seismic risk of buildings with rc frames and masonry infills from timisoara

9th International Masonry Conference 2014 in Guimarães

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International Masonry Conference, Guimarães 2014 1

Seismic risk of buildings with RC frames and masonry infills from Timisoara, Banat region, Romania

MOSOARCA, MARIUS1; PETRUS, CRISTIAN2; STOIAN VALERIU3; ANASTASIADIS, ANTHIMOS4

ABSTRACT: The paper briefly presents the seismic vulnerability of RC frame structures with

masonry infills from Banat region, Romania. The largest city from this region is Timisoara, which has an extensive heritage of old masonry buildings. With the evolution of design codes, the construction techniques evolved towards a trend of buildings with RC frames and masonry infill panels. After 1990 several structural interventions were made on already constructed buildings, which altered mainly the rigidity and thus increasing their seismic vulnerability. The Banat region is a medium to high seismic zone subjected by shallow crustal earthquakes; it is characterized by an increased action of the vertical components. Lacking for ductility detailing as well as uncontrolled strength and rigidity distribution, the buildings with RC frames and masonry infill walls present in Timisoara can develop unfavourable failure modes. Consolidation measures are proposed which can improve the overall seismic behaviour and provide a degree of sustainability.

Keywords: Seismic vulnerability, masonry infills, Banat region, design codes

1 INTRODUCTION

Timisoara is the largest western city from Romania, being situated in Timis County from the Banat region. Serbia and Hungary border this region at the west part, while at the east there are the inferior Carpathian Mountains. Formerly part of the Habsburg Empire, Timisoara has a very large architectural masonry heritage. The first RC structures can be traced as far back as the beginning of the 20th century, but only after World War 2 this material has started to be implemented on a large scale, as a result of increasing the construction rate during the communist regime. In order to accommodate the incoming work-force, in Timisoara there were constructed large numbers of residential and apartment buildings between 1960 and 1975 [1], having simple construction details which were very easy to be implemented. Generally, two structural systems can be distinguished: RC frames with masonry infills, and a mixed system consisting of RC frames with perimeter diaphragms and central tubes. The latter can be characterized by a good rigidity and dissipation capacity of seismic energy, while the RC frames with masonry infill represent the most vulnerable structural system. At the end of the 1970s, a new constructive system was introduced, consisting of large RC panels which were proved to be fast, efficient and having a good seismic behaviour. As a result of the market demand, a lot of these apartment buildings suffered modifications, such as: increasing of living space by removing partition walls, introducing additional loads by constructing an extra level over the

1) Assoc. Prof. Dr. Eng., S.C. H.I.Struct S.R.L. & “Politehnica” University of Timisoara, [email protected]

2) PhD. Stud. Eng., S.C. H.I.Struct S.R.L. & “Politehnica” University of Timisoara, [email protected]

3) Prof. Dr. Eng., S.C. H.I.Struct S.R.L. & “Politehnica” University of Timisoara, [email protected]

4) Dr. Eng., S.C. H.I.Struct S.R.L. & ASA Structural Consultants Thessaloniki, [email protected]

Mosoarca, M.; Petrus, C.; Stoian, V.; Anastasiadis, A.

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roof, leading to an increase of seismic vulnerability. Figure 1 presents the zones in which apartment buildings with masonry infill RC frames were built in Timisoara.

2 SEISMIC ACTIVITY OF BANAT REGION

From seismic point of view, Romania has been classified as a country having a large seismic risk. Considering the energy and number of seismic events, the Banat region is regarded as the second most important seismic zone, being subjected to shallow earthquakes of crustal type. The earthquakes which occur in this area are characterized by a small depth of the seismic source, somewhere between 5 and 15 km and having a reduced surface of the epicentre area where the effects are maximum. A relative small number of pre-shocks, followed by a large number of after-shocks can describe the seismic activity from the Banat region. The main faults have different orientations and depths. The seismic sources of the largest earthquakes from this region are usually located at the intersection of seismic faults or near geological faults of different ages. The spatial scatter of epicentres of the recorded earthquakes from the Banat region indicates a relative large number of zones with a large seismic risk, as it can be seen in Figure 2.

a) b)

Figure 1. City of Timisoara – apartments built between 1960 and 1975 with RC frames and masonry infills [1]

Figure 2. a) location of Banat region, Romania; b) earthquakes from Banat region

Seismic vulnerability of RC frames with masonry infill walls from Timisoara, Banat region, Romania

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Earthquakes of VI and VII intensity on the MSK scale from 1802, 1838, 1940, 1977, 1986 and 1990 recorded in Vrancea region, the most important seismic region from Romania, were also felt in the Banat region. Being close to the border, earthquakes from Serbia can also affect Timis County and the city of Timisoara.

In the Table 1 there are given data regarding strong earthquakes which occurred in the Banat region.

Table 1. Zones with most important earthquakes, intensities and year of occurrence [2]

Epicentre zone Maximum recorder intensity Year

Sanicolaul Mare VII 1879

Barateaz VII 1900

Periam – Varias VII 1859

Jimbolia – Bulgarus VII 1941

Carpinis V 1889

Sanandrei – Hodoni V 1950

Recas V 1896; 1902

Timisoara (Mehala) VII 1879

Sanmihai – Sacalaz VI 1973

Sag – Parta VII 1959

Rudna – Ciacova V 1907

Liebling – Voiteg VII – VIII 1991

Banloc – Ofsenita VII – VIII 1915; 1991

Moldova Noua VIII 1879

Regarding the intensity on MSK scale, Timis County includes zones with the mean recurrence interval for earthquakes of 50 and 100 years, as it can be seen in Figure 3, which can be evaluated in terms of peak ground acceleration, between ag = 0.10g and ag = 0.25g. As a result, a lot of existing buildings were not designed to seismic action, or were designed for much smaller values of ground acceleration.

Regarding the seismic characteristics, one can see that the Banat region earthquakes are similar to those from L’Aquila, Abruzzo, Italy [3]. Both regions have seismic activity characterized by shallow earthquakes of crustal type, in both cases the depth of the seismic source is found between 5 and 15km and after the main shock, a large series of aftershocks occur. The L’Aquila earthquake had a magnitude of 5.8 measured on the Richter scale, while the largest earthquake recorded in Timisoara, had a magnitude of 5.6 on the Richter scale. Large vertical components were recorded for the earthquakes from the Banat region and a similar characteristic was also observed in the case of the earthquakes from Abruzzo region, Italy.

https://www.researchgate.net/publication/233808532_Structural_performance_of_the_historic_and_modern_buildings_of_the_University_of_L'Aquila_during_the_seismic_events_of_April_2009?el=1_x_8&enrichId=rgreq-eafc6ccfdcd38e5713e987436529dfcf-XXX&enrichSource=Y292ZXJQYWdlOzI2NTY1MTU0NDtBUzoxNDE5ODI1MDQ1OTEzNjBAMTQxMDg2MjY3NTk0MA==


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In Table 2 there are presented data regarding the number of dwellings and inhabitants from the most populated cities exposed to the seismic risk from the Banat region.

Table 2. Macro-seismic characteristics of most important cities from the Banat region in the year 2009

City No. of

dwellings (2011 Census)

No. of inhabitants (2011 Census)

Seismic intensity

(MSK) Tc (sec)

ag for

MRI=100

years

Timisoara 124777 304467 VII 0.7 0.20g

Lugoj 14369 37321 VII 0.7 0.15g

Buzias 2273 6504 VII 0.7 0.15g

Deta 2209 5963 VIII 0.7 0.20g

Jimbolia 3615 10048 VII 0.7 0.20g

Sanicolau Mare 4079 11540 VII 0.7 0.20g

Ciacova 1608 5028 VIII 0.7 0.25g

Gataia 1839 5449 VII-VIII 0.7 0.15g

Recas 2679 7782 VII 0.7 0.20g

Several geotechnical studies were made in Timisoara based on which a geotechnical map was established. As it can be seen in Figure 4, several soil types were mapped: clays and silty clays were mainly found on the North part, while on the South part, the dominant soil type was a mixture of clay with sand. Also, the positioning of the seismic fault lines was illustrated on the aforementioned map. One can observe that the fault lines cross the city of Timisoara, as well as the local soil conditions can strongly affect the behaviour of the RC frames with masonry infill walls due to similar periods of vibration, thus increasing the risk of such type of structural systems.

Figure 3. Intensity on MSK scale and mean recurrence interval


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3 EVOLUTION OF ROMANIAN DESIGN CODES

Regarding the evolution of the technical provisions for the seismic design of buildings, we can observe different modifications of the design codes as a result of major earthquake events with large impact on human life.

A first reference regarding this matter is made in a Romanian design guideline “Temporary instructions for preventing the deterioration of buildings due to earthquakes and restoration of the degraded ones” from 1943, as a result of a major earthquake from 1940. A first seismic zoning map was established for Romania in 1952, which included the city of Timisoara in a zone with a very low seismic risk [4]. Based on some studies, a decision was made to include the effect of the seismic action in the design multi-storey buildings from Timisoara.

Later modifications on the design codes from 1963 included Timisoara in a zone with a degree of intensity 6 on the MSK scale, from a total of 12. Despite this fact, between 1963 and 1977, a lot of buildings from Timisoara were designed according to a degree of intensity 7 on the MSK scale [5]. In the design code P13-63, calculation of the seismic action was based on a response spectrum characteristic for crustal earthquakes, having the control period of Tc = 0.3 sec.

As a result of the deep earthquake of Vrancea from 1977, the value of the control period was modified to Tc = 1.5 sec and introduced in the design codes P100-78 and P100-81 [6]. In order to better cover the response of crustal earthquakes from the Banat region, a corner period of Tc = 0.3 and 0.4 sec was implemented. The later P100-92 [7] seismic design code introduced new values for the seismic intensity from Timisoara, considering an increased degree of intensity from 7 to 7.5 on the MSK scale. These modifications further influenced the peak ground acceleration and the dynamic amplification factor β. On the seismic zoning map, Timisoara had a level of seismic intensity of 7.5 with ag = 0.16g, being very close to the intensity zone 8 with ag = 0.20g.

Figure 4. Geotechnical map of Timisoara and location of seismic fault line


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The 2006 seismic design code P100-2006 focused on the level of peak ground acceleration ag [8]. On the zonation map, the Banat region had values ranging from ag = 0.08g to ag = 0.20g. Further modifications of these values were introduced in P100-2013, which increased the peak ground acceleration to values between ag = 0.10g to ag = 0.25g [9].

Table 3 presents the evolution of the seismic zonation map of Romania in terms of seismic risk class on MSK scale and Table 4 presents the zonation maps in terms of peak ground acceleration ag.

Table 3. Seismic zonation maps of Romania – MSK

1952 [4] 1963 [4]

1978 [4] 1992 [7]

Table 4. Seismic zonation maps of Romania – ag

2006 [8] 2013 [9]

Generally, P 100-81 specifies that the RC frame-masonry panel interaction should be based only after an experimental and analytical justification. However, in case of the unfavourable action of the aforementioned influence stated that for buildings with open spaces at the ground level, the efforts


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resulted from the seismic action are multiplied by 1.5 in order to compensate the effect of the reduced ductility [6]. To this end, the beams of the frame also are reinforced at the upper flange due to the truss effect.

In the 1992 Romanian seismic design code, the effect of the masonry infill in RC frame structures is accounted for, by introducing a different coefficient of reduction, q, recognizing the influence of the masonry infill panel on the global ductility of the RC frame. Thus, the value of q ranges between 4 for infill walls considered to be structural elements, ensuring a good collaboration with the RC frame, and 5 when the infill panels are treated as non-structural elements. In order to take into account the interaction between the masonry infill panel and the RC frame and avoid negative effects, fragmentation of the infill panel is proposed or an adjustment of the elastic modulus and resistance of the of the masonry panel [7].

In the later seismic design codes, the masonry infill panel is considered to be a non-structural part of the structure, which can affect the local and overall behaviour of the building. The value of the design seismic force for the non-structural components depends on: the importance of the non-structural component, the peak ground acceleration and characteristics of the response spectrum, the ground motion amplification at the level of the connection with the non-structural component, the dynamic amplification factor of the non-structural component, the reduction of the seismic force effect due to the energy absorption by the non-structural component and its connections with the main structure and on the total weight in exploitation of the non-structural component. In order to design the non-structural components, the method of equivalent forces is used.

Concerning the P100-2006 a reduction of 20%-30% of the basic q-factor is prescribed in function of plane, elevation, or both irregularities. Furthermore, the P 100-2013 is very similar to EN 1998 - Part 1; however it provides, both conceptually as well as analytically, more information regarding how to handle the design of RC frames with masonry infill. In this direction the current Romanian seismic design code provides formulae for evaluating the resistance of the masonry infill panels. Focused on the q-factor it prescribes a 20% reduction as compared with the values provided to the Table 5, treating the interaction phenomena as an unfavourable one. In the case of the column next to an opening in the infill panel some supplementary measures will be taken, Figure 5. a) the design shear force in the columns will be determined considering a calculus model with plastic hinges developed at both extremities of the opening. For high ductility class columns, the capable design moments will be multiplied by 1.3; b) the transversal shear reinforcement will be disposed on the entire length of the opening plus an additional length equal to hc (dimension of the column section) in the contact zone with the masonry; c) if the length on which the column interacts with the masonry column is smaller than 1.5hc, the shear force will be overtaken by inclined reinforcement [10]. Table 5 presents the evolution of the behavior factor q, from current and previous design codes.

Figure 5. Column influenced by the RC frame-infill masonry panel interaction


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Table 5. Evolution of behaviour factor q

-20% -30% P100-81 RC frames with several spans

RC frames with one span 5 4

- -

- -

P100-92 RC frames with structural masonry infills

RC frames with non-structural masonry infills 4 5

- -

- -

P100-2006 HD RC frames with one level

HD RC frames with several levels and one span

HD RC frames with several levels and several spans

MD RC frames with one level

MD RC frames with several levels and one span

MD RC frames with several levels and several spans

5.75 6.25 6.75 4.02 4.37 4.72

4.60 5.00 5.40 3.20 3.50 3.78

4.02 4.37 4.72 2.81 3.06 3.30

P100-2013 HD RC frames with one level

HD RC frames with several levels and one span

HD RC frames with several levels and several spans

MD RC frames with one level

MD RC frames with several levels and one span

MD RC frames with several levels and several spans

LD RC frames

5.75 6.25 6.75 4.02 4.37 4.72

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4.60 5.00 5.40 3.20 3.50 3.78 1.60

- - - - - - -

4 SEISMIC VULNERABILITY OF RC FRAME BUILDINGS WITH MASONRY INFILLS

All buildings situated in a seismic zone are prone to earthquakes and potential damage. The seismic vulnerability of a building represents its ability to be affected by the seismic action, which can be quantified by the damage level of the entire building or of a certain building element.

Considering the large number of apartments built in Timisoara using RC frames with masonry infill panels, it is important to assess their vulnerability correctly. Changes of the structural systems due to architectural needs which require more open space, mixed-use developments tend to affect the building vulnerability to seismic action. A first category of modifications is represented by constructing balconies at the ground level, extending the living space. A second category of interventions is given by the repartitioning of the apartment, by eliminating the wall between the kitchen and the hallway, or by removing the pantry room. Changing the use of the apartment, by transforming it into commercial space represents a third category of intervention, mostly done for the apartments from the ground level. The most common type of intervention is represented by the improvement of the building thermal insulation. This is achieved in two ways: applying polystyrene on the exterior envelope of the structure, minimal intervention, and by constructing roof framing systems which represent an additional level over the usually flat roof.

All the aforementioned structural interventions can increase the seismic vulnerability of the masonry infill RC frame buildings by introducing additional loads, creating weak zones in the structure where the efforts are concentrated. Removing the infill panels in order to increase the living space or make room for storage areas can create a soft-storey effect, which in case of an earthquake can prove to be very dangerous.

The presence of infill panels can increase the rigidity of flexible structures from seismic zones with small values of Tc, which can lead to an increase of the seismic forces over the normal level. Removing such panels can reduce the seismic forces, thus increasing the flexibility of the structure; however such type of structure dispose reduced re-centering capacity. Moreover, structural plan alterations can also introduce torsion effects on the structure, by modifying the centre of rigidity, leading to unfavourable failure mechanisms.


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a) b)

As it can be seen in Figure 6, the most common type of RC frame with masonry infill walls built in Timisoara has a basement, ground floor and four stories. The clear height of a current level is 2.5m, while for the ground floor there are 4.5m, being ideal for commercial space. The building has two spans of 4.25m and three bays of 3.35m and 5.2m respectively. The dimensions of the columns are 40x40cm on which there are supported transversal beams of 40x70cm and longitudinal beams of 30x70cm. The dimensions of the beams are hidden in the thickness of the masonry walls. Masonry walls of 40cm thickness are placed at the exterior and 30cm thickness are used for interior walls together with compartment walls of various thicknesses. No special anchoring of the masonry panels to the RC frame was provided. The RC slabs over each storey have 12cm thickness. Isolated foundations are present under each column.

Structural damages have been recorded in buildings after the earthquake from 1977 from Bucharest. In Banat region there were no damages recorded after this earthquake due to the fact that the distance between the columns is relatively small (3 – 4m) and the thickness of the walls are larger than 30cm.

4.1. Constructional detailing of RC frame with the masonry infill panel

A most common and easily applied construction technique is represented by the interaction between the masonry infill panel and RC columns. The practice used is given by anchor bars starting from the column which are placed within the horizontal mortar joints of the masonry panels, providing a degree of collaboration between the two elements, as it can be seen in Figure 7. The connection between an anchor bar and a RC diaphragm is made with the help of a shot bolt, detail presented in Figure 8. A constructive spacing between these bars of maximum 80cm is usually adopted and the length on which the bars are placed in the mortar joint is usually kept at 50cm. Due to simplicity, no anchor bars are provided between the RC beams and the infill walls, as it can be seen in Figure 9. In the case in which the masonry panel has several openings, additional reinforcement is placed within the

Figure 6. Example of RC frame with masonry infills designed according to P13-63 seismic provisions: a) plan view; b) vertical section


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horizontal mortar joints, between and under the openings, at 60cm spacing as it can be seen in Figure 10. The horizontal joint reinforcement is made with diameter 6mm bars or steel wire mesh.

Figure 7. Example of RC columns and diaphragms connected to masonry walls [11]

Figure 8. Connection detail [11]

Figure 9. Placement of anchor bars between columns and masonry infill panels [11]

Figure 10. Reinforcement in horizontal mortar joint [11]


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4.2. Consolidation measures

A set of consolidation measures were proposed by the student architects from the Faculty of Architecture and Urbanism from Timisoara, and by PhD students from the Civil Engineering Faculty. One of the proposed measures refers to an exterior consolidation with steel profiles of the building frame, method generally used for retrofitting damaged buildings [12]. However, the interventions are based on the concept of sustainability and further defined in terms of lighting, energy consumption and aesthetics. Beside all the energy efficiency, these types of interventions targeting to secure the exterior veneer panels to unfavourable out-of-plane failures in case of seismic action.

Focuses on the proposal illustrated in Figure 11, one can observe that it consists of a 4 layer curtain wall system which will support the existing facade and provide sustainability for the building. A first layer which provides shade during the summer period, together with small reservoirs for cleaning the facade, followed by a second fixing layer made of steel profiles. The third layer consists of a perforated steel profile which protects the building from external actions and acts like a cooling system for the facade during summer time. The fourth layer represents the structure of the curtain wall in order for it to work on two directions while strengthening the existing structure. In order to obtain some financial benefits, some advertisements can be placed on this system.

The proposals of consolidation of RC frames with masonry infill wall attempt to provide a seismic protection of buildings by increasing mainly the rigidity, thus limiting the degradations, displacements and damages as well as trying to avoid the collapse of structural and non-structural elements. Another benefit factor of these proposals is given by the architectural expressivity and ease of implementation of these systems without affecting the ongoing activities inside the building. Further experimental and analytical investigations, towards this direction, should be carried out.

5 CONCLUSIONS

In this paper there was performed an analysis of RC frame buildings with masonry infill walls from an urban area erected until 1977, prior to the implementation of seismic provisions. By studying the large number of these buildings, in function of particular structural systems, the vulnerability should be evaluated and how it affects the overall city resiliency [2]. The buildings from Timisoara, so far did not record damages after Banat region earthquakes, possibly do to closely spaced columns,

Figure 11. Proposal of a four layer curtain wall in order to work together with the building facade


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however the seismic risk is great due to the fault lines that cross the city as well as the increased seismicity of the country.

Moreover, due to the fact that the Banat seismic region is subjected to similar earthquakes as those from Abruzzo, Italy, the existing structures in Timisoara can develop similar brittle failure mechanisms which can lead to important human life and economical loss. Therefore, lessons should be learned and accordingly collected in order to be implemented in future studies.

In order to improve the overall seismic behaviour of the buildings, the concept of sustainable consolidation measures was tried to be introduced, in a way to provide a support for the facade, hence preventing it from overturning and out-of-plane failures. Within the INSYSME research program, corresponding consolidation measures are going to be studied and there will be sought out an efficient way of implementing these measures.

ACKNOWLEDGEMENTS

This project has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No 606229.

REFERENCES [1] Radoslav, R.; Bădescu, Ș.; Branea, A.M.; Danciu, M. & Găman, M.S.: Sistemul urban Timișoara

în epoca modern, Urbanismul. Serie nouă, Nr. 12-13/2012. (in Romanian)

[2] Budău, R. N., Vulnerabilitatea seismică a zonelor de locuit din Timişoara. Clădiri în cadre din beton armat construite până în 1977, Master Thesis, Politehnica University of Timisoara, 2014. (in Romanian)

[3] Ceci A.M., Contento A., Fanale L., Galeota D., Gattulli V., Lepidi M., Potenza F. (2010), Structural performance of the historic and modern buildings of the University of L’Aquila during the seismic events of April 2009, Engineering Structures, Vol. 32, Issue 7, pp. 1899-1924.

[4] http://inforisx.incerc2004.ro/index.htm, website of National Institute for Research and Development in Construction (INCERC).

[5] Anastasescu D., Aspecte ale evoluţiei reglementărilor tehnice privind protecţia antiseismică a construcţiilor din municipiul Timişoara, Buletinul AGIR Nr. 3/2013 iulie-septembrie. (in Romanian)

[6] Seismic design code: Normativ privind proiectarea antiseismică a construcţiilor de locuinţe, social-culturale, agozootehnice şi industriale P.100 - 81, ed. Institutul de Cercetări în Construcţii şi Economia Construcţiilor (INCERC), 1981. (in Romanian)

[7] Seismic design code: Normativ privind proiectarea antiseismică a construcţiilor de locuinţe, social-culturale, agozootehnice şi industriale P.100 - 92, ed. Institutul de Cercetări în Construcţii şi Economia Construcţiilor (INCERC), 1992. (in Romanian)

[8] Seismic design code: Cod de proiectare seismică - Partea I - Prevederi de proiectare pentru clădiri, indicativ P 100-1/2006, ed. Monitorul Oficial al României – Partea I, Nr. 803bis/25.09.2006. (in Romanian)

[9] Seismic design code: Cod de proiectare seismică - Partea I - Prevederi de proiectare pentru clădiri, Revision of P 100-1/2006, 05.2013. (in Romanian)

[10] INSYSME - FP7-SME-2013-2 – Project number: 606229, Delivery report: D3.1, March 2014.

[11] Masonry design code: Normativ privind alcătuirea, calculul şi executarea structurilor din zidărie, indicativ P 2-85, ed. Institutul de Cercetări în Construcţii şi Economia Construcţiilor (INCERC), 1985. (in Romanian)

[12] Daraban R.I., Solutii de consolidare a constructiilor collective in cadre din beton armat si panouri de zidarie, Aesthetics of structures project, 2014 (in Romanian)

http://inforisx.incerc2004.ro/index.htm

http://www.incerc2004.ro/index-e.htm

http://www.incerc2004.ro/index-e.htm




seismic risk of buildings with rc frames and masonry infills from timisoara

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