1 INTRODUCTION

Since the Eighties, pseudo-dynamic and dynamic experimental tests were performed [i.e. (Benedetti et al. 1980), ( et al. 1993), (Spence et al. 1992), (Benedetti et al. 1998), (Paquette et al. 2003), (Mazzon et al. 2009)] with the purpose to study the dynamic behaviour of masonry building models, as well as to assess the effect of several intervention techniques on the seismic re-sponse of historic masonry structures.

Within the framework of the project NIKER, test-ing of subassemblies, as well as of scaled buildings is envisaged. More specifically, two 2-storey build-ings (scale 1:2) are tested on the earthquake simula-tor of the Laboratory of Earthquake Engineering, NTUA. The two buildings are made of three leaf rubble stone masonry. The difference between the two buildings is that in the first one, masonry is plain, whereas in the second one, timber ties are pro-vided at various levels along the height of the struc-ture. This type of construction simulates real historic structures in earthquake prone areas (Eastern Medi-terranean, Asia and South America). In both build-ings, timber floors are provided. The models are first tested as-built, until they are damaged. Subsequent-ly, they are strengthened (grouting of masonry and enhancement of the diaphragm action of the floors) and retested, with the aim to assess the efficiency of the interventions. In parallel, the behaviour of the buildings is being modeled using FEM, with the aim

to predict their pathology, as well as the efficiency of the intervention techniques. In this paper, the exper-imental and the analytical results related to the plain masonry model are briefly presented and commented upon.

2 EXPERIMENTAL PROGRAMME

2.1 Description of specimen without timber ties

The specimen is a 1:2 scale model of a prototype building. Details for the geometry of the building model are showed in Figure 1. In detail, the plan of the typical floor of the specimen is 3.65x2.30m

2.

The height of each floor is 1.60m, whereas the total height of the specimen is equal to 3.20m. The thick-ness of the walls equals 0.25m.

Masonry walls consist of three (approximately equal in thickness) leaves. For the construction of the exterior leaves, stones (limestone from Paramythia-Epirus) with thickness not exceeding 80-90mm were used. The mean compressive strength of the limestone is approximately equal to 100 MPa. The mortar is a lime-pozzolan one with a mixed ag-gregate matrix composed of siliceous river sand and limestone gravels. Its composition and main me-chanical properties (determined according to the EN196:1:1994) are presented in Table 1. The inner part of the walls consists of small stones and mortar in a proportion of 2/1. The aim was to reach a filling material with a percentage of voids of approximately

ABSTRACT: A 2-storey (three-leaf) rubble stone masonry building with timber floors was tested on the earthquake simulator. The 1:2 scaled model simulates typical historical buildings in Europe. The model was tested twice (as-built and after repair and strengthening). The techniques applied to improve the seismic be-haviour of the model are grouting and enhancement of the diaphragm action of floors. The model was subject-ed to several accelerograms (with gradually increasing maximum acceleration). The damages observed in the as-built model have confirmed the vulnerability of the three-leaf masonry, as well as the negative effect of flexible floors. Significant improvement of the seismic behaviour of the building was recorded after the strengthening of the model. Parameter analyses are still in course; the analytical results obtained up to now seem to confirm the observed behaviour of the model before and after strengthening. In this paper, a summary of the selected experimental and analytical results is presented.

Strengthening of historical stone masonry buildings:Experimental testing and modeling of a 2-storey plainmasonry building

E. Vintzileou, H. Mouzakis, C.-E. Adami & L. KarapittaFaculty of Civil Engineering, National Technical University of Athens, Athens,Greece

Life-Cycle and Sustainability of Civil Infrastructure Systems – Strauss, Frangopol & Bergmeister (Eds)© 2013 Taylor & Francis Group, London, ISBN 978-0-415-62126-7

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40%. Thus, the abovementioned mix was positioned without any compaction. It is noted that no connec-tion between the leaves was provided.

Figure 1. Typical plan and picture of the two storey building model.

Table 1. Composition and mechanical properties of mortar. Composition

Mechanical properties

%-wt age fm,c*** fm,fl****

Lime putty P* A** water days MPa MPa

20 20 60 0.50 28 3.50 0.70

60 4.80 0.80 90 4.60 0.60

*Pozzolan

***Compressive strength ** Aggregates

**** Flexural strength

Figure 2 shows the typical floor construction: Tim-ber beams (60x100mm

2) are placed every 340mm

(Fig. 2a). They are supported by masonry through a collector beam (Fig. 2c). Timber pavement, made of 100x10mm

2 timber planks (nailed on the timber

beams) is provided (Fig. 2b). Timber lintels are pro-vided at the top of all openings (Fig. 2d). All timber elements were made of coniferous wood (strength class C22).

a) b)

c) d)

Figure 2. Structural details of the wooden floor.

2.2 Test setup

The tests were carried out on the shaking table facili-ty of the LEE/NTUA. The specimen was securely fastened on the shaking table through a rigid steel base. The instrumentation used in the shaking table experiments was designed to measure the accelera-tions and absolute displacements along X and Y di-rections at both levels during each test; their loca-tions are shown in Figure 3.

a) b)

Figure 3. Measurement points of a) acceleration and b) dis-placement.

The self weight of the model was approximately equal to 14.5Mgr. Additional masses (7.5Mgr) were placed on the two floors, namely, 4.5Mgr and 3Mgr on the floor of the 1

st and 2

nd level respectively.

2.3 Test procedure. Seismic input and test protocol

The specimen was tested on the shaking table under excitation along two horizontal axes (X and Y direc-tions, long and short side of the model, respectively). The as built model was subjected to a sequence of the Kalamata (South Peloponnese) signals (Sept. 13

th, 1986 earthquake, Ms=6.2), as shown in Table

2. The same sequence was followed also in the case of the strengthened model, reaching however higher maximum accelerations (up to 160% Kalamata earthquake, Table 3). Due to the fact that the model after strengthening was significantly stiffer than in its as-built state, it became obvious that another seismic input (with smaller predominant period) would be needed, in order to significantly damage the model. Thus, as shown in Table 3, a sequence of the Irpinia earthquake signals (November 23

rd, 1980,

Calitri record, Ms=6.9) was also imposed to the strengthened model.

Needless to say that the seismic signals were ade-quately scaled. Before the application of the selected seismic inputs, the dynamic properties of the speci-men were measured through sine logarithmic sweep excitation of low amplitude (0.02g). Sine sweep tests were performed separately along x and y directions. Subsequently, the as built model was subjected to a

SW2

LW1

SW1

LW2

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series of scaled motions with increasing maximum acceleration, until significantly damaged. The model was then removed from the shaking table, it was strengthened and retested (following the sequence of Table 3) until failure occurs.

a) b) Figure 4. Base acceleration time histories along X direction and Y direction for a) Kalamata earthquake and b) Irpinia earth-quake.

Table 2. As-built model (BS). Test protocol. No. of test

Excitation Direction of excita-tion

Scale of orig-inal record

Base acceleration

g

X Y

1BS Sine sweep X - – – 2BS Sine sweep Y - – –

3BS Kalamata X&Y 15% 0.04 0.037

4BS Kalamata X&Y 30% 0.10 0.09

5BS Kalamata X&Y 45% 0.14 0.13

6BS Kalamata X&Y 60% 0.18 0.16

7BS

Kalamata X&Y 75% 0.22 0.21

8BS Kalamata X&Y 90% 0.29 0.24

Table 3. Strengthened model (AS). Test protocol. No. of test

Excitation Direction of excita-tion

Scale of orig-inal record

Base acceleration

g

X Y

1AS Sine sweep X - – – 2AS Sine sweep Y - – –

3AS Kalamata X&Y 15% 0.04 0.04

4AS Kalamata X&Y 30% 0.10 0.09

5AS Kalamata X&Y 45% 0.14 0.13

6AS Kalamata X&Y 60% 0.19 0.17

7AS

Kalamata X&Y 75% 0.23 0.20

8AS Kalamata X&Y 90% 0.29 0.25

9AS Kalamata X&Y 100% 0.33 0.27

10AS 11AS

Kalamata X&Y 120% 0.40 0.32

11AS 12

Kalamata X&Y 140% 0.49 0.37

12AS Kalamata X&Y 160% 0.55 0.39

13AS Irpinia X&Y 100% 0.16 0.16

14AS Irpinia X&Y 200% 0.34 0.29

15AS Irpinia X&Y 300% 0.48 0.43

16AS Irpinia X&Y 400% 0.62 0.72

17AS Irpinia X&Y 400%R 0.54 0.66

2.4 Description of the strengthening techniques

After the completion of the as-built model (BS) test-ing, the model was strengthened (AS). Perimeter masonry was grouted.

A grout developed within NIKER project was used: A natural hydraulic lime grout made with 90% pure St Astier NHL 5 and 10% superfine S&B natu-r p zz n c rc y b s μ-silica, W

type. For the preparation of the grout, an ultrasound dispersion mixer assisted by a mechanical device of low turbulence was used (Miltiadou 1990), (Vintzileou et al. 2008). In order to check the pene-trability and the fluidity of the grout, the standard-ized sand column test method (NFP18-891) was ap-plied. Furthermore, the fluidity and the stability of the grout were measured according to the French standards NF P18-358 and NF P18-359, respective-ly. Table 4 summarizes the main data related to the grout that was applied to the model. The grouts were applied to the masonry following the procedure established within the Hellenic Ministry of Culture (Miltiadou et al. 2005) used also by (Vintzileou et al. 2008). Table 4. Main mechanical and injectability charac-teristics of grout. Composition

Mechanical properties

%-wt age fgr,c***

fgr,fl****

NHL5 P* SP** water days MPa MPa

90 10 0.70 82.5 28 1.70 0.60 90 2.90 0.60

Injectability characteristics

T36

Sand column 1.25/2.00 mm

Marsch Cone td=4.7mm

Bleeding

s s %

16.39 25.30 2

*Pozzolan

***Compressive strength ** Superplasticizer

**** Flexural strength

The total volume of the grout injected in the model was equal to 955lt. The consumption of grout shows that the achieved percentage of voids of the filling material was approximately equal to 32%. Such a percentage of voids is representative of three leaf stone masonries.

a) b)

Figure 5. Grouting of three-leaf masonry walls.

The second intervention technique was that of en-

hancement of the diaphragm action of the floors. For

this purpose, a second pavement was placed-on top

0 5 10 15 20

Time (sec)

-2.00

-1.00

0.00

1.00

2.00

Ba

se

Accele

ratio

nY

(m/s

2 )

0 5 10 15 20

Time (sec)

-2.00

-1.00

0.00

1.00

2.00B

ase

Accele

ratio

nX

(m/s

2 )

0 5 10 15 20

Tim e (sec)

-0.30

-0.15

0.00

0.15

0.30

Bas

eA

ccele

ration

X(g

)

0 5 10 15 20

Time (sec)

-0.30

-0.15

0.00

0.15

0.30

Base

Acce

lera

tion

Y(g

)

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of the existing one-with the planks arranged at an

angle of 45degrees with respect to the initial ones.

This technique has proved to be satisfactory, during

the testing campaign at the Univ. of Padova (Valluz-

zi et al. 2010). The new pavement was connected

through the initial one to the timber beams, using

nails (Fig. 6b). The stiffened floors were connected

to the perimeter walls, using bolts 10mm diameter

with 100x100x5 steel bearing plate and L80/80/8

steel section 200mm long, placed in both sides of

masonry (Figs. 6a and 7). The connection was de-

signed to resist a shear force three times as high as

the maximum one recorded on the as-built model

during its testing.

a) b)

Figure 6. Strengthened model: a) general view and b) view

from the top.

a)

b) c) d)

e) f)

Figure 7. Construction details of the wall-to-timber floor con-

nection.

3 TEST RESULTS

3.1 Observed Damages

After each step of testing, the model was carefully inspected and the observed damages were reported on drawings. Figures 8 and 9 show the damage pat-tern of two walls of the model, namely, one long and one short wall (for definition see Fig. 3) before and after the application of the strengthening techniques. These damages occurred after reaching the maxi-mum acceleration of each test of series, ie. after completion of Test 8BS (Table 2) and Test 17AS (Table 3) for the as-built and the strengthened spec-imen respectively.

a) b)

Figure 8. As built specimen: Observed damages at a) the long wall 1 (LW1) and b) the short wall 2 (SW2); at 90% of Kala-mata earthquake.

a) b)

Figure 9. Strengthened model: Observed damages at a) the long wall 1 (LW1) and b) the short wall 2 (SW2); at 400%R of Irpinia earthquake).

In the as-built model (Fig. 8), the short and the long walls presented significant differences in their crack pattern. Actually, the short walls, having smaller as-pect ratio than the long ones and being connected with the floors, were more vulnerable to shear (see shear cracks, Fig. 8b). However, due to the defective diaphragm action of the floors, vertical cracks were also recorded close to the corners of the model, as well as at mid-length of the short walls close to their top. Expectedly, the opening of the cracks due to out-of-plane bending decreases from the top to the base of the building. After Test8 (Table 2), the crack width at the top of the model is approx. equal to

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8mm. At the level of the lintels of the top floor, it decreases to 6.5-7mm, while at the bottom level of the window, the total width of the cracks is approx. equal to 5.0mm.

The long walls of the model are characterized by out-of-plane flexural behaviour. Although there are shear cracks at the corners of the openings (Fig. 8a), the main cracks are the vertical ones tending to sepa-rate the long from the short walls.

Moreover, the tests on the as-built model have proven the vulnerability of the three-leaf masonry. Actually, pronounced detachment of the leaves is apparent at all elements of the as built model (Fig. 10). This separation (caused by out-of plane bend-ing) was initiated in the long walls, at a PGA of 0.12g to 0.16g. The separation is more pronounced close to the corners of the building. Moreover, the separation of the leaves of masonry is apparent also in the regions of openings (Fig. 11).

Figure 10. As built specimen. Observed damages at the level of the roof.

Figure 11. As built specimen. Observed damages at the level of the windows.

The strengthened model exhibited a significantly improved behaviour, in terms of the maximum ac-celeration. Moreover, the improved bond between masonry leaves (due to grouting) and-mainly-the en-hancement of the diaphragm action of the floors and their connection with the perimeter walls has led to a significant modification of the behaviour of the building. Thus, (a) the model was free of damages after the completion of Test12AS (160%Kalamata earthquake), (b) no separation between the leaves of masonry was observed, even after the application of the strongest motion, and (c) due to the enhanced

box-action of the building, its vulnerability to out-of-plane bending was significantly reduced.

Although vertical cracks (due to out-of-plane bending were recorded close to failure (Fig. 9), the major failure mechanism of the model is character-ized by model is characterized by shear (under the Irpinia earthquake). As the seismic input increased to 400% of Irpinia earthquake (Test 16AS), com-bined rocking and sliding mechanisms were formed and large deformations occurred, without significant force-response degradation though. The building proved to be relatively resilient to earthquake excita-tion, although extensive cracking occurred. With the repetition of the same seismic input (Test 17AS), cracking was generalized in the specimen, and the cracks that were initially formed at the corners of the openings extended towards the floors, leading to the formation of horizontal cracks at the level of the en-hanced diaphragms.

3.2 Dynamic properties before and after interventions

Natural frequencies and damping ratios were calcu-lated from the response of the tested model during sine logarithmic sweep excitations. The damping ra-tio was calculated using the half-power bandwidth method (Clough et al. 1975). The resonance fre-quency (first mode) and the corresponding damping ratio of each structure are summarized in Table 5.

Table 5. Dynamic properties. No. of test Frequency Damping

(Hz) (%)

X Y X Y

BS 6.10 4.35 4.6 7.0 AS 10.40

0 9.86 6.6 6.1

For the BS-building, the frequency of the first X-mode (along long wall) is equal to 6.10Hz, while that for Y-mode (along the short wall) is equal to 4.35Hz. The corresponding damping ratio is 4.6% and 7.0% along X and Y direction respectively.

For the AS-building, the frequencies of the two fundamental modes in X and Y direction are very close (box-type behaviour), indicating the efficiency of selected strengthening techniques in both princi-pal directions. The interventions have led to in-creased the stiffness, as shown by the comparison of the values of frequencies of the strengthened with those of the as-built specimen. On the other hand, the initial damping is higher (by approx. 60%, in both directions) than that determined for the strengthened building. In Figure 12a, the transfer functions (TRF) between base acceleration and re-sponse acceleration at midlength of the long wall (A4X) and accelerations close to the corners (A3X, A5X) are shown for sine sweep test in X-direction for BS building. The difference in the response is

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due to the bending of the walls along Y axis, which were not connected to the timber floor. This feature is not present for AS building as shown in Figure 12b.

(a)

(b)

Figure 12. Comparison of Transfer functions at points A3X, A4X and A5X: (a): before strengthening; (b): after strengthen-ing.

3.3 Top acceleration-relative displacements

In Figure 13a, the time histories of accelerations measured at the second storey at points A3X, A4X and A5X (see Fig. 3) for Kalamata earthquake scaled to 90% of original record are presented for BS build-ing, whereas in Figure 13b, the recorded acceleration at the same points are shown for the AS model. It is apparent (Figure 13b), that the recorded acceleration at corners and mid-length of the short wall are closer for the AS, as compared to those recorded for the BS model. This effect is due to the enhanced diaphragm action of the floor, which was connected with the walls along its entire perimeter.

For AS building, the maximum horizontal top ac-celeration that was recorded was around 1.0g and it occurred for Irpinia earthquake scaled to 400% of original records (Fig.14). For BS building, the max-imum horizontal top acceleration attained for Test 8BS and was 0.6g (Fig. 15).

The top, relative to the base, displacement at point D11 is shown in Figure 16. A permanent dis-placement of 5mm was recorded at the end of test.

a) b)

Figure 13. Time histories of the acceleration at point A3X, A4X and A5X; a) BS building and b) AS building.

Figure 14. AS model: Time history of the acceleration at point A4X, for Test 16 (for testing sequence, see Table 3).

Figure 15. BS model: Time history of the acceleration at point A4X, for Test 8 (for testing sequence, see Table 2).

Figure 16. AS building: Time history of top displacement, rela-tive to the base, at point D11.

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3.4 Hysteretic response

In Figure 17 the diagrams of the base shear in X di-rection versus top, relative to the base displacement are presented for AS building and for the Irpinia base motion. The base shear was calculated from the acceleration record located at the points A4X (se-cond storey) and A10X (first storey) using the corre-sponding effective mass of each floor. The response (for increasing intensity of the base motion) is shown in Figure 18. The maximum base shear that was computed was around 130kN and was attained for Test 16, for a base motion scaled to 400% of the Irpinia original record. The deterioration of strength and stiffness is more evident in repetition of base ac-celeration scaled to 400% (for test sequence see Ta-ble 3).

Figure 17. Shear force vs top relative displacement for AS building and Irpinia earthquake.

Figure 18.Comparison of shear force vs top relative displace-ment for AS building subjected to Irpinia earthqauke for step-wise increasing PGA.

4 NUMERICAL ANALYSIS

The numerical part of this research being on-going, only some preliminary results regarding the as-built model are presented in this paper. The as-built stone masonry building was modeled using the finite ele-ment software Abaqus 6.10. A 3D model, accurately

reproducing the main geometrical features of build-ing was developed (Fig. 19). Masonry walls and timber elements (planks, joists, and lintels) were dis-cretizated using 3D triangular solid elements (Fig.20 nd fr sh ng chn qu A h d f’s f nodes belonging to the base of the building were ful-ly restrained.

Figure 19. 3D geometrical model of building.

Figure 20. 3D numerical model of building.

Concerning the masonry material properties, the as-sumptions made refer to a composite isotropic mate-rial. The mechanical properties of stone masonry used in the analysis were E (Young Modulus) =0.45GPa, ν (Poisson ratio) =0.20 and ρ (density) =2.0Mg/m

3. The additional masses were taken into

account by increasing the density of timber floor ar-eas where the masses were fixed.

A linear time history analysis was carried out us-ing the same base motion as the one recorded during shaking table Test 6BS (Table 2), where the first vis-ible damage occurred. Figure 21 shows the distribu-tion of damages (exceedence of tensile strength in terms of maximum principal stress) at time t=4sec of load history. It is worth noting that the damage dis-tribution is similar to the one obtained from the biax-ial excitation (see Fig. 8).

At present, a 3D nonlinear analysis of building model before and after strengthening interventions is in progress.

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a) b)

Figure 21. Distribution of damage at time step 4sec: (a): East

view; (b): Back east view.

5 CONCLUSIONS

The work presented in this paper allows for the fol-lowing conclusions to be drawn: 1. Testing of the as-built two storey model on the

earthquake simulator has proven the vulnerabil-ity of typical historical buildings to seismic ac-tions. Their vulnerability is due both to the de-fective connection between the leaves of masonry and to the pronounced effect of the out-of-plane bending of walls (due to the flexible floors).

2. The strengthening techniques applied to the mod-el, namely grouting, enhancement of the dia-phragm action of the floors and connection thereof with the perimeter walls, have led to a significant improvement of the behaviour of the model both in terms of maximum imposed ac-celeration and in terms of reduction of their vul-nerability to separation of the leaves and to out-of-plane bending.

3. The repair and strengthening of the damaged BS building, resulted to an increase of the frequen-cies, whereas the damping ratios were decreased. This reduction was of the order of 60% as com-pared to the values recorded for the as-built spec-imen.

ACKNOWLEDGMENTS

This research is funded by the EU FP7-ENV-2009-1, Contract No. 244123 (website: http://www.niker.eu/).

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