811

SEI SMIC BEHAVIOUR OF MIXED, REINFORCED CONCRETE, REINFORCED MASONRY STRUCTURAL SYSTEMS

MIRA TOMAZEVIC( l) and CLAUDIO MODENA(2) ( l)In s titute for Tes ting and Resea r ch in Mater iaI s

and Structures, (2) Dimiceva 12, 6 1109 Ljubljana, Yug oslav i a

Facul t y of Civil Engineering, Univers ity of Padova, Via Marzolo 9, 35131 Padova, Italy

ABSTRACT

The seismic behaviour of two three-s toreyed r einforced masonry building models has been investiga ted. The first model represented a mixed reinfor­ced masonry, reinforced-concrete building with peripheral masonry wal l s and a cen tral r.c. column. The second model , however, was pure reinforced masonry building with a cross - wall r eplac ing th e central r.c. column. Both mode ls were subjected to a ser ies of simu l ated earthquake motions on a simp l e earthquake simul a t or. It has been found that good se i smic behaviour of mixed struc tural systems - if adequately conceived and designed - can be expec ted. Pr operly designed mixed structural systems will behave elastical­ly when subjected to moderate ear thquakes, and will possess suffic ient duc­tility to withstand repeated severe shak ing befo r e collapse.

INTRODUCTION

Since there i s a growing interest in residential mason r y const ruct i on, where flexibi lity of architectural design plays an impo rtant role, mixed structural systems, cons isting of peripheral reinforced masonry walls and cen tral r . c . co lumns as principal load-bearing s truc tural elements became very pop ula r. Ho\vever, s ince the pos t-earthquake observa tions in many cas es i ndica ted a rather poor seismic behaviour of such buildings, severe limitations fo r t he construc tion of mixed s tructural sys tems in seismic zones have been introduced in many seismic codes .

In order to modify the sometimes conse r va tiv e r equirements of se ismic codes, th e knowing of actual behaviour of specif ic, three- to f ou r-storeyed masonry buildings, where i nternaI masonry wa ll s a re substituted wi th r.c. columns , i s of r e l 'evan t importance. The r efore, a multi-year cooperative research project between It aly and Yugoslav i a has been initiated to provide experimental da t a on seismic behaviour of t his specific t ype of mason r y construction. Within t h is projec t, two three- storeyed reinforced masonry

812

building models, one with peripheral walls and a central r.c. column (model 1), and the other one with a cross - wall replacing the central column (model 2), have been tested on a simple earthquake simulator at the Institu t e for Testing and Research in Materials and Structures (ZRMK) in Ljubljana, Yugoslavia (1, 2, and 3). This paper discusses the observed seismic behaviour of the tested models.

DESCRIPTION OF TESTS

Construction of Models and Model MateriaIs The tested models represented part of a three-storeyed building (20.7x11.7m in plan) which consists of peripheral load- bearing walls, internal walls which separa te the building into two, symmetrical pa rts, and internal r.c. columns whi~h support the floor slabs. Walls are constructed of light­weight ceramic perforated blocks t ype POROTON, 280 mm thick, and are reinforced with vertical (minimum 10 mm diameter bars at 0.3 - 0.4 m in­tervals, and 16 mm diameter bars at the edges of window and door openings) and horizontal reinforcement (minimum two 6 mm diameter bars, placed in each second horizontal mortar joint). Deformed steel (yield stress 440 MPa) and smooth steel (yield stress 320 MPa) bars are used for vertical and horizontal reinforcement, respectively. Floors are of massive t ype : usually, r.c. slabs a re made lighter by using ceramic or polistyrene infill elements. Slabs are supported by peripheral walls and r.c. beam at their midspan.

Since testing facilities at the Institute are of limited capacity, decision was made to test the 1:5, reduced scale models. In order to avoid difficulties wh i ch would occur if t he materiaIs conforming to the laws of complete model similarity were manufactured, prototype materials have been used and the laws of general model similarity have been followed. Prototype blocks have been simply cut into correspondingly sized model blocks. Concentrated rather than distributed vertical reinforcement has been placed at vertical edges of walls (two 2.3 mm diameter bars), at corners and at wall intersec tions (four 2.3 mm diameter bars) . Horizontal reinforcement, however, was uniforrnly distributed in mortar joints (1.1 mm diameter wire in the form of closed stirrup in each second mortar joint). The so call ed "burned wire", commercially available on the market, has been used.

Floor slabs have been modelled as one- way acting ribbed slabs , with r.C . beam at their mid-span, and were confined with r.c. tie-beams, r einforced with four 3.1 mm diameter bars, atop of a lI peripheral walls. The beam was supported either by internal r.c. column, reinforced with four 4.2 mm diameter bars and 1. 1 mm stirrups at 20 mm intervals (model 1) or by a cross-wall, which r eplaced r.c. column (model 2). Slabs were pre­fabricated, with openings left for splicing and anchoring of vertical reinforcement of walls or columns, which were poured with micro-concrete afte r the positioning and levelling the individual slabs on the top of walls.

Mor t a r used for the construction of \<lalls and grou ting of r e inforcement consisted of Portland cement, lime and sand (aggregate size 0-2 mm) in th e proportion of 1:2:9. Micro-concret e consisted of Portland cement and sand (aggregat.e size 0-4 mm) in the proportion of 1:3.

The dimensions of the two models in plan and eleva tion are shown in Fig.1. The mechanical properties of model materials, however, are g iven in Tables 1 and 2 .

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ElEVATION ElEVATION

~

" ' I

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" ~ 'I II

M II ~ II

' I o

~I II

" II I I II

P 815 60 815 55 55 4755 739 4755 55

H =1 r 'I 1800mm 1800 mm

PlAN PlAN

::; lO

60 0 ill 't+---=--==- -=- -=--= -= =~l::=- =--.-:-.::- =-=-~ ~ --- - - ----

~

'" N ~ ~ r' 309 '.

325 4n 325 309 55 55 4755 342 n_~R-\ 4755 55 .J~ F ==r r 't 1800mm 180Qmm

MODEL 1 MODEL 2

Figure 1. Dimensions of modei buildings

TABLE 1 Mechanical properties of modei masonry materiais

(average vaiues)

Compress i ve st r ength of blocks (MPa) Compressive strength of mortar (MPa)

Compressive strength of micro - conc r ete (MPa) Compressive strength of masonry (MPa)

Tensile strength of masonry (MPa) Moduius of elasticity (MPa)

TABLE 2 Mechanical properties of re inforcing

(average vaiues)

6.0mm 4 . 2mm 3.1mm

Tensi l e strength (MPa) 382 448 443 Yie ld limit (MPa) 253 391 323 Yield strain (%, ) 1. 21 1.86 1.54

Ultimate strain (%) 31.45 21.70 39 .10

Model 9.45 2.74

27 . 95 6.33 0.40 6450

s t eei

2.3mm

11 3 93

0.44

~n

J I

~[ '3.-

M M ~

~

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'" M

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Mode l 2 9.45 1. 85

23 . 43

1.lmm

43

814

In order to achieve the similarity of mass and stiffness distribution in prototype and model structures, additional mass (300 kg at each floor leveI) has been added to both models (Table 3). In order to achieve the similarity of working s tresses in load-bearing wal ls of prototype and model struc ture, which is the condition for similari t y of failure mechanisms of prototype and model bui l dings , peripheral walls of both models were prestressed by means of steel knitted ropes (4 mm in diameter), fixed on the top slab and anchored into the foundation slab. Soft springs a t the top end of the ropes have been used to control the pres tressing forces during shaking tests (6000 N in each rope). It has been found by testing and by calculation, that prestressing of walls did not significantly influence either the dynamic behaviour of models or the magnitude of ba se shear , acting on the model during shaking tests.

TABLE 3 Distribution of masses, concentrated at floor leveIs (kg)

Mode l 1 Mode l 2 3-rd· floor 520.0 536.9 2-nd floor 603.4 658.1 l- st floor 603.4 658. 1

Weight of the model 1811. O 1966.4

Simulated Earthquake Loading and Testing Procedure Ea rthquake simulator at ZRMK consists of two main parts: horizontally and vertically r ai l-gu ided , roller-supported steel platfo rm, onto which the foundation slab of the model was fixed by means of s tee l bolts and nuts, and two way acting displacemen t programmable actuator type Schenck Pl 250P (or 160N), connected to the platform. A Hewlett-Packard micro-computer ser i es 98 16S has been used to control the actua t or.

The dynamic behaviour of both models has been studied in linea r and non-linear range of vibrations. In order to make possible sta ti stical considerations of models' response to seismic exci t ations (4) in the linear range, three different se ts of synthetic accelerograms (5) have been used for the generation of earthquake simulator motion (6): two sets of five earthquakes each, derived from Eurocode 8 respons e spectra (AGEl and AGE2) and one set of five earthquakes derived from the draft Italian seismic code soil type 11. r esponse spectrum (GNDT2). In add ition, one earthquake of the fourth set (AGE3) has been also used at the beginning of tests.

Shape of the response spectra, used in the calculat ions is shown in Fig.2a. Their characteristics, however, are given in Tab le 4 :

TABLE 4 Characteristics of code response spectra used for the

gene ration of synthetic earthquakes

Group T1 T2 K Ro Fractile

AGEl 0.2 0.4 3.0 86 % AGE2 0.2 0.6 3.0 86 % AGE3 0.3 0.8 2.5 86 % GNDT2 0.15 0.8 2.2 50 %

Shape of the time modu lating functi on considered for the generation of

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synthe ti c earthquakes is shown in Fig .2b. Spec tra ,vere arbitrarily scaled to a maximum gr ound acceleration equal to O.lg, thus l'epresenting earthquakes of mode r ate intensity .

L--L __ ~__________________~

T, T

a, b,

Figure 2. Shape of the response spect ra and time modu lating function considered for t he generation of synthetic earthquakes

In the non-l inear range , however, two (one in the case of model 2) records of April 15, 1979 Montenegro , Yugos lavia, earthquake gr ound accelera tions (7) have been used , with 0 . 43g and 0.28g peak acce lera t ion, respectively. Characte rist ics of earthquake s are given in Table 5 :

TABLE 5 Characteristics of earthq uakes us ed for contro lling the

earthquake simula tor motion

Group Range of maximum_2 Range of maximum accelerations (ms ) displacements (mm)

AGE 1 -0.832 - 0.919 -99.5 - 91.5 AGE2 -1. 294- 0.992 -120.1 - 128.0 AGE3 -1.072 - 1.6 12 -164.4 - 129.4

ACC 10 - 4.273 - 3. 985 -96.0 - 77.0 ACC13 - 2 .316 - 2.794 -99.5 - 82.7

Duration (5)

25.52 25.52 25.52

48.40 48.40

The models were instrumented wi th two sets of transducers (Fig.3): with accelerome t ers to measure absolute accelerations a t each floor leveI, and LVDT-s t o measure horizontal displacements relative to the foundation at each floor leveI and at mid-height leveI of transverse wa ll s . Of course, shaking table motion (horizonta l and vertical acce lera tions and horizontal displacements) has been also controlled. AlI data have been r ecorded by means of a 14-channe l Honeywell t ape recorder. Characteristic data, however, have been plotted by means of a Hewlett-Packard series 7090A plotter for immediate testing controlo General view of models during shaking tests is shown in Fi g .4.

Both models we r e subjected to seismic excitations in diff eren t t est ing phases, in a lltoge ther 74 (model 1) or 67 (model 2) test runs. In alI t est runs, the models were first subjected to shaking , and the pos sible changes in first na tural frequency an d damping have been determined from records of

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Model

N

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@

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Figure 3. Instrumentation of models

Model 2

Figure 4. Models during shaking tests

G)

1-=- 0

I

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1--=-0

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I--=-®

free vibrations induced by hitting the top slab of the models with plastic hammer. Then, damage propagation in the models was inspected, and eventual cracks marked and photographed. In the non-linear range, the behaviour of models during most severe shaking was filmed with two S-8 movie cameras. The intensity of shaking was controlled by ad justing the magnitude of maximum ground displacement, obtained by integration of the chosen accelerogram. Of course the duration of earthquakes was compressed in time. In order to compare the effec t of different frequency contents of shaking table motion to the models' response, not a lI earthquakes have been

817

modelled ~n time according to the laws of general model similarity. The characteristics of shaking table motion in different testing phases are presented in Table 6.

.;';

TABLE 6 Input characteristics of shaking table motion

Model

Phase 1,8 runs: 10-25 %, time scale 2.24

Phase 2, 30 runs: 10-25 %, time scale 3

Phase 3, 10 runs: 10-100%, time scale 3 and 6

Phase 4, 10 runs: 25 %, time scale,3 Phase 5, 16 runs

H

10-100%, time scale 3 and 6

Model 2

Phase 1, 8 runs: 10-25 %, time scale 2.24

Phase 2, 30 runs: 10-25 %, time scale 5

Phase 3, 10 runs: 25 %, time scale 5 Phase 4, 10 runs:

10-25 %, time scale 3 Phase 5, 4 runs:

25-100%, time scal* 5 Phase 6, 5 runs :

25-100%, time scale 5

recorded earthquake accelerograms have been used as the input!

TEST RESULTS

Damage Propagation and Failure Mechanism

Model 1: A combination of shear and flexural cracks has first occurred in both middle walls and web parts of corner walls (part of corner Ivalls in the direction of seismic action) of the first storey of model building. As indicated by crack patterns, walls behaved as vertical cantilevers, coupled together with flexible floors and bond-beams. Consequently, cracks and obvious plastic hinges in the slabs and bond-beams have developed at vertical edges of walls at alI floor leveIs. With the increased intensity of shaking table motion, damage in Ivalls, however, remained concentrated in the first storey: corner walls failed in flexure, either with typical crushing of masonry blocks and buckling of vertical reinforcement at the compressed side of the web or with crushing of masonry blocks at the compressed side of the flange (part of corner walls orthogonal to the direction of seismic action) after the tensile reinforcement has yielded. Horizontal cracks at the ends of the internaI r.c. columns have been observed at the same time, indicating plastic hingening of column's ends in the first storey of the modelo With the progressive damage of model's walls, crushing of column's concrete became visible. Middle walls, however, failed in shear, horizontal reinforcement preventing the complete disintegration of those walls.

Model 2: As expected, shear cracks have first developed in the cross­Ivall and one of . the corner walls of the first storey of the modelo During the following test runs, cracks between peripheral walls and floor slabs, and cracks in bond-beams at vertical edges of walls in the direction of seismic action could be observed. With the increased intensity of shaking, cracks in the walls have propagated into the upper storeys: on the second floor, shear cracks have developed in the cross-wall, but peripheral walls were not damaged. On the third floor, however, shear cracks developed in

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TABLE 7 Damage propagation in dependence on the intensity of shaking

Model 1 Model 2

Damage First visible First visible Description cracks cracks

Maximurn Shaking Tab l e Acce l era tion (rns-2) 16.22 13.92

First Darnage Max irnurn Top Floor

Acce leration (rns-2) 19.94 25.28

Base Shear Coefficient 1. 29 1. 52

Propagation of

Darnage Description

Propagation of cracks in the walls , c r acks in bond-bearns

c r acks in the walls of 1-st and 2-nd storey, c r acks between walls and slabs, cracks in

Max i rnurn Resis -t ance

Maxirnum Shaking Tab le Accele r at i on (rns - 2) 23.0 2

Maxirnurn To p Floor Accele rat ion (rns-2) 39 . 88

Base Shear Coefficient 2 . 62

Darnage Desc ripti on

Crushing of blocks and colurnn's concrete,

pl.hinges in bond- bearns, tea ring and buckling

of r einforcernent Be fore Maxi mum Shaking Table

Col laps e Acceleration (rns - 2) 21.16

Maxirnum Top Floo~2 Acceleration (rns ) 22 . 74

Base Shea r Coefficient 1. 73

TABLE 8

bond-beams

31. 20

40.16

2 . 57

Crushing of blocks, pl.hinges in bond­

beams, tearing and buckling of r ein­

forcement

48.00

16 . 99

1. 81

First natura l frequency and equivalen t viscous dampin g

Frequency f (s -1 ) Darnping v (%) Model 1 Model 2 Model 1 Model 2

Virg in model 9.58 13.06 6.6 4.8 Virgin model after

prestressing 11.90 13.29 5 . 6 3.8 First damage 9.52 11.09 5.6 6 . 6

Maximurn resistance 9.15 7.J8 4 .5 6 .1 Before col lapse 4.77 5.54 7 .0 6 . 5

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the middle peripheral walls, with no damage in the cross-wall. Of course, cracks between walls and floor slabs have been observed at alI floor leveIs. Al though cracks in bond-beams opened and crushing of concrete took place, with obvious yielding of reinforcement, indicating plastic hingening of bond-beams at alI floor leveIs, the most severe damage was concentrated in the walls of the first storey.

Whereas typical shear behaviour has been observed in the case of the cross-wall, a combination of shear and bending governed the in-plane behaviour of peripheral walls and corner walls. Horizontal reinforcement prevented the walls to fail in shear, so crushing of masonry blocks has occurred at the compressed side of walls, resulting into buckling of compressed reinforcement and yielding and tearing of tensioned reinforcement at the same time.

Typical crack patterns in structural walls before collapse of both models are shown in Fig.S. Damage propagation in dependence on the intensity of seismic excitations is presented in Table 7.

north face north face cross - waU

....-c=-

~

i

~

rr-

~ ~ ,......-.; IC I I ~

MODEL 1 MODEL2

Figure 5. Typical crack patterns in structural walls before collapse

Based on the analysis of models' response to seismic excitations, three limit states have been defined in models' behaviour: elastic limit, defined with first damage of models' structural elements, maximum resistance, defined ,vith maximum base shear, acting on the models (calculated on the basis of mass distribution - Table 3, recorded mode shapes - Table 9, and maximum recorded accelerations), ultimate limit, representing the dynamic characteristics of the model before collapse, or at the moment of the termination of tests.

Dynamic Characteristics The changes in parameters "hich define the dynamic behaviour and seismic resistance of the tested models, such as first natural frequency and mode shape, as well as equivalent viscous damping, are summarized in Tables 8 and 9.

First natural mode shapes at characteristic limit states are also plotted in Fig.6, where - as the extreme possibilities - pure bending and shear mode shapes of systems with uniformly distributed masses and stiffnesses are also presented.

Whereas significant natural frequency decay has been observed as a re­sult of the increased damage of both models, practically no ~hanges in equivalent víscous damping could be noticed. The differences in damage

3-rd floor 2-nd floor l-st floor

820

TABLE 9 First natural mode shapes

First damage Mode l 1 Model 2

Maximum resistance Made l 1 Model 2

Before collapse Model 1 Model 2

1.000 0.714 0.330

3 - rd floor

firs\ domoge

1.000 0.660 0.297

1.000 0.692 0 .379

1.00

moximum r€'sis1ance

1.000 0.713 0.348

1.000 0.811 0 .568

MO DEL 1

MODEL 2

! !

! !

/

b efore collopse

Figure 6. First natural mode shapes

1.000 0.762 0.385

distribution over the models' height can be clearly noticed in Table 9 and Fig.6: concentrated damage in the first storey of model 1 resulted into the deformed mode shape before collapse. In th e case of model 2, however, the uniform distribution of damage over the model's height and predominant shear behaviour of model's walls can be clearly seen.

In Table 10, stiffness degradation has been evaluated on the basis of calculated base shear and measured first storey relative displacements. It is to note that maximum values of top accelerations and first storey displacements have been used in the calculations of base shear and storey stiffness, respectively, which, however, did not always occur a t the same time!

CONCLUS IONS

The values of the base shear acting on the model buildings should be extrapolated to prototype structures. In order to do this, the laws of general model similarity have been taken into account, and the shaking table accelerations and base shear coefficients in Table 7 have been multiplied by accele r ation modelling factor (Sa = 0.2) The resulting va­lues a re given in Table 11.

l

Base shear (kN)

Disp l acemen t (mm)

Stiffness (N/mm)

St iffnes s (% of initial)

Maximum ground acceleration (g)

Base shear coeff icient

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TABLE 10 First storey stiffness degradation

First damage Model 1 Model 2

22 . 93 29.89

3 .1 8 2 . 46

72 10 12150

Maximum r esis t ance Model 1 Mode l 2

46.5 1 50 . 54

10 . 55 15.7 1

4408 32 17

61.0 25.7

TABLE 11

Before collapse Mode l 1 Model 2

30 . /5 35 . 59

17.24 21.43

1/83 1660

L5 . 0 13.7

Seismic resist ance of prototype buildings

First damage Bldg . 1 Bldg . 2

0.33 0.28

0.26 0.30

Maximum resistance Bl dg . 1 Bldg . 2

0.47 0.64

0 .52 0.51

Before collapse Bldg . 1 Bldg . 2

0.46 0.98

0.35 0 .36

It is of cours e not pos sib le t o draw any definite conclusions as regards the dynamic behaviour of th e t es t ed models ~vi thout a detailed analysis ot t est results. The ana l ys is is under way and i s partly presented at this Conf erence (8). However, by comparing the dynami c behaviou r and t he observed damage propaga tion and failure mechanisms of the two models, no significant difference as r egards the capabi lity of the t wo different t ypes of buildings to resist se ismic loading could be observed. By extrapolating the results to the prototype buildings, the fo llowing conclus ions can be drawn as r ega rds the seismic behaviour of mixed, reinforced masonry, reinforced-concrete structural systems wi th peri pheral masonry wall s and central r.c. columns :

the buildings will behave elastically (no damage is expected in their structura l system) when subjected to mod erate int ensity earthquakes, with peak ground accel era tion not exc eeding 0 . 25g;

if r einforced with minimum vertical and hori zont a l r e info rc emen t, those buildings will possess sufficient ductility to withstand repeated severe shaking , i.e. earthquakes with peak gr ound acceleration up to 0.4g, before collapse.

The following conclusions can be drawn as r egards the design of the tested type of masonry bui ldings to withstand the described l eveI of seismic loading:

at least minimum vertical and horizontal r e inforcement must be present Ln the wa l ls in order to ensure the flexural, duc til e behaviour of structural sys t em ;

structural walls behave as v e rtica l cantilevers, coupled togethe r with flexible floors. Bending moments are carried by the floors and tie­beams, 50 flexural capacity of ti e-beams and floors can be t aken into account when designing the reinforced masonry wa lls for eartquake loading;

no out-of-the-plane phenomena can be expected if the wa lls a r e reinforced with minimum vertical and horizontal reinforcement.

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ACKNOWLEDGEMENTS

The research described in this paper is part of a mu lti-year research project, sponsored by the Research Community of Slovenia and by the Consorzio POROTON, Vicenza, Italy. Their financiaI support is gratefully acknowledged. The authors would also like to express their gratitude to Mr. Oreste Bursi from University of Trento, Italy , for h is help in the phase of preparation of earthquake simulator input data, and to Messrs.Tomaz Velechovsky and Ljubo Petkovic, ZRMK Ljubljana, for the many improvements in earthquake simulator functioning and data acquisition.

REFERENCES

1. Tomazevic, M. , Modena , C., Velechovsky, T. and Zarnié, R., Shaking table study of a reinforced masonry building model with central r.c. column -test re sults. Report ZRMK/IKPI- 87/02, Ljubljana, 1987.

2. Tomazevic, M., Modena, C. and Petkovic, L., Shaking table study of a reinforc ed masonry building model with cross - wall - test results. Report ZRMK/IKPI- 88/02 , Ljubljana, 1988.

3. Tomazevic , M. and Modena, C., Earthquake simulator study of a reinforced masonry building model with central r.c. column . Conference on Civil Engineering Dynamics, Bristol, 1988 (to be presented).

4. Barel, S., Bernardini, A., Bursi, O. and Modena, C., Analisi statistica della risposta di sistemi lineari ad eccitazioni sismiche a piu compo ­nent~ ~n regime stazionario e transitorio - metodologia e programma di calcolo. Report of Istituto scienza e tecnica delle costruzioni, Padova, 1986.

5 . Modena , C. and Barel, S., Studies of the seismic response of masonry wall constructions. Pacific Conference on Earthquake Engineering, Wei rakei, 1987.

6. Ballio, G., Castiglioni, C.A., Perotti, F . and Zandonini, R., Generation of artificial accelerograms for assessing q factors. Report of Politecnico di Milano , Milano , 1986.

7. Bulletin of the Strong-Motion Earthquake Accelerograms. No.l, IZIIS, Skopje, 1984-.-- -- ---------

8. Bursi, O., Modena, C. and Tomazevic, M., Experimental and numerical study of seismic behaviour of a mixed, r.c. and reinforced masonry three-storeyed building structure. 8-th International Brick/Block Masonry Conference, Dublin, 1988 (to be presented) .