Earthquake behavior of flat slab structures, particularities and opportunities for use in the building stock of the Chisinau municipality

The Republic of Moldova is characteristic for active seismic phenomena and there is always a direct danger of strong earthquakes. The building stock of the country has a complex character, this aspect is manifested in the municipality of Chisinau, where we have different buildings in terms of materials used, structural systems and height regime. New trends in construction are focused on buildings with a load-bearing frame structural system and a frame with a flat slab structure. The usual design practice is characteristic for the implementation of beam-column systems, where the plates are supported by the beams and the beams are supported by the columns. Nowadays, for the residential buildings design, a more elegant and ergonomic floor system is used - the flat slab system. Therefore, the slabs are subjected to increased loads, which requires special attention in the design and implementation of these types of structures.


1.
Introduction The construction sector is an important field in the economy of the Republic of Moldova.It represents an active sector of the economy, even if last year, according to the NBS data, it showed a decline.Of the total amount of 15.7 billion MDL, which constituted the total cost of the construction works completed in 2022, about 52.5% is accounted for by the newly executed constructions.[1] The housing fund has the largest share in this volume of completed works.In rural areas, the residential stock is mainly composed of individual dwellings with a low height regime.In cities, the presence of sectors with individual dwellings can be detected, but at the same time, there are urban regulated areas for collective development with an average height regime or development with a large number of levels.But these multi-storey buildings, in most district centers, give an insignificant share of the total building stock.The municipalities of Chisinau and Balti have a higher share of multi-storey buildings in the structure of the building stock.The Chisinau municipality is the largest city in the country and one of the largest urban agglomerations in the region.Performing multiple roles, it is an attractive city center for investments and, accordingly, for the construction of new buildings.Analyzing the housing fund of the municipality of Chisinau, we reveal its complex structure.Individual residential houses stand out for the variety of materials used, whether local materials such as small blocks of lacustrine limestone, natural stone, brick or newer materials such as ACC blocks, ceramic blocks, etc.By analyzing the height regime of these types of housing, a thing can be highlighted, they vary from ground floor structures to structures with multiple floors, most commonly ground floor and 3 levels.Due to the rise in prices for land intended for construction and of building materials, single attached houses, as well as terraced houses, have gained popularity.Thus, in the territories of the municipality, where, according to urban planning regulations, the construction of such structures is allowed, we received the percentage of land occupancy (LPP) and the maximum regulated land use coefficient (LUC), as a result, an increased number of inhabitants in these areas.For multi-level individual dwellings, these values vary, LPP=35-55 % and LUC=0.5-1,while for residential multi-storey buildings, LPP=65-8 % and LUC=2-8.[2] Collective buildings or residential quarters in the municipality of Chisinau are differentiated 1304 (2024) 012010 IOP Publishing doi:10.1088/1757-899X/1304/1/012010 2 depending on the period of construction according to several criteria: -the dominant height regime; -main material of the load-bearing structure; -interior design solutions; -urban planning solutions for the location of buildings on the land; -aesthetic appearance.Residential buildings built in 1950-1970 in the municipality of Chisinau are of medium height and with load-bearing walls structural system.Thus, the structural frame consisted of reinforced concrete frames with masonry of small limestone blocks.The main structural elements were made of small blocks of lacustrine limestone with dimensions of 190mm x 188mm x 390mm or large blocks with dimensions of 190mm x 240mm x 490mm.For the interior systematization of the residential buildings the flats with small rooms were common.The need to provide the population with housing and the difficult economic situation determined this trend in the construction industry.
Residential buildings built in 1970-1990 are distinguished by the use of new technologies of that period, namely the use of precast concrete elements.This method reduced construction lead time and thus managed to cover much of the high housing demand.The residential buildings were made using standard design projects, which were adjusted according to the construction conditions in certain areas (Figure 1).Therefore, the serial flat buildings became the dominant housing typology, and a group of serial projects were designed as flats serial number 143, 102, MS etc. Simultaneously with the described above building systems, structures with a load-bearing skeleton made of precast elements or tower-type buildings made of monolithic reinforced concrete were built.The construction of new residential areas during the period 1990-2020 was dominated by buildings with load-bearing frame structural system.The main load-bearing structural elements of the frame being columns and beams.This constructive system has proved its efficiency and resistance over time to seismic actions.At the same time, the load-bearing structures with a system of flat slabs have become popular since the 2000s.It is distinctive by the fact that the connection between the column and the slab is not carried out through a monolithic beam, but directly through a column-slab joint.

2.
Calculation method for a classic structural frame and a flat slab system The Republic of Moldova is located in an area where seismic activities occur with a certain frequency.In the period 1940-2023 there were a series of earthquakes that had disastrous economic consequences.The intensity of these seismic activities was 5.0 on the Richter scale in 2000, 7.2 on the Richter scale in 1977 and 7.4 on the Richter scale in 1940.[3] According to the seismic zoning map of the Republic of Moldova, the territory is divided into 3 seismic zones, therefore the southern part having a magnitude of 8, the center having a magnitude of 7, and the northern and eastern regions with a magnitude of 6.According to the seismic intensity the city of Chisinau is located in the area with a magnitude of 7 degree.The design of buildings and their construction is carried out considering this important factor.Thus, seismic design of concrete structures will provide sufficient energy dissipation capacity under cyclic loads without significantly reducing the resistance to horizontal and vertical forces.[4] The design and construction of buildings must be done in accordance with the current rules, and the execution process must be controlled and carried out in compliance with regulations, technological provisions that do not allow deviations in the construction process.
Earthquake-resistance strength is a characteristic of buildings and structures to be able to withstand the sudden ground shaking that characterizes the degree of their resistance to earthquakes.The mentioned above characteristic describes the resistance degree of the buildings to the earthquake.This index is very important in building seismic exploration, a branch of civil engineering that specializes in the behavior of buildings and structures under seismic impact.
In this sense, in order to obtain a clear picture of behavior of buildings, calculations were made of the resistance of the building structures, a building with a classic load-bearing frame, beam-column and a building on flat slab system (Figure 2).The classic load-bearing frame system demonstrated good stiffness, stability and good behavior during seismic actions, the biggest disadvantage of this system would be the flares in the lower part due to the beams (Figure 3).The structural calculation for these 2 buildings was carried out in the calculation program LIRASAPR2016.The flat slab structure systems are significantly more flexible than traditional frame structures, thus becoming more vulnerable to seismic loading conditions [5] (Figure 4).In U.S.A, C.A.P. Turner constructed flat slabs in 1906 mainly using intuitive and conceptual ideas, which was start of this type of construction.Many slabs were load-tested between 1910-1920 in U.S.A. [6] This type of structure is widely used in European countries where seismic phenomena are frequent, being traditionally used in Italy, Spain, Portugal, both, for residential buildings and for socio-cultural purposes [7].They are very popular among architects, designers, engineers and customers.Such an increased demand for this type of buildings is since this system offers freedom in the internal systematization of the houses, the possibility and flexibility in changing the floor plan, the easy drawing of engineering networks and advantages in aesthetic aspect (Figure 5).For floors of residential buildings, offices or car parks, a maximum opening of 7 m is recommended, and walls resting directly on floor slabs should be made of lightweight elements.[8] Figure 5.Sections depending on the structural system.
The general dimensions of the analyzed building with a classic load-bearing frame system are 15m IOP Publishing doi:10.1088/1757-899X/1304/1/0120106 x 24m.The bearing elements of the construction have the following dimensions: columns -300mm x 300mm; beams -300mm x 400mm; slabs -150mm and the floor height of 3300mm (Figure 3).The general dimensions of the analyzed building with a flat slab structure system are 15m x 24m.The bearing elements of the construction have the following dimensions: columns -300mm x 600mm; flat slab -230mm; and floor height of 3300mm (Figure 4).Analyzing the diagrams and calculation schemes with load-bearing structural system, can be noticed the classical way of working of the structure under the action of loads and seismic load.The maximum values of the loads to which the beam is subjected appear in the lower part, in the middle of the span (Figure 6), and in the superior part at the beam-column joint (Figure 7).According to the same principle, the loads act on the monolithic slab, in the lower part the maximum load values are noticed in the middle of the span (Figure 8) and at the superior part in at the spans or at the colums (Figure 9).The values of axial forces in the columns decrease vertically, having the maximum value in the embedded zone.
Analyzing the diagrams of the internal forces to which the load-bearing elements are subjected, the more loaded areas at the joint between the column and the plate in the upper part of the flat slab are highlighted.Additionally, the maximum loads are noted in the middle of the flat slab from the bottom side.For the vertical elements, the most stressed area is the one between the part anchored in the ground and the ground floor, vertically, the value of the loads to which the elements are subjected shows a decrease.
In order to improve the performance of the building with flat slab system slabs to the seismic loading, it is necessary to evenly distribute the piers with the working areas in all directions, so that possible displacements are compensated without major stresses.
The displacements of the structure are analyzed with reference to the base of the construction and the highest elevation of the last floor.The horizontal movements of the structure in the longitudinal and transverse direction at the floors level shall not exceed 1/500 of the height.[9] Analyzing the displacement of the building under the action of seismic loads, we ascertain the following: -In case of the flat slab structure system: on the x direction: -x direction 0…………..3.62mm;-y direction -0.123……...0.0202mm; on the y direction: -x direction -0.179……...0.315mm; -y direction -0.00769…...4.47mm.

Conclusion
In order to obtain structures that meet durability requirements, it is necessary to take into account the proper design of the structure, the choice of materials, the recommended constructive provisions, execution, quality control, inspections and classifications.[10] Analyzing the results obtained out of the calculations, we reveal the good behavior of the structure with a flat slab system.The displacements of the flat slab system structure are smaller than in the case of the load-bearing frame structural system.The stress values in the load-bearing elements describe classical stress diagrams.
Flat slab structures have advantages over the classic load-bearing frame structure, such as useful floor height, shorter lead times, and lower steel consumption.As a result of calculation, a lower consumption of reinforcement for the structure with the flat slab system compared to the load-bearing frame system was obtained.Also, in the case of skeleton structures, it is necessary to reinforce additionally the beam.
In order to obtain an efficient flat slab structure, it is necessary to evenly place the vertical bearing elements in all directions.Furthermore, depending on the opening between the vertical bearing elements and the loads to which the slab is subjected, the thickness of the slab is determined.With openings of 4.0x -6.0m, the thickness of the slab is recommended to be 160mm -230mm.The recommended ratio between the opening and slab thickness is 1/30.

Figure 6 .
Figure 6.Inferior loads values in the Mx slab

Figure 7 .
Figure 7. Superior loads values in the Mx slab

Figure 8 .
Figure 8. Inferior loads values in the Mx slab

Figure 9 .
Figure 9. Superior loads values in the Mx slab.Analyzing the diagrams and calculation schemes with load-bearing structural system, can be noticed the classical way of working of the structure under the action of loads and seismic load.The maximum values of the loads to which the beam is subjected appear in the lower part, in the middle of the span (Figure6), and in the superior part at the beam-column joint (Figure7).According to the same principle, the loads act on the monolithic slab, in the lower part the maximum load values are noticed in the middle of the span (Figure8) and at the superior part in at the spans or at the colums (Figure9).The values of