International Journal of Architectural Heritage: Conservation, Analysis, and Restoration

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Shape Memory Alloy Devices for theSt ructural Improvement of Masonry

Heritage StructuresMaurizio Indir l i

a& Maria Gabriel l a Cast el lano

b

aENEA (Ente Nazionale per le Nuove Tecnologie, l'Energia e

l 'Ambient e) ACS Depart ment (Dipart iment o Ambiente, Cambiament iGlobali e Svilu ppo Sost enib il e) PREV Unit (Unit Preven zione deiRischi Nat ural i e Mit igazione Eff et t i ), Bologna, I talyb

FIP Indust riale s.p. a., Research and Development Depart ment ,Selvazzano Dent ro ( PD), It aly

Availabl e onli ne: 31 May 2008

To cite this art icle: Maurizio Indirl i & Maria Gabriella Castellano (2008): Shape Memory Alloy Devicesfor t he St ructur al Improvement of Masonry Heri tage St ruct ures, Internat ional Journal of Archi t ecturalHeri t age: Conservat ion, Analysis, and Rest orat ion, 2:2, 93-119

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SHAPE MEMORY ALLOY DEVICES FOR THESTRUCTURAL IMPROVEMENT OF MASONRY HERITAGESTRUCTURES

Maurizio Indirli1 and Maria Gabriella Castellano2

1ENEA (Ente Nazionale per le Nuove Tecnologie, lEnergia e lAmbiente)

ACS Department (Dipartimento Ambiente, Cambiamenti Globali e Sviluppo

Sostenibile) PREV Unit (Unita Prevenzione dei Rischi Naturali e

Mitigazione Effetti), Bologna, Italy2FIP Industriale s.p.a., Research and Development Department, Selvazzano

Dentro (PD) Italy

The development of innovative techniques based on shape memory alloy devices (SMADs),

for the purpose of seismic protection for cultural heritage structures, began in Italy within the

framework of an European Commission-funded project and continued with further studies

and applications. These devices exploit a special property of shape memory alloys: their

superelasticitythe ability to recover very high deformation without residual strain, asso-

ciated with a non-linear constitutive behavior, making it possible to keep the force constant in

a wide range of displacements. Consequently, SMADs can be used as special ties capable of

limiting the forces transmitted between the structural elements that they connect (e.g., achurch facade and the roof), at the same time allowing small displacements. Compared with

very rigid conventional steel ties, SMADs are able to reduce accelerations and forces, and

thus increase the seismic capacity of the structure. This article provides a brief overview

about the research and development that led to the worlds first applications of SMADs for

seismic protection, within the framework of the post-earthquake restoration of the upper

basilica of St. Francis in Assisi, Italy.

KEY WORDS: cultural heritage structures, masonry structures, earthquake engineering,

anti-seismic devices, shape memory alloy, strengthening and reinforcement, experimental

tests

1. INTRODUCTION

The development of innovative techniques based on the use of shape memory

alloy devices (SMADs), for the purpose of seismic protection for masonry cultural

heritage structures (MCUHES), began in Italy in 1996 within the framework of the

European Commission-funded ISTECH Project (Development of Innovative

Techniques for the Improvement of Stability of Cultural Protection, in Particular

International Journal of Architectural Heritage, 2: 93119, 2008

Copyright Taylor & Francis Group, LLC

ISSN: 1558-3058 print /1558-3066 online

DOI: 10.1080/15583050701636258

Address correspondence to Maurizio Indirli, ENEA (Ente Nazionale per le Nuove Tecnologie,

lEnergia e lAmbiente), ACS Department (Dipartimento Ambiente, Cambiamenti Globali e Sviluppo

sostenibile)PREV Unit (Unit a Prevenzione dei rischi naturali e mitigazione effetti), Via Martiri di

Monte Sole 4, 40129 Bologna, Italy. E-mail: [email protected]

Received 6 March 2007; accepted 16 August 2007.

93


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Seismic Protection) and was subsequently continued with further studies and applica-

tions. This article provides a brief overview about this development, including the

worlds first SMAD applications to MCUHES, offering the scientific and professional

community a powerful tool (together with other techniques) capable of harmonizing

reinforcement and protection and overcoming the drawbacks of the traditional meth-odologies usually employed.

The issue of correct and reliable antiseismic measures for MCUHES is of crucial

importance in the Mediterranean countries, which are characterized by a large num-

ber of ancient (and frequently precious) buildings and non-negligible seismicity over

much of their territory. It has been unfortunately demonstrated that earthquakes

(even those of moderate intensity, often magnified by local soil conditions and

structural vulnerability) can cause collapse or heavy damage. In addition, several

existing still-standing MCUHES, even though not yet severely damaged, have been

at least weakened by previous earthquakes, and their resistance has been lowered by

other factors (poor maintenance, incorrect restoration and expansion work, andchemical attacks on masonry materials from air pollution and traffic-induced vibra-

tions). Furthermore, after the seismic event that hit Italys Molise Region in 2003, a

very important change occurred in the Italian legislative scenario. This change was due

to the revision of the seismic areas classification (generally stricter than before 2003)

and the adoption of an updated seismic code, based on Eurocode 8 (1998), and also

included a specific section for existing buildings (Presidenza del Consiglio dei Ministri

2003a; 2003b; 2005).

In this new context, it was necessary to carefully examine further the subject of

the antiseismic improvement of MCUHES, to avoid a possible conflict between

structural safety and architectural conservation. Thus, a specific Italian working

group produced dedicated guidelines for cultural heritage (Presidenza del Consigliodei Ministri 2006), establishing that antiseismic measures cannot be evaluated with the

same approach used for new constructions built with modern materials (reinforced

concrete or steel, or even masonry), according to a large body of scientific studies and

on the basis of periodic past earthquake experiences. The previously mentioned guide-

lines state that the MCUHES improvement can be performed by using both tradi-

tional devices (TDs), and/or innovative techniques and materials. To obtain a

satisfactory global behavior of the structure, it is necessary to enhance the connections

between the masonry walls and between these walls and the floors, mainly achieved by

the insertion of ties, confining rings, and string courses, preferably in reinforced

masonry, steel, or wood (Modena et al., 2006). Thus, in the authors opinion, thephilosophical approach can be summarized in these following statements:

1. Because MCUHES repair is much more difficult to carry out than that on modern

structures, measures must be defined as a controlled structural improvement

instead of retrofit i.e., accepting an antiseismic protection level lower than

required by the codes for modern structures, thus limiting invasiveness; at the

same time, great attention must be devoted to the buildings final intended use to

accurately manage the importance factor used in seismic standards, which is

higher for public and strategically important constructions (Modena et al., 2006);

2. For each limit state, the improvement effectiveness must be quantified, evaluating

the peak ground acceleration (PGA) levels generating local collapse mechanisms,before and after intervention;

94 M. INDIRLI AND M. G. CASTELLANO

INTERNATIONAL JOURNAL OF ARCHITECTURAL HERITAGE 2(2): 93119


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3. Detailed survey and investigation campaigns are mandatory for MCUHES, the

characteristics of which are frequently not well known;

4. The rehabilitation must be designed in a specific way since the use of standardized

procedures is not possible;

5. The observance of the regola dellarte (the unwritten construction rules for work-manlike masonry elaborated by architects and bricklayers in centuries of work

practice) is fundamental for protection, conservation and restoration;

6. The use of modern techniques and materials can be very useful in reducing seismic

vulnerability, but it must be philologically correct (i.e., respect as much as possible

the original structural behavior), compatible, and reversible.

Therefore, the use of SMADs may be an innovative solution, not yet widely applied,

for harmonizing reinforcement measures and conservation criteria. In the following

discussion, the main research and development results are summarized as well as the

main applications.

2. MAIN PROPERTIES OF SHAPE MEMORY ALLOY DEVICES

AS ANTISEISMIC DEVICES

Although shape memory alloys (SMAs) have been known since 1932 and

research into both the metallurgy and practical uses has increased since the 1960s

(when a nickeltitanium[NiTi] SMA was discovered), their potential use in the

earthquake engineering field has only been studied since the 1990s (Aiken et al.,

1992; Bondonet et al., 1996; Clark et al., 1995; Graesser et al., 1991; Krumme

et al., 1995; MANSIDE Project, 1999; Witting et al., 1992). SMAs are metallicmaterials endowed with special thermo-mechanical properties due to a reversible

transformation between two crystalline configurations (known as austenite and

martensite) when cooled or heated, as well as in the presence of stress, without

degradation of the crystal structure. This behavior enables their use in many

different fields, from orthodontic and orthopedic applications to pipe couplings,

and from eyeglass frames to cellular telephone antennas (Perkins, 1975;

Duerig, 1990).

Specifically, the idea of protecting MCUHES against earthquakes with

SMADs was hatched during the previously mentioned ISTECH project. The

SMA superelasticity (the ability to recover very high deformations, from 6% to10%, more than 10 times the possibility with a conventional metal) was identified

as the most useful property for antiseismic purposes. The stressstrain curve,

measured during a monotonic tension test on an SMA wire, shows two plateaus

during the loading phase, i.e., sections where stress remains nearly constant with

increasing strain (Figure 1). After an initial, almost linear portion (corresponding

to the elastic deformation of the material in its austenitic phase), the curve

describes the usually called superelastic loading plateau. Despite its similarity

with the curve observed during yielding (e.g., mild steel), here the curve is due to the

stress-induced phase transformation from austenite to martensite. A new elastic

deformation takes place when all the material has completed the transformation in

the martensitic phase (i.e., when the imposed strain exceeds a certain value, calledmaximum superelastic strain).

SHAPE MEMORY ALLOY DEVICES FOR MASONRY STRUCTURES 95



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Afterwards, another plateau appears, this time related to the alloys true yield in

its martensitic phase. Figure 2 shows the hysteresis loop generated by loading and

unloading paths of an SMA wire, to which a tension strain has been applied up to the

maximum superelastic strain and then removed. The stress removal induces a reversephase transformation from martensite to austenite that allows almost complete strain

recovery. In view of this reversible phase transformation, a very large number of

similar cycles can be applied without any damage to the material. The absence of

sensible residual deformations allows the creation of SMADs where no permanent

displacements are present when they stop working. Conversely, using TDs (e.g., steel

ties), it could be possible to exploit their yielding and the consequent force-limiting

characteristic, but permanent residual displacement would be present when the forces

are removed.

In case of SMADs in series with vertical steel bars (see Sec. 3.1), the main

expected advantage is the control of the pre-stressing force imposed by the bar on

the masonry walls.In case of SMADs in series with horizontal steel ties (see Sec. 3.2),to be used for the connection of roofs or floors to the walls, or for the connection

Figure 1. Monotonic tension test up to failure on an SMA wire.

Figure 2. Cyclic tension test in the super-elastic range on an SMA wire.




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between transversal walls, the results of the research carried out within the ISTECH

project have shown that a SMAD should behave as follows:

1. Under service loads, the SMAD does not apply any static force to the structural

elements to which it connects (and consequently it is called self-balanced); underlow intensity horizontal actions (wind, small-intensity earthquakes), the SMAD is

rigid, as a TD, and no displacements are allowed;

2. Under higher intensity horizontal actions (stronger earthquakes) the SMAD

stiffness lessens, allowing controlled displacements; they should permit the

masonry to dissipate part of the energy transmitted by the earthquake, mainly

thanks to the formation of microcracks in the brick walls, taking care to avoid

dangerous macrocracks; in the meantime, due to the reduced SMAD stiffness,

lower forces are transmitted to the MCUHES which, consequently, should be able

to sustain a high-intensity earthquake without collapse;

3. Under extraordinary horizontal actions, the SMAD stiffness increases to preventexcessive displacements and instability.

Different SMAD types have been conceived, designed and manufactured for use as

horizontal ties:

1. The self-balanced single plateau SMADas noted in the previous first point, it

becomes active only under dynamic actions, inducing horizontal loads greater

than the initial force. Such a device also offers symmetric behavior for positive

or negative displacements, even though it is based on SMA wires always working

under tension. Figure 3 shows the constitutive behavior of one device of this type,

measured in tests on a prototype, and illustrates its very stable behavior undercycling;

2. The multi-plateau self-balanced SMADan evolution of the device just described;

its behavior is shown in Figure 4 (referring to a three-plateau SMAD). The

advantage gained is its capacity to work at increasing force levels, induced by

different earthquake intensities; in addition, an SMAD conceived in this manner is

less sensitive to the masonry tensile strength, on which the optimal design force of

Figure 3. Single-Plateau SMAD.




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the single-plateau SMAD depends, increasing with it (Biritognolo et al.,

2000). With the multi-plateau SMAD, the risks deriving from over- or

underestimating the masonry tensile strength can be either avoided or sub-

stantially reduced; the design engineer can select two or more force levels and

corresponding displacements to take into account a wide range of masonry

mechanical properties, achieving a good level of optimization. An example of

multi-plateau SMAD developed for horizontal connections is shown in

Figure 5.

Figure 4. Multi-Plateau SMAD.

(a)

(b)

Figure 5. Multi-plateau self-balanced SMADs developed for horizontal connections.




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The SMA selected in the research (NiTi) has very good corrosion-resistance char-

acteristics and this, apart from ensuring their durability, prevents deterioration

phenomena that could affect the materials (masonry) composing the MCUHES.

Moreover, the use of SMADs as structural connection elements is consistent with

the criteria of operation reversibility and compatibility with the original structures.As regards integrity, it must be noticed that the SMAD installation, being an

additive measure, produces just minimal disturbance to the MCUHES.

3. STRUCTURAL CONFIGURATIONS SELECTED FOR SMAD APPLICATIONS

The study of the applications of SMADs started from the choice of some TDs for

which their effectiveness was confirmed by experience. Among the different ideas that

initially arose for the use of SMADs, the main efforts of the research were focused on

the following techniques.

3.1. Tall and Slender Buildings: Bell Towers

Generally, a problem that frequently occurs in slender buildings subjected to

seismic actions is the need to improve their structural behavior, seen as a vertical

cantilever fixed at the base. A typical measure is the insertion of vertical bars,

connecting the top to the ground, tensioned to apply pre-stress to the masonry.

This measure can prevent the typical collapse shown in Figure 6, and the effective-

ness can be improved using SMADs in series with steel bars (Figure 7). The main

expected advantage is the control of the force imposed by the bar on the masonry

walls, in particular during the earthquake: actually, with a proper SMAD design, the

force transmitted to the building should not be higher than the upper plateau of

the SMA elements of the device. This use of SMADs was implemented in the

restoration of the Campanile of San Giorgio at Trignano (Figure 8), heavily

damaged by the October 15, 1996, earthquake and located within the most affected

area, near San Martino in Rio, Reggio Emilia (Italy).

Figure 6. Typical flexural collapse of a bell-tower.




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The earthquake mainly hit the Reggio Emilia and Modena districts in the

Emilia-Romagna Region (magnitude [M] 4.8 and VII MCS). The recorded peak

acceleration values near the epicenter (approximately 5 km) were: NS 0.142 g, EW

0.203 g and UP 0.094 g. Although the earthquake may be classified as moderate, its

energy was particularly evident in the low frequency range (typical of tall buildings).

This fact can explain the widespread damage (in many cases close to collapse) noticed

in several bell towers (Indirli, 1997a).

The Church of San Giorgio in Trignano (Figure 9a) was an ancient Romanesque

chapel, built in 1302. During the following centuries, the building underwent many

changes and additions. In 1700, the currently connected houses were built and, in the

second half of 1800, the campanile (18.5 m high, with a 3 m square base, see Figure 9b)was heavily modified, until it reached the recent configuration; the belfry, now formed

Figure 7. Proposal of SMADs use on slender buildings.

Figure 8. SMADs application in the Trignano Bell-Tower.




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by mullioned windows and an octagonal spire, was heightened and the original

one filled with bricks. The campanile mainly consists of four masonry columns,located at the corners, sustaining the whole building. The curtain walls (approximately

0.4 m thick, Figure 9c), located among the columns and also in the masonry, have a

negligible structural function. More than one half of the campanile rises freely from

the church and the parsonage. Materials are very poor (typically Bolognese burnt

bricks, mortar of common lime with sand and pozzuolana). During numerous on-site

surveys performed by experts from the ENEA (Ente Nazionale per le Nuove

Tecnologie, lEnergia e lAmbiente, Bologna, Italy) after the earthquake, serious

damage was noted on the campanile, which showed severe transverse and through

cracks in the columns and in the masonry infills, with a global fracture at the roof level

of the adjoining buildings (the campanile was very close to collapse, see Figure 10ab).

The free portion of the campanile, under the seismic motion, oscillated to the point ofrupture and then settled again: a rotation of approximately 30 mm was evident

(Figure 10c shows the rotation at the roof level). The structure was declared unsafe

and entrance to it was forbidden; it was then made safe, and restoration was con-

sidered necessary.

The rehabilitation of the Trignano campanile was carried out as the demo-

intervention specified in the ISTECH project itself, after an agreement with the local

authorities and the property owners (Indirli et al., 2001). After a necessary geometric

survey, the retrofit, completed in November 1999, consisted of two parts:

1. The conventional measures (cracked masonry global consolidation, re-plastering,floor reconstruction using light and typical materials, belfry rehabilitation, see

Figure 11d), funded by the regional reconstruction program;

2. The innovative measures (Figure 8 and Figure 11ac), to increase antiseismic

performance; these consisted of the insertion of vertical pre-stressing steel tie

bars in the internal corners of the structure, without drilling masonry, to increase

the flexural resistance of the tower; the tie bars, formed by six tight-screwing

segments (to facilitate their assembly), were placed in series with 4 SMADs; each

SMAD comprised 60 Ni-Ti superelastic wires of 1 mm diameter and 300 mm

length; appropriate anchorages (building top and foundation, see Figure 11a-b)

were applied to support the tensile forces from the tie bars; in fact, the SMADs

were post-tensioned in order to guarantee the constancy of compression acting onthe masonry, keeping the applied force below 20 kN.

(a) (b) (c)

Figure 9. The San Giorgio Church, its Bell-Tower, and the masonry infills.




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(a)

(b)

(c)

Figure 10. Cracks on the Bell-Tower (a-b) and rotation of the upper structure (b-c).




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A new seismic event, with the same epicenter, occurred on June 18, 2000 (M 4.5, VI-

VII MCS). Immediately after the main shock, the campanile was again investigated

with great accuracy, but it showed no damage of any kind.

3.2. Connections of Walls Against Out-of-Plane Seismic Forces: Church

Facades and Tympana

Another type of TD deals with the effective connection of orthogonal walls

subjected to horizontal forces directed perpendicularly to their own plane. In fact,

a typical damage mechanism in masonry buildings, and in particular churches, is

the out-of-plane collapse of peripheral walls (Figure 12), due to inertia forces

generated by a seismic event. A simple and efficient TD to prevent this manner

of collapse is the improvement of the wall connection at floor level, usually using

steel ties. Unfortunately, as seen from past earthquake damage, it is not sufficient

in many cases, because the high rigidity of the ties causes the transmission of

strong forces to the masonry. Consequently, the connection may also fail due to

the punching effect of the anchorage, especially in cases when the masonry

materials used are of poor quality or deteriorated. Furthermore, the high stiffness

of a structure connected in this way can significantly amplify ground accelerations.

This is particularly true of structural elements such as the tympana of churchfacades.

(a)

(b)

(c) (d)

Figure 11. Anchorages: building top(a) andfoundation (b); SMADs beforeassembling (c); Bell-Tower after

rehabilitation (d).




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The successful results of the research and its implementation led to the following

applications in Italy: the transept tympana of the upper basilica of St. Francis in

Assisi, the San Feliciano Cathedral facade in Foligno, and the San Serafino Church at

Montegranaro, all heavily damaged by the 19971998 UmbriaMarche earthquake

(Figure 13). Figures 14 and 15 (and sections 3.2.1 and 3.2.4) show the SMAD inser-

tions in the previously mentioned MCUHES.

This earthquake sequence (Indirli et al., 1997b) started on September 26, 1997 and

took place in a complex deforming zone, along a normal fault system in the central

Apennine Mountains. The seismic event left significant ground effects, which were

mainly concentrated in the Colfiorito intermountain basin. The crustal events generated

extensive ground motion and caused great damage in several urban areas. The extent of

the macroseismic data and the abundance of recorded ground motions provide good

knowledge of the source and structural parameters for better understanding the nature

of the ground shaking and the resulting damage patterns. Three main shocks (time 2:33,local M (ML) 5.5 and VIII MCS; time 11:40, ML 5.8 and VIIIIX MCS; time 11:46, ML

(a)

(c)

(b)

Figure 12. Rocking of gable-end walls in a church: (a) main facade; (b) transept facades; (c) tympana.




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(a)

(b)

(c)

Figure 13. Earthquake damage in churches where shape memory alloy devices have been applied: (a) the

San Feliciano Cathedral in Foligno; (b) the transept tympanum (upper basilica of St. Francis in Assisi); and

(c) the San Serafino Church at Montegranaro.




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4.7 and VII MCS) hit with epicenter near Cesi and Collecurti, towns located on the

border between Marche and Umbria Regions. These shocks were also responsible for

the collapse of the vaults in the upper basilica of St. Francis in Assisi. The seismic crisis

lasted several months; during subsequent events in 1997, other towns were struck,causing heavy damage to many MCUHES (October 3: (MD) 4.8 and VII MCS;

October 4: MD 4.3 and VI MCS; October 7: MD 4.9 and VIIVIII MCS, MD 4.1 and

VVI MCS; October 12: MD 4.5 and VIVII MCS, MD 4.9 and VIIVIII MCS). Before

the seismic sequence, probabilistic and deterministic maps were available for the Italian

territory, indicating PGAs not exceeding 0.4 g for the UmbriaMarche region.

3.2.1. The Reinforcement Measures The Upper Basilica of St. Francis in

Assisi The result of the UmbriaMarche earthquake was the destruction of the vaults

close to the facade and close to the transept and of a portion of the left transept.

Furthermore, the presence of large cracks and permanent deformations was noted allover the vaults, leaving them in a very precarious and dangerous condition. Some

(a)

(b)

(c)

(d)

Figure 14. Schematic illustrations of the shape memory alloy devices insertion for the: (a) upper basilica of

St. Francis in Assisi, tympanum; (b, c) San Feliciano Cathedral, facade; and (d) San Serafino church, facade

(drawing courtesy of the structural engineer).




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factors increased vulnerability: 1) for the transept tympanum, constructed with a

cavity wall with two faces and an inner fill, one of the causes of the partial collapse

was the decay of the mortar connecting the stones of the external face with the inner

fill; another cause was local interaction with the roof top; 2) for the vaults, the collapse

was produced by a large volume of fill (mainly broken tiles and other loose materials

accumulated over centuries of roof repairs in the springing zones); during the seismic

activity, this fill, without any cohesion, alternatively acted only on one side, while on

the other side the fill was detached. Moreover, the loose fill followed the vault

movement, preventing their recovery and facilitating the increase of permanentdeformations (Croci 1998a; 1998b).

(a)

(b)

(c)

Figure 15. Photographs of the shape memory alloy device insertion for the: (a) Assisi Basilica tympanum;

(b) San Feliciano Cathedral facade; (c) San Serafino Church.




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In addition to the very complex conventional restoration, 47 SMADs were used

in the basilica to connect the roof to the two transept tympanums: 24 on the left

(south) side, and 23 on the right (north) side (Bonci et al., 2001). Figures15a and 14a

show details of this SMAD connection, such as: 1) the anchorage plate, inserted in

the facade masonry wall during its partial reconstruction, attached to a threaded bar;and 2) a new reinforced concrete rib, built at the end of the existing reinforced concrete

roof (built in the 1950s), to stiffen it and transfer the load of the new connections.

The SMADs were connected on one side to the threaded bar and on other side to

the rib, through a plate bolted to a counter-plate embedded in concrete. Three

different sizes of three-plateau self-balanced SMADs (recognized by their differ-

ent lengths in Figure 15a) were applied, with design forces ranging from 17 to 52

kN and maximum displacements ranging from 8 and 25 mm, to take into

account the different properties required, because the distance from the transept

lateral walls gradually increases toward the roof top. This was the worlds first

application of SMADs to a building to improve earthquake resistance. It isworth noting that the cost of the SMADs installed in the upper basilica of St.

Francis totaled less than 1% of the total restoration cost (including the cost of

shock transmission units, other innovative devices used to connect the different

parts of a steel truss installed at an intermediate height along the perimeter of

the basilicas nave to increase side wall rigidity and thus avoid the aggravation of

vertical cracks created by past earthquakes [Bonci et al., 2001]).

3.2.2. The Reinforcement Measures: The San Feliciano Cathedral in

Foligno During the UmbriaMarche earthquake, Folignos Cathedral suffered

heavy damage, including the detachment of the facade, with a horizontal dis-placement of 8 cm off the covering vaults. Such a detachment was caused by an

incipient overturning collapse mechanism due to the absence of restraints on the

orthogonal vertical walls and the roof. Thus, an improvement of the facade wall

connection to the vertical walls and the roof was deemed necessary to increase

safety levels against the facade overturning. This improvement was accomplished

by inserting TD steel ties at a height of approximately 15 m, approximately 3/5

of the facade height, while the one between facade and roof was accomplished

with 9 SMADs (Figures 15b, 14b, and 14c). These devices are of the same type

as those used in the upper basilica of St. Francis in Assisi, with two- instead of

three-plateau self-balanced SMADs. Each device is characterized by a design

force of 27 kN and maximum displacement of 20 mm, and ends in tang plates,connected through pins to the clevis of the anchor frames. On one side, the

anchor frame is welded to a new V-shaped beam, welded in turn to the existing

beams of the modern steel roof (built in the 1950s). On the other side, the

anchorage frame is connected to the masonry facade wall through anchor

rods. A gap around the ends of existing steel beams, bearing on a PTFE

(Polytetrafluoro-ethylene) -stainless steel sliding plate, permits relative displace-

ments between facade and roof (which are controlled by the SMADs).

3.2.3. The Reinforcement Measures: The San Serafino Church in

Montegranaro Another significant SMAD application was carried out in 2002 inthe Church of San Serafino at Montegranaro (Ascoli Piceno, Italy), also severely




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properties for which a sufficient body of information was not yet available from either the

literature or the manufacturers and that were important to SMAD applications in

MCUHES; and 2) compare SMAs of different chemical composition, thermo-mechanicaltreatment, and diameter, leading to a final selection of the most suitable alloy(s).

The research included quasi-static tests at different strain rates (both cyclic and

monotonic), cyclic tests at different strain values, and dynamic cyclic tests at different

amplitudes/frequencies, etc. All the tests were tension stress-conducted under controlled

strain conditions. Figures 1819 show typical stress versus strain graphs, resulting from

two of these tests: the cyclic test with increasing strain (Figure 18), and the cyclic test at

the maximum superelastic strain (also called stabilization test; (Figure 19). The reduc-

tion in loading stress, increase in residual strain, and consequent reduction in the energy

dissipated per cycle in subsequent cycles are evident (Figure 19). However, the changes

are rapid during initial cycling but become very slow after several cycles. Thus it is

possible to achieve the desired stability by cyclic-training the SMA wires prior to service.

However, a design approach could even use SMA wires without any training, opting for

a higher energy dissipation capacity rather than greater hysteretic stability.

4.2. Tests on Shape Memory Alloy Devices

The behavior desired for SMADs to be used in series with horizontal steel ties (as

discussed previously in point 13 in Sec. 2) was achieved through groups of suitably

connected SMA wires, i.e., through elements provided with high modularity. This

modularity makes it quite easy to adapt the constitutive behavior of SMADs to

different design requirements, e.g., to achieve single- or multi-plateau SMADs. TheSMADs were checked by a series of dynamic tests.

Figure 17. The intervention with shape memory alloy devices at the San Pietro in Feletto Church.




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Figure 3 shows the constitutive behavior of a self-balanced single-plateau

SMAD prototype, measured by dynamic cyclic testing where displacements

were imposed with a sinusoidal law, at 25 mm amplitude and 1 Hz frequency. It is

worth noting that the hysteretic behavior of said SMAD under cycling is very stable.

An evolution of the aforesaid SMAD is the multi-plateau, self-balanced type.

An example of the force versus displacement behavior of a three-plateau SMAD is

shown in Figure 4. It is worth noting that the frequency dependence of SMADs is

negligible, as can be observed by comparing the two graphs in Figure 20, referring to

tests at different frequencies (1 and 4 Hz). This is due to a particular system, developed

within the ISTECH project and patented by FIP Industriale (Selvazzano Dentro,Italy). In effect, without this system, the frequency dependence of the superelastic

Figure 18. Typical example of stress versusstrain graph resulting from the cyclic test at increasing strainon a

nickeltitanium (NiTi) shape memory alloy device sample.

Figure 19. Typical stabilization test on a nickeltitanium (NiTi) shape memory alloy device.




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behavior of SMAs, consisting of an increase of the slope of superelastic loading and

unloading plateaus and a decrease in energy dissipation capacity, can be substantial. It

is also worth noting that the graphs shown here refer to results of tests carried out up

to the design displacement; that is why the stiffening effect (as discussed previously in

point 3 in Sec. 2) is not visible in these graphs.

4.3. Shaking Table Tests

A set of shaking table tests on brick masonry wall mock-ups (Castellano et al.,

1999; Indirli et al., 2000) were performed (Casaccia ENEA center, Rome, Italy, and

Seriate Ismes Laboratories, Bergamo, Italy), simulating a MCUHES portion, inparticular a church facade with a tympanum (Figure 21). The aim of the tests was

(a)

(b)

Figure 20. Force versus displacement loops measured on a multi-plateau self-balanced shape memory alloy

device during a sinusoidal test carried out at the FIP Industriale Laboratory (Selvazzano Dentro, Italy) at

amplitude 15 mm and frequency 1 Hz (a) and 4 Hz (b).




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(a)

(b)

(c)

Figure 21. Masonry mock-ups used in the shaking table tests (a and c) and shape memory alloy

devices (b).




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to evaluate the effectiveness of the SMADs for the prevention of out-of-plane

collapse of masonry walls. Two identical masonry wall mock-ups were constructed

(Table 1), each connected to a stiff steel frame at approximately three-fourths of

their height. In fact, both brick walls simulated a portion of a historic building

facade, while the steel frames represented the remaining part of the structure. The

only difference between the two walls was the type of connection: the first wall was

linked up to the rigid frame by two traditional steel ties (TDs), and the second wasconnected by a pair of SMADs.

The two mock-ups were placed together on the shaking table. The SMADs

were of the multi-plateau, self-balanced type, with a design initial force of 3.5 kN,

and a first plateau force equal to 5.2 kN. Both TDs and SMADs were anchored to

the walls using steel plates. Load cells were interposed between each device and the

anchorage. Both identification tests and simulated earthquake were conducted, the

latter at eight different intensity levels. The reference earthquake was synthetic,

derived from the EC-8 spectrum for soft soil and also modified to achieve a rather

flat spectrum in the range of the first two modal frequencies expected for both the

mock-ups (230 Hz).The difference between SMADs and TDs became evident under higher excita-

tions. The test results confirmed the ability of SMADs to substantially reduce the risk

of earthquake-induced out-of-plane collapse of masonry walls. The TD wall collapsed

with the mechanisms predicted by the numerical analyses; the first collapse mechanism

was the overturning of the upper part, after a horizontal crack had formed just

above the anchorage plates, occurring during test No. 6 (PGA: 0.6535 g). In test

No. 7 (PGA: 0.7790 g), the same wall showed a crack at approximately 2 m height.

This crack opened up completely (i.e., crossed the wall width) during the subsequent

test No. 8 (PGA: 0.9390 g), thus causing the definitive collapse of the TD wall. It also

showed evident cracks at its base, again after test No. 8. Conversely, the SMAD wall

did not suffer any visible damage, even when subjected to an earthquake characterizedby PGA almost 50% higher than the earthquake causing the first collapse in the TD

wall. The different behavior of the two walls can be understood by comparing the

peak accelerations measured at different points along the walls (Figure 22ad). In

particular, Figure 22b shows such a comparison with reference to test No. 6, which

caused the collapse of the upper part of the TD wall. In fact, the acceleration reduction

provided by SMADs is impressive: for example, almost 50% at the top, and more than

60% at the connections level, in test No. 6. It is worth noting that the top maximum

acceleration reached in the TD wall in test No. 6 (2.9 g) is even higher than that

reached (2.8 g) in the SMAD wall subjected to a PGA 1.4 times higher (test No. 8,

Figure 22d). Amplifications of acceleration between the shaking table and the wall topare 4.4 (TDs) and 2.3 (SMADs); instead, at the connections level (again in test No. 6),

Table 1. Mock-ups features

masonry brickwork height 403 cm mortar course thickness $1 cm

three-leaf brickwork thickness 36.5 cm reinforced concrete base height 22 cm

masonry brickwork width 99 cm reinforced concrete top curb height 35 cm

brick dimensions 5.5 11.5 24 cm SMAD and TD connections height 342 cmmock-up total height 460 cm




24/28

the table acceleration is amplified by 3.6 and 1.4 (TDs and SMADs respectively).Figure 23 shows the SMAD force reduction and displacement increase, compared to

the very rigid TD connections. The maximum force peak in the SMAD wall was

reduced by 45% in test No. 6, when the TD wall collapsed due to a crack above the

connections (force 13.3 kN); the SMAD wall did not show any damage until an

equal force was reached in test No. 8. This shows the effectiveness of the new SMAD

tying technique in reducing the acceleration amplification, owing to the reduced

stiffness, force limitation, and energy dissipation offered by the SMA superelasticity.

The SMADs remain in their first plateau up to test No. 6. In test No. 7, an increase in

stiffness was noted. The second plateau was reached only in test No. 8, where a

displacement of about 15 mm was recorded.

Figure 23. Force versus displacement loops measured in the connections at increasing seismic intensity.

(a) (b)

(c) (d)

Figure 22. Peak acceleration values on the walls with shape memory alloy devices and traditional devices,

measured at different heights.




25/28

To conclude, shaking table tests demonstrated that a new tying technique using

SMADs can be highly effective in preventing the out-of-plane collapse of peripheral

masonry walls, such as church facades, poorly connected at the floor/roof level.

Compared with TDs, the SMADs can increase resistance against out-of-plane seismic

vibrations of such masonry walls by at least 50% (in terms of maximum PGA bearablewithout damage), owing to a reduction in top acceleration of at least 50%. Unlike

TDs, SMADs can also prevent the collapse of tympanum structures.

5. NUMERICAL ANALYSES OF MCUHES WITH SMADS

Several numerical analyses were performed during the research, many of them

specifically devoted to MCUHES configurations with SMADs.

First, discrete element method (DEM) models were applied. Suitable for study-

ing masonry vulnerability and damage/collapse mechanisms, these models are able to

accurately simulate progressive failure and large displacements/rotations betweenblocks, including the enhancing capability of TDs and SMADs. The selection of the

case studies (bell tower, tympanum, arch) was done coherently with respect to the

targets described in paragraph 3, adopting material mechanical properties in agree-

ment with building codes, investigation results, and experience. Consulting references

(Azevedo et al., 1999; 2000; Sincraian et al., 1998a; 1998b; 1999) for detailed informa-

tion, it is worth noting that the DEM models highlighted the benefits of both TDs and

SMADs and considerably contributed to the final results.

Moreover, various finite element model (FEM) analyses were performed during

the research, to study both experimental configurations and application solutions.

The behavior of the mock-ups subjected to the shaking table tests (Sec. 4.3) was

analyzed (in this case, also, provided with reinforcement, either TDs or SMADs), inorder to predict their behavior under seismic excitations and suggest SMAD data for

design (Biritognolo et al., 2000).

Finally, a specific FEM model was applied for the Trignano Campanile under

static/dynamic conditions (Cavina 1997), in order to evaluate SMAD pre-stressing, tie

bar size and forces at the constraints (top and foundations). The results of in situ

experimental tests (ambient and forced vibration) performed on the bell tower

(Bongiovanni et al., 2000) have been successfully compared with the numerical ones

(numerical values: two flexural modes, 2.70 and 2.90 Hz, a third torsional value, 6.90

Hz; experimental frequencies: 2.25, 2.60, and 5.50 Hz respectively).

6. CONCLUSIONS

Theoretical, numerical and experimental studies have permitted the development

of innovative rehabilitation techniques based on the use of SMA technology. The

feasibility of using SMADs with different behaviors was demonstrated through the

construction of a number of prototypes that underwent extensive testing. Shaking table

tests showed that the SMAD-based tying technique can be highly effective in preventing

the out-of-plane collapse of peripheral masonry walls, e.g. church facades and tympa-

nums, poorly connected at the floor and/or roof level. Compared with traditional steel

ties, the SMAD ties can increase resistance against out-of-plane seismic vibrations of

such masonry walls by at least 50% (in terms of maximum PGA bearable withoutdamage), owing to a reduction in top acceleration of at least 50%.




26/28

The results of the research and its implementation led to several applications (for

the first time in the world) of SMADs to MCUHES, in order to reduce earthquake

damage, harmonize reinforcement measures with conservation criteria, eliminate some

of the drawbacks of TDs, and increase the respect of each monuments specificity,

complying with reversibility, compatibility and durability criteria. However, sinceSMADs are a modern evolution of traditional steel ties, likewise they cannot compensate

for all the structural deficiencies of MCUHES in terms of earthquake capacity. Thus,

they should usually be used in combination with other techniques, whether innovative or

conventional. The inclusion in guidelines and/or codes of specific design rules for inter-

vention with SMADs would be beneficial for further use of this innovative technique.

ACKNOWLEDGMENTS

ISTECH Project funding by the European Commission is gratefully acknowl-

edged. Thanks to all the research partners and teams, in particular to: R. Chiarotto,P. Galeazzo, S. Infanti (FIP Industriale, Padua, Italy), G. Manos (University of

Thessaloniki, Greece), J. J. Azevedo, G. E. Sincraian (Istituto Superior Te cnico of

Lisbon, Portugal), V. Renda, D. Tirelli (Joint Research Centre of the European

Commission, Ispra, Varese, Italy), G. Croci, M. Biritognolo, A. Bonci, A. Viskovic

(La Sapienza University of Rome, Italy), A. Martelli, B. Carpani, M. Forni,

M. Muzzarelli, B. Spadoni, G. Venturi (ENEA, Bologna, Italy), G. Bongiovanni,

G. Buffarini, P. Clemente, G. De Canio, P. Funaro, G. Rienzo, and D. Rinaldis

(ENEA, Rome, Italy). The contribution of S. Viani and other colleagues from ISMES

laboratories (Bergamo, Italy) is also gratefully acknowledged.

Special thanks to Mayor M. Mariani and the technical office of the Municipalityof San Martino in Rio (Reggio Emilia, Italy) at the time of the earthquake (1996), as

well as to the parish priest of the San Giorgio Church in Trignano.

After the 1997 earthquake, the restoration project of the St. Francis Basilica in

Assisi was designed by G. Croci and P. Rocchi, with the participation of G. Carluccio

and A. Viskovic, under the supervision of A. Paolucci (the artistic coordinator of the

Ministry of Cultural Heritage), C. Centroni (Superintendent of Umbria), and G. Basile.

Furthermore, special thanks go to L. Marchetti (Superintendent of the Umbria Fine

Arts Office at the time of the operations), L. Tortoioli, and S. Costantini (Umbria

Regional Government). The on-site SMAD installation was done by FIP Industriale

under the supervision of R. Paggetta (Umbria Fine Arts Office); the mathematical

models and the analyses of the operations were done by A. Bonci and A. Viskovic.Designers of the rehabilitation of the Cathedral of San Feliciano (Foligno, Perugia,

Italy) were G. Carini, G. Colombatti, and L. Radi. Designer of the rehabilitation of the

Church of San Serafino in Montegranaro (Ascoli Piceno, Italy) was R. Mariani.

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International Journal of Architectural Heritage: Conservation, Analysis, and Restoration