G. Carluccio', S. Meluzzi, A. Viskovic* · 2014. 5. 20. · The vault has extended cracks (figure 5), consisting of a principal longitudinal lesion on the intrados of the vault and

The crypt of St. Carlo alle Quattro Fontane

in Rome

G. Croci*, P. Degni\ G. Carluccio', S. Meluzzi", A. Viskovic*

"Faculty of Engineering, University of Rome 'La Sapienza', Italy

*>Architectural Heritage of Rome, Italy

'Tecnocontrolli S.r.L, Italy

Abstract

The Church of "St. Carlo alle Quattro Fontane", designed by FrancescoBorromini, is a masterpiece of the Italian Baroque. The appearance ofdeformations and cracks in the vault of the crypt called for the insertion ofprops. Historical research was carried out to ascertain the original shape andcompare it with the deformations visible today. Investigations revealed thatthe vault was built in good quality pozzolanic concrete mixed with irregulartuff stone blocks. Structural analyses were carried out using finite elementmodels, taking into account the non-linear behaviour of the vault's material.

The results show how the peculiar shape of the vault reduces the safetymargins with partializations of sections; cracks have expanded slowly overthe years, probably also in relation with the high sensitivity to horizontaldisplacement produced by little soil settlements. Interventions were designedtaking into account also the high artistic and historical value of the building,respecting Borromini's original concept. It as therefore decided to use asystem of prestressed tie bars, placed along the radius through the vault,anchored at the edges and connected, in the central area, by a stainless steelelliptical ring placed at the intrados of the vault itself.

1 Introduction

The Church (figure 1) was built between 1634 and 1636 on a quasi-ellipticalplan, generated by the joining of four secondary ellipses to a principal andcentral ellipse (figure 2). This complex form of the nave rises to the corniceand is then covered by an elliptical dome base on four pendentives and foursemi-elliptical domes.

Transactions on the Built Environment vol 15, © 1995 WIT Press, www.witpress.com, ISSN 1743-3509

386 Architectural Studies, Materials & Analysis

The vault of the crypt is based on thesame plan as the nave above and it isvery flattened with an upwardlongitudinal concavity in the centralzone. If on the one hand this designgives exceptionally plastic results, onthe other it penalizes the bearingcapacity, which can only develop alongthe low diagonal arches (figure 3); thisis the main reason for the staticproblems that have arisen since itsconstruction. In effect, the hollowstructure implies very high thrusts,which are extremely sensitive to evensmall horizontal movements of thesupports, so that substantial bendingmoments are generated and as a resultan extended pattern of cracks hasdeveloped. figure 1: facade of the Church

figure 3: longitudinal sectionof the vault

figure 2: view from above

2 Historical analysis and description of the vault

Borromini began work on thechurch, which is part of a largercomplex for the Trinitarians, in1634 and the crypt wascompleted almost immediately.Here the formally unitarytreatment of space is matched bythe composition of the vaultwhich, as endoscopic tests haveshown, was built from a massivecasting of pozzolanic concretemixed with tuff stones and brick

figure 4: endoscopy of the thicknessof the vault


Architectural Studies, Materials & Analysis 387

fragments (figure 4); the resulting structure is highly compact, has no cavitiesand mortar is extremely cohesive.

3 Description of the damage

The vault has extended cracks (figure 5), consisting of a principallongitudinal lesion on the intrados of the vault and several cracks on theextrados of the springings (figure 6). There are also a number of probablytransient transversal lesions near to the apse and a settlement of about 3 cm.in the central part of the vault.

Via Quattro Fontane

figure 5: crack pattern of the vault

It is not possible to determinethe precise development overtime of these lesions. It is,however, probable that thecentral longitudinal crack inthe intrados and the cracks inthe springings of the extradoshad already been caused bythe weight of the structurewhen the scaffolding wasremoved. The fact that thelesions on the intrados areclearly visible even after theplastering and painting operations carried out in recent years, suggests thatthe phenomenon continues.

figure 6: cracks in the extrados near the altar



4 Structural Analysis

4.1 GeneralFinite element mathematical models were used for the dual purpose ofidentifying the principal causes of the above damage and providing aqualitative and quantitative evaluation of the measures needed to restore thenecessary levels of safety.

4.2 Overall original conditionThe study envisaged first of all a three-dimensional model aimed at analysingthe whole vault in an elastic field. The results showed that the weight of thestructure alone causes high levels of traction both on the intrados of thekeystone (30 t/sq.m., figure 7) and on the extrados near to the side altars (43t/sq.m., figure 8).

figure 7: 3D model - transversal figure 8: 3D model - transversalstresses in the extrados stresses in the intrados

The model also revealed that, on account of the little curvature and high ratiobetween the major and minor axes, the central band of the vault behavesessentially like a flattened arch, for a width of about 4 metres. A second two-dimensional finite element model was thus prepared and both a linear and anon-linear elastic analysis carried out.

4.3 Damaged conditionThe non-linear analysis was carried out in stages, submitting the vault toincreasing loads. At each increase of the load, a post-processor reduced therigidity of all the elements for which the points representative of the state oftension were outside the strength dominion, redistributing the correspondingtensions to the adjacent elements. The first breaking finite elements werereached on the intrados, in the centre-line, with 63% of dead weight(formation of the longitudinal crack in the keystone of the vault); thestructure then achieved a new configuration of equilibrium until at 93% ofthe load, further cracks occurred and the diagram tent to an horizontal line.When all the dead weight was applied other cracks were formed in theextrados near to the side altars (figure 9).



Further increases in theload leaded to rapidlyincreasing deformations,making it possible toidentify an ultimate loadcorresponding to a value ofthe order of 25-30% overthe working load (figure10). The safety margin wasthus extremely narrow andthe calculation amplyexplained the actualdamage situation.

figure 9: 2D model - non linear analysis

NON LINEAR STEP BY STEP ANALYSIS

125%

0,00 0,30 0,60 0,90 1,20 1,50VERTICAL DISPLACEMENT (cm)

2,10

figure 10: load - vertical displacement curve

In order to check the safetylevels found, a second typeof calculation was carriedout using a three-dimensional elastic modelin which the actual crackswere directly inserted; thisshowed high level of tensilestresses (figure 11)confirming that the vaulthas essentially come to theend of its bearing capacity.It was therefore judgedabsolutely necessary toproceed with the planned reinforcement measures described below.

figure 11: 3D model with crackslongitudinal stresses



4.4 Situation after interventionThe reinforced structure was analysed by applying to the vault forcesequivalent to the prestressing supplied by the tie bars. The effectiveness andadvantages of the measures adopted can be seen in the results obtained byboth the three-dimensional and two-dimensional models (figures 12-13-14).In both cases there is a noticeable reduction in stress peaks. The pressure fallsfrom 60 to 38 t/sq.m. and the traction from 45 to 14 t/sq.m. on the extradosnear to the side altars and from 30 to 5 t/sq.m. on the keystone of theintrados.

figure 12: 2D model - situationafter intervention

figure 13: 3D model - the extradosafter intervention

figure 14: 3D model - the intrados after intervention



5 Criteria for intervention

The reinforcement meas-ures were designed so as toassure the necessary safetylevels while keeping to aminimum any alterations tothe original design. It wastherefore decided that thebest solution was to adopt aradial system of tie barsinserted in holes madethrough the thickness of thevault (figure 15).The bars are connected inthe central part by astainless steel ring (figures 16-17) and anchored at the ends at the level of thefloor of the premises adjacent to the chapel.

figure 15: arrangement of the bars in thethickness of the vault

figure 17: detail of the anchoringarrangement

The effect of the bars is not only toprovide vertical forces which, asalready noted, are directed upwards inthe region of the central ring,lightening the load on the vault, butalso thanks to the radial arrangement,to encircle and connect the springingsand perimeter walls so as to counterall lateral movements to which the

vault is particularly susceptible on account of its extremely flattened design.The stages of intervention can be summarised as follows: plastering and

injection of structural mortar into the main cracks; precision drilling with adiamond-headed core barrel placed radially and inclined into the thickness of

figure 16: the anchor ring



the vault (figure 18); insertion of 22 stainless steel bars with a diameter of 18mm; prestressing to a value of approximately 6 t (figures 19-20).

figure 18: perforation of the vault figure 19: anchor ring duringprestressing

figure 20: detail of prestressing mechanism

To keep the structure under control,both during the prestressing and laterduring the first stage of the works, aseries of strain gauges wereconnected to a data recording unitable to monitor the stressing valuescontinuously (figure 21).

We are grateful to theSuperintendent Francesco Zurli forhis support.

figure 21: data recording unit


Documents

G. Carluccio', S. Meluzzi, A. Viskovic* · 2014. 5. 20. · The vault has extended cracks (figure 5), consisting of a principal longitudinal lesion on the intrados of the vault and