The Restoration of Ancient Brickwork Chimneys
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Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 The restoration of ancient brickwork chimneys D. ~osia',G. pistone', G. ~iva~& A. violante' ' Department of Structural Engineering, Politecnico di Torino, Caste110 del Valentino, Turin, Italy ' Departnietit of Construction Processes in Architecture, Venice University Institute of Architecture, Tolentirzi, Verzice, Italy Abstract The debate concerning themes of the conservation of the obsolete building heritage is particularly lively today, ranging from interests more strictly associated with museums (the safeguarding of machines, devices and tools; the documentation of productive cycles; period photography exhibitions) to the problem of the reuse of large empty containers - originally factories - in the urban fabric of large towns. Chimneys, much more than large building volumes? are fascinating "signs" of human activity in a past that is still recent, yet concluded. Unlike the large factory containers, these "points of reference" can no longer be recovered for functional use, at the most they can only act as a reminder of the past. But the conservation comes up against quite difficult problems: stood abandoned for decades, they are nearly always in a state of advanced decay, against which the poor attention that has been devoted to them has prevented the development of an effective consolidation technique that would respect their original conception. They are continuously being demolished or cropped; only in rare cases are steps taken to reinforce them, applying means of consolidation that are frequently invasive and eye-catching. The research presented here aims to document what is left in two areas that have a large number of these structures (Piedmont and Veneto), and tries to develop a method of investigating their state of conservation which would allow prompt intervention to deal with their pathological condition, respecting the original structures as far as possible. Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 2 10 Structural Studies, Repairs and Maintenance of Historical Buildings 1. Introduction In a previous research presented at the STREMAH Conference in 1997, three chimneys were examined out of the many encountered in the study carried out on this aspect of the building heritage in Northern Italy [l] [2], as they were considered to be significant examples of a wide panorama as regards their structural characteristics. The chimneys were selected for their representative nature with reference both to the period of their construction and to the particular constructive features, to be subjected to F.E.M. analysis. The oldest of the structures analysed, the one in Venice, dates back to 1879, but was probably rebuilt in 1903 when the premises were enlarged, and was originally 35 metres tall, while the second chimney examined, of 1907, is one of a pair of twin chimneys located at Codigoro (Ferrara), each 55.0 metres tall; the third, at Valle Mosso (Biella), built in the period between 1920 and 1935, is 47.5 metres tall. Our interest was particularly focussed on certain typical sources of stress, such as wind and extremes of temperature, which could have influenced the durability of the structures: an adequate understanding of their structural behaviour enables us to identify characteristic aspects of weakness and consequently to take action with maintenance work and reinforcement aimed at limiting the typical damage observed in chimneys, such as vertical cracks and the loss of straightness of profile. With ths aim another chimney of medium height was recently studied, which can be compared with the others. Before examining in details the results obtained in this last case, we shall briefly recall the conclusions reached with the structures previously examined. 2. Chimneys in Venice, Codigoro and Valle Mosso Although all three chimneys examined are made with a double stack, as may be seen from the project drawings available for all the structures, and built with the shafts solidly set into the massive base ('justifying the assumption of the constraint condition of a truss embedded in the ground), their structural characteristics present significant differences which are also expressed in different structural behaviour. The chimneys in the nord-est of Italy region, of an earlier date, have external stacks characterised by a greater thickness (the chimney in Venice is the most squat). This circumstance allows the structures examined to be assimilated rather to the simple flue type, considering the reduced section of the inside flue and the fact that it does not present significant connections with the outside stack. The Piedmontese chimney at Valle Mosso, which is the most recent, has instead a structure composed of two very thin concentric rings, connected by vertical ribs and horizontal circular crowns of masonry so as to form a very light honeycomb structure. This results in very different forms of structural behaviour: a considerable rigidity and stability against overturning for the chimneys in Venice and Codigoro and, on the other hand, high flexibility with reduced overturning margins for the chimney at Valle Mosso. The vibration periods, in seconds with Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 respect to the first vibrating mode, were respectively 1.26 for the chimney in Venice, 1.59 for the one at Codigoro and 2.34 for the one at Valle Mosso. From the point of view of their state of preservation, none of the three chimneys presented significant lesions or signs of failure of the foundation at the time of investigation. The chimney at Codigoro was in an excellent state of repair, due to the maintenance work carried out and to the attention that it receives regularly, with the precaution of putting the two flues into operation from time to time, thus keeping at bay the well-known processes of decay caused by the frost and by the differences in temperature between parts that are differently exposed. Of the other two chimneys, the one in Venice especially presented accentuated phenomena of decay of the bricks and mortar on the northern side and at the base; the top of both chimneys had also been demolished for safety reasons (the one in Venice has been lowered to about 27.5 metres). For all the examples considered, numerical simulation was carried out, checking - both individually and by overlapping the effects - the consequences of the following possible different actions: dead load, wind according to Italian rules, second category seism (corresponding to a conventional acceleration of a/g = 0.07) and temperature range in limit conditions when not in operation (winter shade -20°C and sun +20°C). For the materials, the following hypotheses were made, based on the experimental results obtained from similar masonry of the same period: Young modulus E = 3000 MPa, Poisson coefficient l/m = 0.2 and specific weight 18 KNI~~. All three chimneys are stable at their dead weight, confirming the good workmanship of their masonry. The thermal stress to which they are subjected after having been abandoned, above all in winter, produces quite high states of tensile stress which could have caused localised cracks, particularly on the north wall of the stack. This type of phenomenon has frequently been encountered in abandoned chimneys and suggests a possible explanation for the appearance of the characteristic vertical lesions found on the north wall of many chimneys. The analysis of the horizontal loads due to wind and above all to seism indicates perturbing states of stress which, while they could probably be absorbed by masonry in a good state of preservation during operation, allowing the redistribution of peaks of tension, lead us to be very prudent in considering the structural reliability of stacks that have remained out of use for a long time if they are subject to dynamic actions of this kind. 3. The chimney at Ceva The brick chimney at Ceva (Piedmont - Italy) (fig. 1) was built in 1908 to serve the new mill of the "Anonima Cotonificio di Ceva" for the mechanical weaving of cotton, which initially provided employment for 400 male and female workers. The company had to close in 1931, after which the premises were used for other production purposes until 1979. Since then it has had no precise function but, in memory of its productive past, there remains on the territory the sign of this elegant brick chimney, 42.35 metres tall. Transactions on the Built Environment vol 55, © 2001 WIT Press, www.witpress.com, ISSN 1743-3509 2 12 Structztrai Studies, Repairs and Maintenance of Historical Bttildings It is a double stack chimney with a circular section and truncated conical vertical development, made of shaped solid bricks and, with all probability, using external scaffolding, as is shown by the clear traces of "scaffold holes" on the whole height of the shaft, sealed with mortar. The chimney is made up of a base (h = 2.05 m), only slightly pronounced, containing the access door for maintenance work, the shaft itself (h = 34.32 m) and the top (h = 5.98 m), complete with coping. The base, which is cylindrical, has a constant diameter of 3.82 metres and is a few centimetres wider than the shaft. The internal flue remains cylindrical for the whole height and has a diameter of 1.34 metres. A service ladder made of metal rungs embedded in the internal masonry is still present, though unsafe. Today the truncated conical shaft is tilted towards the south, with a shift of the axis, from the base to the join with the top, assessed at 40 cm: the inclination causes a rigid rotation and is due to settlement of the foundations. The survey was very accurate as regards the external geometry, in particular the present deformation and the verticality; on the other hand it was not possible to inspect the inside of the flue, but only to view it very approximately through the bottom opening, as the access ladder is now unsafe.