new beams which, instead of being made of wood, are
in pultruded fibreglass. (Fig.15)
Fig. 13-14 – New and old steelwork (left) and conservation treatments of old steelwork (right).
Fig. 15-16 – The pultruded fibreglass beam (left) and surface strengthening of the planks (right).
The reasons of this choice, discussed at length, are related to the reduced weight of these
elements, to their manoeuvrability on site - especially in this case in confined spaces such as the
attic - to their durability and independence from biotic degradation, their recognizability, expressive
actuality, authenticity, compatibility and reversibility. We haven’t been used structural resins.
Advanced Materials Research Vol. 778 869
In going forward it was necessary to use a consolidating material to return part of the surface
resistance to the wooden planks and thereby ensure better maintenance. (Fig.16)
In order to ensure the maintenance and periodic monitoring of the structures from the attic, was
also prepared an electrical installation and lighting totally independent of the wooden structures of
the floors and roof of the building.
The final intervention was that of valorisation of the ceilings and of the rooms with LED
lighting light floor lamps specially designed. (Fig.17, 18)
Fig. 17-18 – The final intervention with new illuminations of the hall of Ottagoni's room (left) and
of Leoni's room (right).
Acknowledgements
A special thanks to the following people: Andrea Marchini architect of the technical office of
Sabbioneta’s Municipality; Giovanni Sartori Councillor for Culture of Sabbioneta’s Municipality;
Marco Bondani architect coordinator of safety on yard; Massimo Cherido technical director of
Lares restorations Company of Marghera (VE)-Italy; Anna Maria Basso Bert architect of the
Superintendence of Brescia, Cremona and Mantova.
870 Structural Health Assessment of Timber Structures
References
[1] F. Augelli, Diagnostic operations as well as detailed analysis of the detailed design, construction
management, accounting and safety coordination in the design phase of the work of "Restoration of
the wooden floors of the Palazzo Ducale in Sabbioneta" - General Report, Municipality of
Sabbioneta (MN), 2010.
[2] F. Augelli, V. Nicola, F. Petracco, L. Ronchi, The structural concept of the sixteenth century
floors of the Ducal Palace in Sabbioneta (Mantua, Italy), proceedings of 2nd SHATIS’13
International Conference on Structural Health Assessment of Timber Structures, Trento (Italy),
September 4-6, (2013).
[3] F. Augelli, A. Bordina, J. Migliavacca, Diagnosis for the conservation of wooden ceilings in the
Ducal Palace in Sabbioneta (MN), Italy, proceedings of 2nd SHATIS’13 International Conference
on Structural Health Assessment of Timber Structures, Trento (Italy), September 4-6, (2013).
[4] F. Augelli, C. Amigoni, The conservation and strenghtening project of the wooden structures of
the Ducal Palace in Sabbioneta (MN), proceedings of 2nd SHATIS’13 International Conference on
Structural Health Assessment of Timber Structures, Trento (Italy), September 4-6, (2013).
[5] UNI 11138:2004, Cultural heritage - Wooden artefacts - Building load bearing structures -
Criteria for the preliminary evaluation, the design and the execution of works.
Advanced Materials Research Vol. 778 871
ResearchGate has not been able to resolve any citations for this publication.
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The paper will concerns the design choices agreed between the designers responsible for the structural consolidation and the implementation of safety of the wooden floors of the Ducal Palace in Sabbioneta near Mantova in Italy. The consolidation project and the implementation of safety has been carried out in full compliance with the structural design and materials of existing Renaissance structures with a focus on compatibility, durability, recognizability and reversibility of the process. For the characteristics of these floors and the work done in the design phases, please refer to the contributions of Augelli, Nicola, Petracco, Ronchi [1], to that of Mastropirro [2], to that of Augelli, Bordina, Migliavacca. [3]
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Diagnostic operations as well as detailed analysis of the detailed design, construction management, accounting and safety coordination in the design phase of the work of Restoration of the wooden floors of the Palazzo Ducale in Sabbioneta
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The conservation and strenghtening project of the wooden structures of the Ducal Palace in Sabbioneta (MN), proceedings of 2 nd SHATIS'13 International Conference on Structural Health Assessment of Timber Structures
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F. Augelli, C. Amigoni, The conservation and strenghtening project of the wooden structures of the Ducal Palace in Sabbioneta (MN), proceedings of 2 nd SHATIS'13 International Conference on Structural Health Assessment of Timber Structures, Trento (Italy), September 4-6, (2013).
Cultural heritage -Wooden artefacts -Building load bearing structures -Criteria for the preliminary evaluation, the design and the execution of works
UNI 11138:2004, Cultural heritage -Wooden artefacts -Building load bearing structures -Criteria for the preliminary evaluation, the design and the execution of works. Advanced Materials Research Vol. 778 871