Historic Masonry Buildings

Structural Analysis of Historical Constructions, New Delhi 2006 P.B. Lourenço, P. Roca, C. Modena, S. Agrawal (Eds.)

A New Code Approach to the Seismic Vulnerability Assessment of Historic Masonry Buildings D. Sonda University of Padua, Department of Construction and Transportation Engineering, Padua, Italy

F. da Porto University of Padua, Department of Construction and Transportation Engineering, Padua, Italy

M.R. Valluzzi University of Padua, Department of Architecture,Urbanism and Survey, Padua, Italy

ABSTRACT: The new seismic code currently in force in Italy has been completely renovated, especially with regard to the approach to existing masonry structures. Particular attention is paid to proper diagnosis, assessment and intervention measures. The paper presents an example of the application of the code on a historic aggregate building that suffered several modifications during the centuries, characterized by the presence of retrofitted structural units with rigid floors and structural units with lack of connections at each storey. The main difficulties in the interpretation of the seismic behaviour of the building are described.

1 INTRODUCTION 1.1 General The current Italian seismic code has been recently updated after the first tentative issue in 2003 (OPCM n. 3274 2003 and n. 3431 2005). Those changes have been based on the most recent scientific research results on the seismic response of existing masonry buildings, which allowed recognizing a specific role of masonry in the field of the construction materials, even for earthquake-resistant constructions. The main changes concern the proposal of a new approach to existing masonry buildings, taking into account: i) the fundamental preliminary knowledge of the building (historical and damage information, modification and developments during time, application of investigation techniques for masonry qualification and mechanical characterization) (Binda et al. 1999 and 2004); ii) the reliability of simplified kinematic models based on the possible loss of equilibrium of structural macro-elements (for local behaviour assessment, both for in-plane and out-of-plane mechanisms) (Giuffrè 2003, Bernardini et al. 1990, Valluzzi et al. 2004a); iii) the adoption of improvement interventions instead of generalized structural upgrading solutions, based on the current conservation criteria (compatibility, durability, minimum intervention, etc.) (Carbonara 1997). Particular attention is paid to aggregates and complex buildings, which have been finally recognized as one of the most spread morphological features of the historical heritage. For them, reliable assessment procedures still need to be setup and/or properly adapted (Tomazevic 1999). Moreover, the new code imposes a penalization in the structural estimation when scarce preliminary diagnosis is available, by means of the “confidence factors”, which reduce the mechanical strength of materials depending on three different knowledge levels (limited, normal and full). The compulsoriness of local assessment for masonry buildings, by checking the evolution of kinematic mechanisms until failure, is also requested by the code. Actually, the knowledge level imposes also the possibility to use different methods for the global assessment of the structure. In order to choose the admissible type of analysis and the appropriate partial safety factor values, three knowledge levels are defined: LC1 (limited knowledge), LC2 (normal knowledge), LC3 (full knowledge) (OPCM n. 3431, 2005 and prEN1998-3, 2004). Structural

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evaluation based on a limited knowledge allows the use only of linear analysis methods. Finally, most of the upgrading interventions commonly used in the past, such as substitution of timber floors with heavier reinforced concrete slabs, jacketing, reinforced injections, insertion of r.c. tie-beams, are considered to be obtrusive in comparison with improvement interventions. Very often the latter, such as tying, interlocking, limited rebuilding, properly designed and controlled injections, preservation of timber components, etc., are also more compatible with traditional construction techniques, diffused also in non-monumental heritage buildings (Baronio et al. 1993, Penazzi et al. 2001, Modena et al. 2004, Valluzzi et al 2004b). The requirements of the new seismic code have been applied on a large complex located in Schio (Vicenza, Italy), which suffered numerous modifications and interventions since it construction, during the past century. The resultant building is irregular under the point of view of morphology, materials and mechanical behaviour. The in-situ investigations allowed calibrating a numerical model able to interpret qualitatively the overall behaviour of the complex and to simulate the global seismic response of the structure. The study is also completed with the local assessment of the main macro-elements identified in the building. 2 CASE OF STUDY 2.1 The structure The case of study is the main building where the offices of the Schio municipality are located (Vicenza province, Italy). The building is a complex construction located in the historical centre of Schio (Fig. 1), and resulting from the subsequent connection of three buildings: Palazzo Tomasi - Palazzo Garbin - Palazzo Romani-Rossi probably built in different periods. The three portions are aligned in the Est-West direction with a lower body in North one. Originally, the buildings were constructed with stone or brick masonry walls and timber floor and roof, then between 1990 and 1998 they were strengthened and retrofitted. According to the recent updated seismic code, the complex building was split up in five Structural Units (SU) having more homogeneous structural characteristic (Fig. 2, Tabl2 1), on the basis of the structural morphology and the unitariness of the interventions.

Figure 1 : View of the aggregate building.

Figure 2 : Structural Units split up from the aggregate.

D. Sonda, F. da Porto and M.R. Valluzzi

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Table 1: Structural Units. Building Structural Units label Palazzo Tomasi SU 01 Palazzo Garbin SU 02 Palazzo Romani Rossi SU 03-04 Palazzo Garbin avant-corps SU 05

All the structural units are regular in elevation but not all of them are regular also in plan. In particular, Palazzo Garbin is irregular in plan due to the presence of timber floors with scarce in-plane stiffness, which are not able to distribute horizontal loads to the main masonry walls. The buildings are based on superficial foundations but, due to the fairly compacted soil properties, no cracks related to possible settlements can be detected. 2.2 Recent structural modification All the buildings were strengthened and retrofitted in recent years. Unfortunately, not all the works are well documented; some descriptions are given in the following. 2.2.1 Palazzo Garbin A strengthening intervention was carried out in the past decades to improve the load bearing capacity of the timber beams at the first floor. New steel plates or trusses were connected to the existing timber beams. None of the steel elements was connected with the walls, thus evidencing the aim of strengthening the beams only for bending. At the East corner of the floor, near palazzo Romani-Rossi, a concrete lift-shaft connected to the main walls by concrete floors was built. The main alteration in the building was, around 1990, the introduction of a new horizontal structure added to the existing timber floor for fire-prevention. It is a compound steel-concrete floor with high in-plane stiffness, onto which the archives and the thermal station were placed to guarantee them higher fire protection (Fig. 3a). This intervention was extended only on a portion of the last floor, causing a high increase of the dead-loads at the top of the building. The walls were connected with steel bars to the floor slab (Fig. 3b).

Figure 3 : Palazzo Garbin: view of strengthening interventions on the floor.

2.2.2 Palazzo Tomasi Palazzo Tomasi was subjected to the generalized strengthening of all the timber floors, carried out by adding collaborating concrete slabs at the existing floors extrados (Fig. 4a). Some walls were also reinforced by jacketing. Foundations were thickened by the integration of new concrete beams (Fig. 4b). 2.2.3 Palazzo Romani-Rossi Palazzo Romani-Rossi was interested by the complete substitution of the existing timber floors with new concrete slabs (Fig. 5). The walls were strengthened with local injections and reinforced with concrete slabs.

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(b) (a) Figure 4 : Palazzo Tomasi: strengthening intervention on timber floors (a) and foundations (b).

Figure 5 : Palazzo Romani-Rossi: new concrete floors.

2.3 In-situ investigations Instrumental investigations were carried out to obtain a realistic characterization of the material properties. According to the code, tests were integrated by visual inspections of the walls after removing the plaster. The following procedures were adopted: soil drillings, to recognize available stratigraphy related to neighbouring areas; flat-jack tests on walls (single and double), to estimate stress values and to assess the elastic modulus and compressive strength of the masonry (Fig. 6); core boreholes in the most representative portions of the walls; sampling of mortar and bricks for laboratory chemical and petrographic analyses; non-destructive tests to detect reinforcing bars in concrete. STRESS / STRAIN TEST M1 35 30 25 20 15 10 5

-8,0

-6,0

-4,0

-2,0

0,0

2,0

4,0

0 6,0

STRAIN

(b) (a) Figure 6 : Double flat-jacks test a) and results b). Table 2 : Masonry strength (MPa) estimated in different Structural Units SU 1 SU 2 SU 3-4 Mean compressive strength (fm, N/mm2) 1,75 1,41 1,82 Design compressive strength (fd, N/mm2) 1,46 1,17 1,51 Design shear strength (τd, N/mm2) 0,41 0,41 0,53

SU 5 1,41 1,17 0,41


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The design compressive and shear strengths of masonry were defined dividing the mean values by a “confidence factor” (FC = 1,2) related to the intermediate (“normal”) knowledge level (Tab. 2). 2.4 Numerical modelling of the complex The investigated constructions are part of a complex building in a historical town. Each building included in the complex, and sometimes different walls in the same building, differ for age, materials, construction techniques etc. The new code approach recommends considering the interactions between neighbouring structural units. The structural unit interactions can lead to local and/or global effects. In order to determine the global response of the building complex, an overall simplified linear numerical model was performed (Fig. 7).

Figure 7 : FE model of the buildings complex.

A dynamic seismic analysis was carried out by means of the finite element model, in order to show qualitatively the structural response. The analysis highlights very low frequencies associated to local out-of-plane behaviour of the masonry walls (Fig. 8).

1st frequency: 0,46 Hz

2nd frequency: 0,48 Hz

Figure 8 : First two natural frequencies of the complex

2.5 Modelling of structural units 2.5.1 Palazzo Garbin This building has timber floors without in-plane stiffness. As indicated by the new code overall approach, a non-linear analysis of a single masonry wall was carried out. A preliminary simplified linear static analysis was also done. The results of the non-seismic analysis in terms of stresses distribution highlighted an overstress in the ground floor masonry elements. Theoretical values of principal compressive stresses, due to permanent gravitational load, are in good agreement with the experimentally recorded stresses (single flat-jack test) (Fig. 9).

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Stress (DaN/cm2

Figure 9 : Numerical compressive stresses distribution due to gravitational load.

2.5.2 Palazzo Tomasi – Palazzo Romani-Rossi These two buildings were completely retrofitted by replacing the existing timber floors with new concrete slabs in Palazzo Romani-Rossi and with the timber-concrete composite structural technique in the Palazzo Tomasi. In the latter, the presence of a thick concrete layer (>5 cm) and the use of shear connectors between timber and concrete caused an improvement of the floor inplane stiffness. Therefore, a rigid floor diaphragm behaviour may be assumed and a simplified non-linear static analysis (POR) of each storey can be carried out in order to assess separately their seismic performance. The analysis was conducted in the direction orthogonal to the adjacent buildings (Fig. 10). The overall shear behaviour of each single floor was represented by a shear-displacement curve. GROUND FLOOR

HORIZONTAL LOAD (daN)

250000

200000

0,166 0,111

150000

OVERALL BEHAVIOUR

100000

50000

0 0 0,000

WALLS BEHAVIOUR

0,100

0,200

0,300

0,400

0,500

0,600

0,700

0,800

0,900

DISPLACEMENT (cm)

(b) (a) Figure 10 : Shear-Displacement curve (a) to ground floor (Palazzo Romani-Rossi) (b)

2.6 Local mechanisms The methodology used for the assessment of the seismic vulnerability of buildings in historical centres considers the application of simply kinematics models able to describe the mechanical behaviour of structural components and assemblages. The analyses are performed locally, on the most relevant elements in the building, by applying the models describing the single mechanisms. This check is compulsory for existing masonry buildings at the ultimate state. The most significant parameter describing the kinematics models is the collapse coefficient c= a/g, which corresponds to the seismic masses multiplier characterizing the attainment of the limit conditions in the equilibrium of the considered element, evaluated by means of linear kinematic analyses. The code requires also the control of the evolution of the mechanism until the occurrence of failure (Fig. 11). This can be done by means of non-linear kinematic analyses, which allow evaluating the displacement capacity of the studied mechanism until collapse. These analyses give the generalized relationship between the seismic mass multiplier or the seismic spectral acceleration a* of the equivalent SDOF system and the displacement of the control point on the real structure or the equivalent system. They thus allow describing the evolution in the seismic response after the propagation of damage and the activation of the kinematic mechanism in the structural elements, and to compare it with the seismic demand.

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0,90

Acceleration Sa(g)

0,80

∆u = 0,073

0,70 0,60

Demand Spectrum

0,50

du* = 0,202

0,40 0,30 0,20 0,10

Secant period

0,00 0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

Displacement (m)

(a)

(b) Figure 11 : Local mechanism (a) and determination of the performance point (b) (Palazzo Garbin)

The secant period (Ts) and the demand displacement (∆d(Ts)) are defined as follow: Ts = 2π

d *s a *s

Ts < 1.5T1 1.5T1 ≤ Ts < TD TD ≤ Ts

(1) ⎞ Ts2 ⎛ 3 (1 + Z H ) ⎜ − 0 .5 ⎟ 2 2 ⎟ 4π ⎜ 1 + (1 − Ts T1 ) ⎝ ⎠ 1.5T1Ts ⎛ Z⎞ ∆ d (Ts ) =a gS ⎜1.9 + 2.4 ⎟ H⎠ 4π2 ⎝

∆ d (Ts ) = a gS

∆ d (Ts ) =a g S

1.5T1TD ⎛ Z⎞ ⎜ 1.9 + 2.4 ⎟ 2 H⎠ 4π ⎝

(2) (3) (4)

The mechanism is verified if the ultimate displacement (du*) result greater than the demand displacement (∆d). 2.7 Damage scenarios By using the previously defined procedure to evaluate the damage limit state the PGA for ultimate (SLU) and serviceability (SLD) limit states were determined. Results are listed in Table 3. As can be seen, all the buildings, even the strengthened ones, are in an unsafe condition, due to the high performance required from the code. For all the buildings the factor which relates to the consequences of a structural failure (importance factor ) is 1,40. Table 3 : Peak Ground Acceleration at the ultimate limit state (SLU), defined by means of global and local analyses, and at the damage limit state (SLDL), and comparison with code required values. Limit state Palazzo Garbin Palazzo Tomasi Palazzo Romani-Rossi Code required PGA (g) (g) (g) (g) SLU 0,012 0,063 0,033 0,187 Global analysis SLU 0,047 0,215 0,104 0,187 Local mechanisms SLDL 0,013 0,118 0,071 0,075

Palazzo Garbin, the unstrengthened building, exhibited the lower PGA for ultimate limit state (SLU) due to the past interventions that reduced the seismic performances, as extensive cuts in the ground floor masonry walls (fig.9), and the high increase of the dead-loads at the top of the building. Palazzo Tomasi and Palazzo Romani-Rossi, the two strengthened ones, exhibited a PGA for ultimate limit state (SLU) as expected for non-seismic strengthened buildings. The lower performance of Palazzo Romani-Rossi can be attributed to the high weight of the new concrete floors.

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3 CONCLUSIONS The new Italian seismic code represents an innovative and more suitable instrument for a proper approach to masonry existing buildings. Retrofitting of damage, interventions, changes in use, structural and functional updating, often create irregularities in the configuration of the buildings, which can affect significantly their seismic behaviour. They cannot be disregarded in the structural assessment of the aggregates and for the suitable choice of possible interventions, as previous experiences in Italy have unhappily demonstrated. Investigation methodologies must integrate properly at least a minimum knowledge level, to calibrate suitable models and to suggest proper intervention measures. Nevertheless, this approach still needs specific further in-depth integrations, to make available practical tools to professional people, able to correctly interpret the standard requirements. Numerical models demonstrated to be often inadequate to represent the overall behaviour of masonry buildings, especially for irregular configurations as in complex aggregates, where structural subdivisions are unavoidable. Their usefulness is limited to the possibility to evidence interactions among the Structural Units and to address for the proper selection of the local mechanisms. REFERENCES Baronio G., Abbaneo S., Binda L., 1993, Le murature in pietra: malte per consolidamento. Proc. Le pietre da costruzione: il tufo calcareo e la pietra leccese, Bari, 747-761 (in Italian) Bernardini, A., Gori, R., Modena, C., 1990, Application of coupled analytical models and experiential knowledge to seismic vulnerability analyses of masonry buildings, In: Earthquake Damage Evaluation and Vulnerability Analysis of Buildings Structures, A. Kortize Ed., INEEC, Omega Scientific. Binda, L., Cardani, G., Saisi, A., Modena, C., Valluzzi, M.R., 2004, Multilevel Approach to the Analysis of the Historical Buildings: Application to Four Centers in Seismic Area Finalised to the Evaluation of the Repair and Strengthening Techniques, Proc. 13th IBMaC, RAI Amsterdam, the Netherlands, 4-7 July 2004. Binda, L., Gambarotta, L., Lagomarsino, S., Modena, C., 1999, A multilevel approach to the damage assessment and seismic improvement of masonry buildings in Italy, In: Seismic Damage to Masonry Buildings, Bernardini Ed., Balkema, Rotterdam. Carbonara G., 1997, Avvicinamento al Restauro. Teoria, storia, monumenti, Liguori. Giuffrè, A., 1993, Sicurezza e conservazione dei centri storici. Il caso Ortigia, Laterza, Bari. Modena, C., Valluzzi, M.R., Garbin, E., Da Porto, F., 2004, ‘A strengthening technique for timber floors using traditional materials’, Proc. of SAHC2004: IV Int. Conf. on Structural Analysis of Historical Constructions - possibilities of experimental and numerical techniques, Padova, Italy, 10-12 November 2004, 911-921. O.P.C.M. n. 3274, 20/03/03: Norme tecniche per il progetto, la valutazione e l’adeguamento sismico degli edifici – e successivi aggiornamenti (in Italian) O.P.C.M. n. 3431, 03/05/05: Norme tecniche per il progetto, la valutazione e l’adeguamento sismico degli edifici (in Italian) Penazzi, D., Valluzzi, M.R., Saisi, A., Binda, L., Modena, C. 2001. Repair and strengthening of historic masonry building in seismic area, Proc. Int. Conf. ‘More than two thousand years in the history of architecture safeguarding the structure of our architectural heritage’, Bethlehem, Palestine, Vol. 2, Section V (7 pp.). prEN 1998-3, Eurocode 8 - Design of structures for earthquake resistance Part 3: Assessment and retrofitting of buildings, FINAL DRAFT Tomazevic M. 1999. Earthquake-Resistant Design of Masonry Buildings (Series on Innovations in Structures and Construction), Vol 1,London, Imperial College Press. Valluzzi, M.R., Cardani, G., Binda, L., Modena, C. 2004a, Seismic vulnerability methods for masonry buildings in historical centres: validation and application for prediction analyses and intervention proposals, Proc. 13th WCEE, Vancouver, B.C., 1-6 August 2004, Canada (on CD-ROM) Valluzzi M.R., da Porto F., Modena C., 2004b, “Behavior and modeling of strengthened three-leaf stone masonry walls”, RILEM Materials and Structures, MS 267, Vol. 37, April 2004, pp. 184-192.