SEISMIC ASSESSMENT, REHABILITATION AND RETROFIT OF … 11981 final corr.pdf4Associate Professor,...

SEISMIC ASSESSMENT, REHABILITATION AND RETROFIT OF A

CULTURAL HERITAGE CHURCH THROUGH SIMULATION

Anastasia K. ELEFTHERIADOU1, Sotirios K. MELLIS

2,

Georgios-Alexandros PALASKAS3, Aikaterini D. BALTZOPOULOU

4

ABSTRACT

The current paper presents a procedure for earthquake resistant assessment of Cultural Heritage masonry

structural systems using rehabilitation and retrofitting measures. The Monastery of St. John the Baptist (1506-

1507 A.D.) on the Island of Ioannina was selected as a case study for illustration. The seismic vulnerability was

assessed after the: (a) historical investigation of the building, (b) detailed geometrical relieves, (c) identification

of materials by means of surveys and of the physical and mechanical properties of the stone blocks and mortar,

(e) foundation soil characterization, (f) dynamic identification of the structure by means of a refined Finite

Element model. The FE model was used to assess the safety level of the building according to the provisions of

Eurocode 8 using planar (masonry walls) and linear (steel ties) finite elements. The rehabilitation and retrofitting

measures aimed at eliminating the causes of the damages and securing the structural elements with irreversible

damages. These measures complied with the constraints imposed by the limitations of preservation and

intervention techniques. The foundation of the entire structure was strengthened using root piles for the

minimization of the differential settlements and the foundation of the eastern wall was further strengthened with

a new R/C footing. Steel ties were used for the arches’ cracks and the through cracks in the walls were treated

with a combination of wall stitching, grouting and steel tie bars. Useful conclusions are drawn regarding the

effectiveness of the intervention techniques for the reduction of the vulnerability of the case-study structure,

through the produced results.

Keywords: Rehabilitation; Retrofit; Seismic Assessment; Cultural Heritage Buildings; Structural Damage

1. INTRODUCTION

Cultural heritage assets consist part of the history and the identity of civilizations. It is presently

acknowledged and scientifically proven that seismic hazard has the potential to substantially affect the

lifespan, the serviceability or even destroy European cultural heritage buildings. The preservation and

valorization of historical buildings is a major social and economic concern of modern societies. The

devastating impacts of the seismic events of the last century, especially in the Mediterranean region

with earthquake prone countries, proved that earthquake is a major threaten appraising social and

economic losses. The mitigation of seismic vulnerability and the adequate management of the

provoking risk aim to maintain and strengthen the cultural heritage buildings resilience with safety,

economic, social and historical benefits.

Cultural Heritage (CH) buildings, mainly of unreinforced masonry (URM) structures, was the

prevalent method of construction up to the early 20th century and constitute the historical centers of

many European cities or entire towns. These structures are particularly exposed to seismic risk,

because they were conceived according to empirical rules, considering only gravitational loads. The

destructive impacts of seismic risk are directly and closely related with the vulnerability of the

1Postdoctoral Researcher, Department of Civil Engineering, D.U.Th., Xanthi, Greece, [email protected]

2PhD Candidate, Department of Architecture, D.U.Th., Xanthi, Greece, [email protected]

3MSc Civil Engineer, Thessaloniki, Greece, [email protected]

4Associate Professor, Department of Architecture, D.U.Th., Xanthi, Greece, [email protected]

mailto:[email protected]:[email protected]:[email protected]:[email protected]

2

building exposure. The United Nations Educational, Scientific and Cultural Organization (UNESCO)

encourages within the Organization of Protection of the World Cultural and Natural Heritage the

identification, protection and preservation of cultural heritage around the world considered to be of

outstanding value to humanity: “Heritage is our legacy from the past, what we live with today, and

what we pass on to future generations. Our cultural heritage is irreplaceable source of life and

inspiration”. Many European towns belong in the UNESCO World Heritage Sites. Additionally, the

EU Internal Security Strategy aim at increasing Europe’s resilience to crises and disasters.

The current research presents a procedure for earthquake resistant assessment of Cultural Heritage

masonry structural systems using rehabilitation and retrofitting measures. The Monastery of St. John

the Baptist (1506-1507 A.D.) on the Island of Ioannina was selected as a case study for illustration.

The seismic vulnerability was assessed after the: (a) historical investigation of the building, (b)

detailed geometrical relieves, (c) identification of materials by means of surveys and of the physical

and mechanical properties of the stone blocks and mortar, (e) foundation soil characterization, (f)

dynamic identification of the structure by means of a refined Finite Element model. The FE model

was used to assess the safety level of the building according to the provisions of Eurocode 8 using

planar (masonry walls) and linear (steel ties) finite elements. The rehabilitation and retrofitting

measures aimed at eliminating the causes of the damages and securing the structural elements with

irreversible damages.

2. STRUCTURAL MODELING AND ASSESSMENT

2.1 General Description of the Cultural Heritage Structure

The Monastery of St. John the Baptist (figure 1) on the Island of Ioannina was built in 1506-1507 A.D.

by Nektarios and Theofanis Apsarades brothers. The only surviving part of the monastery is the main

church (“Katholikon”), for the construction of which removal of rocks of the nearby cave and

backfilling of the lake was used. The main church is a rectangular building with stone masonry walls

and approximate inner dimensions 6,00m x 3,60-5,60m x 6,75m (length x width x maximum height).

Its main morphological characteristics are the cross-shaped roof and the two semi-hexagonal niches at

the north and south walls. Due to the lower foundation level of the east wall, an underground room,

the so called crypt, was built. The crypt is located below the altar, has inner dimensions 2,95m x

1,85m x 1,70m and is accessible from the altar through a trap door. Extending the nave to the west,

there is a narthex, an also rectangular stone masonry building. Although narthex and nave were

contemporarily built, they are not statically connected. The masonry walls have thickness 35 to 80 cm,

founded on rock and partially on embankments. The roof was of stone plates.

Figure 1. The Monastery of St. John the Baptist (1506-1507 A.D.) before the rehabilitation,

northern and eastern view.

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2.2 Recorded Damages

The structural system of the Katholicon, located in the cold and humid climate of Ioannina during the

centuries of its service life, along with the disintegration of the materials due to ageing, has also

suffered from damages due to additional causes, such as differential settlements and earthquakes. The

most recent documented damages refer to the earthquakes that took place in 1967,1969 and 1984.

The damage pathology of the church focused on:

1) Severe drifts from the vertical axe, especially noticed in the eastern part of the building caused from the ground subsidence due to the existence of differential foundation in the specific part

of the church. In particular, on one hand there is different level in foundation and on the other

hand there are different soil conditions (from rock changes in embankment fills) (figure 2a).

2) Severe cracking in the inner surface of the altar arch due to absolute lack of tensile components (figure 2b).

3) Severe cracking in the central (lateral) arch and vertical recesses, probably owed to the ground subsidence due to the differential soil conditions (figures 3,4).

4) Bending of the structural wooden beams of the ground floor of shrine (roof of the crypt). The, so far, rehabilitation attempts, according to documented reports dating back from 1960, were

based on obsolete techniques with doubtful results.

Figure 2 a) Severe drifts from the vertical axe in the eastern part of the building

b) Severe cracking in the inner surface of the altar arch

Figure 3. Severe cracking in the central (lateral) arch

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Figure 4. Severe cracking in the vertical recesses

2.3Structural Analysis

In order to determine the static and dynamic behavior of the structure before and after the

interventions, elastic static and dynamic analyses of spatial finite element (FE) models were carried

out. The structure was discretized and analyzed using planar (masonry walls) and linear (steel ties)

finite elements and the F.E. programme SAP 2000 v.16. In order to obtain good result accuracy and

geometry description, an appropriate F.E. mesh refinement was made. The maximum size of the

planar F.E. used was about 0,45x0,45m, while in specific places of the model, where further detail was

needed (openings, connection of arches), denser f.e. mesh was chosen (figure 5).

The seismic actions were considered according to the design response spectrum of EN 1998-1 and the

Greek National Annex.

Two model cases were applied for the part of the main church, except for the narthex which had no

damages and was not statically connected with the nave:

Model case 1 [all_loadcases]: It represents the entire structural model of the church. All foundation joints were considered hinged.

Model case 2 [steel_ties]: This analysis considered only the vertical loads and was used for the design of the steel ties. The hinges at the joints of the foundation in the area of the

crypt were replaced by springs, modeling the soil factor equal to Κ=5000 kN/m3. This way

the non-likely possibility of the subfoundation failure is investigated, which results in

larger loads on the tension components.

Figure 5. Finite element discretisation : a), b) Northwest and Southwest elevations of the structure

c) Position and discretisation of steel ties.

a) b) c)

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As no special code exists for cultural heritage structures, the structural design was based on analytical

methods with the application of the contemporary Eurocodes. Safety factors in analysis and design,

and mean values instead of characteristic ones for material properties were used, in order to decrease

the uncertainties of the behavior of the structure.

For the church modeling the following assumptions were made:

o Planar finite elements of thick shell for the stone walls; o Linear finite elements for the steel ties; o Uniformly distributed surface load for the snow action and cladding; o Seismic loads are applied through the modal response spectrum analysis with the application

of the EC8 design spectrum. The results needed for the design of the rehabilitation and

retrofitting measures, are extreme joint reactions (design of the subfoundation) and extreme

axial forces (design of steel ties) and can be calculated using response spectrum analysis as a

single parameter problem.

o After performing a modal analysis 24 modes of vibration have been taken into account representing more than the 90% of the structure’s mass in each direction of vibration.

o From experience and field data, material mechanical properties of the stone blocks and mortar were determined , after sampling, at the Building Materials Laboratory of Aristotle University

of Thessaloniki as follows:

- Compressive strength of masonry unit (stone brick): fb = 38,00 ΜPa; - Masonry Unit Group: Not classified; - Masonry Unit Category (in terms of manufacturing control): Category II (natural stone

units);

- Class of execution control: 5 (ELOT EN 1996-1-1:2005); - Constant K=0,50; - Compressive strength of mortar: fm = 1,00 ΜPa; - Safety factor γm = 2,70; - Characteristic compressive strength of masonry: fwk = K∙fb

0,7∙fm

0,3 = 6,38MPa;

- Design value of masonry compressive strength: fwd = fwk/γm =2,36MPa; o For the new materials applied in the interventions:

- Timber strength class: C24 (EN 338:2003); - Concrete strength class C20/25: fc=20N/mm

2 uniaxial strength of concrete in

compression;

- Reinforcing Steel B500c: Yield stress fy=500 MPa for the reinforcement used in general; - Steel ties: Type MK B500 (yield stress fy=500 MPa); - Stitching staples from stainless steel; - Compressive strength of new mortar fm = 4,00 ΜPa; - Drilled holes on masonry units holes are filled with epoxy resin-based bonding system.

o Rehabilitation and retrofit of the structure included both grouting and deep jointing. The increase of the compressive strength of the stone wall due to grouting, depends on the amount

of the grout the stone wall will absorb, which makes the calculation of the increase highly

inaccurate. For this reason, the increase of the compressive strength of the stone wall was

calculated considering only the impact of deep jointing, using the formula:

- fwc = 1/γRd ∙ ζ ∙ fwc,0 , where: - 1/γRd = 0,80. - fwc,0 = Characteristic compressive strength of masonry (before jointing) = 6,38 MPa. - ζ = 1 + ω ∙ ((volume of new mortar) / (Total mortar volume)), where

- ω = 4-8 for stone masonry and 1-2 for clay masonry.

- Assuming ω=6, mean wall thickness 50 cm and jointing 2.5cm deep at both surfaces:

ζ=1+6∙ (2∙2,5/50) = 1,60, finally

- fwc = 0,80 ∙ 1,60 ∙ 6,38 = 8,17 MPa. - The modulus of elasticity of the masonry after the rehabilitation measures is:

E=1000∙ fwc = 8,17 Gpa (ELOT EN 1996-1-1:2006 §3.7.2). This value was used in the computer design models

- Poisson’s ratio : ν=0,15.

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For the applied loads on the structure, the following assumptions were made:

- Reinforced concrete: 25,00 kN/m³. - Structural steel: 78,50 kN/m³. - Natural stones masonry: 26,00 kN/m³. - Structural timber: 4,20 kN/m³. - Roof permanent loads (gravel and stone plates): 6,00 kN/m². - Snow loads (ELOT EN 1991-1-3:2003 and N.A.) :

- Characteristic ground snow load at sea level: sk,0 =0,80 kN/m². - Shape coefficient μ1=0,80. - Altitude 500m and snow zone B. - Characteristic ground snow load: sk=0,80∙0,80∙ [1+(500/917)

2]=0,83 kN/m

2. For

safety and simplicity is taken sk=1,00 kN/m2.

- Altar variable loads: q=5,00 kN/m²

3. REHABILITATION AND RETROFIT

3.1 General description – philosophy of interventions

Main goals of the rehabilitation and retrofitting measures were, to eliminate the causes of the damages

and subsequently secure the structural elements with irreversible damages, upgrading the structural

performance, though maintaining the architectural character of the monument. Taken these into

consideration the following measures were taken (figure 6):

For the minimization of the differential settlements, the foundation of the eastern part of the structure (altar) was strengthened with a new reinforced concrete footing combined with root

piles.

For the arches’ cracks five steel ties were placed in carefully selected places. The through cracks in the walls were treated with a combination of wall stitching, grouting

and steel tie bars.

The old wooden floor of the altar was totally reconstructed. These measures had to comply with the constraints imposed by the objectives of preservation and

reversibility of interventions. In order to achieve this:

No visible concrete was used. The anchor plates of the steel ties were completely covered with stone blocks, making them

not visible.

The steel ties were carefully placed into the mural-free space between successive murals, or over the existing wooden ties.

The existing masonry was carefully drilled using waterjet cutting, not impact drills, so as to avoid any disturbance of the masonry or murals.

Figure 6. Plan design with the rehabilitation and retrofit measures and techniques.

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3.2 Analytical description of interventions

1) Subfoundation and root piles

The existing foundation of the eastern part of the structure was further strengthened by a new RC

footing in combination with root piles (figure 7):

- The eastern walls, which were initially founded on embankment, were excavated inside of the church. Then root piles were constructed in order to fix the existing stone walls on the bedrock.

The root piles had 4cm diameter, were 3m long and reinforced with one Φ20 bar B500c. They

were constructed as vertical as possible, so as to minimize (to eliminate – if possible) the

resulting, due to eccentricity, bending moment. In order to achieve the best possible connection

to the bedrock, grouting of the root piles was very meticulously carried out. The cement/sand

ratio of the grout used was 700kg cement/m3 sand.

- The relatively loose ground between the existing stone foundation and the bedrock was removed and replaced by a RC footing at least 1m wide. The excavation under the existing foundation

was executed in parts (about 1m each), in order retaining works to be avoided.

- The new footing was decided to be reinforced (reinforcement ratio was 0,25% for both top and bottom reinforcements), so as to be able to work independently, in the unlikely case that the root

piles would not be able to cooperate.

- No special means were needed for the construction of the root piles or the new RC footing.

2) Steel ties (at arches)

In general, steel ties were placed at the lowermost point of the arches, in order to prevent crack

widening. Five (5) steel ties Φ20 type MK B500 were used in total, as follows:

- At the eastern arch, into the stripe between the murals, two (2) steel ties were placed over the existing wooden ones, which would be restored.

- At the central (lateral) arch two (2) new steel ties were placed, into the mural-free space between successive murals.

- At the western arch one (1) steel tie was placed, into the mural-free space between successive murals.

Figure 7. Detail of the foundation strengthening.

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3) Wall stitching

This method was applied for the treatment of the wide through cracks of the northern wall and vertical

recesses. The procedure followed is shown in figure 8 and is described below:

- The existing plaster was removed creating a zone 50cm on either side, and along the entire length of the crack.

- In this zone, the stone blocks (and the mortar between them) around the crack, at the outer face of the wall were removed creating a recess. The distance between the recesses was 80cm along

the crack. The removal of the stone blocks was executed carefully and gradually (one by one).

- At the inner layer of the wall, in the created recesses, holes Φ12 and 8cm deep were drilled on either side of the crack. Extreme caution was taken during the drilling of the holes for the

protection of the church murals.

- The drilled holes were filled with epoxy resin-based bonding system. Then stainless steel stitching staples (Π-shaped reinforcements) Φ8 were placed into the drilled holes. These

reinforcements varied in length, location and orientation, so as to uniformly distribute the

developing forces on the wall.

- The created, after the removal of the stone blocks, recess, was thoroughly cleaned and wetted up to saturation.

- Injection and vent tubes were then placed at the perimeter of the recesses. The tubes reached 1/2 and 1/3 of the total wall thickness respectively.

- The new stone block was properly placed to bridge the crack. Finally, a rectangular 3mm-thick stainless steel plate 80x15cm (PL 800x150x3) was placed into the joint, just before the

hardening of the grout, in order to further reinforce the joints.

- The same procedure was repeated for every stitching point (every 80cm along the crack). - Joints and cracks were sealed before grouting. The composition of the new grout was

determined after thorough analysis of the existing ones, in order to ensure total collaboration

between old and new materials. The analysis of the existing grouts was made by the Laboratory

of “Ancient Materials” of EKEFE “DEMOCRITOS”. The new grout consisted of hydraulic

lime (no cement was used), and had compressive strength fm=4,00MPa.

Figure 8. a) Stone wall before interventions, b) Wall-stitching procedure – Phase 1, c) Wall –stitching procedure

– Phase 2

a)

b)

c)

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4) Repair of the detached southwest corner

Due to the murals at the southwest corner of the church, the repair of the detached corner inside from

the church was ruled out. Taken this constraint into consideration, the only reliable way of repairing

the detached corner was the use of steel ties, applied from the outside face of the wall. Thus, either

side of the detached corner was carefully drilled (using waterjet cutting, not impact drills). Afterwards,

steel ties type M12 B500C immersed in resin type Hilti HIT-HY 70 were placed into the drilled holes.

The steel ties were properly anchored on the outer wall face via steel anchor plates, placed on a 2mm

thick layer of nonshrink cement grouting. The previous procedure is presented in figure 9.

5) Reconstruction of altar wooden floor

The new altar floor consisted of parallel timber beams of rectangular cross section 85x175mm placed

every 0,50m (figure 10). The timber strength class was C24 (pine or spruce), the most commonly used

timber class in Greece.

Figure 9. Repair of the detached corner. Figure 10. Detail of altar new wooden floor.

3.3 Design of interventions

1) Root piles

The total bearing capacity of one root pile is assumed to be equal to its friction capacity (the piles’ tip

capacity is neglected):

QRk = π∙Φ∙L∙qsk , where

- Diameter of pile : Φ= 4,0 cm; - Length of pile : L= 3,00 m and - Friction of rock according to DIN 1054: qsk = 500 kN/m

2

So, QRk = π∙0,04∙3,00∙500 = 188,50 kN.

The design value of the root pile bearing capacity is taken by dividing QRk with the safety factor

γRd=2,00: QRd = QRk / 2,00 = 94,25 kN.

Figure 11 schematically shows the maximum reactions measured in KN at the nodes of the altar

foundation. The distance between successive nodes, according to the structure analysis model, was

about 40cm. Therefore, root piles were in general placed every 40cm, except for the eastern corners of

the altar (shaded area), where the required distance between successive root piles was 25cm.

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Figure 11: Maximum base reactions at the nodes of the altar foundation.

2) Steel ties and anchoring plates

Figure 12: Design of steel ties anchor plates

In reference to figure 12:

S = masonry wall thickness = 0.50 m (at the anchoring places of the steel ties)

Square anchor plates 15x15cm and 10mm thick are chosen, therefore:

l = length of anchor plate = 0.15 m.

s = width of anchor plate = 0.15 m.

fvko = 0.10 MPa (assumed for natural stones masonry with mortar Μ1-Μ2).

The maximum tensile force of the steel ties (derived from the analysis of the structure) is Nsd = 9,56

kN.

The control was carried out for the least favorable case. At the least favorable case, all four sides of

the truncated pyramid base shown in figure 12 b), coincide with stone block joints. The bearing

capacity of the joint against slip of the stone block, under the force transmitted on it by the steel ties

anchor plate was verified.

ΝRd,slip = fvko∙(4S + 2l + 2s)∙S∙ = 0,10∙1000∙ (4∙0.50 + 2∙0.15 +2∙0.15) ∙0.50∙1.414 = 183.85 kN >>

Nsd = 9.56 kN.

Therefore, steel ties Φ20 of the MK B500 system (yield stress 500 MPa), type MEKANO with tensile

strength 175,0kN, were chosen.

3-4) Wall stitching and repair of the detached southwest corner

The methodology is analytically described in §3.2 of this paper.

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5) Reconstruction of altar wooden floor

The design of the new altar floor was carried out according to the provisions of EN 1995-1-1:204/NA

and EN 1991-1-1:2002. The timber beams were checked at ultimate and serviceability limit states for

the combinations stated in EN 1990 (formulae 6.10 and 6.14, 6.15 respectively). Finally, a vibration

check for the floor was carried out according to EN 1995-1-1 (§7.5, §7.6).

4. CONCLUSIONS

The “Katholicon” of St. John the Baptist Monastery, an early 16th century stone masonry structure,

over the centuries of its service life, has suffered damages due to various causes, such as disintegration

of materials due to ageing, differential settlements and earthquakes. Due to the poor foundation

conditions of the eastern part of the structure, the differential settlements were progressively growing,

widening the already existing cracks and undermining the stability of the entire structure. Main goals

of the rehabilitation and retrofitting measures were to eliminate the cause of the problems and

subsequently to repair the existing damages improving the structural performance, though maintaining

the architectural character of the monument. The selected methods of interventions were completely

consistent with these requirements. The applied rehabilitation and retrofitting measures included: 1.

Subfoundation with a new reinforced concrete footing combined with root piles, which fixed the

unstable eastern foundation into the bedrock, and stopped further settlements; 2. Steel ties, which

secured the arches, preventing cracks from further widening; 3. Wall stitching, which successfully

treated the existing gap through cracks at the northern wall and vertical recesses, restoring the

cooperation between stone blocks; 4. The detached southwest corner was fully repaired by steel ties

into the masonry; 5. Last, the construction of a new timber floor made the church again totally

operational. These intervention measures not only faced the existing problems and damages, but they

significantly upgraded the structure’s seismic behavior. Furthermore, it must be mentioned that the

selected rehabilitation and retrofitting measures were successfully tested and proved effective during

the recent earthquakes of 2017 that occurred in the region of Ioannina, which represent a real

experiment in scale 1:1.

5. REFERENCES

Asteris P.G., Chronopoulos M.P., Chrysostomou C.Z., Varum H., Plevris V., Kyriakides N., Silva V. (2014).

Seismic vulnerability assessment of historical masonry structural systems. Engineering Structures 62–63: 118-

134.

Ceroni F., Pecce M., Sica S., Garofano A. (2012). Assessment of Seismic Vulnerability of a Historical Masonry

Building. Buildings, 2: 332-358; doi:10.3390/buildings2030332.

Eleftheriadou A.K., Karabinis A.I., (2006). Analytical Estimation of Vulnerability Functions for RC Structures.

Proceedings of the 1st European Conference on Earthquake Engineering and Seismology, September, Geneva,

Switzerland.

EN 1991-1-1:2002/ ΝΑ: Eurocode 1 - Actions on structures - General actions. - Densities, self-weight, imposed

loads for buildings.

EN 1991-1-3:2003/ ΝΑ : Eurocode. 1 - Actions on structures - General actions - Snow loads.

EN 1998-1:2005/ ΝΑ : : Design of structures for earthquake resistance - General rules, seismic actions and rules

for buildings.

EN 1995-1-1:2004/ ΝΑ: Eurocode 5 - Design of timber structures - General - Common rules and rules for

buildings.

EN 1993-1-8:2005/ ΝΑ: Eurocode 3 - Design of steel structures -. Design of joints.

EN 1996-1-1:2005/ ΝΑ: Design of masonry structures - General rules for reinforced and unreinforced

masonry structures.

ELOT EN 1991-1-1:2002: National Annex.


ELOT EN 1998-1:2005: National Annex.




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ΕLΟΤ ΤΠ 1501-14-02-05-02: 2009: Repair of wide masonry cracks with wall stitching.

ΕLΟΤ ΤΠ 1501-14-02-04-00: 2009: Masonry retrofitting with grouting.

Karantoni Fillitsa, Tsionis Georgios, Lyrantzaki Foteini and Fardis Michael N. (2014). Seismic fragility of

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