Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

New studies on Brunelleschi’s Dome in Florence

L. Giorgi & P. MatracchiDepartment of Restoration and Conservation of Architectural Heritage, University of Florence, Italy

ABSTRACT: The recent investigations on the Brunelleschi’s Cupola of Florentine Cathedral were devotedabove all to identify masonry structure and to analyse the construction of the two domes (inner and external shells)in connection with the real building problems found in the works. Exact information on the characteristics of brickmasonry has been obtained. Other important data have been supplied by georadar investigations, demonstratingthat the inner dome is made of two (intrados and extrados) curtains with interposed a different masonry. Thesenew studies have been put in relation with the surveys of the dome, reaching original conclusions, far from thecommon mythicized interpretations of Brunelleschi’s Dome. In the construction a fundamental role was playedby the executive slowness.

1 INTRODUCTION

Twenty years ago a first book on Santa Maria del Fiore,the Florentine cathedral, was published, as a result ofnew investigations started from the facade, the aislesand the nave (Rocchi 1988). In some way, the con-structional sequence carried on by the builders wasfollowed, as the works in Arnolfian period started justfrom the facade demolishing part of the naves of SantaReparata, the ancient cathedral.

Recently a fourth volume of the series has been pub-lished and this includes the studies on Brunelleschi’sdome (Rocchi C.d.Y. 2006). These recent investiga-tions were related above all to define the masonrystructure and the execution of the two domes (the innerand the outer ones) in relation with the real problemsfound in the works.

2 THE DURATION OF THE DOME BUILDINGPHASES

In the initial segment, including the first corridor, thedome was built in pietraforte (a local sandstone). InApril 1422, above this level, the dome was made onlyof brick masonry and in June 1425 (Saalman 1980) themacigno (another Florentine sandstone) tie of the sec-ond corridor was under construction. Presuming thatthe faces of these eight sided cloister vault were raisednearly uniformly, the execution of this first brick domesegment was completed in about thirty-nine months.

From March 1426 building proceeded above thesecond corridor; from June 1429 the macigno ties of

Figure 1. Schematic cross section of the dome indicating,from archival records, the levels achieved in the differentbuilding periods.

the third corridor started to be supplied and in July1433 were all be put in place. Given the fact that thedate 1430 has been found scratched on the plaster justbelow the floor of this passage, 1430 can be assumed

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Figure 2. Cast of the brick masonry taken after the removalfrom dome intrados of some plaster with Zuccari’s muralpaintings. A black vertical line marks the corner of the dome.

as the year of completion of the second dome segment.So, including the year 1430, the dome portion betweenthe second and third corridors was built in fifty-eightmonths.

The construction of the upper section, between thethird corridor and the lantern, started in July 1433, wasconcluded by March 1435, as the following year theserraglio (the lantern ring base) was begun. So thisfinal segment was built in twenty-one months.

It has been possible to evaluate the average raise ofthe wall per month in each of the three dome sections,putting in relation the building period with the heightof the built masonry. For the first section of the dome,high 5.50 m and built in 39 months, this average valuecorresponds to a 14 cm masonry per month; for thesecond section (12 meters in 58 months) the value isnearly 21 cm per month, in the third section (15 metersin 21 months) 71 cm per month. On this basis, the brickcourses laid each month would result about two, threeand ten respectively in the three sections.

Limiting the working period to ten months per year,due to weather conditions, the values of dome ele-vation are nearly the same: respectively 17, 27 and94 cm masonry per month in the three sections. Theresults would not be different from the previous ones.In the years 1422–1430 the dome works proceededvery slowly: up to the third corridor the wall was raisedless than a brick course per week; on the contrary, arelevant increase is recorded in the upper part builtbetween 1433 and 1435.

It is necessary to consider that above the third cor-ridor the relevant narrowing if the dome reduces thebuilt masonry volume to nearly 1/3 in comparison withthe masonry of the second section. However, even con-sidering such a difference, which caused a consequent

Figure 3. The survey of part of dome intrados masonrypoints out discontinuities in herring-bone bonds pattern atthe corner. Bricks different in size were used, frequently cutto fit them to the space left between the herring-bone bonds.

reduction of the employed masons due to the less spaceavailable on the scaffoldings, it is evident the increasedspeed in the construction of the upper part of the dome.

It could seem contradictory that works proceededquicker just where the brick beds inclination wasgreater. However at this level a centring was probablylaid resting on a system of beams inserted in open-ings correspondent to the small passages opened to theinside of the cupola. The openings were then partiallywalled up to build the present oculi.

3 THE HERRING-BONE APPARATUS

Most of the recorded herring-bone bonds is locatedbetween the second and third corridor, where a veryirregular laying of the bricks and relevant variation oftheir inclination are visible.

After the dome intrados survey, the zenith angleof the herring-bone traces has been measured. It hasbeen verified a very wide range of the inclination (fromcirca 31◦ to 55◦) among the eight faces of the dome andamong the herring-bone bonds of the same face, withthe prevalent inclination from 36◦ to 40◦. It depends onthe different way of laying the bricks, using more orless quantity of mortar in the joints, as an evidenceof the unequal way of working of the different teamsof masons.

In particular, in the South-East face of the domemuch more inclined herring-bone bonds than else-where have been recorded. It seems to prove the

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Figure 4. The eight faces intrados (by stereophotogrammetric survey) with the traces of herring-bone lines to point out theirdifferent inclination, ranging from 31◦ to 55◦.

Figure 5. View of dome intrados with the lines of identifiedherring-bone traces.

permanence of the same team in the same worksitearea for a long construction period.

The knowledge of the wall apparatus has been deep-ened thanks to the detachment of plaster parts executedduring the restoration works of the wall paintings.The detailed survey drawings (Dalla Negra 2004) andthe casts of the masonry prove the irregularity of theherring-bone bricks, laid with continuous and varyingrotations on the plane of the dome face, using mortarjoints often thick as much as the bricks. The rows ofthe vertical bricks are discontinuous: somewhere theinclined herring-bone row is interrupted by a numberof horizontally disposed bricks, and then it starts againinclined. The interruption of the herring-bone patternis located both in the middle of the dome faces, andin the corners. In this point there is nearly no walltoothing: nearly all the bricks, unbroken or fragmented,arrive just at the corner. In the few cases where a singlebrick bridges the faces, it is not a special element but aordinary brick cut to adapt to the corner; so there is noevidence that special V-shaped bricks have been used

to build the corners, at least where the plaster has beendetached and the wall apparatus has been visible.

It has to be considered that the stone ties of the thirdcorridor probably determined some interruption of theinclined lines of the herring-bone bricks. The sameinterruptions should be caused by the correspondentstone ties of the second corridor; but at this level it wasimpossible to record them because of the absence ofwide visible herring-bone bond segments.

Also the extrados of the inner dome has been exam-ined, at least in many parts with no plaster. The samecharacteristics as the intrados have been found: oftenthere is no continuity in the herring-bone lines, adjoin-ing bricks have different rotations, and thick mortarjoints are used.

The removal of part of the tiles, made possible totake a stereo-photogrammetric survey of a meaning-ful sample of the outer dome extrados. The resultinginformation on the herring-bone bonds is comparablewith that previously acquired: different inclinations,discontinuities, variable thickness of the joints, irreg-ular brick disposition even in short segments of theherring-bone bonds.

The information now available on the herring-bonebonds is insufficient to make a comparison betweenthe wall apparatus of the intrados and the extradosof the same dome portion. The available surveys dis-continuous, limited to only few samples and relatedto parts located on different levels. Nevertheless theyappear really meaningful to make some considerationson the herring-bone structure in the whole of the dome.It is interesting the wide variation in the horizontaldistance (measured along the brick courses) of twoadjacent herring-bone bonds. At the intrados of theinner dome this measure is known in all the parts wherethe plaster was detached: in the corner between theEast and the North-East faces, at the height of about15 meters above the inner gallery, these distances arecm 95, 100 e 110; in the North and the North-West cor-ner, at the height of 25 meters above the gallery, are

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Figure 6. The georadar diagram of the inner dome dis-plays two symmetrical discontinuities (at the intrados andthe extrados) in the brick masonry, and a different inner core.

Figure 7. The drawing represents the wall structure of theCu-pola, basing on georadar investigations. The inner domeis made by two outer layers where herring-bone bonds arepresent, and by an inner core where bricks are probably laidon simple curved beds (corde blande) (Rocchi C.d.Y. 2006).

cm 45, 60, 90; in the North and the North-East corner,at the height of 27 meters above the gallery, are cm 45,50, 60; in the North face, at the height of 29 meters,the greatest variability has been recorded, as the her-ring bones are distant cm 10, 15, 25, 30, 45, 50, 55. Atthe extrados, above the third corridor, at the height ofthe second horizontal arch (circa 26 meters above thegallery) the distances among the herring-bone bondsvary from 85 to 110 cm, so they are much greater thanthe ones recorded on the intrados at about the samelevel.

Also the wall apparatus recorded on the outershell has great variability (in the same sub-horizontalbrick course two adjacent herring-bone bonds may bedivided by half brick or many bricks) and discontinuityin the herring-bone lines (short segments made by few

bricks, sudden changes in their inclination, splitting ofherring-bone bonds, . . .).

In the case of herring-bone bond crossing the domemasonry through its total thickness following a uniquecurve centre, very different dimensional data should beexpected: two adjacent herring-bone bonds distant 60cm on the intrados of the inner dome, on its extradosthe distance should be 65 cm and on the outer domerespectively 68 and 70 cm, distances only limitedlygreater than the ones recorded on the inner intrados.The dimensional heterogeneity that has been recordedin the building is totally different, even if the rele-vant slowness of the works should have made easy tocontrol the geometry of the herring-bone lines.

For these reasons it seems improbable that theherring-bone bonds cross radially the whole of thedome following a unique alignment to the centre. It ismore reasonable that the intrados and extrados herring-bone bonds of the inner dome are not continuous, butbelonging to two different masonry layers. The exis-tence of two distinct layers, the inner and the outerparts of the wall, has been proved by georadar inves-tigations, and can also be supported by considerationsrelating to the building technique used in the works.Moreover, it seems proved that the herring-bone bondsof outer dome are totally independent from those of theinner dome.

4 THE BRICK MASONRY FACING THE RIBSAND OCULI PASSAGES

In the space between the two shells, at each corridorlevel, very refined brick masonry can be observed onthe walls of both the passages through the ribs and tothe oculi opening inside the cupola. Here the bricks,homogeneous in size, are laid with regular and verythin joints and likewise the herring-bone bonds arevery accurate.

Examining in detail the bricks of the passages, somemisleading wall patterns can be detected: some bricksapparently part of a herring-bone row are illusory, asa single brick, laid using its largest face outwards, hasbeen carved in order to simulate the thin sides of fourdifferent bricks. Other special brick elements of thepassages are used to form the vertical corner of theabutments in masonries laid on inclined beds.

The georadar investigations carried on in the cor-ridors revealed a difference between the masonry ofthe outer part of the wall and of its internal nucleus.So it can be affirmed that, apart from the herring-bonebonds, on the wide faces of the dome a nearly usualmasonry (thick joints, irregular bricks, somehow irreg-ular laying of the elements, . . .) was executed, whilethe very refined brick facing was limited to the pas-sage walls; hence some diffused misunderstandings onthe real wall quality, supposing that the whole dome

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Figure 8. The accurate brick pattern visible in the serraglioat the top of the dome (left) and in the passages throughthe ribs (right). Here the flat brick line carved to suggest adifferent pattern is pointed out.

Figure 9. Side wall of a passage to an oculus open to theinner space of the dome. Basing on georadar investigations(diagrams on the right) the very accurate visible wall appara-tus is just a facing, uneven and different from the remainingdome masonries.

brick masonry was represented by that one of the pas-sages. On the contrary, it seems even possible that thepassage facing was placed during further restorationworks.

Next to the serraglio the situation is the same: theintrados masonry of the inner dome is very confus-ing, with irregular and segmented herring-bone bonds,whilst in the passage at the base of the serraglio thevisible brick masonry is as accurate and refined as theone just examined in the lower passages. So this kindof masonry appears once more to be only a facing,and this is also confirmed by the outer face of thesame serraglio wall, visible at the top of the spacebetween the two domes: under the plaster a usual thickjoint masonry, the same found everywhere but in thepassages, emerges.

Figure 10. Axonometric scheme with the sub-horizontalcanti-lever-arches connecting corner and intermediate ribs.

5 THE ARCH CONNECTIONS BETWEENTHE RIBS

It is meaningful the superimposition of the detailedsurvey of the just described extrados masonry ofthe outer shell on the measured drawing of the sub-horizontal cantilever-arches located on the intradosbetween the corner and intermediate ribs.

Above all it has to be underlined the peculiarityof their masonry structure: in the lower part head-ers are laid to form a segmental arch thick one brick,while above this arch some courses of stretchers fol-low the brick laying of the ribs and of the outer dome.The bricks of these cantilever-arches, however laid,are connected to the ribs and the outer shell; thoseones above the segmental arch are laid together withthe bricks of the shell (at least at the lower levels,as proved by the presence of herring-bone bonds insome of them). The segmental arches continue beyondthe extrados up to the external surface of the shell,as documented by the superimposition of the uppercantilever-arches and the extrados survey. The plasterabsence in the extrados of the outer dome gives theopportunity to identify nearly the total straight-arch ofa cantilever, and partially a second straight-arch placedat the same level on the opposite side.

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The reducing cross section of those arches, typi-cal of cantilevers, and the consequent wider sectionin correspondence of the corner ribs, transfers on thelatter ones most of the stresses. Furthermore, it can beimagined that the thickening of the cantilever-archesat the corner rib could allow to cross them diagonallyand to make possible the laying of a continuous irontie, shaped as a multi-sided polygon and so much moreeffective.

6 THE CORDE BLANDE

The corde blande, the curved laying of brick courses,progressively increasing from the base to the top ofthe dome, are well documented by the survey of thedome extrados. This special laying system was causedby works needs, as it was necessary to give continuityto the differently inclined brick beds in the two cor-ner ribs and in the central part of a single face. Theconsequent curved laying of the bricks was got andmade progressive not only with varied thickness ofthe mortar joints, but introducing discontinuities in the

Figure 11. Zenith view to the third corridor intrados of thespace between the two domes, showing the cantilever-archesat the intrados of outer dome.

Figure 12. Detail of a cantilever-arch. The vertical bricks at the intrados (left) form a segmental arch supporting the abovemasonry, laid on sub-horizontal curved beds (a corda blanda) homogeneous with the outer dome masonry.

herring-bone row and in the sub-horizontal masonrybetween them as well as.

A further aspect has to be kept in mind. At the intra-dos the dome corner, measured from the gallery to thebase of the lantern oculus, is only a little more than39 meters, while at the middle of a face the corre-sponding length is 3 meters shorter. In the works, thisdifferent length (nearly 10%) required a greater num-ber of brick courses in the corners, and this necessarilycaused further irregularities.

During the construction these work adjustmentswere made easier by the masonry discontinuity givenby the herring-bone bonds, that caused the includedparts of masonry to be independent the one from theother. For this reason such adaptations are nowadaysnearly invisible when examining the wall.

The marble mouldings at the external base of thelantern are apparently curved, and so they seem toreply the curved line of the corde blande. In fact,the masonry which supports them is straight and thecurved line of the mouldings is caused by the brickmasonry settling, which was not followed by a cor-respondent subsidence of the marble covering of thecorner ribs. Here the joints between the elementsare extremely reduced and so subject to only lim-ited dimensional reductions for plastic deformation orshrinkage. On the contrary, these phenomena are rele-vant in all the walls of the dome, built with thick jointmasonry (Petrini 1989).

7 IRON BARS EMPLOY

In the masonry of each aisle vault and of each navetransverse arch of Santa Maria del Fiore iron ties wereinserted.This can be considered the start point of a newconstruction technique (iron reinforced brick vaults)carried on and increased in the construction of theCupola of Florentine cathedral.

Many iron elements are visible at the intrados of thedome: eyes, different in size and located on the facesat regular levels, or metal straps at the corners. Up to

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now their presence has been put in relation with thenecessity to fasten intrados suspended scaffolding, orto fix corner wooden centrings (Dalla Negra 1995).The same works praxis has been proved to be previ-ously utilized in the construction of aisles and nave ofthe church, where iron elements used to fix centeringshave been found in transverse arches and diagonal ribsof the nave vaults.

Some iron bars inserted in the dome masonry arevisible. They could be part of a more extended tyingsystem, which probably has played a relevant role inbuilding static condition.

At the dome spring, just above the inner gallery,where should be located the first stone tie, many eyediron bars stick out of masonry among stone cantilevers.The bars probably cross nearly the whole of the domemasonry.

Detailed observation has been made at the level ofthe third corridor in the space between the shells atthe North-Eastern side of the dome. Here many ironelements are visible: a radial bar laid aside a centralrib; two transversal bars inserted at the intrados ofstone ties; a third bar, protruding from the masonryof the inner dome in coincidence of its set back. Inthis last case, the bar position, almost parallel to dome

Figure 13. Survey of the extrados summit of the outer North-Eastern shell. The projection on the outer surface of inner ribsand canti-lever-arches points out the exact coincidence between the intrados arch of the latter and the vertical bricks of thesurvey. The herring-bone bonds, though irregular, also go on intermediate ribs.

extrados, indicates that probably this element is theanchor of an iron tie crossing radially the main shell.

Broader investigations to find out iron presence inthe dome were carried on by metal-detectors by ItalianArmy (Compagnia Genio Guastatori della BrigataMotorizzata “Friuli”). Exterior wall stripes and floorswere examined at the level of the three corridors. Geo-radar investigations made later on the inner domerevealed signs, probably related to the presence ofiron bars.

Basing on these investigations, it comes out thatthe builders kept in mind the different behaviour ofthe two shells, placing reinforcing iron bars mainlyin the corridors. In any case it has been ascer-tained the conspicuous use of radial iron bars in thestone masonries at the dome spring. Here the pres-ence of large stone elements could have required awidespread use of iron cramps, unnecessary in brickmasonry.

The late introduction of the cantilever-arches, start-ing from the second corridor, probably suggested alsothe usage of almost continuous perimetral iron bars.This kind of hooping, present in a more recogniz-able form at this level, seems absent above the thirdcorridor.

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Figure 14. Masonry at the connection of the outer shell,corner rib and cantilever-arch. Everywhere very thick mortarjoints are visible.

Figure 15. Survey detail of the outer shell at the base of thelantern. At the centre, the marble moulding bends fitting tothe slight lowering of the dome. At the corners, where themarble covering of the rib act as a strut, a hollow space (inblack) is formed.

Figure 16. Near the edge between the rib (left) and the extra-dos of the inner dome (right) a metal bar comes out the setback. The bar is probably the anchor of an iron tie crossingthe main dome.

The use of metallic connections along the rib cor-ners is probably a consequence of the difficult toothingof ribs and herring-bone masonry of the dome. Any-how all that proves, even if in outstanding structuresas the dome and nave of Santa Maria del Fiore, thewidespread usage of iron reinforcements in Medievaland Early-Renaissance buildings.

REFERENCES

Dalla Negra R. (ed.) 2004. La Cupola di Santa Maria delFiore. Città di Castello: Sillabe.

Dalla Negra R. 1995. La cupola del Brunelleschi: il cantiere leindagini, i rilievi. InAcidini Luchinat C. & Dalla Negra R.(eds.). La cupola di Santa Maria del Fiore. Il cantiere direstauro, 1980–1995: 21–22. Roma: Istituto Poligrafico eZecca dello Stato.

Giorgi L. & Matracchi P. 2006. Santa Maria del Fiore, fac-ciata, corpo basilicale, cupola. In Rocchi 2006: 277–324.

Petrini G., 1989. Su alcuni recenti rilievi architettonici per ilrestauro dei monumenti. In XY. Dimensioni del disegno,V, n. 11–12, pp. 147–149.

Rocchi Coopmans deYoldi, G. (ed.) 2006. S.Maria del Fiore.Teorie e storie dell’archeologia e del restauro nella cittàdelle fabbriche arnolfiane. Firenze: Alinea.

Rocchi, G. et al. 1988. S.Maria del Fiore: Rilievi, documenti,indagini strumentali. Interpretazione. Il corpo basilicale.Milano: Hoepli.

Saalman, H. 1980. Filippo Brunelleschi.The Cupola of SantaMaria del Fiore: 263. London: A. Zwemmer.

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