Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Seismic resistance assessment of heritage masonry buildings inpublic use in Ljubljana

M. LutmanSlovenian National Building and Civil Engineering Institute, Ljubljana, Slovenia

ABSTRACT: Recently developed parametric approach for seismic vulnerability and seismic resistance assess-ment and its correlations with previous methodologies are described in this article. The methodology is based onthe previously collected parameters of seismic resistance analysis of existing, mainly heritage masonry buildingsall over Slovenia. Using this methodology, seismic resistance of several groups of masonry buildings has beenassessed. Most of them are in public use in the Municipality of Ljubljana (MOL): fire stations, health centres,older buildings of elementary schools and kindergartens, some buildings of authorities and administration andolder residential buildings. The results are presented in dependence of the year of construction, as well as theshare of a certain vulnerable level in each group. The results again showed the reflection of the professionalknowledge into design, construction practice and the level of seismic resistance.

1 INTRODUCTION

A huge part of building heritage in Slovenia was builtin the period before 1895, when the most intense earth-quake hit Ljubljana region. Till then building practicewas based upon the experience, but from then onfirst seismic code was prepared to prescribe mea-sures for adequate seismic safety. It was an ordinancefor earthquake resistant construction of buildings, avery prescriptive-type code based mainly upon theobservation, how a structure suffers damage duringearthquake. From that first, most prescriptive typeof seismic code the vital professional knowledge hasbeen incorporated into the following codes. Not reallyearlier than the destroying earthquakes hit Friuli andMontenegro in 1976 and 1979 the design-engineersbecame more aware of destructive potential of earth-quakes. Noticeably higher seismic resistance level ofbuilding stock was enabled by the application of lastnational code from 1981, which is now being replacedby Eurocode 8.

Adequate seismic resistance and general safety forexpected earthquakes is more or less assured for newbuildings with careful design and construction. But wemust take care also for older buildings of our architec-tural heritage and protect them from the consequenceof earthquakes. In spite of all technical knowledge anddifferent tools for accurate seismic analysis are avail-able, that kind of approach for seismic behaviourassessment would require too much time and expenses.Therefore more simple, but reliable enough method-ologies have been developed to assess the seismic

vulnerability and seismic resistance of existing build-ings. These methodologies are usually applied to acertain group of buildings, that have either the com-mon owner (manager) or they have the same specificuse. The results are first of all the general informationof a group and afterwards on that base certain priorityplan for aseismic strengthening can be prepared.

2 METHODOLOGIES FOR SEISMICVULNERABILITY AND SEISMICRESISTANCE ASSESSMENT

2.1 Previous methodologies

When assessing the seismic risk of a certain area theexpected intensity of earthquake has to be definedon one hand and the seismic resistance of built-uparea on the other hand. Entire territory of Slovenia isearthquake-prone, but the earthquakes of the highestintensity are unfortunately expected just in the centralregion around Ljubljana where the population den-sity is the highest as well (Fig. 1). The buildings thatwere constructed before the application of first seismiccode are expected to suffer the majority of earthquakedamage.

Since the strategy for seismic vulnerability reduc-tion should begin with seismic resistance assessmentof buildings in a certain area, some method-ologies have been developed also at SlovenianNational Building and Civil Engineering Institute inLjubljana, Slovenia (ZAG). The experience of original

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Figure 1. Design ground acceleration map of Slovenia for T = 475 years return period earthquakes (2001).

methodologies used in the world and specific typeof residential and public buildings in Slovenia hasbeen taken into account. A simple method for seismicvulnerability assessment of masonry buildings wasprepared in 1986. It was a parametric method withfive main parameters as: type and quality of walls,quantity of walls, layout of walls in plan, connected-ness of walls and the height of building. The values ofthese parameters were numerically estimated withinthe range from 1 to 5. The sum of these values, eachmultiplied by a given weighting factor, was the finalvalue of seismic vulnerability Vs.

2.2 Seismic vulnerability assessmentmethod RAN-Z

The method RAN-Z, where the seismic vulnerabil-ity is assessed as well, was developed in 1995 (Perušet al. 1995). This method uses similar parameters asthe previous one: the structural type, the regularity,the amount of structural walls, the existing damageextent, the confinement, the height of the building andthe seismic zone. Beside the difference in the range oftheir values, the method is based upon the use of neu-ral network and this represented the main step further.The data base, that the neural network uses to identifythe correlation between the values of input parametersand the values of seismic vulnerability assessment, hasbeen prepared with the results of a questionnaire, ful-filled by six top experts of earthquake engineering inSlovenia. The vulnerability is numerically evaluated

within the range from 1 to 9 and it is directly connectedto the 9-grade expected damage scale, previously usedin Slovenia.

The buildings with low vulnerability level arelabelled with “green”. In the case of design earthquakethey are supposed to suffer no damage or minor dam-age as damage in plaster, cracks at chimneys, gablewalls and sliding to falling of roof tiles:

– green I: no damage,– green II: non-structural damage,– green III: minor structural damage.

After an earthquake they can remain in use. Thebuildings with medium vulnerability level are labelledwith “yellow”. In the case of design earthquake struc-tural damage may be expected – from thinner cracksto crushed elements:

– yellow I: slight structural damage,– yellow II: medium structural damage,– yellow III: heavy structural damage.

Certain repair of the building structure of this levelis going to be needed to get the previous structuralsafety. For this reason, these buildings may temporarynot be in use. The high vulnerable buildings, labelledwith “red”, are expected to be damaged to such exten-sion where the repair is not reasonable and they shouldbe demolished:

– red I: serious structural damage,– red II: partial collapse,– red III: total collapse.

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2.3 Seismic resistance assessmentmethod PO-ZID

In 2001 the newest method PO-ZID was developed(Lutman et al. 2002), where seismic resistance is pri-marily assessed instead of seismic vulnerability. Thismethod also uses neural network, where the modellingsuccessively consists of the choice of parameters, col-lection of available data, creating the database and theproof of the model effectiveness. In order to deter-mine the correlation between the input parameters andthe output parameters, a sufficiently large database ofwell-distributed and reliable data is needed. It consistsof all masonry buildings, that have been systematicallyinvestigated and their seismic resistance has been ana-lytically evaluated at ZAG during last 30 years. Thebase is being simultaneously enlarged with new resultsin order to increase the accuracy of the method.

The choice of input and output parameters was themost vital part of the modelling. The set of parametershave been namely selected on the basis of experi-ence and in order to approach the analytical resultsas much as possible. After many trial and error pro-cedures, it has been found that the best results canbe obtained by taking into account the derived inputparameters instead of the basic ones. The main param-eter is derived with the original expression for the shearresistance of a wall:

where σo−avg = average compression stress due to ver-tical load; ρ∗

in = equivalent wall/floor area (A∗in/Afloor).

Since the walls have different tensile strengths, A∗in is

total equivalent wall cross-sectional area (referred tothe reference tensile strength ft−ref ); and gtot = totalweight to floor area (Gtot/Afloor).

In order to upgrade this basic estimation, additionalnon-dimensional input parameters have been included:the participation parameter of shear walls in the direc-tion under consideration (Eq. 2), the participationparameter of the walls perpendicular to the directionunder consideration (Eq. 3) and the non-dimensionaleccentricity of the layout.

Considering the available database, acceptable cor-relation with the chosen output parameter has beenobserved only for the ultimate seismic resistance coef-ficient SRCu, while the ultimate ductility factor q hasbeen found too sensitive.

The ultimate seismic resistance coefficient SRCu isassessed independently for both two directions in thelayout. The final result is the SRCu of minor value,multiplied with the confinement factor knp:

3 SEISMIC RESISTANCE ASSESSMENT OFHERITAGE MASONRY BUILDINGS INPUBLIC USE IN LJUBLJANA

3.1 Fire stations

The authoritative services in the Municipality ofLjubljana (MOL) started to establish and develop thedatabase of seismic vulnerability of existing buildingstock in order to prepare a plan for measures for thecase of earthquake. Due to high importance of fire sta-tions and health centres these buildings were assessedat the very beginning.

In 2001 the seismic resistance of 42 fire stations inLjubljana has been estimated. The year of construc-tion varies a lot: the oldest fire station is from thetimes of Maria Teresia and the newest is about 10years old. Usually these buildings have two storeys,some built of stone masonry, the majority built ofbrick masonry (full or perforated blocks), the newestare even of reinforced-concrete. For all this buildingsthe available technical documentation about the designand the construction has been inspected. The buildingsthemselves have been afterwards inspected visually.

First, the seismic resistance of every fire stationbuilding was assessed by using PO-ZID method. InFigure 3 the seismic resistance is presented in depen-dence on the year of construction. The values ofseismic resistance are relative: it is the ratio betweenthe estimated seismic resistance coefficient of thebuilding and the seismic resistance coefficient, whichis required by the seismic code. The code, chosen forthis comparison, is still valid national seismic code

Figure 2. Typical older fire station in Ljubljana.

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Figure 3. Fire stations in MOL: Seismic resistance(SRCu/VK).

Figure 4. Fire stations in MOL: Seismic vulnerability(RAN-Z).

from 1981. If SRCu−np/VK > 1, the required seismicresistance by national seismic code is exceeded.

Besides and independently, the seismic vulnerabil-ity was assessed with the method RAN-Z. The esti-mated values are shown in Figure 4, also in dependenceon the year of construction.

Since the seismic vulnerability level varies from lowto medium, serious damage is not expected at any firestation due to an earthquake of VIII degree on EMSintensity scale, expected in Ljubljana according toseismic hazard map of Slovenia. Fire stations are esti-mated as low vulnerable (63%) or medium vulnerablebuildings (37%) (Fig. 5).

From both two diagrams (Fig. 3 and Fig. 4) it maybe summarized that newer buildings are generallymore resistant and less vulnerable than the olderones, as the consequence of growth of professionalknowledge.

Figure 5. Fire stations in MOL: Seismic vulnerability(RAN-Z).

Figure 6. Seismic resistance/Seismic vulnerabilityrelationship.

Since seismic resistance and seismic vulnerabilityhave been assessed separately for this first group ofbuildings, the correlation between these two param-eters has been found out: one for masonry and onefor the reinforced-concrete buildings (Fig. 6). Theobtained correlations are sensible: higher seismicresistance means lower seismic vulnerability and theopposite. These two correlations were then used toassess seismic resistance from seismic vulnerability,previously assessed with RAN-Z method.

Similar correlations have been also found betweenthe results of the newest method PO-ZID and theresults of the oldest method from 1986. Afterwardsthese correlations were applied to the results of allprevious methodologies.

3.2 Health centres

The next group includes buildings of health centresand some hospitals as well. Some of them are about100 years old, some were built between WW I andWW II, the majority in the fifties and sixties (Fig. 7)

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Figure 7. Typical building of health centre in Ljubljana.

Figure 8. Health centres in MOL: Seismic resistance(SRCu/VK).

and some in the nineties of last century. The olderbuildings are usually constructed of masonry and thenewer buildings of reinforced-concrete.

The results of seismic resistance assessment arepresented in dependence on the year of construction(Fig. 8). Higher seismic resistance has been againfound for newer buildings comparing to older ones.Buildings with reinforced concrete structure are lessvulnerable than buildings of masonry type. The totalshare of buildings, usable after an earthquake of VIIIdegree intensity, is only 44%, temporary unusablewould be 48% of them and even 8% of health centresare expected to suffer damage above reparable (Fig. 9).

3.3 Elementary schools and kindergartens

Older buildings of elementary schools and kinder-gartens, altogether 96 buildings, were assessed in 2003with the newest method for masonry and reinforced-concrete buildings. A lot of them were built in times

Figure 9. Health centres in MOL: Seismic vulnerability(RAN-Z).

Figure 10. Typical urban school building from beforeWW I.

of Austria-Hungary (Fig. 10), some between WW Iand WW II. After WW II the first reinforced-concreteschool buildings were built, in most cases designedwithout consideration of seismic load.A lot of them areolder village schools. Some kindergartens are formerdwelling houses from before WW II. In previous groupall fire station buildings were included, the newest aswell, while this group includes only buildings builtbefore 1970, when the seismic code from 1963 and1964 have consistently been incorporated in the designpractice.

The results (Fig. 11) are not very optimistic: thebuildings from before WW II have quite similar seis-mic resistance, usually beyond the requirements ofthe 1981 national seismic code. Besides we should beaware that the seismic load according to this code isnot adequate for the masonry buildings. In the case ofreinforced concrete school buildings from this periodserious structural damage can be expected in the caseof design earthquake. Only 11% of all buildings areestimated as low vulnerable, the majority of them aremedium vulnerable and even 4% would suffer heavydamage from the earthquake of VIII degree EMSintensity scale (Fig. 12).

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Figure 11. Older buildings of elementary schools andkindergartens: Seismic resistance (SRCu/VK).

Figure 12. Older buildings of elementary schools andkindergartens: Seismic vulnerability (RAN-Z).

3.4 Buildings of authorities and administrationof the municipality of Ljubljana

This is a small group. Quite a lot of these buildings areparts of the old Ljubljana downtown (Fig. 13), manyof them built after WW II, less of them are new. Mostof them were assessed directly with newest methodPO-ZID in 2004, while the results of previous meth-ods were transformed into the new output parameterswith the application of derived correlations (Fig. 14).All known structural interventions were taken intoaccount. Unfortunately not many renovation interven-tions included measures for seismic strengthening,although they could be carried out in phases. Manyrestorations of old buildings’ facades were carried outeven without placing simple steel ties to confine thebuilding.

Many older administration buildings are very vul-nerable (Fig. 15): even one third of them would be

Figure 13. The city hall of Ljubljana.

Figure 14. MOL authorities and administration buildings:Seismic resistance (SRCu / VK).

Figure 15. MOL authorities and administration buildings:Seismic vulnerability (RAN-Z).

heavy damaged by an earthquake of VIII degrees, agood half would suffer moderate damage and only8% of buildings would suffer only non-structuraldamage.

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Figure 16. Older residential building from before WW II.

Figure 17. Older residential buildings in MOL: Seismicresistance (SRCu/VK).

3.5 Older residential buildings in Ljubljana

It is a large group of 186 older residential buildings(Fig. 16), mainly in quarters of old Ljubljana town.For most of them seismic vulnerability was assessedwith the older method about 20 years ago.Those resultswere transformed into the present form. Recently somenew assessments were done for residential buildingswith reinforced concrete structures. The results of thisgroup are characteristic for these quarters and not forthe whole region of Municipality of Ljubljana.

The results of this group (Fig. 17) are obviously notso widely spread as the results of previous groups. Themain reason may be found in the fact that old resultswere transformed into the new form. The seismic vul-nerability of the majority was assessed with “yellowIII” level, where heavy damage could be expected(Fig. 18). The exceptions are the results of newestmethod PO-ZID and the results of the analytical push-over method, which are also included. Among lowvulnerable buildings in this group we find the firstconfined masonry buildings – labelled with “yellow I”

Figure 18. Older residential buildings in MOL: Seismicvulnerability (RAN-Z).

Table 1. Shares of seismic vulnerability levels for groupsof buildings under consideration.

Vulnerability level

Group n YCavg Green Yellow Red

Fire stations 46 1949 63% 37% –

Health centres 23 1962 43% 48% 9%

Older buildings of 96 1946 12% 84% 4%elementary schoolsand kindergartens

Buildings of MOL 13 1895 8% 58% 33%authorities andadministration

YCavg . . . average year of construction.n . . . number of buildings.

and of course the buildings with reinforced concreteshear walls, assessed with level “green III”.

3.6 Seismic vulnerability – comparisonof the results

The comparison of the share of a certain vulnera-ble level in all assessed groups has been drawn out(Table 1). Beside each group, there are the averageyear of construction and the number of buildings ineach group.

In the case of fire stations far and away the leastdamage is expected (63% of “green”). The elementaryschools, where the average age is similar as in the caseof fire stations, the results are quite different – only

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11% of low vulnerable buildings. The results of healthcentres are somewhere between, although they werebuilt later. For these three groups all seismic codesrequire higher seismic resistance than for common res-idential and business buildings. Far more vulnerableare the buildings of Ljubljana administration, whichare also the oldest.

4 CONCLUSIONS

The majority of buildings representing cultural her-itage in the Municipality of Ljubljana are built ofmasonry. Many of them still have timber floors oflow weight, which are not causing high inertial forces.Improper confinement and inadequate in-plane rigid-ity are consequently the main drawbacks resultingfrom this reason. Insufficient rigidity is also a defi-ciency of early reinforced-concrete floor structures –hollow tile floor structures, which transfer vertical loadmainly in one direction. Inadequate seismic resistanceis typical for buildings with lower amount of struc-tural walls, irregular in plane, for buildings with morethan three floors and mainly for buildings built beforeWW II.

Early reinforced-concrete structures are typicalirregular in plane, being much more resistant in onethan in the other direction. The reinforced-concretecolumns are often very slender, with minor size ofthe cross-section of only 12 cm, besides they are verypoor with stirrups. The lack of structural redundancy,potential plastification and seismic energy dissipationzones are their main structural deficiencies. It can beconcluded once more, that the professional knowl-edge reflected in design, construction practice andconsequently in the level of seismic resistance.

The results of these assessments are basically usedas general information of seismic resistance of a groupof certain type or in certain area. Upon this informa-tion a priority lists are made, first, in order to makeplans for seismic risk reductions, and, second, to pre-pare activities for seismic strengthening of a certainbuilding.

By presented methods only few percents of entirebuilding stock of Municipality of Ljubljana have beenassessed. It has been done this way in the case of build-ings of higher importance for public safety and civil

protection in the immediate post-earthquake period,as well in the case of buildings with higher social andeconomic importance.

In the process of seismic resistance and seismic vul-nerability assessment of all other buildings the useof existing data base and even more rapid and moresimple methods are planned. Reliable seismic vulner-ability maps are going to be filled with these resultsand updated after every structural strengthening of aparticular building.

ACKNOWLEDGMENTS

The research, presented in this article has been car-ried out within the framework of several researchproject, financed in the last decade by the Authoritiesof the Municipality of Ljubljana and its Departmentfor Research and Culture.

REFERENCES

Eurocode 8. Design of structures for earthquake resistance,Part 1: General rules, seismic actions and rules forbuildings. EN 1998-1. 2004, Brussels, CEN.

Tomaževic, M. & Turnšek, V. 1982. Verification of theseismic resistance of masonry buildings. Proceedingsof the British Ceramic Society No.30, (Shelton House,Stoke-on-Trent, 1982): 360–369.

Sheppard, P. & Lutman, M. 1988 Estimation of expectedseismic vulnerability: A simple methodology for mediumsized groups of older buildings. Seismic Risk Assessmentand Design of Building: 47–62.

Peruš, I., Fajfar, P. & Grabec, I. 1994. Prediction of the seis-mic capacity of RC structural walls by non-parametricmultidimensional regression. Earthquake Engineering &Structural Dynamics, 23, 1994: 1139–1155.

Peruš, I., Fajfar, P. & Reflak, J. Seismic vulnerability method-ology for concrete and masonry buildings, Report n.1,1995 (in Slovenian).

Lutman, M., Peruš, I. & Tomaževic, M. Seismic vulnerabilitystudy of buildings in the Municipality of Ljubljana, FinalReports, 1999–2007 (in Slovenian).

Lutman, M., Peruš, I. & Tomaževic, M. 2002. Parametricapproach to seismic vulnerability evaluation of existingbuildings. The Twelfth European Conference on Earth-quake Engineering, September 2002, London, Elsevier,2002 Amsterdam: 9–13.

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