Case study on reinforced concrete frame structure adaptability

The limited space to build from scratch significantly increased the need to rehabilitate for the re-functionalization of different buildings. The reinforced concrete frame structure is easily adaptable and has several advantages in this direction. When changing the function of a building an evaluation of the existing damages is done as well as a numerical simulation with the new loads. The article presents the case of changing a neighbourhood clinic into a kindergarten. The seismic risk coefficients are computed (R1, R2 and R3) in order to establish the seismic risk class for the existing building and also an evaluation of the damage state. A comparison between the seismic force computed when the building was designed and according to current codes is performed. In order to identify the failure mechanism pushover analysis was performed in both X and Y direction.


Introduction
The constant change in modern society and the limited space to build from scratch significantly increased the interest to reuse old buildings, no matter their original destination.Instead of tearing down a structure, some beneficiaries are opened to restore them to a purpose that is of interest today.This trend contributes beside the economic development, to urban revitalization by giving abandoned buildings a new life.Such an example is the transformation of La Fabrica, an out-of-use monument to Catalonia's industrial past, into Ricardo Bofill home and office [13].The building was initially constructed during Catalonia's first golden period of industrialisation in the early 1920s, then new structures and extensions were added gradually when production demanded, finally serving as a repair shop during the World War I. Contributing to this strand of the architectural canon was of great interest to Bofill, with his refreshing take on urbanism [Figure 1].In the underground there is a gallery, meanwhile at the ground floor a huge conference and exhibition room, with 10-metre-high ceilings which was initially the factory's processing area.The raw cement walls which retain the memories of the previous use of the building were kept unchanged [14].In order to be able to do that professionals should reinvent themselves and acquire new skills for the reconfiguration of old buildings.Buildings are designed to withstand the time challenge.This is the main reason why, sometimes during their lifecycle functional change is needed in order to adapt to the community's needs or the beneficiary's view.In order to support the community, special funding are offered by the Government and by the European Committee for the rehabilitation of existing infrastructure.An example of such funding is the National Recovery and Resilience Plan of Romania, which is a strategic document regarding the investment priorities and reforms needed for recovery and sustainable growth, in alignment with the green and digital transitions of the European Commission.In order to access these funds a particular documentation needs to be submitted which follows national and international regulation.In the Romanian legislation, among the essential documents is the expertise report which is done by an authorised expert.The report gives information regarding the qualitative and quantitative state of the construction and, if it is the case, some recommendations regarding the structural rehabilitation measures that need to be taken into consideration to strengthen the structural system.This report is done following the P100 part III, from 2019 regulations [4].The article presents the Romanian methodology for an expertise report which studies the damaged state of a neighbourhood clinic in order to convert it into a kindergarten hence in the area several private medical offices were opened in the last years and the building has not been used for some time.On the other hand, the need for a new kindergarten in the area is stringent since the city of Iasi has extended a lot in that direction, in the Copou hill direction and further to Breazu and Rediu villages.In the second part of the paper, a nonlinear static analysis is performed for one of the buildings in order to compare the results with those obtained through the simplified computation method.

2.
Romanian methodology to evaluate existing buildings According to P100-3/2019 there are three methods to assess the building state, differentiated by the detailing degree of the computation methods and the considered checking: level 1 (low complexity), level 2 (medium complexity) and level 3 (high complexity) [4].The corresponding method is chosen based on the following criteria: technical requirement from the design and execution period of the project, building complexity from structural point of view (openings, number of floors, horizontal and vertical regularities), available data which gives the knowledge level, the function of the building, its importance and value, seismic hazard conditions according to P100-1/2013, structural system, fundamental requirements for the buildings, the purpose of the technical report and other relevant conditions.Level 2 methodology is most frequently used for current buildings.It contains a qualitative assessment and a quantitative one.The qualitative evaluation implies to check a list of elements referring to the structural layout based on the used materials, meanwhile the quantitative one is based on an elastic structural computation which uses specific coefficients for each material.The earthquake effect is approximated by a set of conventional forces which are then applied on the structure.The lateral forces are chosen so that the obtained displacements are obtained through the linear computation to simulate the displacements imposed to the structure by the seismic load.The checking relation depends on the failure mode (ductile or brittle) of the structural elements to different internal forces.The structural computation can be done considering one of the methods presented in P100-1/2013 meaning the equivalent static seismic force or the modal computation with response spectrum [3].The qualitative assessment measures the extent to which the rules for general compliance of structures and detailing of structural and non-structural elements are respected in the analysed constructions.The nature of the structural deficiencies and their extent represent essential criteria for the decision of structural intervention and rehabilitation solutions.The following aspects are considered: the loading path towards the foundation, building redundancy check, construction layout, the interaction with other constructions and elements.Each criteria is based on the scientist's experience and the visual inspection points, which are in the end summed up.Depending on the final score, the R1 (risk coefficient) is defined for the building which includes it in one of the four risk classes.R1 represents the degree of fulfilment of the conditions of structural compliance, of the composition of the structural elements and of the constructive rules for structures that withstand effect of the seismic action.R2 risk coefficient represents the degree of structural damage that expresses the proportion of structural damages produced by seismic action and other causes.The damaged state is evaluated in a table, based on the visual inspection considering the following criteria: the earthquake action effect, the vertical actions, deformations effects, poor execution, and environment effect.In the end a total score between 0 and 100 is obtained which includes the building in one of the four seismic risk classes [3,4].The degree of seismic structural insurance represents the ratio between the capacity and the seismic structural requirement, expressed in terms of strength for level 1 and 2 methodologies or in terms of displacements for level 3 methodology.R3 risk coefficient is determined for the ultimate limit states.

3.
Building description, initial results and discussions The analysed structure was designed initially to be a day nursery, which was finished in 1971.The building was designed according to P13-63 seismic regulation.In 1983 it was converted into a neighbourhood clinic, which was closed in 2013.Between 2013 and 2022 the building was closed and abandoned.For the initial space conversion, no structural rehabilitation was performed on the building, only lightweight modification of the interior space.The benefits of a reinforced concrete frame structure were extensively used in this case.In 2020, the building was studied to establish the damage degree and the rehabilitation measures necessary in order to convert it into a kindergarten.The building is a complex one, made of 2 bodies, A and B, with a technical basement, ground floor and one floor.The structural system is represented by a reinforced concrete frame structure with precast reinforced concrete elements as exterior envelope.The dimensions of the main structural elements are: 25x45 cmxcm beams, 35x45 cmxcm columns and slabs of 13 cm thickness.In case of the seismic force, the literature review showed that the seismic norms had improved along the building lifecycle [5,7].If initially the building was designed after a code which considered the seismic force a percentage from the building weight, the today evaluation is performed according to P100-2013, which is based on Eurocode 8, improved by the recent research results in the field [1,3,6].Table 1 presents synthetically the coefficients considered in both codes for the seismic load assessment (P13/63 and P100/2013) [2, 3,4].It can be noticed that some coefficients had been introduced, some of them had been replaced and some values had been changed.If a comparison is made between these coefficients, it can be seen that with respect to the P13-63, the current code is experiencing a reduction of the seismic force with only 4.24%.These results are consistent with the findings on site regarding the limited damage suffered by the building along the years.After the visual inspection only, minor damages were observed.Among these were cracks of the exterior and interior plaster, the woodwork of the openings and the terrace finishes which lead to water infiltration in the top slab, Figure 2. Significant problems were identified at the sidewalk which also led to infiltrations to the foundation.Based on the visual inspection R1 and R2 coefficients were computed.When checking the structural integrity of building B, a R1 coefficient of 85 points was obtained.In assessing this factor the following parameters were considered: conditions referring to the structural configuration (the loads path, the presence of soft stories, irregularities in plan and elevation, the torsional effect, the foundation layout), conditions referring to the interactions with other structures (distances from other buildings, the presence of short columns, the non-structural elements are adequately connected to the structural system), conditions referring to the elements formation and conditions referring to the slabs.According to the received grading, the building is considered to be in IIIrd seismic risk class.In case of R2 coefficient, when the different possible structural and non-structural damages were assessed and based on the visual inspection a value of 87 was obtained.Considering that the maximum value is 100, it is noticed that, from this point of view the structure is also included in IIIrd seismic risk class.

Figure 2. Initial building state
The structure was introduced in SAP2000 computer software and the static analysis was performed.In the numerical simulation a characteristic snow load of 2.5kN/m 2 was considered.The self-weight of the structure was computed automatically by the software, meanwhile the considered dead load for the current floor was of 1.5 kN/m 2 and 8.5 kN/m 2 for the top floor, and the live load was of 3 kN/m 2 for the current floor and 1.5 kN/m 2 for the top floor.The results were represented by the internal forces magnitudes.These values were used to compute the R3.A value of 67 was obtained, which is at the lower level of IIIrd seismic class.
According to P100-2019, the IIIrd seismic risk class contains structures that withstand seismic load and can present structural damage, without affecting the structural safety, but the non-structural damages may be significant.
The modal analysis was performed in order to assess the deformed shapes and the modal participation factor.Figure 3 presents the structural deformation for the first two eigen modes.It can be noticed that for eigen mode 1 the deformation is represented by translation on the short direction, in eigen mode 1 torsion appears, meanwhile in eigen mode 3 translation on the long direction was obtained.
Considering this low value of R3 coefficient and the fact that the building will be rehabilitated in order to build a kindergarten, a pushover analysis was performed in SAP2000 software in order to assess the most vulnerable elements and the capacity curves on X and Y direction of the analysed building.

Nonlinear static analysis, results and discussions
The pushover analysis is used in the evaluation of the seismic performance of existing structures, but also in the design stage.It is considered an appropriate method for the Performance based design of structures.The method involves the application of a series of inelastic static analyses on a structure, using a loading pattern defined on the basis of the first eigen mode or other shapes of loading or in the terms of accelerations.The loading pattern remains unchanged in the analysis process, but the intensity increases until the structure reaches the maximum displacement imposed by the designer [8][9][10].The results of the pushover analysis are represented by the failure mechanism, respectively the evaluation of the structure's capacity, plotting the variation of the displacement from the top of the structure with respect to the shear force at the base of the structure [11][12].The capacity curve for the analysed building, on X and Y direction are presented in Figure 4.It can be observed that the maximum efforts are withstood in X direction due to both the geometric configuration of the structure and the positioning of the cross-sections.A maximum force of around 6000kN can act on the structure, leading to a displacement of 12mm in the longitudinal direction of the building, causing damage in the ground floor columns.As expected, in the short direction -Y, the building is considered to be more vulnerable, as the maximum displacement is only 0.5mm.In case of the failure mechanism, it was observed that for both load cases -pushover in X and Y direction, the first cracks are developing in the beams from the first floor, and then continue to the rest of the beams.As the force increases, as well as the displacement of the structure the cracks for the immediate occupancy level are obtained at the base of the ground floor columns, Figure 5.The failure mechanism that was observed is in accordance with the EC8 and P100/2013 concept which recommend to design soft beams-strong columns.

Conclusions
Considering the changes, a society undergoes in order to adapt to current trends, the built environment should be designed in order to offer the possibility for the buildings to take different functionalities, depending on the necessities.Moreover, the limited space to build from scratch in major cities and the restrictions from the local authorities regarding new buildings, as well as the available funding possibilities for the rehabilitation of existing buildings encourages the revitalisation of existing ones which are currently left abandoned or are not used.In the case study presented it can be seen that the reinforced concrete frame structures can be considered to be very easily adaptable no matter what functionality is considered.The large open space between the structural elements offers a wide space which can be arranged easily in any shape with lightweight materials, as is the autoclaved concrete.In the present study it was also observed that, even though the structure was built according to a 1963 regulation, the restrictions applied then are similar to those from current codes which are in accordance to the European legislation, Eurocode 8.A decrease of the seismic force with 4.24% was obtained from the computation.Due to this similarity, and to the fact that the building was well maintained, no structural rehabilitation was required.The works involved for the re-functionalization of the space referred to re-plastering, changing the woodworks and some changes in the layout of the walls adequate for a kindergarten, Figure 6.

Figure 1a .
Figure 1a.Old photo with the La Fabrica

Figure 5 .
Figure 5. Failure mechanism from pushover analysis in X and Y direction respectively

Table 1 .
Coefficients used for the seismic load assessment.