Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9
The Nidaros Cathedral in Trondheim. The Norwegian National Symbol. Stabilization of the Choir
Tonny Jespersen Jorgen Nielsen Consulting Engineers A/S, Copenhagen, Denmark
ABSTRACT: The Nidaros Dome had its great period in the Middle Age, when the cathedral stood in ali its glory as a gothic cathedral in English stile. A fatal fire in 153 I destroyed the cathedral, and it was only rebuild in a frugal way. But in 1869 the great restoration began, and the Norwegians could soon again be proud of their national symbol. Unfortunately the choir was reconstructed too weak and cracks and deformations were the results. An intervention was made in 1986, but the cracks apparently continued. In 1999 a panel in Trondheim was arranged with the staff from NDR (Nidaros Domens Restaureringsarbejder) and 4 foreign structural engineers. They should contribute to an assessment ofthe stability similar to what happened in Milan 600 years earlier.
HISTORY
King Olav Haraldsson was killed in 1030 in the Battle of Sticklestad, which is situated c. 100 km north of Trondheim. His body was brought to Trondheim, where he was buried. King Olav was canonised in 1031. A wooden chapel was built above his grave, but it was replaced by a large stone church in early Romanesque style in the end of the II th century, Ekroll (1995). From c.1150 transepts were added to the north and south sides, but in the period from I 180 to 1320 the Romanesque church was changed to a gothic cathedral. Several fires have ravaged the cathedral, but the ultimate fire was in 1531 , when the church and the entire town did bum. The cathedral was in ruins. The church was only restored in a poor way, the vault in the chance I was replaced by a flat roof and the nave was totally destroyed. The recovery to really greatness belonged to the future. The cathedral has an important meaning to the Nor Figure I. The Nidaros Dome from the south west. wegians. According to the constitution the kings have to be blessed here and the crownjewels are kept here. The restoration of the cathedral took place in a The cathedral is Europe's northernmost medieval period of growing Norwegian nationalism and in that cathedral. way the church is not only a religious symbol but also a symbol to the Norwegian state: 2 THE RESTORATION "When the Nidaros Dome lies in ruins, Norway is hit by poverty, but ifthe cathedral stands in ali its glory, The architects H. E. Schirmer (1869- 1871) and later prosperity will apply to Norway". Eilert C. B. Christie (1872- 1906) did manage the
1341 restoration works. Schirmer did work with the Chapter deformations of the walls, and it was close to hit some House, Christie with the Octagon, chancel, transepts people. and central tower. The cracks in the Kings Porch on the south side of lt may have been difficult to rebuild a gothic cathe the choir were found to be worrying. dral c. 600 years afier the end of the gothic period in In 195 1 a repair programme for the porch began, but Europe. Christie did a very careful restoration work, the cracks apparently continued the subsequent years. and he used the old traces in the walls from the medieval cathedral to reconstruct the vault. But he had no traces to follow, when he constructed 3 THE INTERVENTION IN 1986 the buttressing system, Bjorlykke 2004. The aisle walls, especially in the south side, were For several years the cracks and the deformations were leaning 400 mrn outwards, Ekroll & Storemyr ( 1998). ascribed to bad foundations. The old heavy vaults had made their influence on the But in 1986 a Norwegian engineering firm, walls. Therefore an important step for Christie was to Reinertsen was hired to make investigations of the right the aisle walls to be plumb, which he did in a possibilities to improve the stability in relation to the risky and successful intervention. horizontal forces from the vault. But though the new vaults were made much li ghter They made several suggestions, among others to than the old vaults, the buttressing system was con place steel trusses in the 10ft room above the vault, structed too weak to resist the forces from the vault. which became the preferred solution, see figure 3 The stability of the choir in Nidaros Cathedral (Reinertsen 1986). therefore has been a matter of interest since th e It was important to avoid visible anchors through reconstruction. the choi r. Cracks in the vaults, the flying buttresses and the The steel trusses were tensioned to about 20 kN. piers have been developed for severa I years. The ai sle Afier the intervention the monitoring was nearly walls seemed again to lean outwards. brought to an end, but it was obvious, that the cracks In 1935 a smallmarble colurnn (without structural continued to grow. importance) did fali down, maybe due to the horizontal Especiallya bi g crack in the vault in th e south east ern corner seemed to be serious, beca use the crack was quite through the vault and could be seen both from the choir and from the 10ft. lt seemed to separate the corner. In addition the south eastern corner ofthe choir was leaning outwards in a diagonal direction. In 1999 and 2000 the Norwegians arranged two seminars with participants from NDR and 4 for eign engineers, Jacques Heyman (England), Eberhard Stüwe (Germany), Krister Berggren (Sweden) and Tonny Jespersen (Denmark). The stability ofthe choir was the main topic for the seminar. The conclusion from the seminars was a recornnlendation to make new analyses of the sta bility of the choir, not only fo r the weight from the vau lt itself, but also from the forces from the wind.
Figure 2. The choir seen from wes t. Figure 3. The sleel trusses from 1986.
1342 "Ad quadratum" and "Ad triangulum" to The Nidaros Dome, see figure 5, We can see that the top of the vault easy can be placed within a square with a base Iike the width of the choir, but the vault will rise above the top of the equilateral triangle with the same base, According to these observalions we can not con clude, that the Nidaros Dome is very different from many other medieval cathedrals, It is of course impossible to conclude whether the choir is stable or not in the Iight ofthe geometric rules, but we can see, that questions and disagreements could arise, if the geometric rules \Vere lhe only tools to analyse the stability, Figure 4, The crack lhrough lhe vaull in lhe easlern comer.
5 THE MEASURING
Sefore the analysis could be done measuring of 4 cross sections in the choir and aisles were carried oul. This measuring gave important information's about the geometry and the deformalions of the arcade and aisle walls, From lhe measuring it appeared, that lhe pressure from the vault once more hacl caused hori zontal cleformations of lhe aisle walls, however in a smaller exlent than before the restoration, 1I is not known whether the clisplacements hacl increasecl after the intervention in 1986, The top of the arcade walls however had only small displacemenl. The reason may be, that the roof structure supportecl the arcade walls in a horizontal clirection, see below,
5,1 The roa! The roof construclion consists oftrusses macle of steel. The pitch is about 58°, the roofing is copper, The drawings from the restoration show, that the roof structure originally was planned and carried out to have a sliding support in the south sicle, probably Figure 5, The "Ad quadrarum" and lhe "Ad triangulum", with consicleration for movements from changes in temperature, Sut fortunately the slicling support has not worked In addition the pane I suggested an intervention to in that way! stabilize lhe eastern corners, In the first place the roof structure would be unable The present writer was asked to perform the task, to support the walls ancl prevent cleformations from It was recommended toa to continue the monitoring the pressure from the vault, if it had worked with a ofthe cracks and deformations, slicling support in one sicle, in the seconcl place the horizontal forces from the wincl on the roof could only 4 MEDIEVAL RULES be resistecl in the side with a fixecl support, i,e, the north side, ancl when this side was the shell sicle, ali the loacl Was the cathedral constructed in a way much different from wincl and from the vault would work in the same from other medieval calhedrals in Europe, since cracks direction, and deformations apparenlly indicated a lack in the There is no horizontal latticework in the 10ft room stability? above the vault in the choir, so ali the horizontal loads As mentioned in Heyman (1995) the medieval from wind from the south together with the pressure master builders used geometrical rules to design a from lhe vaull shoulcl in that way be resisted from the cathedral. It could be interesting to use the rules arcade walls and the aisle walls in the north sicle,
1343 Figure 6. The plan ofthe Nidaros Cathedral.
It is not probable, that the aisle walls would have 5 A The flying buttresses been able to resist that load. As mentioned above the buttressing system was con In the situation without sli ding support, the greater structed toa weak. part ofthe wind load will be transmitted to the wind The horizontal support from the flying buttresses is ward side, which is much better for the traverse varying between 7 kN (the passive state of the thrust stability of the choir. line) and 32 kN (the active state). Compared with some other goth ic cathedrals it 5.2 The vaul! seems to be very little, for instance the cathedral in Lichfield, where the corresponding support amounts The vault is a pointed groined vault with a rectangu to 30- 1000 kN, with reference to Heyman (1995). lar bay. The web consists of bricks with a thickness of 120 mm, plastered on both sides, the groins of a thickness of350 mm. There are 6 bays in the choir; the 5.5 The pieI' and lhe aisle wall length of the bays is c. 4.3 m. The width of the choir is varying from c. 1004 m to lI A m. The reason to the The new measuri ng showed, that the aisle wall did lopsidedness is unknown. lean more outwards than the arcade wall, which was The passive thrust line is used to determine the hor surprising, in the light of the weakness of the flyi ng izontal forces from the vault, which amounts to 68 kN buttress. per bay. The active force amounts to about 79 kN. Monitoring du ri ng a period indicated that the dis The corresponding vertical load from the vault is tance from the arcade wall to the aisle wall in the 153 kN. triforium seemed to increase. Since the flying but tresses were unable to support the arcade wa ll in an acceptable way, it could be expected, that the arcade 5.3 The arcade wall wall would move more outwards than the aisle wall The arcade wall in the 10ft room is about 1300 mm with the resul t, that the distance between the arcade thick and consists ofbricks except for the facing wall , wall and the aisle wall instead would decrease forming which are made of soapstone. cracks in the top of the vault above the ais le. In the choir the arcade pill ars consist of sol id But the observation was different. The cracks in the sandstone and the arcade wall consist of soapstone. vault above the aisles were on the underneath of the While the aisle wall s are leaning outwards, the vault. arcade pillars stay in a practically vertical line. However cracks were fo und in the tiles in the trifo In the top of the arcade walls leaks from the para rium. But the reason for these cracks was the increasing pet have caused severe frost damage in the brickwork, distance between the aisle wall and the arcade wall, see which just now is going to be repaired. figure 7.
1344 Th e thrust lin e for the vault
Figure 7. Cracks in the floor tiles in the triforium. 7 kN ,~ The thrust line for the flying buttress
6 kN / e~ 0,063 1208 kN 1 ~29a.1-k~N~~------
Figure 9. The lhrusl line in lhe arcade wall and lhe pier.
Figure 8. The steel rafters in the roof above the aisle. 6 THE THRUST UNE
5.6 The roo! above lhe aisle With the help from both the steel trusses in the 10ft The statically calculations of the load from the vault and the steel rafters in the roof above the aisle the demonstrated, that the choir was standing in a strained thrust line now could be constructed in a statically equilibrium, and that it was a good idea to erect the acceptable way. steel trusses in 1986. The analysis demonstrated, that it would be optimal But even with the help from the steel trusses ten to tension the trusses in the 10ft room from 20 kN to sioned to 20 kN, it was dilficult to construct a thrust 50kN. line with a proper way from the top ofthe walls to the This operation was planned to be made in two steps bottom. with 30- 35 kN as the first target, which was done In addition the clerastorium passage caused split in 2002, and on that occasion measuring nuts were ting forces in the arcade wall. mounted on the steel trusses, so its now easy to follow Further investigations however revealed, very sur the actual forces in the trusses. prising, that the roof above the aisle vault was carried The second step has not yet been realised, because by steel rafters well bolted to the clerastorium wall and the Norwegians wanted to evaluate the results from the to the aisle wall, and it seemed to be probable, that the first tensioning. rafters took part in the structural system and relieved The last step can be accomplished if necessary. the flying buttresses, see figure 8. The steel rafters were able to resist both pressure and tension, and they have done great usefulness in con 7 STABIUZATION OF THE EAST CORNERS nection with both pressures from the vault and perhaps especially the forces from the wind. The corners were fixed with a new transverse steel Could Architect Christie have had this aspect truss near the eastern gable, which should prevent in view? movements ofthe comer in the north- south direction,
1345 and two longitudinal anchors to prevent movements in the eastern direction. The new eastern steel truss is designed for 35 kN, while the longitudinal anchors are designed for 16 kN. The temperature in the 10ft room can change from-IO°C in the winter to +20° in the surumer. This results of course in considerable movements of the anchors. With a length of 26 m from the east to th e west gables, the change in dilatation and con traction will amount to 4,7 mm related to the mean temperature 5°e. Ifthe anchor is rigidly connected to the two gables, the strain in the anchor will change as follows:
E = 11 x.1.T (1)
6 Figure 10. The box with the spring. E = 12 X 10- x 15 = 1.8 x 10-4 (2)
4 2 (J = E x E = 2 X 10sx 1.8 X 10- = 36 N/mm (3) P kN 30 kN The anchor is 020 rum, which means, that the force 1- - i-27P- IOioC will change: 25 ~- --- I f 4,=2 15 N m~ I ! 2 20 ti -' M = 1t /4 X 20 x 36 = 11.3 kN. (4) I 1 kN I I ;;;.., - 10 o :;;f- !6 I p...!<~ c 15 'l 7i ~~ I I
1346 8 WIND ON THE CHOIR The new horizontal latticework has been designed and will soon be realized. 8.1 A comparison between lhe choir and lhe nave The horizontal latticework is not designed to resist ali the wind from the south, but it has to relieve the As mentioned in 4.6, there is no horizontallatticework arcade wall, the aisle wall and the steel rafters in the in the 10ft room above the choir contrary to the nave. roof above the aisle and in that way transmit some of From the Middle Age the masonry in the nave has the wind load to the gables in the choir. been more powerful than in the choir, and Christie Many cracks in the cathedral come of the pressure restored it in that way. from the vault. Sut also the roof structure was made stronger, espe Sut some of them might be a result of the forces cially with regard to the transverse stability, as the from the wind. roof structure here was provided with a horizontal The new horizontal latticework will reduce the latticework. number of that kind of cracks in the future. The difference in structure between the choir and the nave is thought-provoking, since the load from the wind practically is the same. 9 THE RESTORATION IN THE FUTURE The nave was restored about 40 years later than the choir. Why was the nave constructed much stronger? The Norwegians have been working with the cathedral There is no actual answer for that questiono practically since the restoration in 1869. For the moment many soapstone in the top of the 8.2 The loadji-om lhe wind arcade wall will be replaced owing to decay from the weather. The choir is situated in the traditional direction east Sut the Norwegians, and especially the people fram west. The most serious intluence from the wind is Trondheim ought to be happy, that a lot of restoration consequently wind from south- north. work still have to be done, because the legend say: NMI (Norwegian Meteorological Institute) has "The actual day when the restoration of the Nidaros made anemometry for some years. The wind speed Dome is finished, the church and the whole town will from the south is the most dangerous. disappear in the inlet". NMI has assessed the wind speed to 39.6m/s in a levei of 30 m above the ground corresponding to the levei for the top of the roof. REFERENCES The total horizontal characteristic load from the 2 south wind on the roof is 0.98 kN/m , which is Bjorlykke; K. 2004: Investigations in the archives of NDR 11.5 kN/m or 49 kN per bay. (Nidaros Domens Restaureringsarbejder). The roof in the choir has no connection to the adja Ekroll, E. 1995: Nidaros Cathedral and the Archbishops cent gables, so it is impossible to imagine, that some Palace, page 28- 31. ofthe load from the roof could be directly transmitted Ekroll, 0 & Storemyr, P. 1998: The Nidaros Cathedral to the gables by the roof structure. The horizontalload Choir, Norway: 750 Years of Structural Problems. (The therefore has to be resisted by the walls alone. First International Congress on Restoration of Gothic Cathedrals, Vitoria-Gasteiz, Spain, May 1998). Figure 3 is from Consulting Engineers Reinertsen in 8.3 Th e rein{orcemenl o/lhe Iral1sverse slability Trondheim. Figure 6 is from (Gerhard Fischer 1942-43). The calculations demonstrated that the choir was Heyman, Jacques 1995: The Stone Skeleton. Cambridge unable to resist the above mentioned forces with a sat University. isfactory safety, and the author of this paper therefore Figure 1 and 2 are from the NDR publication: "Nidaros Cathe suggested building in a horizontal latticework in the dral and the Archbishops Palace" 1995 (Editors: 0ystein 10ft room above the vault, as in the nave. Ekroll. Jill Krokstad, Tove S0reide).
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