Comparative analysis of two strengthening solutions for the structural rehabilitation of a historical masonry building

The paper presents a comparative analysis based on the Finite Element Method (FEM) for a masonry building located in Iasi, the North-Eastern side of Romania. This specific region is known to be found in a seismic area having a design peak ground acceleration of 0.25g. The building was assessed as being in the 1st seismic risk class, according to the Romanian norms, with a high probability of collapse in case of future earthquakes. Being a historical monument, the structural strengthening solutions are very limited. The study consists of modal and seismic analyses for the masonry structure. A comparison of the advantages and disadvantages of two approaches of structural rehabilitation solutions is done. The structural strengthening solution “A” consists of the addition of reinforced concrete thin wall elements, while the other - called solution “B” - consists of an interior steel profile skeleton mechanically and chemically connected to the adjacent masonry walls.


Introduction
For centuries, masonry buildings represent an important problem to be investigated by civil engineers.The number of masonry buildings is high due to the abundance and accessible manufacturing process of the main used materials, like bricks and mortars.Since Roman times, we have had bricks and different types of mortars, which made possible the construction of the greatest buildings in history, e.g. the Pantheon, Roman aqueducts, Hagia Sophia a.o.In Europe, including Romania, masonry was the main building material for an important period of time.A lot of public buildings and/or individual homes were made using brick masonry.Considering the limited regulation for these types of buildings in seismic areas, the intermittent occurrence of medium and large earthquakes, at a recurrence interval of 20 or 50 years has produced a lot of damage to structural systems.Some of the buildings with a poor structural system have already collapsed due to the inadequate design of the masonry walls and floors to horizontal actions.The problem derived from poor structural engineering knowledge at that time.In Romania for example, in the last 50 years, seismic action was better approached, so today civil engineers have a higher degree of knowledge about masonry structural behaviour to seismic action.In the last decades, European Norms were adopted and local annexes were elaborated for masonry structures [1,2], seismic actions [3,4] and for the investigation and strengthening processes [5].The preservation/conservation of some historical buildings is important because the city's value is given by the originality and integrity of these old constructions.
To identify the optimal and most appropriate solution for strengthening masonry, appropriate analyses are required.At the same time, numerical analyses of masonry structures are a complex problem, due to the special mechanical characteristics of the masonry.Being a laborious process, as it is presented in literature [6][7][8][9][10][11][12][13] and not accessible to all engineers, the regulations provide analytical instruments utilized to assess the seismic vulnerability of the buildings.Moreover, they give recommendations to improve the structural behaviour of the masonry building including numerous types of strengthening solutions, e.g.reinforced concrete elements, injected mortars, composite materials or adding steel profiles.The engineers have to evaluate and propose a balanced solution based on their experience [14][15][16][17][18], the requirements of the regulations and the client's possibilities.Starting from previous experiences with experimental research [19][20][21][22] and numerical analyses, the authors investigated a historical masonry building located in Iasi, in North -Eastern part of Romania, almost 200 km from the Vrancea's epicentre seismic area -the origin of the most important Romanian earthquakes (figure 1).The building has over 100 years since its construction.In the last decade, it was not used.In the South and North parts, there are other similar buildings, but their period of construction is different.The building from the South part was strengthened 15 years ago with reinforced concrete elements.The studied building named C22 -C24 has multiple visible damages observed on the façades and interior.Therefore, immediate seismic risk assessment is required.

Materials and structural elements integrity
Geotechnical drillings and diggings had to be made with the purpose of gathering all the necessary information regarding site's soil characteristics and the existent foundation system.Following the laboratory determination, the soil stratification: contains two layers: from 0 to -3.20 m -a filling soil layer, and from -3.20m to -10.00m -a yellow dusty clay/silty clay, plastic to consistent, moisture sensitive.The maximum value of the plastic pressure is ppl=245kPa.The C24 building presented in Figures 3 and 4, has an underground level, with a foundation system made of stone masonry and a maximum foundation depth of -3.90 m.The C22 building has no underground level, the existing foundation, in this case, is constructed under the minimum permissible value of approximately 60 cm and is composed of stone masonry and a wood beam.In Table 1 the main structural elements are presented with their characteristics.In the evaluation process of the seismic risk factor for the building, a series of visual and quantitative assessments have been made.The structural conformation of the building has 90 cm thick walls at the underground level and a floor made of masonry vaults, which rest on I-shaped steel profile beams, as presented in figure 5.At the first level, the thickness of the walls is 70 cm and 50 cm, and the above slab is made of wood beams.On the second level, the masonry walls have a thickness of 50 cm with the slab made of wooden beams.The roof of the building is made up entirely of timber beams.

Seismic risk evaluation
As per the Romanian seismic assessment code of existing buildings, [5], the score for each seismic factor was computed considering a "limited knowledge" of the structure.The mechanical properties of the materials correspond to a trust factor of 1.35.The evaluation sought to identify the highly vulnerable areas of the structure and to verify the criteria regarding the requirements for stability, strength, rigidity and ductility.
The value of the degree of fulfilment of the seismic composition conditions, R1 from 1 to 100, is established based on the score assigned to each category of composition conditions.In the case of the analysed structure, the score was 28, corresponding to a high seismic risk noted RsI.In the case of R2, the degree of structural integrity allocation is established based on the score assigned to each category of conditions, regarding the assessment of the degradation state for the structural elements.The factor range is between 1 and 100.The obtained score for the studied building was 30 which corresponds also to a high seismic risk RsI.
The degree of structural seismic response R3 characterises the strength and ductility of the structure, as a whole, and the strength and stability of the non-structural components, in relation to the seismic requirements for the present and future.For the evaluation of the R3 index, a linear numerical model of the structure was analysed, which led to a low value of the factor of 18% from 100%.That means the analysed structure does not have the sufficient structural capacity to withstand a future earthquake and a strengthening intervention is absolutely necessary.In this case, two solutions were proposed and they were compared based on linear numerical analyses.

Numerical modelling of the building
Generally, the linear modelling of masonry structures with 2D shell elements does not provide accurate results along the thickness of the walls but in the case of historical monuments, this information sometimes is necessary.However, when a global analysis is required to be conducted for obtaining a general tendency and stress distribution, with some hypotheses and assumptions, a linear numerical model with shell finite elements for masonry walls can represent a good instrument.In this paper, the structural system of the building, presented in figure 7, was modelled using 2D finite elements in Robot Structural Analysis software [25].First, the actual state of the structure was considered, with wood elements added as linear finite elements, and masonry walls -as 2D shell elements, Figure 7.The load cases and load values selected according to Romanian design codes are presented in Table 2.These were combined also according to CR 0-2012 Romanian code [26].For the seismic analyses, a peak ground acceleration of 0.25g was considered, with a corner period of 0.7s.The structural behaviour factor q was selected to be equal to 1, for a low dissipative masonry structure.Seismic Z direction -Figure 8 and 9 present the schematic arrangement of the 2D finite element models of the strengthened structure.In the first solution called "A" (figure 8), additional reinforced concrete elements were added in the vertical and horizontal direction, with the purpose of achieving a spatial skeleton of RC elements bonded by masonry walls.In case "B" (figure 9), the solution consists in adding an interior steel skeleton made of "U" and "IPE" profiles.The vertical steel profiles are linked to each other on both sides of the walls with threaded metal rods, which assure the tightening and best contact with the existing structure.
In order to achieve a horizontal diaphragm, a composite slab was proposed, made of steel profiles and reinforced concrete.

Results and discussion
From the numerical analyses, the following results were observed and compared: modal analysis results, the pressure on the foundation soil, stress map distribution and deformations.From the modal analyses, the dynamic characteristics are presented in table 3.In figure 17 it is shown a comparative graph between maximum values of normal stress for each analysed scenario.Figure 18 presents a comparative graph between maximum absolute displacements in seismic combination.Comparing the numerical analysis results, it was observed that in both proposed solutions, the maximum normal stress obtained using the seismic combination decreased by 287% compared to the initial state of the existing building, so the strengthened building will be in seismic risk class III.From the economic and technical point of view, solution "A" is more accessible to local entrepreneurs, but it takes more time and more labour.However, it has the advantage of higher fire and corrosion resistance.On the other hand, the second solution with steel profiles needs a smaller amount of time to be completed and it is demountable.Thus, the original structure will be better preserved and will not be affected by additional work.The main disadvantages of the steel profile solution are corrosion risk and relatively small fire resistance.Besides these, workers must have special certified technical skills in order to mount the structure.From the structural point of view, both solutions are appropriate, but future economic analyses are necessary in order to choose the most suitable one for the client.This aspect is part of future work and was not analysed in the current paper.

Figure 1 .Figure 2 .
Figure 1.Location of the building: a) Vrancea seismic area and Iasi location in N-E of Romania; b) Building location in the city centre

Figure 4 .
Figure 4. Front view of the building

Figure 5 . 5 Figure 6 .
Figure 5. a) Underground level of the building; b) upper building masonry wall2.1 Degradations of the buildingIn the visual inspection, numerous types of damages were observed, as follows: water infiltration, vertical and inclined cracks, steel deep corrosion of the beams on top of the underground level, areas with missing plaster, wood elements deformations.Figure6presents different types of damages observed at the first and second floor of the building.

Figure 7 .
Figure 7. 2D shell finite element model of the existent building

Figure 8 . 8 Figure 9 .
Figure 8. 2D finite element models for the strengthened structure using solution A with RC elements

Figures 10 Figure 10 . 9 Figure 11 .Figure 12 . 10 Figure 13 .Figure 14 . 11 Figure 15 .Figure 16 .
Figures 10 present the first vibration mode for each analysed model.A small difference of the fundamental frequency it was observed.Figures 11-16 present results with map stress distribution for each scenario.

Table 2 .
Load cases