مجله علوم و مهندسی زلزله
سال نهم شماره 1 (پیاپی 30، بهار 1401)

Conducting seismic geotechnical microzonation studies from the point of view of site effects requires an assessment of dynamic properties and especially the natural frequency of site in the study area.Common methods for estimating these parameters include using microtremors measurements and real earthquakes. In addition, in order to study the exact behavior of the site and determine the exact seismic geotechnical model of the region, it is necessary to perform numerical analysis and use the results of microtremor experiments and use geological information in the region. Without using numerical methods, solving problems of the frequency domain is usually impossible. For this purpose, numerical methods obtain the relations and equations governing the domain by examining the differential behavior of the problem (the model under study). In this regard, finite element methods and modeling of energy absorbing boundaries to reduce the reflection data of seismic waves have been used in numerical analysis. Also, in microzonation studies, the scale of work must be selected from valid instruction like TC4. For this purpose, the method of grade 3 in TC4 has been used.In this research, the study area of Esfahan has been selected, which is one of the important cities in terms of the existence of numerous historical monuments, relatively high population and strategic industries. Unfortunately, in this region, due to its reputation for safety against earthquakes, very few seismic studies and site effect studies have been conducted. Considering seismic characteristics of site in urban planning and design of structures can provide the basis for sustainable and safe development in cities. Carrying out studies in this regard can prevent the construct of structures in any area, which can be in the resonance period or take the necessary measures to minimize damage.The main geology texture of Esfahan is influenced by two elements. Firstly, the Zayandehrood River, which flows east-west direction in the city of Esfahan, and next the mountains around Esfahan, especially Sofeh Mount, which the main part of the city are affected by its slope.In this study, using one-dimensional and two-dimensional numerical analysis and microtremor results, geotechnical and seismic studies, the seismic geotechnical behavior of Esfahan region has been studied in the north-south direction (parallel to the topographic changes of the city) and east-west along the river.One of the significant points identified in this study is the different seismic geotechnical behavior of the north-south and east-west directions with each other and the fact that the behavior of the north-south profile of Esfahan should be considered due to changes of the surrounding environment.The results show that the behavior of Esfahan site in the north-south direction, which is affected by topographic changes, should be considered as two-dimensional and the east-west direction one-dimensionally can be considered. The results also show that the natural frequency values ​​of Esfahan increase from southern area to northern area respectively. In this study, after related analyzes and comparison of seismic geotechnical results and dynamic analysis, the study area was classified into three categories: The southern areas of the city with a natural period less than 0.4, the central areas with a period range of 0.4 to 0.8 and the northern areas. The city is in the natural period above 0.8 seconds.

Considering the human and financial costs resulting from the destruction of dams, studying the seismic behavior of these structures and controlling their stability against events like earthquakes is crucially important. Numerical models are utilized to carry out this vital process. However, these models possess many assumptions that differ between mathematic models and reality. As a result, experts use system identification methods to ease such problems. Generally, system identification consists of two parts; experimental tests and signal processing. The first part refers to field experiments like ambient vibration tests, and the latter section means analyzing records gathered from the field tests. From the estimated dynamic properties, the mathematical models can be calibrated to evaluate the possible responses of the building under study to future earthquakes.The ambient vibration test, used in this paper, is one of the dynamic structural tests which records natural vibrations such as wind and human activities on the structures. These oscillations stimulate the structure’s dynamic properties on a marginal scale which can be seen and analyzed in seismic records. Recently, an ambient vibration experiment was conducted on the double curvature concrete Dez dam, which is one of the highest dams in the country. The dam is currently 203 meters high, and a project is being undertaken to add around 8 meters to its current height. For this reason, evaluating the dynamic characteristics of its body and calibrating the existing dam’s numerical models is vital. In this regard, the ambient vibration test with 19 seismometers and accelerometers was conducted when the height of the dam reservoir was at its maximum water level (MWL). Sensors were placed on the dam in four different layouts to obtain dynamic mode shapes from the plan and the dam’s height.In this paper, some stationary records have been selected from data recorded within several days, and the data are analyzed with three different methods. These methods are 4-Spectrum, Frequency Domain Decomposition (FDD), and Enhanced Frequency Domain Decomposition (EFDD). Each of the techniques used in this research has advantages and disadvantages. The 4-spectrum method is a reliable method for obtaining the modal frequencies owing to the simultaneous control of the four spectral with each other. However, since all sensors' spectrums must be separately controlled by different source points, some possible errors lie in this approach. In the FDD, the power spectrum density and correlation spectrum are automatically controlled by the computer, and a powerful mathematical tool called SVD is used to analyze the data matrix. The EFDD has a similar structure to the FDD method, while it uses a special algorithm to select the peaks in the spectrums. This makes it slightly more accurate than FDD. Furthermore, to calculate modal damping, the Half Power method was used in all these methods.After analyzing all the records with different system identification methods, natural frequencies in six modes were found in the frequency range of 2.63 Hz to 13.92 Hz and mode shapes in five modes were drawn. Likewise, the damping ratio was estimated at 1.8% to 0.6%, which shows the half-power method is not an accurate way to calculate the damping ratio in concrete dams. Finally, the process of system identification, using field experiments, is an experimental process, for obtaining more accuracy, the results extracted from the Dez dam were compared with the outcome of Birkeh Dam's essay, which has the same features as the Dez dam. The comparison of the system identification results shows that the extracted outcomes are relatively precise, which is suitable for calibrating the existing computer models.

In many vibration-based methods, stiffness matrix variations play an important role in determining the location and extent of damage. In this research, through a bilinear crack model, stiffness matrix components are directly linked to the cracking parameters with the aid of seven coefficients. An innovative method for identifying the number of breathing cracks is developed, in a manner that the information of the initial structure is not needed. To model structures in MATLAB software, a nonlinear dynamic analysis program with the ability to model bilinear cracks is developed. Breathing crack modeling has been done using the softness method and the concept of curvature of the member. In order to determine the open or closed state of the crack, at each moment of vibration, an indicator was applied based on the moment curvature of each element. Due to the crack opening and closure, frequency varies continuously over time. If there is only one crack in the structure, two frequency values for the first mode of vibration are obtained. It is clear that one is related to when the crack is open and the other corresponds to when the crack is closed. By increasing the number of cracks, the number of frequency bands increases nonlinearly. This feature applied to determine the number of damaged points, so that the vibration frequencies were determined and arranged throughout the vibration time. By plotting the ordered frequencies, the points of the same frequency were plotted at a certain level and formed the frequency steps. A significant relationship was found between the number of frequency steps and cracked points. By applying different crack topography in buildings of one, two and five stories, a relationship between frequency steps and number of cracks was formulated. In each incident scenario, the effect of different crack intensity and distribution is investigated through three different cases. The first case is, 20 different crack layouts with a depth of 0.1 of the cross sectional area and random distribution, the second case is, 20 different crack layouts with a depth of 0.3 of the cross sectional area and random distribution, and the last case is, 20 different layouts in which the location and depth of the crack as well as the crack propagation are considered random. The average and standard deviation of frequency steps for each 20 tries are obtained. This study shows that, the variation of frequency steps in terms of the cracks number is similar in different structures. The uniform change of the depth of all cracks is ineffective in increasing the number of frequency steps. However, diversifying cracking due to the combination of asymmetric distribution, increases the number of frequency steps. When about half of the members are cracked, the maximum standard deviation of frequency steps occurs. This dispersion reduces the identification accuracy up to two crack numbers in the studied structures. The curve of variation of frequency steps in terms of the number of cracks is divided into two parts. The first part of the curve is as long as the number of cracks is less than half of the number of members. At this region, the exponential relationship between the two parameters is established. The second part of the curve is when more than half of the number of members are cracked. In this case, the frequency steps show little changes with the change in the number of cracks. In this region, the relationship between the two parameters is different for each structure, but it has a trend of quadratic order. Some of the factors such as axial force which can change the stiffness of the frames at any moment of vibration can affect the number of frequency steps. Investigating the effect of these factors can be the subject of future research.

Generally, in the analysis and design of buildings, the foundation is assumed to be rigid and the effect that soil subsidence under the foundation and the flexibility of the foundation may have on the response of the structure is not considered. If the interaction between the structure, the foundation and its supporting soil environment significantly changes the actual behavior of the structure compared to the study of the behavior of the structure alone, and one of the most important issues in soil-structure interaction is the Diagrid Rocking motion of the structure.Examination of the behavior of structures in past earthquakes shows that asymmetric torsion has been one of the causes of severe vulnerabilities. Considering the advantages of modern seismic design methods in which energy dissipation additives such as dampers are used to control the responses in an earthquake, it is possible to control the seismic torsion in the structure. However, recent earthquakes have shown that Steel structures are damaged by earthquakes, making them very difficult and even impossible to repair. For this reason, after relatively severe earthquakes, these buildings have been damaged and destroyed, and in order to reuse the structure, it is necessary to spend a lot of time and money due to the extent of damage to the structure, and this issue creates a new idea to limit damage specific points of the structure.  In this way, buildings can be exploited more quickly by replacing damaged elements. One of the new methods to improve the seismic performance of steel buildings is the use of systems that limit damage to the structure. Among these methods, we can mention systems with rocking motion. In these systems, the main building behaves elastically so that the energy absorption and nonlinear performance occur only in certain parts of the building that have been predicted. Therefore, in this study, a new system has been introduced, using the mechanism of Rocking movement in the shear walls of the structure, transmits damage to the structural fuses and makes the steel structure safe during and after earthquake, and very repairable. The systems used in this research include Diagrid structural system considering the interaction of soil and structure on the foundation and its comparison with Diagrid structural system. To better model this new system, first design post-tensioned cables as well as the exact details of the column connections in ABAQUS software are designed considering the connection dimensions for the 12th story. Then, in SAP2000 software, structural members were designed for 12-story and 16-story diagrid structures with rocking motion. The Results show the high performance of rocking motion on the foundation in reducing the stresses distributed in the diagrid structure. Because with the post-tensioned cables, the displacement of the structure is greatly reduced, while in the diagrid structure, with the movement of the Diagrid, the displacement of the structure is increased by 20%, but due to high attenuation, it has good seismic performance. It has been shown because the number of plastic joints in the LS area has reached less than that. The use of a controlled rocking motion system significantly reduces axial force in structural members by about 30 % and retractable cables in the rocking drive system have an effect more than 70 % in reducing the deformation of the structure and then the flow damper is placed.

One of the most critical parameters in the process of analysis and design of structures is determination of the fundamental period of vibration. The fundamental period depends on the distribution of the mass and stiffness of the structure. However, the building codes propose some empirical equations based on the observed period of real buildings during an earthquake as well as ambient vibration tests. Furthermore, the majority of these proposals do not take into account the presence of infills walls into the structure despite the fact that infill walls increase the stiffness and mass of structure leading to significant changes in the fundamental period numerical value. These equations are usually a function of type and height of the buildings. The different values between the empirical and analytical periods are due to the elimination of non-structural effects in the analytical methods. For this reason, the presence of non-structural elements such as infill panels should be carefully considered. Another effective parameter on the fundamental period is the effect of Soil-Structure Interaction (SSI). It is obvious that soil flexibility increases the fundamental period of the structure. In most of the seismic building codes, the role of the SSI is usually considered beneficial to the structural system under seismic loading since it increases the fundamental period and leads to higher damping of the system. Recent case studies and post-seismic observations suggest that the SSI can be detrimental and neglecting its effects could lead to the unsafe design for both the superstructure and the foundation, especially for the structures located on soft soil. The current research deals with the effect of infill panels on the fundamental period of steel moment resisting (MRF) and eccentrically braced frames (EBF) considering the influence of soil-structure interaction. In this study, all of the buildings designed under gravity and earthquake loading have been evaluated by utilizing 3-D FE modeling incorporating Eigenvalue-analysis to obtain the elastic periods of vibration. For this purpose, the effect of building height, the infill wall panel stiffness, various percentage of infill openings as well as the effect of soil structure interaction in 3, 6, 9, 12, 15 and 18-story 3-D frames were investigated. The studied frames were modeled and analyzed in SeismoStruct software. The calculated values of the fundamental period were compared with those obtained from proposed equation in the seismic code. The results have shown that the number of stories and the soil type are critical parameters that influence the fundamental period of steel frames. Also, it has been found that the height of structures significantly influences the fundamental period. As it is known, in the absence of infill walls, the numerical fundamental periods have generally higher values than those obtained from the empirical formula recommended in building codes such as Iranian Standard No. 2800 and FEMA450 and the other codes. The presence of infill wall leads to a considerable decrease on the fundamental period of steel frames. This decreasing is strongly dependent on the infill wall panel stiffness. In other words, an increase of the infill wall panel stiffness reduces the fundamental period. Also as the infill opening percentage increases, the fundamental period of the structure almost linearly increases. The soil-structure interaction strongly affects the fundamental period of structures. The fundamental period is higher for more flexible soil types. Furthermore, the influence of soil-structure interaction is higher when the infill wall stiffness is higher. Based on the results, one can conclude that the fundamental period of a building cannot be predicted by only using the height of the building. Finally, new equations, which are a function of the selected parameters (building height, modulus of elasticity and the infill wall thickness, infill wall percentage and soil type), are also proposed for predicting the fundamental period of MRF and EBF buildings.

After the Northridge earthquake in 1994, numerous reports of failures due to brittle fractures of the weld joint for steel structures were presented in various Codes. One of the ideas to solve this problem was to promote the plastic hinge away from the weld through the intentional weakening of the beam at a certain distance from the column. In this idea, by weakening the top and bottom flanges of the beam at a certain distance from the column, the ductility of the connection is increased and brittle failure in the weld area is prevented. The existing steel frame buildings that are designed according to Pre-Northridge seismic provisions need to be rehabilitated to prevent the connections from experiencing brittle fracture at their welds. The presence of concrete slab in existing steel buildings imposes economic problems in retrofit projects. Asymmetrically weakening the beam is considered as an appropriate method for seismic rehabilitation of steel frame connections in which the rehabilitation action is conducted through intentional weakening the bottom flange of the beam and without the difficulty of removing concrete slab. Two techniques “reduction” and “heat induction” are among suggested methods for asymmetric weakening of the beam. In the “reduction” technique, the weakening action is conducted by cutting some parts of the beam bottom flange at a certain distance from the connection. In the “heat induction” technique, the weakening action is conducted by applying a special process of heating to the bottom flange of the beam at a certain distance from the column. In this heating process, which reduces the yield and ultimate strength by 35% and 25%, respectively, in other words the steel is annealed. This drop in strength in the heated area causes the plastic hinge to move over the beam. The main purpose of this study is to investigate and compare the seismic behavior of low-, medium-, and high-rise 2-D steel frames improved through these two techniques “reduction” and “heat induction” under far-field and near-field earthquakes, numerically. Two types of verification are conducted to ensure the accuracy of numerical modeling. First, 2-D rehabilitated connections through two “reduction” and “heat induction” techniques are verified with experimental results. Then, three 2-D frames are verified with Gupta & Krawinkler results. The results of the frames analysis showed that inter-story drift and total rotation of rehabilitated frames by “reduction” technique were on average 15 percent more than rehabilitated frames by “heat induction” technique. This indicates the defect of the improved connection through the “reduction” technique in low elastic stiffness and torsional-lateral instability compared to the "heat application" technique. In addition, it was determined, as the height of the frame increases, the effectiveness of far-field earthquakes decreases and near field earthquakes show their effects on the structure more. As the ratio of the earthquake pulse period to the main period of the structure increases, the imposed deformation on the frames increase. So that, if this ratio is equal to one, the maximum relative drift for the frames is estimated.

In recent years, the use of helical screws in reinforced concrete components has been developed. Practical results of these experiments have shown that spiral reinforcement (SR) has improved seismic performance compared to other conventional methods. Experience has shown that by applying torsion in concrete members, due to the principal tensile stresses in the diameter of each element, the member elements of torsional structures with spiral patterns will expand. Due to the design of the expansion of torsional cracks in the cross-section, these cracks are relatively ideally perpendicular to the cross-section of the torsion reinforcement. The research results so far clearly show that the use of rectangular helical bolts increases the bearing capacity. Also, in this method, with a constant ratio of transverse reinforcement, the ductility condition is improved compared to conventional bends in workshops. Finally, the use of helical reinforcement in structural members increases the shear, flexural and axial strength. Commonly used braces require two end hooks to support the anchor. For the length of these two hooks, a large amount of steel material is needed for each closing brace, which increases the weight and price of the steel. This process is not required, and this type of reinforcement reduces the weight of the steel and saves money due to the reduction of steel consumption compared to conventional braces. The effect of SR on the formation of cracks and the member's behaviour after cracks has also been tested.According to the researches, it is observed that so far, the effect of continuous closures as well as the effect of changing different parameters in the enclosure in columns with a square cross-section with continuous rebars has not been done. Regarding the confinement of concrete members with reinforcement, considering the cases mentioned about the importance and role of transverse struts in the performance of structural components, the purpose of this study is to investigate the performance of helical reinforcement in comparison with conventional transverse reinforcements in square reinforced concrete columns.  Also, the effect of changing various parameters such as yield stress of reinforcements (high-strength steel with normal strength), cross-section reinforcement and cross-sectional column dimensions on behavioral performance, including load-bearing capacity and ductility, in columns with this type of arch and analysis. In this study, the finite element analysis method has been used. The results show that the capacity and ductility of the column enclosed by the winding is higher than that of the column retained by the conventional tension. Also, in the column held by the twist, the column withstands less stress after loading, so it can be said less damage in this column will be obtained. In the load-displacement curves drawn for the column, at the beginning of the vertical branch up to a certain load, the behaviour of the columns is the same. Until the peak is reached, the stiffness decreases with increasing stiffness, and in the area after the apex, the softening behaviour increase. As the spacing of the bolts and the pitch of the windings increases, the behaviour of the columns in the two modes becomes closer to each other. It shows a relatively similar behaviour, and the difference in their capacity also becomes much smaller. Ductility is also reduced. Increasing the pitch of the windings has a greater effect on the capacity of the column than increasing the distance of the windings. By increasing the distance of the turns, the capacity of the column enclosed by the tortoise decreases sharply. However, in these conditions, the column enclosed by the tortoise performs better than the column enclosed by the arch. The results show that by increasing the cross-sectional dimensions of the column, the rebars withstand less stress, and the column capacity also increases. Also, keeping the number of rebars constant and increasing the cross-sectional dimensions to a certain extent increases the capacity, and then increasing the dimensions without increasing the number of rebars does not have much effect on the column capacity. The results show that with the increasing yield stress of the rebars, the capacity of the column has increased. Increasing the strength of the rebars reduces the stress in the concrete, resulting in less damage.

Confined masonry buildings, in which all or part of gravity loads and all of lateral loads on both main directions of the building are resisted by unreinforced masonry walls, have been widely used in Mediterranean Europe, Latin America, the Middle East, south Asia, and the Far East. Experiences obtained from past earthquakes and experimental results indicate that confined masonry buildings, if properly built, exhibits an adequate seismic response. Consequently, it represents a good choice in those seismic regions where masonry is widely used due to economical or traditional reasons.In this paper, first, the functions of tie beam and tie column and the similarity of seismic behavior of the confined masonry walls and the filled frame have been studied. The structural response of confined masonry and infilled frames under in-plan lateral loading is similar, despite the different construction techniques. In both cases, structural separation occurs at the initial stage. After this separation, a diagonal compressive stress field is formed in the masonry. In the following, the field study of five masonry buildings after the Sarpol-e Zahab earthquake have been presented. The buildings have been selected in such a way that all types according to the scope of this research have been investigated. These types include: 1. Two-story masonry building with tie-beam, 2. Two-story masonry building with tie beam and tie column, 3. Two-story masonry building with tie beam and tie column in first story and tie beam in second story, 4. One-story masonry building with tie-beam, 5. One-story masonry building with tie beam and tie column. The results of this study show that the seismic behavior of confined masonry buildings in comparison with a lot of severely damaged buildings with steel and reinforced concrete structures, is much more desirable. In the following, the behavior of two main types of two-story masonry building in Sarpol-e Zahab including masonry wall with tie-beam and masonry wall with tie-beam and tie-column is analyzed as finite element models in Abaqus. By using finite element linear analysis, the load path and cracks in the confined masonry walls can be studied with good accuracy. In the condition that there are only tie-beams in the building and gravity load and seismic force are applied to the wall, the maximum stress is formed in the diagonal path in the middle of the first story wall. Consequently, the first cracks will form in the same path when seismic force creates a shear stress more than shear strength of the wall under gravity load. In the condition that there are both tie-beams and tie-columns in the building, in the initial earthquake cycles, when the wall and ties have a good connection and seismic force is applied from left to right, the shear stress at the junction of the left side of the tie-columns to the right side of the walls is maximum and as a result, the walls will separate from the ties in these locations. In the next cycle, in which the direction of the earthquake is from right to left, the shear stress at the junction of the right side of the tie-columns to the left side of the walls is maximum and as a result, the walls will separate from the ties in these locations. These changes in direction of the earthquake will lead to the separation of the walls from the tie-columns in the initial earthquake cycles. Therefore, the behaviour of confined masonry wall will be similar to the infilled frame in subsequent cycles. Accordingly, the maximum shear stress path will be formed in the diagonal of the bays confined by the tie-beams and tie-columns. Consequently, the first cracks in the bearing walls of confined masonry buildings will form in this path. The results of the study show that the failure modes obtained from the finite element analyses are well matched to the actual cases damaged in the Sarpol-e Zahab earthquake.