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Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 15363799 Seismic protection of monuments by shape memory alloy devices and shock transmitters M.G. Castellano & S. lnfanti FIP lndustriale, Selvazzano (Padova), ltaly ABSTRACT: European seismic-prone countries, and in particular Italy, host an impressive number of histor- ical constructions. Protecting historical constructions from the effects of severe earthquakes by conventional strengthening methods is not always reliable. AIso, conventional methods are often toa invasive as well. A lot of innovative techniques that overcome some ofthe drawbacks oftraditional techniques have been developed in the past few years to improve the seismic resistance ofhistorical constructions. This paper aims to present innovative techniques based on the use ofshape memory alloy devices and shock transmission units. Earthquake protection ofthe aforesaid structures through seismic devices is practical and feasible, which is illustrated in this paper via three examples in Ital y: the Basilica of San Francesco in Assisi, the Cathedral of San Feliciano in Foligno, and the Church of San Serafino in Montegranaro. Ali these churches were recently restored after damage caused by the 1997 Umbria-Marche earthquake. TNTRODUCTION Unfortunate ly, the high seismic vulnerability ofarchi- tectural heritage a li over the world is demonstrated not only by a good number of scientific studies but also by the occurrence of earthquakes in recent years that heavily damaged - and in some cases completely destroyed - ancient structures that are irreplaceable witnesses to past culture and art. This high vulnera- bility is often intrinsic and related to certain structural types and materiaIs, or original mistakes in designo Said vulnerability is also due to cumulative damage from past earthquakes that is often difficult to assess without a thorough survey. AIso, age in g progressively reduces the strength of materiaIs. That is the reason why some monumental structures have recently suf- fered strong damage in the face of earthquakes oflower intensity than those in the past centuries. Seismic resis- tance has sometimes been decreased by faulty past restoration interventions as well. Thus, restoration interventions are necessary to improve the seismic performance ofmost ofthe histor- ical constructions located in seismic prone countries. Conventional intervention techniques do not often endow structures with sufficient resistance against a maximum expected earthquake and operate changes in the original structural configuration that are not acceptable from a cultural standpoint. A lot of innovative techniques have been devel- oped in the past few years to overcome some of the drawbacks of conventional techniques and improve the resistance of architectural and cultural heritage against strong earthquakes. Some of these innovative techniques implement passive control or strengthen- ing of the structure through proper seismic devices, e.g. seismic isolators, shape memory alloy devices, shock transmission units. Seismic isolation is now a mature technique that has been extensively applied in many countries in modem structures such as bridges, viaducts, indus- trial plants, and buildings. It consists of reducing the energy transmitted by an earthquake to a structure by changing the structure's dynamic characteristics; specifically, by increasing its natural period (Skinner et a!. 1993). This change is usually achieved through the use of special devices - isolators - with very low horizontal stiffness and appropriate damping - that "decouple" the structure from ground motion. Thanks to this decoupling and a consequent reduction of the transmission and amplification of horizontal acceler- ations into the structure, seismic isolation is the only anti-seismic technology that allows the mitigation of earthquake effects not only on the structure itself, but its contents as well. That is why it is irreplaceable for ali those structures whose contents or functionality after earthuquake are more valuable than the struc- tures themselves like museums, hospitaIs, buildings with a criticaI civil defence role, etc. Some impor- tant applications to restore cultural heritage structures have also been implemented, in particular in the United 1229

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Page 1: Seismic protection of monuments by shape memory alloy ... · Seismic protection of monuments by shape memory alloy devices and shock transmitters M.G. Castellano & S. lnfanti

Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 15363799

Seismic protection of monuments by shape memory alloy devices and shock transmitters

M.G. Castellano & S. lnfanti FIP lndustriale, Selvazzano (Padova), ltaly

ABSTRACT: European seismic-prone countries, and in particular Italy, host an impressive number of histor­ical constructions. Protecting historical constructions from the effects of severe earthquakes by conventional strengthening methods is not always reliable. AIso, conventional methods are often toa invasive as well. A lot of innovative techniques that overcome some ofthe drawbacks oftraditional techniques have been developed in the past few years to improve the seismic resistance ofhistorical constructions. This paper aims to present innovative techniques based on the use ofshape memory alloy devices and shock transmission units. Earthquake protection ofthe aforesaid structures through seismic devices is practical and feasible , which is illustrated in this paper via three examples in Italy: the Basilica of San Francesco in Assisi , the Cathedral of San Felic iano in Foligno, and the Church of San Serafino in Montegranaro. Ali these churches were recently restored after damage caused by the 1997 Umbria-Marche earthquake .

TNTRODUCTION

Unfortunately, the high seismic vulnerability ofarchi­tectural heritage ali over the world is demonstrated not only by a good number of scientific studies but also by the occurrence of earthquakes in recent years that heavily damaged - and in some cases completely destroyed - ancient structures that are irreplaceable witnesses to past culture and art. This high vulnera­bility is often intrinsic and related to certain structural types and materiaIs, or original mistakes in designo Said vulnerability is also due to cumulative damage from past earthquakes that is often difficult to assess without a thorough survey. AIso, ageing progressively reduces the strength of materiaIs. That is the reason why some monumental structures have recently suf­fered strong damage in the face of earthquakes oflower intensity than those in the past centuries. Seismic resis­tance has sometimes been decreased by faulty past restoration interventions as well.

Thus, restoration interventions are necessary to improve the se ismic performance ofmost ofthe histor­ical constructions located in seismic prone countries. Conventional intervention techniques do not often endow structures with sufficient resistance against a maximum expected earthquake and operate changes in the original structural configuration that are not acceptable from a cultural standpoint.

A lot of innovative techniques have been devel­oped in the past few years to overcome some of the

drawbacks of conventional techniques and improve the resistance of architectural and cultural heritage against strong earthquakes. Some of these innovative techniques implement passive control or strengthen­ing of the structure through proper seismic devices, e.g. seismic isolators, shape memory alloy devices, shock transmission units.

Seismic isolation is now a mature technique that has been extensively applied in many countries in modem structures such as bridges, viaducts, indus­trial plants, and buildings. It consists of reducing the energy transmitted by an earthquake to a structure by changing the structure's dynamic characteristics; specifically, by increasing its natural period (Skinner et a!. 1993). This change is usually achieved through the use of special devices - isolators - with very low horizontal stiffness and appropriate damping - that "decouple" the structure from ground motion. Thanks to this decoupling and a consequent reduction of the transmission and amplification of horizontal acceler­ations into the structure, seismic isolation is the only anti -seismic technology that allows the mitigation of earthquake effects not only on the structure itself, but its contents as well. That is why it is irreplaceable for ali those structures whose contents or functionality after earthuquake are more valuable than the struc­tures themselves like museums, hospitaIs, buildings with a criticaI civil defence role, etc. Some impor­tant applications to restore cultural heritage structures have also been implemented, in particular in the United

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States, e.g.: Salt Lake City and County Building, San Francisco City Hall and Oakland City Hall (Elsesser et aI. 1991, Naaseh 1995, Naaseh & Elsesser 2000). In Europe, the use of seismic isolation in monuments is not yet so established as in the United States, and there are few applications only in ltaly. In effect, despite the many feasibility studies conducted for different types of hi storical structures, only some applications for the protection of statues, such as the famous ancient greek Riace Bronzes in the museum of Reggio Calabria and the Scilla and Netluno marble Statues of Angelo Montorsoli in the regional museum of Messina, have been already completed, while the first application to an historical building (the Church of Santa Croce in Nocera Umbra) will be carried out in next months.

Conversely, other types of seismic devices, such as shape memory alloy devices and shock transmission units, have found application since some years in the restoration of some Italian cultural heritage structures damaged by recent earthquakes. This paper aims to present some ofthese applications.

Shape memory alloy devices (SMAD) have been developed specifically for use in ancient masonry structures (Caste llano 2000). They are axial devices exploiting the superelastic properties of shape mem­ory alloys used in lhe form of small diameter wires (Castellano et aI. 1999). SMADs can be substituted for conventional stee l ties to connect different structural elements, e.g. façades to fl oors or the roof. The advan­tage accrued over lhe use of steel ties is mainly due to their force limitation capacity. For example, when used as horizontal ties to connect façade walls to roof, SMADs permit controlled displacements and canlimit transmitted forces, according to the device cOl1stitu­tive law. Figure I shows an example of such a law for a device of the multi-plateau type with 3 plateaux. The design engineer can select two or more force lev­eis and corresponding di splacements; these are then realized using groups of shape memory alloy wires with different lengths, properly connected within the device.

25,--------------------------------, 20

15 10

~ 5

~ -s .i: -10

-15 -20

-25 -30 -I----------~--------~--------~~

-25 -20 -15 -lO -5 10 15 20 25

DisphH:ement (rum)

Figure I. Force vs. displacement loops measured on a SMAD during a sinusoida l test (amplitude ± 24 mm, frequency 2 Hz).

Numerical analyses and experimental results have shown that SMAD cOl1nection permits reducing accel­erations on the faça de wall , thus preventing the out­of-plane collapse ofthe wall , including the tympanum. In particular, comparison of shaking table tests on wall s tied with SMAD or with conventional steel ties showed that the wall using the SMAD connections did not suffer any visible damage, even when subjected to an earthquake with PGA almost 50% higher than the earthquake causing the first collapse in the wall con­nected by tradi tional steel ties (Castellano et aI. 1999).

The following sections describe the application of this type of SMAD to the Basilica of San Francesco in Assisi, the Cathedral of San Feliciano in Foligno, and the Church of San Serafino in Montegranaro respectively.

Shock transmission units (STU) have been widely used in the last 30 years in new structures - mainly bridges and viaducts or prefabricated buildings - as dynamic or temporary restraints. They are piston/ cylinder devices that utilize fluid flow through orifices to provide for a reaction that is the result of apressure differential across the piston head and is function of the velocity appl ied to the mentioned piston. STU are designed to work as a safety belt for a car driver: at the occurrence of a dynamic movement exceeding lhe so called "activation velocity" (i nduced for example from an earthquake) they react as a very stiff-temporary­restraint, whilst for very slow movements, such those imposed by thermal expansion, they do not provide fo r a major reaction.

During the last decade, STU have also been appl ied in the restorat ion of some historical masonry bllild­ings. The f irst ofthese applications was effected in the church ofSan Giovanni Battista in Carife , Italy, which was heavily damaged by the lrpinia 1980 earthquake (Mazzolani et aI. 1994). Part of the damage included the complete collapse of lhe old timber roof. A new steel truss roofwas built and shock transmission units were used to connect the new roof to the masonry walls. This was done to keep roofthermal elongations from induci ng high stresses on the walls and, at the same time, gllarantee a situation where the roof can share the seismic actions with ali the walls during a seismic attack (owing to the roof high in - plane stiff­ness and lemporary stiff connections to the walls given by the STU).

The most recent application of shock transmission units in monuments was implemented at the Basilica of San Francesco in Assisi , Italy, described below.

2 THE BASILICA OF SAN FRANCISCO IN ASSISI

The Basilica ofSan Francisco inAssisi , Italy, was built in the 13th century. It was hit by many earthquakes in

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its history, but none produced the extensive damage suffered in September 1997.

During said earthquake, a big portion of the vaults of the Upper Basilica, frescoed by Giotto and Cimabue, collapsed and large cracks and permanent deformations were produced throughout the vaults. Also, a portion of the tympanum of the left transept collapsed and the tympanum of the right transept was heavily damaged as well. The damage to the tympana was due to both mortar decay and the pounding ofthe roof against the façade wall (induced by the lack of effective connections between them).

Figure 2. Insta llation scheme of shape memory alloy devices in the Basilica of San Francisco in Assisi (units are mm).

Figure 3. Shape memory alloy devices for the Basilica of San Francisco in Assisi.

The restoration of the Basilica included the use of many innovative materiaIs and techniques (Croci 2000) like the seismic devices described below (Bonci et a I. 2001).

2.1 Application of shape memory alloy devices

In the Basilica of San Francisco, 47 SMAD were used to connect the roofto the two tympana ofthe transept (24 on the left [south] side, and 23 on the right [north] side). Figure 2 shows details of said SMAD connec­tion : an anchor plate attached to a threaded bar inserted in the façade masonry wall during its partial recon­struction; a new reinforced concrete rib built at the end of the existing r.c. roof (built in the 1950s) to stiffen it, support the roof beams and bring the roof verti­cal loads directly onto the transept lateral walls. The SMAD were connected on one side to the threaded bar and on other side to the rib, through a plate bolded to a counter-plate embedded in concrete.

Figures 3 and 4 show the devices as built and as installed, respectively.

Figure 4. Shape memory alloy devices installed in the Basilica ofSan Francisco in Assisi.

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Figure 5. The horizontal stainless stee l truss along the entire Upper Basilica.

Three different sizes of SMAD were applied, with design forces ranging from 17 to 52 kN and maxi­mum displacements ranging from ±8 to ±25 mm, to take into account the different properties required as the distance from the transept lateral waIls graduaIly increases toward the rooftop. SMAD of different sizes can be recognized in the photographs by their different length.

This was the world's first application of shape memory aIloys to a building to improve earthquake resistance.

2.2 Application of shock transmission units

Shock transmission units were used to connect the di f­ferent parts of a steel truss instaIled at an intermediate height along the perimeter ofthe basilica (Fig. 5). The aim of said truss is to increase the stiffness of the si de waIls and thus avoid the aggravation ofvertical cracks created by past earthquakes and reopened by the last one. The STU aIlow the truss to behave as it were con­tinuous and rigidly connected to the masonry during an earthquake, whereas the truss and the masonry waIls can undergo displacements independently under nor­mal service conditions. Thirty-four shock transmission units oftwo types were instaIled (220 and 300 kN max­imum force respectively and maximum displacement of ±20 mm) in groups of two or three in each posi­tion (Fig. 6). Said devices were made of high strength stainless steel to both reduce dimensions and increase durability.

lt is worth noting that the cost ofthe shape memory aIloy devices and the shock transmission units instaIled in the Basilica of San Francesco in Assisi comprised less than 1 % of the total restoration cost.

3 THE CATHEDRAL OF SAN FELICIANO IN FOLIGNO

The Cathedral of San Feliciano in Foligno, Italy, was built on 1133 on the site of an early basi lica, enlarged during the XV century and further modified in the XVI, XVIII centuries as weIl as at the beginning of

Figure 6. A pai r of 220 kN shock transmission units installed in the Basilica of San Francisco in Assisi .

xx century. The main façade, originaIly built in 1133, was modified in 1904, when the mosaic was added.

During the 1997 earthquake, Foligno's Cathedral suffered heavy damage, including the detachment of the façade, with a horizontal displacement of 8 cm off the covering vaults. Such a detachment was caused by an incipient overturning coIlapse mechanism due to the ineffective restraints on the orthogonal vertical waIls and the roof.

Improvement of the connection of the façade waIl to the vertical waIls and the roof was thus deemed necessary to increase safety leveis regarding façade overturning.

The effectiveness of the connection to the vertical waIls was improved through the insertion of tradi­tional steel ties at a height of about 15 m, about 3/5 of the façade height, while the one between façade and roof was effected with 9 shape memory aIloy devices (Fig.7).

Said devices are of the same type of those used in the Basilica of San Francisco in Assisi, i.e. with the force vs. displacement law shown in Figure I, with 2

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instead of 3 plateaux. Each of them (Fig. 8) is char­acterized by a design force of 27 kN and maximum displacement of ±20 mm, and end in tang plates, con­nected through pins to the clevis ofthe anchor frames. On one side, the anchor frame is welded to a new

ROTATION OFTYMPANUM

Figure 7. Arrangemenl of SMAD in lhe Calhedral of San Feliciano in Foligno.

Figure 8. A shape memory alloy device for lhe Calhedral of San Feliciano (F = 27 kN, S = ±20 mm).

SHAP[ M[MORY ALLOY O[VIC[

N[W V-SHAPEO 8[AM

Figure 9. Anchorage delails ofa shape mel110ry alloy device (Calhedral ofSan Feliciano).

v - shaped beam, welded in tum to the existing beams of the modem steel roof (built in the 1950s). On the olher side, the anchorage frame is connected to the masonry façade wall through anchor rods. A gap around the ends of existing steel beams, bearing on a PTFE - stainless steel sliding plate, permits rela­tive displacements between façade and roof (which are controlled by the shape memory alloy devices) (Figs 7, 9).

4 THE CHURCH OF SAN SERAFINO IN MONTEGRANARO

The church of San Serafino in Montegranaro and the adjacent convent were built in 1603 on the ruins of a previous church built in the XIII century and collapsed in 1431. The 1997 Umbria-marche earthquake caused the collapse of one of the wood trusses of the roof. Actually, the lack of proper connections between the roof and both the lateral walls and the façades could have caused even further damage due to out-of-plane collapse ofperipheral walls (Fig. 10). The retrofit was thus based on the installation of two SMAD to con­nect the front façade to lhe lateral walls (Fig. 11) and of cross bracings with steel ties to connect the lateral walls. Each SMAD works in tension only and is char­acterised by a 39 kN load capacity and a 20 mm design stroke.

Figure 10. Hypothesis of oul-of-plane collapse of periph­eral walls of lhe Church of San Serafino (courlesy of R. Mariani).

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Figure li. Sketch ofthe arrangement ofthe shape memory alloy devices connecting the faça de to the lateral walls ofthe Church of San Serafino (courtesy of R. Mariani).

5 CONCLUSTONS

Shape memory alloy devices and shock transmission units can be regarded as powerful tools in the hands ofthe engineer to reduce the earthquake vulnerabi lity of cultural heritage structures. Both of them are axial devices aimed at connecting different structural ele­ments, but through completely different behaviours. Their installation is simple and their cost is compara­tively far lower than the total cost incurred in restoring cultural heritage structures, much like the cost of seis­mic devices used in modem structures when compared to the construction costs.

ACKNOWLEDGEMENTS

The restoration of ihe Basilica of San Francesco in Assisi was designed by G. Croci, P. Rocchi, A. Paolucci, C. Centroni, and G. Basi le.

Design architects and engineers of the restoration of the Cathedral of San Feliciano were G. Carini, G. Colombatti and L. Radi.

Design engineer ofthe structural restoration ofthe Church of San Serafino was R. Mariani, architects were D. Cardamone and L. Tanfani.

Shape memory alloy devices were developed within the framework ofthe ISTECH research project

(Development of Innovative TEchniques for the improvement ofStability ofCultural Heritage, in par­ticular seismic protection), partially funded by the European Commission under Environment and Cli­mate RTD Programme (Contract No. ENV4-CT95-O I 06). Partners of this project, that was coordinated by FIP Tndustriale, were: Aristotle University of Thessaloniki; ENEA-SIEC-SISM ofBologna; Istituto Superior Técnico of Lisbon; Joint Research Centre of the European Commission, Ispra (VA-Italy); and University "La Sapienza" ofRome.

REFERENCES

Bonci, A. et aI. 2001. Use of shock transmission units and shape memory alloy devices for the seismic pro­tection of monuments: the case of the upper Basi lica of San Francisco at Assisi. In 1nternalionai Millennium Congress - More than two thousand years in the history of architecture, UNESCO-lCOMOS, Paris, 10-12 September 2001, Vol. 11.

Castellano, M.G. 2000. Development and experimental char­acterisation ofshape memory alloy devices (SMADs). In Shape Memory AI/oy Devices for Seismic Proteclion of Cultural Heritage Structures, Proc. of Final Workshop of ISTECH Project, Ed. A. Viskovic, Rome: I 1- 19.

Castellano, M.G. et aI. 1997. Seismic protection of cu ltural heritage through shape memory alloy based devices. In Proc. of1nternationai Post-SMiRTConference Seminal' on Seismic Isoiation, Passive Energy Dissipation and aclive control of vibrations of structures, Taormina, Itaiy.

Castellano, M.G. et aI. 1999. Seismic protection of cu ltural heritage using shape memory alloy devices: an EC funded project (ISTECH). In Proc. of the Internationai Post­SMiRT Conference Seminal' on Seismic Isoiation, Passive Energy Dissipation and Aclive Contrai of Vibrations of Structures, Cheju, Korea.

Croei, G. 2000. The restoration of the Basi lica of St Franeis of Assisi. In Proc. of the IASS-MSU In/nl Symposium Bridging large spans from antiquity to the present: 12 1- 134.

EIsesser, E. et aI. 1991. Repair of five historie bui ldings damaged by the Loma Prieta earthquake. In Proc. of The seismic retrajit of historic buildings conference, San Francisco, California: 4-1 /4-40 .

Mazzolani, F.M. & Mandara, A. 1994. Seismie upgrading of churches by means of dissipative devices. In Proc. ofthe In t. Workshop and Seminal' on 8ehaviour of Steel Struc­fures in SeismicAreas (STESSA '94), Timisoara, Romania: 747- 758.

Naaseh, S. 1995. Seismic retrofit ofSan Francisco Ci ty Hall : the role of masonry and concrete. In Proc. ofThe third national concrete and masonry engineering conference, NCMEC 1/1, San Francisco, California.

Naaseh, S. & EIsesser, E. 2000. Seismic protection of domed structures. In Bridging iarge spans from antiquity to lhe present, Proc. , IASS-Symposium 2000, Istanbul: 281 - 290.

Skinner, R.1. et aI. 1993 . An introduction to seismic isolation. Chiehester, England: J.Wiley & Sons.

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