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The restoration of ancient brickwork chimneysD. ~ o s i a ' ,G. pistone', G. ~ i v a ~A. violante ''Department of Structural Engineering, P olitecnico di Torino, Caste110del Valentino, Turin, Italy' epartnietit of Construction Processes in Architecture, VeniceUniversity Institu te of Arch itectu re, Tolentirzi, Verzice, Italy

AbstractThe debate concerning themes of the conservation of the obsolete buildingheritage is particularly lively today, ranging from interests more strictlyassociated with museums (the safeguarding of machines, devices and tools; thedocumentation of productive cycles; period photography exhibitions) to theproblem of the reuse of large empty containers - originally factories - in theurban fabric of large towns.Chimneys, m uch m ore than large building volum es? are fascinating "signs" ofhuman activity in a past that is still recent, yet concluded. Unlike the largefactory containers, these "points of reference" can no longer be recovered forfunctional use, at the most the y ca n on ly act as a reminder of the past.But the conserva tion com es up against quite difficult problems: stood abandon edfor decades, they are nearly always in a state of advanced decay, against whichthe poor attention that has been devoted to them has prevented the developmentof an effective consolidation technique that would respect their originalconception. They are continuously being demolished or cropped; only in rarecases are steps taken to reinforce them , applying means of consolidation that arefrequently invasive and eye-ca tching.The research presented here aims to docu me nt what is left in two a reas that havea large num ber o f these structures (Piedm ont and V eneto), and tries to develop amethod of investigating their state of conservation which would allow promptintervention to deal with their pathological condition, respecting the originalstructures as far as possible.

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210 Structural Studies, Repairs and Maintenance of Historical Buildings1. IntroductionIn a previous research presented at the STREMAH Conference in 1997, threechimneys were exam ined out of the many encountered in the study carried out o nthis aspect of the building heritage in Northern Italy [ l ] [2], as they we reconsidered to be significant examples of a wide panorama as regards theirstructural characteristics. The chimneys were selected for their representativenature w ith reference bo th to the period of their construction an d to the particularconstructive features, to be subjected to F.E.M. analysis. Th e oldest of thestructures analysed, the one in Venice, dates back to 18 79, but was pro bab lyrebuilt in 1903 when the premises were enlarged, and was originally 35 metrestall, while the second chimney examined, of 1907, is one of a pair of twinchimn eys located at Codigoro (Ferrara), each 5 5.0 metres tall; the third, at Va lleMosso (Biella), built in the period between 1920 and 1935, is 47.5 metres tall.Our interest was particularly focussed on certain typical sou rces of stress, s uc h aswind and extrem es of temperature, which could have influenced the du rab ility ofthe structures: an adeq uate understanding of their structural behaviour enables usto identify characteristic aspects of weakness and consequently to take actionwith maintenance work and reinforcement aimed at limiting the typical damageobserved in chimneys, such as vertical cracks and the loss of straightness ofprofile.With t h s aim another chimney of medium height was recently studied, whichcan be com pared with the others. Before examining in details the results obta inedin this last case, we shall briefly recall the conclusions reached with thestructures previously ex amined .2. Chimneys in Venice, Codigoro and Valle M ossoAlthough all three chimneys examined are made with a double stack, as may beseen fro m the project drawings available for all the structures, and built w ith theshafts solidly set into the massive base ('justifying the assumption of theconstraint condition of a truss embedded in the ground), their structuralcharacteristics present significant differences which a re also expressed indifferent structural behaviou r.The chimneys in the nord-est of Italy region, of an earlier date, have externalstacks characterised by a greater thickness (the chimney in Venice is the mostsquat). Th is circum stance allows the structures examined to be assimilated ratherto the simple flue type, considering the reduced section o f the inside flue an d thefact that it does not present significant connections with the outside stack. ThePiedmontese chimney at Valle Mosso, which is the most recent, has instead astructure composed of two very thin concentric rings, connected by vertical ribsand horizontal circular crowns of masonry so as to form a very light honeycombstructure. This results in very different forms of structural behaviour: aconsiderable rigidity an d stability against overturning for the chimn eys in V enic eand Codigoro and, on the other hand, high flexibility with reduced overturningmargins for the chimney at Valle Mosso. The vibration periods, in seconds with

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respect to the first vibrating mode, were respectively 1. 26 for the chimne y inVen ice, 1. 59 for the one at Codigoro and 2.3 4 for the one at Valle M os so .Fr om the point of view of their state of preservation, none o f the th ree chimne yspresented significant lesions or signs of failure of the foundation at the time ofinvestigation. T he chimney at Codigoro was in an excellent state o f repair, due tothe maintenance work carried out and to the attention that it receives regularly,with the precaution of putting the two flues into operation from tim e t o time, thuskeeping at bay the well-known processes of decay caused by the fros t and by thedifferences in temperature between parts that are differently exposed. Of theother two chimneys, the one in Venice especially presented accentuatedphenomena of decay of the bricks and mortar on the northern side and at thebase; the top of both chimneys had also been demolished for safety reasons (theone in V enice has been low ered 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 thefollowing possible different actions: dead load, wind according to Italian rules,second category seism (corresponding to a conventional acceleration of a/g =0.0 7) and temperature range in limit conditions when not in oper ation (wintershade -20C and sun +20C).For the materials, the following hypotheses were made, based on theexperimental results obtained from similar masonry of the same period: Youngmodulus E = 3000 MPa, Poisson coefficient l/m = 0.2 and specific weight 18K N I ~ ~ .All three chimneys are stable at their dead weight, confirming the goodworkmanship of their masonry. The thermal stress to which they are subjectedafter having been abandoned, above all in winter, produces quite high states oftensile stress which could have caused localised cracks, particularly o n the northwall of the stack. This type of phenomenon has frequently been encountered inabandoned chimneys and suggests a possible explanation for the appearance ofthe characteristic vertical lesions found o n the north wall of ma ny chim ney s.Th e analysis of the horizontal loads due to wind and a bove all to s ei sm indicatesperturbing states of stress which, while they could probably be absorbed bymasonry in a good state of preservation during operation, allowing theredistribution of peaks of tension, lead us to be very prudent in considering thestructural reliability of stacks that have remained out of use for a long time ifthey are su bject to dynamic actions of this kind.3. The chimney at CevaTh e brick chim ney at Ceva (Piedmont - Italy) (fig. 1) was built in 19 08 to servethe new mill of the "Anonima Cotonificio di Ceva" for the mechanical weavingof cotton, which initially provided employment for 400 male and femaleworkers. The company had to close in 1931, after which the premises were usedfor other production purposes until 1979. Since then it has had no precisefunction but, in memory of its productive past, there remains o n the territory thesign of this elegant brick chim ney , 42.35 metres tall.

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2 12 Structztrai Studies, Repairs and M aintenance of Histor ical BttildingsIt is a double stack chimney with a circular section and truncated conical verticaldevelopment, made of shaped solid bricks and, with all probability, usingexternal scaffolding, as is shown by the clear traces of "scaffold holes" on thewho le 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 afew centimetres wider than the shaft. The internal flue remains cylindrical for thewho le height and has a diameter of 1 .34 metres. A service ladder made of metalrungs embedded in the internal masonry is still present, though unsafe.Today the truncated conical shaft is tilted towards the south, with a shift of theaxis, from the base to the join with the top, assessed at 40 cm: the inclinationcauses a rigid rotation and is due to settlement o f the foundations.Th e su rvey was very accurate as regards the external geom etry, in particular thepresent deformation and the verticality; on the other hand it was not possible toinspect the inside of the flue, but only to view it very approximately through thebottom opening, as the access ladder is now unsafe. It would also have beeninteresting to investigate the cavity wall between the two stacks, especially tocheck the type of reciprocal connection. It is docum ented that the cavity wall wasfilled with concrete about forty years ago, at least in the bottom part, to improvethe stability of the structure; unfortunately there are no precise data on the levelreached by the filling, which was presumably kept fairly low, considering thedifficulty of reaching high levels with the casting.

Figure 1 Chimney at Ceva

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4. Numerical tests carried out on the chimney at CevaTh e tests were carried ou t with the F.E.M. Algor programme.Since there were no data on the internal parts of the cavity wall, we opted for atype similar to that documented for the chmney at Valle Mosso, which isgeographically not far from the one at Ceva [3]. As we said in point 2, thestructure was found to be very light: in the present case the horizontal connectingcrowns were not reproduced. The two tubular elements that make up thestructure were simulated with brick elements: the outer one has a truncatedconical section with a constant thickness, while the inner one has a cylindricalsection.In the absence of a direct inspection of the cavity wall between the two flues, 4connecting crowns were hypothesised for a first series of models and 8 for asecond series. The documentation available on similar cases often indicates anarrangement with 4 connecting septurns, but there is no lack of cases with doublethat number of elements and even more: the two hypotheses highlight specificfeatures of behaviour that are quite marked in some aspects, to which we shallreturn shortly.

Tensor

Figure 2: State of tension at the base of the chimney

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2 14 Structural Studies, Repairs and Maintenance of Historical Buildings1 Tensor

Figure 3: Cross section show vertical tensileThe models were composed respectively of 7452 nodes and 4440 elements andof 7628 nodes and 4784 elements.The assumptions concerning the simulated isotropic stress-resistant material withlinear behaviour were as follows: specific weight 18 K N / ~ ~ ,oung modulus E =2500 MPa and Poisson coefficient llm = 0,25.The actions considered, besides the dead weight, were, firstly, wind, seism andtemperature.We also wanted to simulate the deterioration of the material, especially on thenorth side, gradually varying the modulus of elasticity in a radial direction, so asto reproduce the deterioration of the mechanical properties of mortar and bricksas a result of exposure to severe atmospheric conditions.5. Results of the researchThe action of the dead load shows stress gradients that increase regularlydownwards, reaching a value of 0.6 MPa, perfectly compatible with the material,for which a breaking stress of about 10 MPa may be estimated. This value isbased on tests on masonry of the same period in the geographic area in which thechimney is located, performed with "strong" bricks and common lime mortar,well organised from the point of view of texture [4].Wind action was presumed in agreement with Italian regulations: having definedthe geographic area and the characteristics of the territory in the vicinity of theconstruction, the wind may be assumed as a static action which presses on theleeward side with 0.7 K N / ~ ~nd decompresses the windward side with half thevalue. The analysis shows a vast surface at the base of the chimney (fig. 2) whichis certainly subject to a state of tension on the material, with stress values

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varying between 0.15 and 0.45 MPa; the situation concerns both the base and,above all, the external stack. The concern produced by this static situation maybe mitigated with the following considerations:

the cross section o f the p lan (fig. 3) shows how the vertical tensile stress islimited to the external circular crown; consequently the centre of gravity of theresistant section still remains sufficiently within the perimeter of the chimney,though at the cost of maximum stress on the compressed edge, reaching valuesof 2 MPa;the circular profile certainly attenuates the state of decompression on thewindward edge, so the value prescribed by the regulations may be consideredslightly overestimated.The result of the analysis is consistent with previous findings on the otherchimneys and clearly stresses how the stability of these structures with respect tothe action of the wind is guarantees in unsatisfactory conditions, due to thepossible restriction of the section and the risk of the formation of horizontallesions at the joins in the mortar. Since the latter are not generally visible innormal conditions, they certainly become a vehicle of aggression for mortar inconditions of damp and frost, favouring the gradual decay of the material, withloss of its mechanical characteristics.From the seismic point of view the chimney, characterised by periods of 2.44 s(fxst mode)and 0.5 S (second mode) has a similar behaviour to the one at ValleMosso, with aggravation o f the stability pr ob le m already mentioned in the eventof wind.

Figure 4 : Base of the chim ney

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2 16 Structural Studies, Repairs and Maintenance of Historical BuildingsThe effect of the progressive deterioration of the material (fig. 4) was considered,bearing in mind that the consequent worsening of the mechanical characteristicsappears, as regards deformation, as a decrease of the modulus of Young: it istherefore reproduced by gradually lowering the modulus of elasticity along theperimeter on the northern exposed half of the chimney. The deformation gives agood visual reproduction of the curved vertical profiles to be seen on someabandoned chimneys (fig. 5). As regards resistance, clearly the vertical tensionvalues, slightly varied with respect to the entire modulus, must however bycompared with breaking tensions that are at times quite reduced with respect tothe value o f 10MP a taken as reference.The behaviour of the chimney exposed to heat effects appear to be veryinteresting. In the investigations carried out in the past, the heat effects wereconsidered with reference to the differences in value that occur between thesector exposed to the sun and the sector exposed to the north, for example onparticularly cold winter mornings when the weather is fine; states of horizontaltensile stress had been observed, which were certainly not very high, butpotentially responsible for the onset of vertical cracks.Even during operation of the chimney the conditions may be no less severe; thetemperature of the furnaces normally exceeds 200C, quickly bringing themasonry of the thin internal masonry cylinder to the same value, while the airchamber between the two flues, even though interrupted by the presence ofseptums, remains at a temperature very close to that of the environment. Theresult of thls is the formation of a layer of joint action b etw een the in terna lelement which tends to expand the external one and the external one whichopposes the movement.

Displacement

Figure 5: Curved vertical profile o n abandoned chimne ys

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The comparison between the two deformations on the plan (fig. 6) shows howthe arrangement with 8 septums guaran tees a far more regular behaviour than theone with 4 septums. However, it is above all with regard to tensions that thegreatest differences are seen . The joining of the sep tum on the internal flueproduces, in the horizontal section of the flue, a state of biaxial compression inboth arrangements, but causes tensile stress on the external flue in a directiontangential to the perimeter where the septums rest on the wall: in the case of 4septums the tensile stress reaches values of 0.18 MPa against 0.05 MPa in thecase of 8 septurns. In the intermediate sections between the septums the situationis inverted: on the external surface of the flue the values reach 2.2 MPa (4septums) and 1.9 MPa (8 septums), while the internal surface remainscompressed (0.35 MP a i 0.3 MPa). On the other hand: in the same sections theexternal surface of the bearing cone is compressed and the internal surface undertensile stress, with maximum stress values of 0.4 MPa in the arrangement with 4septums.In brief we may observe how in both arrangements the flue tends to maintain acontin uous inner ring that is all com pressed , due to the state of joint actionproduced by the expansion prevented by the radial septums. The two principaltensions are both negative on the whole ring, which indicates a regular andfundamentally stable pressure curve. Thls favourable condition in obviouslymore accentuated in the case with 8 septums. Though the external flue is muchmo re rig id, it is in a critical situation, especially where th e septums join the flue,where there is high tensile stress, especially in the case with 4 septums: thepressu re curve , which is very irregular, is no t as stable as that of the internal flue,du e to the free expansion that the external flue may perform.The condition observed may explain the appearance of vertical lesions, widelyreported in the past in masonry chimneys and repaired by fitting metal ringsencircling the outside.

Figure 6: Comparison between two deformations on the plan

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2 18 Structural Studies, Repairs and Maintenance of Historical Buildings6. ConclusionsThe studies carried out on chimneys in Piedmont and nord-est of Italy suggestthe following hypotheses for interpreting the phenomena of obsolescencerepeatedly observed in these structures, in particular th e app earance of the typicalcurved profile, especially when they are no longer in use, and long verticalcracks on the external walls.The first phenomenon, which may be more generally associated with thephysical and mechanical decay of the masonry, would seem to originate in thecyclical stress caused by the w ind, which is able t o cause horizontal micro-cracksin the joins in the mortar and thus favour aggression by atmospheric agents(particularly frost), espec ially in structures that are no long er in use.The second aspect would appear to originate in the strain generated in the twoflues due to the conne cting septum s, when the difference in temperature betweenthe two surfaces is relatively high (really high in normal working conditions).Widely spaced arrangements of ribs with a function of connecting rather thinstacks produce high horizontal strain on the ex terna l flue, with possible breakag eof the material. Subordina tely, the sam e effect co uld be produ ced, as docu men tedby the research in [ 3 ] , y the difference in tempe rature b etween the sou th and thenorth face, when su bjec ted to relatively high At values.References[ l ] Pistone G ., Riva G ., Problemi di recupero delle vecchie ciminiere in operalaterizia: esperienze in Piemonte e nel Veneto, Costruire in laterizio no 79 gen-feb 200 1.[2] Bosia D.,Pistone G., Riva G ., Le ciminiere in opera muraria, AmbienteCostruito, no 1 2001.[3]Pistone G.,Riva G.,Zorgno A., Structural beh aviour o f ancient chimneys, 5Conference on Structural Studies, STREMAH '97, Comp. Mech. Publications,Southampton and Boston, vol. 3, Advances in Architecture Series.[4] Mattone R., Pasero G., Pavano M,, Pistone G., Roccati R., Provesperimentali su campioni di varie dimensioni volte alla determinazione dellecaratteristiche meccaniche delle vecchie murature, Proceedings at 6International Brick Ma sonry Conferen ce, R om a? 1982.