Impact of Groundwater Level Change on Natural Frequencies of RC Buildings
Impact of Groundwater Level Change on Natural Frequencies of RC Buildings
Ratnika, Lasma;Gaile, Liga;Vatin, Nikolai Ivanovich
2021-06-22 00:00:00
buildings Article Impact of Groundwater Level Change on Natural Frequencies of RC Buildings 1 2 3 , Lasma Ratnika , Liga Gaile and Nikolai Ivanovich Vatin * Department of Civil Engineering, Riga Technical University, Kalku Iela 1, Centra Rajons, , , LV-1658 R¯ ıga, Latvia; lasma.ratnika@rtu.lv Faculty of Architecture and Urban Planning, Riga Technical University, Kalku Iela 1, Centra Rajons, , , LV-1658 R¯ ıga, Latvia; Liga.gaile_1@rtu.lv Institute of Civil Engineering, Peter the Great Saint Petersburg Polytechnic University, 195251 Saint Petersburg, Russia * Correspondence: vatin@mail.ru; Tel.: +7-9219643762 Abstract: Structural health monitoring (SHM) provides an opportunity to assess and predict changes in the technical condition of structures during the operation of a building. Structural damage, as well as several operational and environmental conditions, causes changes in modal parameters. Temperature is the most popular environmental condition which is used for research. However, to the authors’ knowledge, this is the first investigation that highlights the effect of groundwater level change on the natural frequencies of the buildings and the impact of possible damage detection features. Groundwater level change can influence structural health monitoring measurements and cause faulty structural damage identification using vibration-based methods. This paper aims to analyse the impact of the groundwater level changes on the modal parameters of mid-rise reinforced concrete buildings. The modal parameters of mid-rise reinforced concrete buildings are determined using finite element (FE) models. Three different FE models of structural system types of nine-storey reinforced concrete (RC) buildings with shallow foundations are used to determine the impact of Citation: Ratnika, L.; Gaile, L.; Vatin, groundwater level fluctuation on the values of the buildings’ natural frequencies. Changes in the N.I. Impact of Groundwater Level groundwater level have an impact on the natural frequencies of the mid-rise reinforced concrete Change on Natural Frequencies of RC buildings. This research proposes a new environmental condition that has to be considered to identify Buildings. Buildings 2021, 11, 265. the structural damage using the vibration-based method. It is found that groundwater level rise https://doi.org/10.3390/buildings causes a decrease in the natural frequency value. In this research, it is established that the influence of the groundwater level on the natural frequencies of the buildings can change abruptly, and there is Academic Editor: Eva O.L. Lantsoght a non-linear correlation between groundwater level change and natural frequencies of the buildings. The natural frequencies of the buildings can change under varying environmental conditions as well Received: 6 May 2021 as in the case of structural damage. To identify structural damage in the long-term structural health Accepted: 16 June 2021 monitoring measurements, it is recommended to select features which are sensitive to structural Published: 22 June 2021 damage but are not affected by groundwater level change. Data normalisation and elimination using linear correlation methods can be used for short-term SHM under varying seasonal groundwater Publisher’s Note: MDPI stays neutral level change. with regard to jurisdictional claims in published maps and institutional affil- Keywords: finite element method; environmental impact; reinforced concrete buildings; natural iations. frequencies; groundwater level; structural health monitoring Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Structural safety is one of the most important issues during the designing and op- This article is an open access article eration of a building. Structural damage can be defined as changes in the properties of distributed under the terms and the materials, structural changes, or changes in boundary conditions that affect structural conditions of the Creative Commons element performance and operational life. Attribution (CC BY) license (https:// The structural monitoring includes observation of structure technical condition by creativecommons.org/licenses/by/ periodic measurements at a specific time and the determination of damaged structures to 4.0/). Buildings 2021, 11, 265. https://doi.org/10.3390/buildings11070265 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 265 2 of 13 assess the technical state of the existing building structures. In the long term, structural monitoring provides periodically updated information on the ability of structures to continue their intended function, taking into account the inevitable aging of structures and damage caused by the operating environment [1]. The development of structural health monitoring (SHM) is closely related to the development of digital computing equipment. Developments in structural monitoring have been observed in the last 30 years. In the investigations of [2], a growing interest in identifying construction damage is demonstrated. Meanwhile, these studies identified many technical challenges for adapting SHM in practice. Structural damage causes changes in modal parameters. These changes are different for each modal parameter. Structural monitoring [3] most frequently investigates changes in frequencies, mode shapes, and damping ratios. In order to use vibration-based methods [4] in the monitoring of building structures, it is important to know the modal parameters of the structures. In [5], Salawu presented an overview of the use of modal frequencies in the identifi- cation of structural damage. The observation that changes in structural properties causes changes in frequency was an incentive to use modal methods to identify damage. However, modal parameters of the structures during construction and operation of the building are also affected by several environmental factors. These factors may vary depending on the type of building structure, location of the building site, and other aspects. Temperature, humidity, geological features, as well as non-bearing structures affect the modal parameters of the structures and may cause incorrect identification of the damage. Suppose the impact of these variables is greater than or comparable to the impact of the structural damage on the modal parameters. In that case, it is difficult to accurately identify the structural damage. Worden et al. [6] points out that natural frequencies provide an effective indication of structural damage; however, one of the disadvantages is that the frequencies may vary due to environmental or operational conditions. The study, which obtained data on the modal parameters of a 22-storey reinforced concrete (RC) building every day for a period of one year, determined the environmental impact. The study observed a high correlation between natural frequencies and ambient temperature. The differences between the minimum and maximum natural frequencies in mode shapes were 15.8%, 11%, and 7.13%. The paper also determined that relative humidity is a factor influencing natural frequencies [7]. Gargaro and Rainieri [8] present the impact of temperature on natural frequencies in structural monitoring of a hospital building in Campobasso, Italy. The results of the study indicate the importance of reducing the impact of environmental factors to effectively detect structural damage. In fact, the impact of the thermal variation is more relevant to the mode shapes in longitudinal and torsional directions than in the transverse direction. Butt and Omenzetter [9] investigated modal parameters of buildings, including soil elasticity and the impact of non-bearing structures, and emphasised the importance of soil and non-bearing structure modelling in the calculation model of a building providing quantitative data for the monitoring of the technical condition of structures. Including soil impact [9] in the calculation model, the natural frequencies were reduced by 21%, 25% and 20% of the reference model variant, therefore, indicating a soil impact on the modal parameters. It was found that soft soil affects the mode shapes and even modifies the modal shapes, thus deriving vertical oscillations [10]. The soil impact on the modal parameters is assessed using flexible base analysis. This analysis considers the compliance of both the foundation elements and the soil. Fixed base structure (Figure 1a) refers to a combination of rigid foundation elements on a rigid base. Then a static force F causes structure with stiffness k and mass m deflection D on a fixed base: D = F/k (1) Buildings 2021, 11, x FOR PEER REVIEW 3 of 14 ∆ = F/k (1) However, a flexible base (Figure 1b) structure is a structure with vertical, horizontal and rotational flexibility at its base representing the impact of soil flexibility. If a force F is applied to a structure resting on a flexible base, then the total deflection [11] with respect to the free field at the top of the structure, ∆fl, is: ∆fl = F/k+F/kx+(F∙h/kyy)∙h (2) kx—horizontal stiffness in the x-direction, kyy—rotational spring stiffness, representing rotation in the x-z plane (about the y-y plane), h—structure height. Then an expression for flexible base period Tfl is given by Veletsos and Meek [10], is obtained as: Buildings 2021, 11, 265 3 of 13 2 0.5 Tfl/T = (1 + 1/kx + (k∙h )/kyy) (3) (a) (b) Figure 1. Schematic illustrations of deflection caused by applied force to: (a) fixed-base structure; Figure 1. Schematic illustrations of deflection caused by applied force to: (a) fixed-base structure; and (b) flexible base structure [11]. and (b) flexible base structure [11]. Foundation springs depend on the characterisation of the soil stratigraphy—soil However, a flexible base (Figure 1b) structure is a structure with vertical, horizontal types, layer thicknesses, groundwater and rock depth. Shear strength parameters vary and rotational flexibility at its base representing the impact of soil flexibility. If a force F is depending on the depth. Below the groundwater level, undrained strength parameters applied to a structure resting on a flexible base, then the total deflection [11] with respect to are re the qu fr ire eedfield . Drain at e the d st top renof gth the pastr ram uctur eters ar e, D e gener , is: ally acceptable above the groundwa- ter level [11]. D = F/k + F/k + (Fh/k )h (2) fl x yy In 1943 Terzaghi intuitively suggested that when dry sand becomes saturated, the soil stiffness (Young’s modulus) reduces by approximately 50%. Effective vertical stress k —horizontal stiffness in the x-direction, on soil under the water level reduces roughly to half, which reduces the effective confin- k —rotational spring stiffness, representing rotation in the x-z plane (about the y-y plane), yy ing stress by 50%. This reduction causes the loss of saturated soil stiffness by 50% com- h—structure height. pared to the dry condition, and settlement of the building in the soil below the water level Then an expression for flexible base period T is given by Veletsos and Meek [10], is is doubled [12]. obtained as: Fluctuations of the groundwater level cause changes in several characteristics of the 2 0.5 T /T = (1 + 1/k + (kh )/k ) (3) fl x yy soil layers. The groundwater level depth from the ground surface affects the bearing ca- pacity and deformation of the soil, and also the stability and solidity of the foundations. Foundation springs depend on the characterisation of the soil stratigraphy—soil types, Primarily, deformations of cohesive soils are affected by the position of the water level layer thicknesses, groundwater and rock depth. Shear strength parameters vary depending [13]. on the depth. Below the groundwater level, undrained strength parameters are required. Changes in the groundwater level are determined by the amount of atmospheric Drained strength parameters are generally acceptable above the groundwater level [11]. rainfall, air temperature, rock lithological composition and the degree of drainage of the In 1943 Terzaghi intuitively suggested that when dry sand becomes saturated, the soil site [14]. The level is also affected by intense water exploitation in urban areas, building stiffness (Young’s modulus) reduces by approximately 50%. Effective vertical stress on material careers, water reservoirs, amelioration systems and other objects. However, the soil under the water level reduces roughly to half, which reduces the effective confining groundwater level is slightly affected by these technogen activities, while the meteorolog- stress by 50%. This reduction causes the loss of saturated soil stiffness by 50% compared to the dry condition, and settlement of the building in the soil below the water level is doubled [12]. Fluctuations of the groundwater level cause changes in several characteristics of the soil layers. The groundwater level depth from the ground surface affects the bearing capacity and deformation of the soil, and also the stability and solidity of the foundations. Primarily, deformations of cohesive soils are affected by the position of the water level [13]. Changes in the groundwater level are determined by the amount of atmospheric rainfall, air temperature, rock lithological composition and the degree of drainage of the site [14]. The level is also affected by intense water exploitation in urban areas, building material careers, water reservoirs, amelioration systems and other objects. However, the groundwater level is slightly affected by these technogen activities, while the meteoro- logical conditions impact the seasonality of groundwater levels. The cyclical nature of feeding changes in groundwater levels is divided into four parts: winter drop, spring rise, summer fall and autumn rise. The ranges of seasonal variation depend on the lithological composition of water-containing sediments. Level changes in sandy and clayey sediments are of a different nature. Fluctuations of the groundwater level are observed faster and are more pronounced in sandy soils with a lower content of clayey sediments. Buildings 2021, 11, x FOR PEER REVIEW 4 of 14 ical conditions impact the seasonality of groundwater levels. The cyclical nature of feed- ing changes in groundwater levels is divided into four parts: winter drop, spring rise, summer fall and autumn rise. The ranges of seasonal variation depend on the lithological composition of water-containing sediments. Level changes in sandy and clayey sediments are of a different nature. Fluctuations of the groundwater level are observed faster and are more pronounced in sandy soils with a lower content of clayey sediments. The study [14] that compiled the data about the difference in groundwater levels of the various observation stations (Figure 2) indicate long-term groundwater level fluctua- tions—periods with low water levels are replaced by periods with rising levels. Also, Buildings 2021, 11, 265 4 of 13 groundwater levels in individual wells of the observation stations indicate the different nature of changes. Some observation stations show local changes in groundwater level that have not been explained by changes in atmospheric precipitation but by local influ- ences. The study [14] that compiled the data about the difference in groundwater levels of the To summarise, it is important to include environmental and operational condition various observation stations (Figure 2) indicate long-term groundwater level fluctuations— effects in SHM to precisely detect structural damage. Environmental impacts on modal periods with low water levels are replaced by periods with rising levels. Also, groundwater parameters are not static, and they are changing under different circumstances. Also, the levels in individual wells of the observation stations indicate the different nature of changes. impact on the variations on the modal parameters may be higher than in the case of struc- Some observation stations show local changes in groundwater level that have not been tural damage. explained by changes in atmospheric precipitation but by local influences. Figure 2. Groundwater level change in the various observation stations during the long-term pe- Figure 2. Groundwater level change in the various observation stations during the long-term riod [14]. period [14]. This paper investigates changes in the natural frequencies under varying environ- To summarise, it is important to include environmental and operational condition ef mental condit fects in SHMions, for example, to precisely detect fluc strtuati uctural ons of the groundwa damage. Environmental ter levelimpacts . on modal It is found that the rise of the groundwater level leads to the decrease of the natural parameters are not static, and they are changing under different circumstances. Also, the freq impact uencies on of the the variations buildingson . The theimpact modal on parameters the naturmay al freq beuenc higher ies d than epends in the oncase the soi of l str stuctural ructure,damage. and the impact on the structural system of the buildings is different. It was ob- This paper investigates changes in the natural frequencies under varying environmen- served that the influence of the groundwater level on the natural frequencies of the build- tal conditions, for example, fluctuations of the groundwater level. ings could change abruptly, and there is a non-linear correlation between the features. ItThis re is found search that proposed the rise ofa n the ew environm groundwater ent level al con leads ditio to n t the hatdecr has t ease o be consid of the natur eredal to frequencies of the buildings. The impact on the natural frequencies depends on the soil identify the structural damage using the vibration-based method. structure, and the impact on the structural system of the buildings is different. It was observed that the influence of the groundwater level on the natural frequencies of the buildings could change abruptly, and there is a non-linear correlation between the features. This research proposed a new environmental condition that has to be considered to identify the structural damage using the vibration-based method. 2. Materials and Methods In SHM it is important to separate changes caused by structural damage from changes caused by varying operational and environmental conditions. Data acquisition and normal- isation in measurements under varying conditions are very important in the identification of structural damage. Changes in modal parameters caused by operational and environmental conditions, such as temperature or humidity, can vary depending on climate characteristics, exact time of the day, and other factors. Many researchers have studied the soil impact and assessment 11.320 m 11.320 m 11.320 m Buildings 2021, 11, x FOR PEER REVIEW 5 of 14 2. Materials and Methods In SHM it is important to separate changes caused by structural damage from changes caused by varying operational and environmental conditions. Data acquisition and normalisation in measurements under varying conditions are very important in the identification of structural damage. Changes in modal parameters caused by operational and environmental conditions, Buildings 2021, 11, 265 5 of 13 such as temperature or humidity, can vary depending on climate characteristics, exact time of the day, and other factors. Many researchers have studied the soil impact and assessment in SHM measurements. In these studies, it was presumed that the soil impact is st in atSHM ic, and i measur t does not ements. cha In nge wit these h studies, in a cert itain was am pr ou esumed nt of tim that e. As them soil entimpact ioned ab isov static, e, soi and l para it meters ca does not change n be cha within nged duri a certain ng short- amount time or of time. long-tAs ime per mentioned iods depending above, soil on parameters various can be changed during short-time or long-time periods depending on various factors. factors. This p This aper paper invest investigates igates varvariable iable soisoil l effe ef ct fect impact impact on on modal modal par parameters ameters of tof hethe buil build- d- ings due to groundwater level fluctuation. Ninety finite element model (FEM) cases were ings due to groundwater level fluctuation. Ninety finite element model (FEM) cases were performed to determine the impact of the groundwater level fluctuation on the modal performed to determine the impact of the groundwater level fluctuation on the modal parameters of the medium-rise buildings. Three different structural system type finite parameters of the medium-rise buildings. Three different structural system type finite el- element (FE) models of nine-storey RC buildings with shallow foundations were developed ement (FE) models of nine-storey RC buildings with shallow foundations were developed and calculated using the structural FE analysis software Dlubal RFEM and RF-SOILIN and calculated using the structural FE analysis software Dlubal RFEM and RF-SOILIN add-on module. Numerical simulations have been developed using linear elastic analysis. add-on module. Numerical simulations have been developed using linear elastic analysis. FE model type A is RC cellular structure building, FE model type B is RC beam and column FE model type A is RC cellular structure building, FE model type B is RC beam and col- structural system with two rigidity cores, and FE model type C is a framed RC beam and umn structural system with two rigidity cores, and FE model type C is a framed RC beam column structural system building with lightweight shear infill walls and two rigidity and column structural system building with lightweight shear infill walls and two rigidity cores. These three types of building are common structural systems for mass housing, cores. These three types of building are common structural systems for mass housing, especially in Eastern Europe. The structural column cross-section is square 350 350 mm , especially in Eastern Europe. The structural column cross-section is square 350 × 350 mm , bearing wall thickness is 300 mm, slab thickness is 100 mm and lightweight shear wall bearing wall thickness is 300 mm, slab thickness is 100 mm and lightweight shear wall thickness is 200 mm. Building types are shown in Figure 3. thickness is 200 mm. Building types are shown in Figure 3. (a) (b) (c) Figure 3. Finite element (FE) model types: (a) FE model type A; (b) FE model type B; (c) FE model type C. Figure 3. Finite element (FE) model types: (a) FE model type A; (b) FE model type B; (c) FE model type C. The stiffness The stiffness and d and ampin damping g properties o properties f the ofsoil the ar soil e mo ar dell e modelled ed using vert using icavertical l and hor- and izontal springs. Springs are calculated for each soil type using RFEM soil-structure inter- horizontal springs. Springs are calculated for each soil type using RFEM soil-structure actinteraction ion analysis analysis. . The progr The am pr det ogram ermine determines s the sprithe ng co spring efficients in coefficients each in fin each ite elfinite ement. For element. FE model t For FE model ype C (fr type amed C (framed RC beam RCan beam d column st and column ructural s structural ystem build system ing building with light with - lightweight shear wall infill and two rigidity cores), masonry infill has been included in the weight shear wall infill and two rigidity cores), masonry infill has been included in the appropriate manner in the finite element model [15]. For the calculation, it was considered appropriate manner in the finite element model [15]. For the calculation, it was considered that the foundation base vertical spring values are variable due to soil parameter changes, that the foundation base vertical spring values are variable due to soil parameter changes, but horizontal foundation base springs are 70% of the vertical spring value. The value of but horizontal foundation base springs are 70% of the vertical spring value. The value of the applied load was equal for all FEM model simulations. Groundwater level fluctuation caused stiffness changes on the foundation base. The range of soil parameters was chosen based on typical characteristics of soil reported in the literature [16,17]. Soil type 1 is dense sand layers soil structure, Soil type 2 is clay and sand layers soil structure and Soil type 3 is dense sand layers soil structure with an attenuation peat layer. Input parameters for FEM calculation are presented in Tables 1–3. 26.110 m 26.110 m 26.110 m Buildings 2021, 11, 265 6 of 13 Table 1. Description of soil type 1. Soil Layer Parameters Soil Description Specific Weight Modulus of Ordinate from Thickness, m Poisson’s Ratio Elasticity, Ground Level, m 3 3 , kN/m
, kN/m sat 2 [MN/m ] Sand, closely 17.0 19.0 30.0 0.30 0.80 0.80 graded Sand 19.0 21.0 30.0 0.30 2.50 3.30 Dusty sand 18.0 20.3 18.0 0.30 3.50 6.80 Sand, gravelly 18.0 20.0 20.0 0.30 7.20 14.0 sand Table 2. Description of soil type 2. Soil Layer Parameters Soil Description Modulus of Specific Weight Ordinate from Thickness, m Poisson’s Ratio Elasticity, Ground Level, m 3 3 , kN/m
, kN/m 2 sat [MN/m ] Sand, closely 17.0 19.0 30.0 0.30 0.80 0.80 graded Sand 19.0 21.0 30.0 0.30 2.50 3.30 Clay, low 19.0 19.5 2.50 0.42 3.50 6.80 plasticity Sand-clay 18.0 19.0 10.0 0.35 7.20 14.0 mixture Table 3. Description of soil type 3. Soil Layer Parameters Soil Description Modulus of Specific Weight Ordinate from Thickness, m Poisson’s Ratio Elasticity, Ground Level, m 3 3 , kN/m
, kN/m 2 sat [MN/m ] Sand, closely 17.0 19.0 30.0 0.30 0.80 0.80 graded Sand 19.0 21.0 30.0 0.30 2.50 3.30 Peat 10.40 10.40 1.0 0.40 0.30 3.60 Dusty sand 18.0 20.3 18.0 0.30 3.2 6.80 Sand, gravelly 18.0 20.0 20.0 0.30 7.2 14.0 sand The groundwater level of the reference FE model was chosen 2.1 m from the building ground level, and the fluctuation step of the groundwater level was 20 centimetres. The im- pact of the groundwater level change on modal parameters for each soil type and FE model type was calculated as a difference between the reference FEM natural frequency and the calculated FEM natural frequency value of the next fluctuation step of groundwater level: Df = f f (4) i 1i 2i Df —natural frequency difference for mode i, f —natural frequency of reference FEM groundwater level for mode i, 1i f —natural frequency of FEM of next fluctuation step of groundwater level for mode i. 2i 3. Results Calculations were undertaken for each groundwater level depth for three types of soil structure and three types of building structural system in order to investigate the ground- water level impact on the first natural frequencies of the reinforced concrete buildings. It was found that groundwater level rise has an impact on modal parameters of the buildings, such as natural frequencies. Impact on modal parameters differs from building mode and soil structure. Various soil structures have a different impact on the natural frequencies of the buildings in a specific mode. For FE model type A and model type C, for Buildings 2021, 11, x FOR PEER REVIEW 7 of 14 Calculations were undertaken for each groundwater level depth for three types of soil structure and three types of building structural system in order to investigate the groundwater level impact on the first natural frequencies of the reinforced concrete build- ings. It was found that groundwater level rise has an impact on modal parameters of the buildings, such as natural frequencies. Impact on modal parameters differs from building mode and soil structure. Various soil structures have a different impact on the natural frequencies of the buildings in a specific mode. For FE model type A and model type C, for comparison, we took natural frequencies in first transverse mode, first longitudinal mode and first torsional mode. However, for FE model type B we took natural frequencies in the first longitudinal mode, first torsional mode and second torsional mode. Groundwater level change has an impact on first natural frequencies in first trans- verse mode and first longitudinal mode for FE model type A, reached 2.5% difference in Soil type 2 (Figures 4 and 5). However, in the first torsional mode difference between ref- erence groundwater level and fluctuation step, FE model natural frequencies reached 0.05%. Groundwater level rise decreases natural frequencies values to 1.1 m level where frequencies value increases. A groundwater level at –1.3 m reached foundation base level and in graphs the change of natural frequencies had a peak value here. After the level – 1.1 m, natural frequencies values continued to decrease (Table 4). Table 4. Difference of the natural frequencies due to groundwater level depth fluctuation for FE model type A. Vibration Mode of FEM of Refer- Groundwater Level Depth from Ground Level, m ence Groundwater Level, (Natu- –1.9 –1.7 –1.5 –1.3 –1.1 –0.9 –0.7 –0.5 –0.3 Buildings 2021, 11, 265 7 of 13 ral Frequency, Hz) Difference of the Natural Frequency ∆fi, % 1st transverse mode (1.348 Hz) 0.22 0.45 0.59 1.26 0.96 1.19 1.34 1.56 1.71 1st longitudinal mode (2.023 Hz) 0.10 0.15 0.25 0.44 0.40 0.44 0.49 0.54 0.64 comparison, we took natural frequencies in first transverse mode, first longitudinal mode 1st torsional mode 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 and first torsional mode. However, for FE model type B we took natural frequencies in the (2.174 Hz) first longitudinal mode, first torsional mode and second torsional mode. 1st transverse mode (1.056 Hz) 0.28 0.57 0.85 1.80 1.33 1.61 1.89 2.18 2.46 Groundwater level change has an impact on first natural frequencies in first trans- 1st longitudinal mode (1.816 Hz) 0.22 0.39 0.50 1.05 0.83 0.94 1.10 1.27 1.38 verse mode and first longitudinal mode for FE model type A, reached 2.5% difference 1st torsional mode in Soil type 2 (Figur 0.00 0. es 400 and 0. 5). However 00 0.,05 in 0. the first05 torsional 0.05 0. mode05 0. difference05 0. between05 (2.167 Hz) reference groundwater level and fluctuation step, FE model natural frequencies reached 1st transverse mode (1.184 Hz) 0.25 0.42 0.59 1.27 1.01 1.18 1.44 1.60 1.77 0.05%. Groundwater level rise decreases natural frequencies values to 1.1 m level where 1st longitudinal mode (1.925 Hz) 0.16 0.21 0.31 0.62 0.47 0.57 0.68 0.73 0.83 frequencies value increases. A groundwater level at 1.3 m reached foundation base level 1st torsional mode and in graphs the change of natural frequencies had a peak value here. After the level 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (2.169 Hz) 1.1 m, natural frequencies values continued to decrease (Table 4). Buildings 2021, 11, x FOR PEER REVIEW 8 of 14 Figure 4. Difference of the natural frequencies of FE model type A in the first transverse mode for Figure 4. Difference of the natural frequencies of FE model type A in the first transverse mode for three different three different soil types. soil types. Figure 5. Difference of the natural frequencies of FE model type A in the first longitudinal mode for Figure 5. Difference of the natural frequencies of FE model type A in the first longitudinal mode for three different soil types. three different soil types. Table 4. Difference of the natural For FE fr mode equencies l type B, due to gro gru oundwater ndwater leve levell c depth hange fluctuation has an impact for FE on model natu type ral A. frequencies in the first longitudinal mode, first torsional mode, and second torsional mode that Groundwater Level Depth from Ground Level, m reached a 1.5% difference in Soil type 2 (Figures 6–8). Groundwater level rise decreases Vibration Mode of FEM of Soil Reference Groundwater Level, 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 natural frequency values as non-linearly related features (Table 5). Type No. (Natural Frequency, Hz) Difference of the Natural Frequency Df , % Table 5. Difference of the natural frequencies due to groundwater level depth fluctuation for FE 1st transverse mode (1.348 Hz) 0.22 0.45 0.59 1.26 0.96 1.19 1.34 1.56 1.71 model type B. 1st longitudinal mode (2.023 Hz) 0.10 0.15 0.25 0.44 0.40 0.44 0.49 0.54 0.64 1st torsional mode Vibration Mode of FEM of Refer- Groundwater Level Depth from Ground Level, m 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 (2.174 Hz) ence Groundwater Level (Natural –1.9 –1.7 –1.5 –1.3 –1.1 –0.9 –0.7 –0.5 –0.3 Frequency, Hz) Difference of the Natural Frequency ∆fi, % 1st longitudinal mode (0.563 Hz) 0.18 0.18 0.36 0.53 0.53 0.53 0.71 0.71 0.89 1st torsional mode 1 0.00 0.00 0.14 0.14 0.14 0.14 0.14 0.29 0.29 (0.691 Hz) 2nd torsional mode (0.789 Hz) 0.13 0.13 0.13 0.25 0.25 0.25 0.25 0.38 0.38 1st longitudinal mode (0.487 Hz) 0.21 0.41 0.41 1.03 0.82 1.03 1.23 1.23 1.44 1st torsional mode 2 0.16 0.16 0.16 0.31 0.31 0.31 0.31 0.31 0.47 (0.643 Hz) 2nd torsional mode (0.722 Hz) 0.14 0.14 0.14 0.28 0.28 0.28 0.42 0.42 0.55 1st longitudinal mode (0.532 Hz) 0.00 0.19 0.19 0.56 0.38 0.56 0.56 0.75 0.94 1st torsional mode 3 0.00 0.00 0.15 0.15 0.15 0.15 0.15 0.29 0.29 (0.678 Hz) 2nd torsional mode (0.773 Hz) 0.13 0.13 0.13 0.26 0.26 0.26 0.26 0.39 0.39 Figure 6. Difference of the natural frequencies of FE model type B in the first longitudinal mode for three different soil types. Soil type Soil type No. No. Buildings 2021, 11, x FOR PEER REVIEW 8 of 14 Figure 5. Difference of the natural frequencies of FE model type A in the first longitudinal mode for three different soil types. For FE model type B, groundwater level change has an impact on natural frequencies in the first longitudinal mode, first torsional mode, and second torsional mode that reached a 1.5% difference in Soil type 2 (Figures 6–8). Groundwater level rise decreases Buildings 2021, 11, 265 8 of 13 natural frequency values as non-linearly related features (Table 5). Table 5. Difference of the natural frequencies due to groundwater level depth fluctuation for FE Table 4. Cont. model type B. Groundwater Level Depth from Ground Level, m Groundwater Level Depth from Ground Level, m Vibration Mode of FEM of Refer- Vibration Mode of FEM of Soil ence Groundwater Level (Natural –1.9 –1.7 –1.5 –1.3 –1.1 –0.9 –0.7 –0.5 –0.3 Reference Groundwater Level, 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 Type No. (Natural Frequency, Hz) Frequency, Hz) Difference of the Natural Frequency ∆fi, % Difference of the Natural Frequency Df , % 1st longitudinal mode (0.563 Hz) 0.18 0.18 0.36 0.53 0.53 0.53 0.71 0.71 0.89 1st transverse mode (1.056 Hz) 0.28 0.57 0.85 1.80 1.33 1.61 1.89 2.18 2.46 1st torsional mode 1st longitudinal mode (1.816 Hz) 0.22 0.39 0.50 1.05 0.83 0.94 1.10 1.27 1.38 1 0.00 0.00 0.14 0.14 0.14 0.14 0.14 0.29 0.29 1st torsional mode (0.691 Hz) 0.00 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 (2.167 Hz) 2nd torsional mode (0.789 Hz) 0.13 0.13 0.13 0.25 0.25 0.25 0.25 0.38 0.38 1st transverse mode (1.184 Hz) 0.25 0.42 0.59 1.27 1.01 1.18 1.44 1.60 1.77 1st longitudinal mode (0.487 Hz) 0.21 0.41 0.41 1.03 0.82 1.03 1.23 1.23 1.44 1st longitudinal mode (1.925 Hz) 0.16 0.21 0.31 0.62 0.47 0.57 0.68 0.73 0.83 1st torsional mode 2 1st torsional mode 0.16 0.16 0.16 0.31 0.31 0.31 0.31 0.31 0.47 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (0.643 Hz) (2.169 Hz) 2nd torsional mode (0.722 Hz) 0.14 0.14 0.14 0.28 0.28 0.28 0.42 0.42 0.55 1st longitudinal mode (0.532 Hz) 0.00 0.19 0.19 0.56 0.38 0.56 0.56 0.75 0.94 For FE model type B, groundwater level change has an impact on natural frequencies 1st torsional mode in the first longitudinal mode, first torsional mode, and second torsional mode that reached 3 0.00 0.00 0.15 0.15 0.15 0.15 0.15 0.29 0.29 (0.678 Hz) a 1.5% difference in Soil type 2 (Figures 6–8). Groundwater level rise decreases natural 2nd torsional mode (0.773 Hz) 0.13 0.13 0.13 0.26 0.26 0.26 0.26 0.39 0.39 frequency values as non-linearly related features (Table 5). Buildings 2021, 11, x FOR PEER REVIEW 9 of 14 Buildings 2021, 11, x FOR PEER REVIEW 9 of 14 Figure 6. Difference of the natural frequencies of FE model type B in the first longitudinal mode for Figure 6. Difference of the natural frequencies of FE model type B in the first longitudinal mode for three different soil types. three different soil types. Figure 7. Difference of the natural frequencies of FE model type B in the first torsional mode for Figure 7. Difference of the natural frequencies of FE model type B in the first torsional mode for three Figure 7. Difference of the natural frequencies of FE model type B in the first torsional mode for three different soil types. three different different soil types. soil types. Figure 8. Difference of the natural frequencies of FE model type B in the second torsional mode for Figure 8. Difference of the natural frequencies of FE model type B in the second torsional mode for Figure 8. Difference of the natural frequencies of FE model type B in the second torsional mode for three different soil types. three different soil types. three different soil types. Building model types have different stiffness and different mass values. Therefore, Building model types have different stiffness and different mass values. Therefore, also groundwater level change impact on natural frequencies of the building has consid- also groundwater level change impact on natural frequencies of the building has consid- erable disparity. For example, the global bending stiffness value difference for cellular erable disparity. For example, the global bending stiffness value difference for cellular structure and frame structure is around 85%. This explains the difference of 1st natural structure and frame structure is around 85%. This explains the difference of 1st natural frequency for model type A and model type B. frequency for model type A and model type B. For FE model type C, groundwater level change impact is similar to FE model A sim- For FE model type C, groundwater level change impact is similar to FE model A sim- ulation. Changes are observed on the first natural frequencies in the first transverse mode ulation. Changes are observed on the first natural frequencies in the first transverse mode and first longitudinal mode, reaching a 2.5% difference in soil type 2 (Figures 9 and 10). and first longitudinal mode, reaching a 2.5% difference in soil type 2 (Figures 9 and 10). Also, in the first torsional mode, the difference between the natural frequencies of the FE Also, in the first torsional mode, the difference between the natural frequencies of the FE model of reference groundwater level and FE model of the next groundwater level fluc- model of reference groundwater level and FE model of the next groundwater level fluc- tuation step reached 0.05% and groundwater level rise decreased natural frequencies val- tuation step reached 0.05% and groundwater level rise decreased natural frequencies val- ues until the 1.1 m level where frequencies value increased. After 1.1 m, natural frequen- ues until the 1.1 m level where frequencies value increased. After 1.1 m, natural frequen- cies values continued to decrease (Table 6). cies values continued to decrease (Table 6). Table 6. Difference of the natural frequencies due to groundwater level depth fluctuation for FE Table 6. Difference of the natural frequencies due to groundwater level depth fluctuation for FE model type C. model type C. Vibration Mode of FEM of Refer- Groundwater Level Depth from Ground Level, m Groundwater Level Depth from Ground Level, m Vibration Mode of FEM of Refer- ence Groundwater Level, (Natu- –1.9 –1.7 –1.5 –1.3 –1.1 –0.9 –0.7 –0.5 –0.3 ence Groundwater Level, (Natu- –1.9 –1.7 –1.5 –1.3 –1.1 –0.9 –0.7 –0.5 –0.3 ral Frequency, Hz) Difference of the natural frequency ∆fi, % ral Frequency, Hz) Difference of the natural frequency ∆fi, % 1st transverse mode (1.303 Hz) 0.23 0.38 0.54 1.15 0.92 1.07 1.23 1.46 1.61 1st transverse mode (1.303 Hz) 0.23 0.38 0.54 1.15 0.92 1.07 1.23 1.46 1.61 1 1st longitudinal mode (1.942 Hz) 0.10 0.15 0.21 0.41 0.31 0.36 0.46 0.51 0.57 1 1st longitudinal mode (1.942 Hz) 0.10 0.15 0.21 0.41 0.31 0.36 0.46 0.51 0.57 1st torsional mode (2.134 Hz) 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 1st torsional mode (2.134 Hz) 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 1st transverse mode (1.02 Hz) 0.29 0.59 0.88 1.76 1.37 1.57 1.86 2.06 2.35 1st transverse mode (1.02 Hz) 0.29 0.59 0.88 1.76 1.37 1.57 1.86 2.06 2.35 2 1st longitudinal mode (1.743 Hz) 0.17 0.29 0.46 0.92 0.69 0.86 0.98 1.15 1.32 2 1st longitudinal mode (1.743 Hz) 0.17 0.29 0.46 0.92 0.69 0.86 0.98 1.15 1.32 1st torsional mode (2.119 Hz) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1st torsional mode (2.119 Hz) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1st transverse mode (1.146 Hz) 0.26 0.44 0.61 1.22 0.96 1.13 1.31 1.48 1.75 1st transverse mode (1.146 Hz) 0.26 0.44 0.61 1.22 0.96 1.13 1.31 1.48 1.75 3 1st longitudinal mode (1.851 Hz) 0.11 0.16 0.27 0.54 0.43 0.49 0.59 0.65 0.76 3 1st longitudinal mode (1.851 Hz) 0.11 0.16 0.27 0.54 0.43 0.49 0.59 0.65 0.76 1st torsional mode (2.125 Hz) 0.00 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 1st torsional mode (2.125 Hz) 0.00 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 Soil type Soil type Soil type No. No. No. Buildings 2021, 11, 265 9 of 13 Table 5. Difference of the natural frequencies due to groundwater level depth fluctuation for FE model type B. Groundwater Level Depth from Ground Level, m Vibration Mode of FEM of Soil Reference Groundwater Level 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 Type No. (Natural Frequency, Hz) Difference of the Natural Frequency Df , % 1st longitudinal mode (0.563 Hz) 0.18 0.18 0.36 0.53 0.53 0.53 0.71 0.71 0.89 1st torsional mode 0.00 0.00 0.14 0.14 0.14 0.14 0.14 0.29 0.29 (0.691 Hz) 2nd torsional mode (0.789 Hz) 0.13 0.13 0.13 0.25 0.25 0.25 0.25 0.38 0.38 1st longitudinal mode (0.487 Hz) 0.21 0.41 0.41 1.03 0.82 1.03 1.23 1.23 1.44 1st torsional mode 0.16 0.16 0.16 0.31 0.31 0.31 0.31 0.31 0.47 (0.643 Hz) 2nd torsional mode (0.722 Hz) 0.14 0.14 0.14 0.28 0.28 0.28 0.42 0.42 0.55 1st longitudinal mode (0.532 Hz) 0.00 0.19 0.19 0.56 0.38 0.56 0.56 0.75 0.94 3 1st torsional mode 0.00 0.00 0.15 0.15 0.15 0.15 0.15 0.29 0.29 (0.678 Hz) 2nd torsional mode (0.773 Hz) 0.13 0.13 0.13 0.26 0.26 0.26 0.26 0.39 0.39 Building model types have different stiffness and different mass values. Therefore, also groundwater level change impact on natural frequencies of the building has considerable disparity. For example, the global bending stiffness value difference for cellular structure and frame structure is around 85%. This explains the difference of 1st natural frequency for model type A and model type B. For FE model type C, groundwater level change impact is similar to FE model A simu- lation. Changes are observed on the first natural frequencies in the first transverse mode and first longitudinal mode, reaching a 2.5% difference in soil type 2 (Figures 9 and 10). Also, in the first torsional mode, the difference between the natural frequencies of the FE model of reference groundwater level and FE model of the next groundwater level fluctua- Buildings 2021, 11, x FOR PEER REVIEW 10 of 14 tion step reached 0.05% and groundwater level rise decreased natural frequencies values Buildings 2021, 11, x FOR PEER REVIEW 10 of 14 until the 1.1 m level where frequencies value increased. After 1.1 m, natural frequencies values continued to decrease (Table 6). Figure 9. Difference of the natural frequencies of FE model type C in the first transverse mode for Figure 9. Difference of the natural frequencies of FE model type C in the first transverse mode for Figure 9. Difference of the natural frequencies of FE model type C in the first transverse mode for three different soil types. three different soil types. three different soil types. Figure 10. Difference of the natural frequencies of FE model type C in the first longitudinal mode for Figure 10. Difference of the natural frequencies of FE model type C in the first longitudinal mode Figure 10. Difference of the natural frequencies of FE model type C in the first longitudinal mode three different soil types. for three different soil types. for three different soil types. 4. Discussion 4. Discussion The collected data show that the groundwater level fluctuation in the soil with clayey The collected data show that the groundwater level fluctuation in the soil with clayey layers impacts the natural frequencies of the buildings. layers impacts the natural frequencies of the buildings. The research results coincided with the study [18] in which the authors investigate The research results coincided with the study [18] in which the authors investigate the modal parameters of a frame structure, including dynamic soil–structure interaction the modal parameters of a frame structure, including dynamic soil–structure interaction effects. In that study, it was also found that the soil–structure interaction affects all of the effects. In that study, it was also found that the soil–structure interaction affects all of the structural modes in the case of soft soil foundation. structural modes in the case of soft soil foundation. According to formula (3) of the article, the fundamental frequency is roughly propor- According to formula (3) of the article, the fundamental frequency is roughly propor- tional to the square root of the building foundation stiffness. However, this stiffness also tional to the square root of the building foundation stiffness. However, this stiffness also is influenced by foundation settlement changes due to the water ground-level fluctua- is influenced by foundation settlement changes due to the water ground-level fluctua- tions. A traditional way to take into account this effect is to use the correction factor Cw tions. A traditional way to take into account this effect is to use the correction factor Cw as a multiplier to the settlement in dry condition [12]. According to the field investigations as a multiplier to the settlement in dry condition [12]. According to the field investigations reported in the literature, e.g., [19,20], settlement has maximum value when the water reported in the literature, e.g., [19,20], settlement has maximum value when the water table reaches the footing level. In Figure 4, where the peak value of the natural frequencies table reaches the footing level. In Figure 4, where the peak value of the natural frequencies is reached, groundwater level (–1.3 m) is precisely at the foundation base level. Further- is reached, groundwater level (–1.3 m) is precisely at the foundation base level. Further- more, when water are above the footing base, the settlement due to water level rise in- more, when water are above the footing base, the settlement due to water level rise in- creases at a slower rate [21]. This explains the peak value of fundamental frequency in the creases at a slower rate [21]. This explains the peak value of fundamental frequency in the results obtained. Nevertheless, it is straightforwardly connected to the adopted method- results obtained. Nevertheless, it is straightforwardly connected to the adopted method- ology of settlement calculations. ology of settlement calculations. However, the groundwater level fluctuation in sandy soil layers has less impact on However, the groundwater level fluctuation in sandy soil layers has less impact on the features, even including the peat layer as a stiffness attenuation layer. the features, even including the peat layer as a stiffness attenuation layer. Natural frequencies of different building structural system-type changes under var- Natural frequencies of different building structural system-type changes under var- iable environmental conditions such as groundwater level show that to avoid faulty re- iable environmental conditions such as groundwater level show that to avoid faulty re- sults in SHM it is very important to include and consider operational and environmental sults in SHM it is very important to include and consider operational and environmental effects in measurements. effects in measurements. To assess all environmental factors in SHM and introduce automatic damage detec- To assess all environmental factors in SHM and introduce automatic damage detec- tion in practice long monitoring and many FEM simulations are needed. Operational and tion in practice long monitoring and many FEM simulations are needed. Operational and environmental effects may affect each structure individually. It can take a lot of time and environmental effects may affect each structure individually. It can take a lot of time and Buildings 2021, 11, 265 10 of 13 Table 6. Difference of the natural frequencies due to groundwater level depth fluctuation for FE model type C. Groundwater Level Depth from Ground Level, m Vibration Mode of FEM of Soil Reference Groundwater Level, 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 Type No. (Natural Frequency, Hz) Difference of the Natural Frequency Df , % 1st transverse mode (1.303 Hz) 0.23 0.38 0.54 1.15 0.92 1.07 1.23 1.46 1.61 1st longitudinal mode (1.942 Hz) 0.10 0.15 0.21 0.41 0.31 0.36 0.46 0.51 0.57 1st torsional mode (2.134 Hz) 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 1st transverse mode (1.02 Hz) 0.29 0.59 0.88 1.76 1.37 1.57 1.86 2.06 2.35 1st longitudinal mode (1.743 Hz) 0.17 0.29 0.46 0.92 0.69 0.86 0.98 1.15 1.32 1st torsional mode (2.119 Hz) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1st transverse mode (1.146 Hz) 0.26 0.44 0.61 1.22 0.96 1.13 1.31 1.48 1.75 1st longitudinal mode (1.851 Hz) 0.11 0.16 0.27 0.54 0.43 0.49 0.59 0.65 0.76 1st torsional mode (2.125 Hz) 0.00 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.05 4. Discussion The collected data show that the groundwater level fluctuation in the soil with clayey layers impacts the natural frequencies of the buildings. The research results coincided with the study [18] in which the authors investigate the modal parameters of a frame structure, including dynamic soil–structure interaction effects. In that study, it was also found that the soil–structure interaction affects all of the structural modes in the case of soft soil foundation. According to formula (3) of the article, the fundamental frequency is roughly propor- tional to the square root of the building foundation stiffness. However, this stiffness also is influenced by foundation settlement changes due to the water ground-level fluctuations. A traditional way to take into account this effect is to use the correction factor Cw as a multiplier to the settlement in dry condition [12]. According to the field investigations reported in the literature, e.g., [19,20], settlement has maximum value when the water table reaches the footing level. In Figure 4, where the peak value of the natural frequencies is reached, groundwater level ( 1.3 m) is precisely at the foundation base level. Furthermore, when water are above the footing base, the settlement due to water level rise increases at a slower rate [21]. This explains the peak value of fundamental frequency in the results obtained. Nevertheless, it is straightforwardly connected to the adopted methodology of settlement calculations. However, the groundwater level fluctuation in sandy soil layers has less impact on the features, even including the peat layer as a stiffness attenuation layer. Natural frequencies of different building structural system-type changes under vari- able environmental conditions such as groundwater level show that to avoid faulty results in SHM it is very important to include and consider operational and environmental effects in measurements. To assess all environmental factors in SHM and introduce automatic damage detection in practice long monitoring and many FEM simulations are needed. Operational and environmental effects may affect each structure individually. It can take a lot of time and effort to develop each structure’s behaviour separately under varying operational and environmental conditions. Due to changing effects, only part of the measurements from similar conditions can be used [22]. For SHM, in order to minimise false structural damage identifications, it is important to obtain simulation data in a wide range of environmental and operations conditions [23]. Reynders et al. [24] present the SHM approach consisting of three steps: 1. Reducing data by identifying damage sensitive features such as modal parameters in short periods; 2. Determination of non-linear environmental model using the damage sensitive features as outputs from the previous stage; Buildings 2021, 11, 265 11 of 13 3. Monitoring the forecasting error of the global model. However, this approach has limited applicability of SHM measurements in practice. It was found that the values of the parameters involved should be selected very carefully to obtain useful results. Several studies present data normalisation or elimination. For a linear relationship between the features and the unknown latent variables, factor analysis can be used for measurements [22]. Cross et al. [25] describe cointegration as a new method to deal with change in structural response induced by the environment. The main idea is response variables that are integrated to create a stationary performance whose stationarity reflects the normal state of the structure could be linearly combined. Nevertheless, the method only works for linearly related variables. However, not all environmental or operational effects have a linear impact on the features. Results show a non-linear correlation between groundwater level fluctuation and changes of the natural frequencies of the buildings. Kullaa in [22] describes a mixture of factor analyses model to compensate for the non-linear effects. This research shows that using non-linear models in SHM, structural damage is more clearly detected. However, at the same time, it results in more false identifications of damage. Erazo et al.et al. [26] presents Kalman filtering for a variable environmental and operational condition induced noise reduction in data. In research [27], which present natural frequencies variations of the Consoli Palace due to changes in ambient temperature, authors considered that correlation between natural frequencies and temperatures are often non-linear. Also, the groundwater level fluctuation impact on natural frequencies of buildings shows a non-linear correlation and reached a maximum difference of 2.5%, which can negatively influence measurement results. Tjirkallis in [28] accentuates that changes in natural frequency can reach 10% from environmental and operational variations and means that changes can be larger than those caused by significant damage. In SHM, measured data will depend not only on the condition of damage but also on the environmental and operational variability. Several methods were presented in the research mentioned above, but almost all of them faced false identification of structural damage. Many methods are restricted to use only on the structures for which the model was created [29]. To accurately identify structural damage, it is recommended to choose features that are susceptible to structural damage but not to environmental and operational changes [30]. It is found that groundwater level change impacts the natural frequencies of the buildings when strong soil–structure interaction (SSI) occurs. That is because in this case changes in soil parameters significantly affect the natural frequencies of the buildings. Strong structural and soil interaction is, therefore, important for low and mid-rise buildings on week soil. Future work is to investigate groundwater level change impact on natural frequencies of buildings with other types of building foundation, structural scheme, and irregularly shaped buildings. In future, it is necessary to perform more experimental studies and compare them with the numerical simulation results. 5. Conclusions In this research, the modal properties of 90 FEM models were analysed to investigate the variable soil effect impact on the buildings’ modal parameters due to fluctuation of groundwater level. Three different structural type FE models of nine-storey RC buildings with shallow foundations were developed. FE model types included a wall-bearing RC structural system building, RC beam and column structural system building with two rigidity cores, and a RC beam and column structural system building with lightweight shear walls and two rigidity cores. The groundwater level of the reference FE model was chosen to be 2.1 m from the building ground level, and the fluctuation step of the groundwater level was 20 centimetres. The impact of the groundwater level change on Buildings 2021, 11, 265 12 of 13 modal parameters for each soil type and FE model type was calculated as the difference between the reference FEM natural frequency and the calculated FEM natural frequency value of the next fluctuation step of the groundwater level. This research determined that groundwater level change affects all modes of analysed types of building structural systems. Natural frequency value decreases due to the rise in groundwater level. Maximum natural frequencies difference reached 2.5% for the RC building with bearing walls and for the RC frame building with lightweight shear walls and two rigidity cores in the case of a clayey soil type. This frequency fluctuation is in the first transverse vibration mode. Natural frequencies of the buildings in torsional mode reached only 0.5% of the frequency difference for all types of buildings and all soil types. Therefore, it is concluded that torsional mode is less sensitive to water ground level changes. Groundwater level change can influence SHM measurements and cause faulty struc- tural damage identification. The seasonal changes in groundwater level are approximately 1 m. As indicated in the graphs, the buildings’ natural frequencies and groundwater level fluctuation have a linear correlation up to 1.2 m. Data normalisation using linear correlation methods can be used for SHM measurements in analysing short-term data. It was observed that groundwater level influence on building natural frequencies could change abruptly, and there is a non-linear correlation between the features in the long term measurements. This research proposed a new environmental condition that needs to be considered in SHM for the vibration-based method. In practice, structural engineers should be aware of this effect on the modal parameters of the buildings to identify structural damage using vibration-based methods. To approach the precise structural damage identification in the long-term SHM measurements, features that are sensitive to structural damage, not environmental or operational changes, including groundwater level fluctuations, need to be chosen, or those effects should be filtered out from data. Further experimental investigations are planned to identify structural damage using the vibration-based method for real structures under variable operational and environmen- tal conditions, including groundwater level changes. Author Contributions: Conceptualization, L.R., L.G. and N.I.V.; methodology L.R. and L.G.; formal analysis L.R., L.G.; investigation L.R., L.G. and N.I.V.; data curation L.R.; writing L.R., L.G. and N.I.V.; visualization LR; supervision, L.G. and N.I.V. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Higher Education of the Russian Federation as part of World-class Research Center program: Advanced Digital Technologies (contract No.075-15-2020-934 dated 17 November 2020). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: The research has been supported by the RTU fund within the activity “Project Competition for the strengthening of the capacity of RTU scientific staff for students 2019/2020”. Conflicts of Interest: The authors declare no conflict of interest. References 1. Li, J.; Hao, H. A review of recent research advances on structural health monitoring in Western Australia. Struct. Monit. Maint. 2016, 3, 33–49. [CrossRef] 2. Farrar, C.R.; Worden, K. An introduction to structural health monitoring. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2007, 365, 303–315. [CrossRef] [PubMed] 3. Fan, W.; Qiao, P. Vibration-based Damage Identification Methods: A Review and Comparative Study. Struct. Health Monit. 2010, 10, 83–111. [CrossRef] 4. Beskhyroun, S.; Wegner, L.D.; Sparling, B.F. New methodology for the application of vibration-based damage detection techniques. Struct. Control. Health Monit. 2011, 19, 632–649. [CrossRef] Buildings 2021, 11, 265 13 of 13 5. Doebling, S.W.; Farrar, C.R.; Prime, M.B. A Summary Review of Vibration-Based Damage Identification Methods. Shock. Vib. Dig. 1998, 30, 91–105. [CrossRef] 6. Worden, K.; Friswell, M.I. Modal-Vibration-Based Damage Identification. In Encyclopedia of Structural Health Monitoring; Wiley: Hoboken, NJ, USA, 2008. 7. Yuen, K.-V.; Kuok, S.-C. Ambient interference in long-term monitoring of buildings. Eng. Struct. 2010, 32, 2379–2386. [CrossRef] 8. Gargaro, D.; Rainieri, C.; Fabbrocino, G. Structural and seismic monitoring of the “Cardarelli” Hospital in Campobasso. Procedia Eng. 2017, 199, 936–941. [CrossRef] 9. Butt, F.; Omenzetter, P. Long term seismic response monitoring and finite element modeling of a concrete building considering soil flexibility and non-structural components. SPIE Smart Struct. Mater. Nondestruct. Eval. Health Monit. 2011, 7981, 79811. [CrossRef] 10. Gravett, D.Z.; Mourlas, C.; Taljaard, V.-L.; Bakas, N.; Markou, G.; Papadrakakis, M. New fundamental period formulae for soil-reinforced concrete structures interaction using machine learning algorithms and ANNs. Soil Dyn. Earthq. Eng. 2021, 144, 106656. [CrossRef] 11. NEHRP Consultants Joint Venture. Soil-Structure Interaction for Building Structures. Nist Gcr. 2012, 12, 917–921. 12. Shahriar, M.A.; Sivakugan, N.; Urquhart, A.; Tapiolas, M.; Das, B.M. A study on the influence of ground water level on foundation settlement in cohesionless soil. In 18th International Conference on Soil Mechanics and Geotechnical Engineering: Challenges and Innovations in Geotechnics; ICSMGE: London, UK, 2013; Volume 2, pp. 953–956. 13. Rethati, L. Soil Mechanics and Classification; ISSMGE: London, UK, 2001; Volume 12, pp. 11–33. 14. Latvian Environment Geology and Meteorology Center. Overview of the State of Surface and Groundwater in 2017; Latvian Environment Geology and Meteorology Center: Riga, Latvia, 2018. 15. Di Trapani, F.; Bertagnoli, G.; Ferrotto, M.F.; Gino, D. Empirical equations for direct definition of stress-strain laws for fiber-sections based macro-modelling of infilled frames. ASCE J. Eng. Mech. 2018, 144, 04018101. [CrossRef] 16. Moon, S.-W.; Ng, Y.C.H.; Ku, T. Global semi-empirical relationships for correlating soil unit weight with shear wave velocity by void-ratio function. Can. Geotech. J. 2018, 55, 1193–1199. [CrossRef] 17. Guerdouh, D.; De Jijel, U.; Khalfallah, S.; Polytechnique, E.N. Soil-structure interaction effects on the seismic performance of frame structures. Rev. Constr. 2019, 18, 349–363. [CrossRef] 18. Papadopoulos, M.; Van Beeumen, R.; François, S.; Degrande, G.; Lombaert, G. Computing the modal characteristics of structures considering soil-structure interaction effects. Procedia Eng. 2017, 199, 2414–2419. [CrossRef] 19. Ferreira, H.N.; da Silva, C.A.F. Soil failure in the Luanda Region, geotechnical study of these soils. In Proceedings of the 5th International Conference on Soil Mechanics and Foundation Engineering, Paris, France, 17–22 July 1961; pp. 95–99. 20. Khanna, P.L.; Varghese, P.C.; Hoon, R.C. Bearing pressure and penetration tests on typical soil strara in the region of the Hirakud dam project. In Proceedings of the 3rd International Conference on Soil Mechanics and Foundation Engineering, Zurich, Switzerland, 16–27 August 1953; Volume 1, p. 246. 21. Jcu, R. Settlement of Shallow Foundations Due To Rise of Water Table in Granular Soils. Doctoral Dissertation, James Cook University, Douglas, Australia, 2014. 22. Kullaa, J. Vibration-Based Structural Health Monitoring Under Variable Environmental or Operational Conditions. In New Trends in Vibration Based Structural Health Monitoring; Springer: Vienna, Austria, 2010; Volume 520, pp. 107–181. [CrossRef] 23. Sohn, H. Effects of environmental and operational variability on structural health monitoring. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2006, 365, 539–560. [CrossRef] [PubMed] 24. Reynders, E.; Wursten, G.; De Roeck, G. Output-only structural health monitoring in changing environmental conditions by means of nonlinear system identification. Struct. Health Monit. 2014, 13, 82–93. [CrossRef] 25. Cross, E.J.; Worden, K.; Chen, Q. Cointegration: A novel approach for the removal of environmental trends in structural health monitoring data. Proc. R. Soc. A Math. Phys. Eng. Sci. 2011, 467, 2712–2732. [CrossRef] 26. Erazo, K.; Sen, D.; Nagarajaiah, S.; Sun, L. Vibration-based structural health monitoring under changing environmental conditions using Kalman filtering. Mech. Syst. Signal Process. 2019, 117, 1–15. [CrossRef] 27. Kita, A.; Cavalagli, N.; Ubertini, F. Temperature effects on static and dynamic behavior of Consoli Palace in Gubbio, Italy. Mech. Syst. Signal Process. 2019, 120, 180–202. [CrossRef] 28. Tjirkallis, A.; Kyprianou, A.; Vessiaris, G. Structural Health Monitoring under Varying Environmental Conditions Using Wavelets. Key Eng. Mater. 2013, 569-570, 1218–1225. [CrossRef] 29. Limongelli, M.P.; Çelebi, M. Seismic Structural Health Monitoring: From Theory to Successful Applications; Springer: New York, NY, USA, 2019; ISBN 978-3-030-13976-6. 30. Worden, K.; Farrar, C.R.; Manson, G.; Park, G. The fundamental axioms of structural health monitoring. In Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences; The Royal Society: London, UK, 2007; Volume 463, pp. 1639–1664.
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