BULLETIN OF EARTHQUAKE SCIENCE AND ENGINEERING
Year:2022 | Volume:9 | Issue:2|Papers Count:8

One of the numerical methods in the mechanics of continuous environments is the boundary element method. In this method, the governing differential equations will be converted to integral equations and applied to the problem boundary. Then the boundary is divided into boundary parts and numerical integration is performed on the boundary elements, from the solution of which a single solution of the problem can be obtained. In the boundary element method, the partial differential equations defined within a space are converted to integral equations at the boundaries of that space, which reduces one dimension of the problem. For example, an elastodynamic problem defined in a two-dimensional space is replaced by an integral equation problem defined at its boundaries that has one dimension. If the problem defined in a three-dimensional space is replaced by a two-dimensional integral equation problem. Finally, the integral equations will be solved numerically by dividing the boundaries into a network of finite element discrete elements. The boundary element method can be easily applied to borders with complex geometry. Boundary element method or boundary integral equation (BIEM) is one of the numerical modeling methods that have many applications in numerical simulation of fault dynamics. Its results provide a broad view of the physics of earthquake rupture. To solve two-dimensional problems, the numerical technique of the boundary element method has been widely used. The boundary element method has been used to model the behavior of faults of overlapping centers, the growth of junction assemblies, veins and seismic analysis of topographic features. One form of BEM is based on separation, which is called the displacement discontinuity method. The theory of detachments in elastic materials has been widely used for more than half a century to evaluate the displacement, stress and strain fields around faults. By integrating Green's functions, the displacement field around the discontinuity surface can be calculated. These displacement fields are the Navier equations that are the governing equations of linear elastic theory. Strain components are obtained from the spatial derivatives of the displacement components, and the stress components can be calculated using Hooke's law for homogeneous and homogeneous elastic materials. Therefore, the mathematical tool of detachment theory is able to calculate the displacement, stress and strain fields around faults in half-elastic space, but it is less accurate compared to geophysical data. In this paper, numerical modeling of faults using the boundary element method has been reviewed, and studies conducted in the field of numerical modeling of faults using the boundary element method have been reviewed. Finally, the results show that the boundary element method is suitable for problems with complex boundaries such as fault geometry and problems with infinite boundaries. It is also possible to predict slip on the fault and surface deformation using numerical modeling using the boundary element method. The results of numerical modeling of the fault using the boundary element method and comparing these results with the observed values ​​show that the boundary element method is a suitable method for predicting issues such as slip distribution on the fault, surface displacement and slope instability. Also, the boundary element method is a suitable method for predicting the location of smaller and secondary faults. Therefore, it can be said that the boundary element method is a powerful tool for numerical modeling of earthquake or fault rupture dynamics. In the articles studied in this study, the slip distribution at the fault surface has been determined using the inversion of surface displacements resulting from the main earthquake. If aftershocks also cause surface displacements, then as a suggestion for future research, the inversion of surface displacements caused by aftershocks can also be used to determine the secondary slips created on the fault surface.

Seismic stress analysis and investigation of soil’s lateral pressure on buried pipelines are among the challenging subjects in the seismic design of lifelines. Buried pipelines are considered as long structures buried in a semi-infinite medium of soil and be modeled as beams on spring support of the soil, in a conventional technique. The specifications of the support springs were determined using the codes’ relations (such as ASCE 1984 and ALA2001 guidelines). The specifications of pipe-soil equivalent springs are presented as a function of the diameter of the pipe and soil’s mechanical properties. Numerous researches were shown that the pipe-soil interaction is highly dependent on other factors; such as the wall thickness of the pipe, material type of the pipe, and the loading condition as well. In this research, in the first step, two full-scale tests on steel continuous pipes buried in dense sandy soil were done. A computer program (BPSIOS) was developed in MATLAB and ABAQUS to generate the specifications of pipe-soil interaction springs through an optimization algorithm. BPSIOS determined the force-displacement relation for the pipe-soil equivalent springs so that the deformed shapes of the pipe were matched with its real values for any given base displacement. The experimental data were analyzed by BPSIOS to present a comparison with the code’s results. The numerical models were validated and verified using the experimental data in the second part of the study. The finite element simulations were employed to develop a numerical database. The database included twenty-four nonlinear 3D shell-solid finite element models. They were constructed and analyzed in ABAQUS. The database consisted of steel buried pipes with four diameters (in the range of 100 to 400 mm) and six-level of wall thickness (from D/t = 11 to 48). The pipes were widely used in supply lines in natural gas distribution networks. The results of the models were fed into BPSIOS software to determine the specifications of pipe-soil interaction springs. Considering the wall thickness of the pipe, a new relation was established for lateral pipe-soil interaction of dense sandy soil and steel pipes in place of a strike-slip fault. The sensitivity and 3D regression analyses were done, using MATLAB Curve-fitting Toolbox, to develop a new form of relations for pipe soil interaction. The sensitivity analyses showed that the wall thickness of the pipe had a great level of effect on the exerted lateral force on the pipe. The results of the numerical models, made by BPSIOS, show that the ASCE1984 presented results are only valid in the case of small pipes with D/t = 48. However, the ALA2001 guideline presents greater values of the interaction forces, which are closer to the results for moderate pipes. The proposed pipe-soil relation is dependent on the diameter and wall thickness of the pipe and exclusively specified for the material properties of the steel pipe, dense sandy soil, and strike-slip loading condition. It was found that the wall thickness of the pipe had a great effect on the initial and secondary stiffnesses of the pipe-soil equivalent springs. However, the yield displacements were found in the previously presented values for most of the cases.

Examination of the behavior of structures in past earthquakes shows that the 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 concrete 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. In this way, buildings can be exploited more quickly by replacing damaged elements. One of the new methods to improve the seismic performance of concrete buildings is the use of systems that limit damage to the structure. Among these methods, we can mention systems with rocking motion, in which 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 cradle movement in the shear walls of the structure, transmits damage to the structural fuses and makes the concrete structure safe in earthquake and after, and very repairable. Precise details of connections, design and nonlinear analysis of this system have been done in ABAQUS and SAP2000. The solid element was used to model the rocking system in ABAQUS and concrete damage palsticity model was used to modeling the concrete, which is used to model the nonlinear behavior of concrete. The contact between the steel bolts and the concrete shear wall is simulated using the contact element.The concrete shear wall in this method remains in the elastic range, but the dampers connected to the shear wall due to the elevation of the shear wall absorb most of the seismic force. It should be noted that the system reversibility is provided by post-tensioned cables. The torsion in the structure is of the concentrated mass type and the exit from the 5%, 10% and 20% axes is applied in the concrete structure. The results show that the use of this new system in comparison with concrete structures without it effectively reduces the damage to the structure and the concrete structure remains intact and by changing the retraction force of retracted cables seismic torsion control is performed. Also, the functional levels of the structure remain in the IO area. The use of a controlled rocking motion system significantly reduces axial force in structural members by about 30%. Post-tensioned cables in the cradle drive system have a more than 70% effect on reducing the deformation of the structure and then the flow damper is placed. According to the written DEMATEL code, the tensile strength and stiffness of the structure and the stiffness of the Gap element are the most effective; besides, the vibration mode and displacement of the structure are the most effective factors in controlled rocking motion. The use of a new repairable shear wall with rocking motion has caused the vibration mode to dominate the structure of the first vibration mode and the distance between the torsion mode, and the first and the second modes are very large.

One of the most important determinants of securing the structures in seismic zone is reinforcement of the structures for resisting against the lateral loads. In the recent years, for reinforcing lateral load-resisting systems, implementing applicable elements such as precast concrete and steel plate shear walls as a new approach has attracted many scholars in the area of construction industry. This scholarly attraction refers to the potential benefits of using these advanced operational techniques from the perspective of being affordable, expediting operations and flexibility. Reviewing the research background in the areas of precast concrete frame and steel shear walls reveals that in the past studies, the aforementioned elements have been studied separately and despite the attention of the researchers in the field of civil engineering, none of them have concentrated simultaneously on the systems of precast concrete frame and the steel shear wall together in their studies. Another aspect to consider is the fact that new and modern cities are in the dire need of industrialized construction. This is an important issue since need for quality habitat grows more and more because of high rate of population in the expanding cities. Use of precast segments is a solution in response to this significant demand. Therefore, the present research focuses on the system of precast concrete structures with steel shear walls. In particular, the purpose of this study is to investigate the effect of steel shear wall as a lateral load-resisting element on precast concrete frames using cyclic analysis. In order to achieve the research objectives and performing cyclic analysis, the OpenSEES software was utilized. There are several options in OpenSEES for nonlinear modeling of structural members. They include distributed and concentrated plasticity options. The distributed nonlinearity is closer to reality as experience shows that nonlinear action is actually distributed along a certain portion of members before total failure. On the other hand, use of distributed plasticity option encounters certain difficulties when calculating with reverse loading conditions. The concentrated plasticity option is preferred for the same reason as it has proved to yield results that are close enough to representative response values determined using the other option. That is why the concentrated plasticity computation framework was selected in this research. Moreover, and to the same extent of discussion, different nonlinear stress-strain models can be picked up in OpenSEES. The Steel02 and Concrete02 material constitutive relations benefit from both simplicity and accuracy. The same relations were chosen to cope with the needs of the current research work. For this purpose, the relative behavior of precast concrete frames with a steel shear wall in buildings with different numbers of floors (3, 5 and 10 floors) and different beam-column connections categorized based on their stiffness, strength and construction details as rigid and semi-rigid, were analyzed. Additionally, the behavior of precast concrete frames with steel shear walls (with semi-rigid connections) and concrete frames without steel shear walls (with rigid connection) in buildings having the stated number of floors were examined on the basis of the aforementioned parameters. In general, the results generated by the analysis confirmed the positive and appropriate effect of steel shear walls on the lateral behavior of the precast concrete frame. The results of the research clearly showed that the use of steel shear walls in precast concrete buildings could reinforce lateral load-resisting system and could increase the safety of the structure in the seismic zones. Therefore, this research contributes to the literature by recommending the use of steel shear walls in precast concrete buildings and suggests further studies on this system, including use of a wider range of buildings and earthquake ground motions on various soil types.

In seismic countries, with regard to the damages and losses caused by ground motion, the earthquake resistant design of structures is a critical matter. Previous earthquakes have shown that the regulation in seismic codes does not guarantee the buildings to resist against seismic forces and may cause severe damage in the structures. This subject goes back to the design philosophy in seismic code and also weakness in the hypothesis of seismic design of structures. Noted that, the earthquake records include many inherent parameters affected the response of the structures. Meanwhile the seismic codes use only a few of them in the designing process such as spectral acceleration at the first mode period, i.e., Sa(T1) and well-known parameter PGA. In recent decades, many studies focus on the correlation between seismic parameters and the response of structures and found out that the used parameters in the codes do not have the best correlation with the response of structures. This finding depends on the databank of the records including the site-to-fault distance intensity of the earthquake, frequency content etc. as well as the type of earthquake resistant structure. This paper deals with a comprehensive study on the correlation between both far-field and near-field earthquake parameters and response of moment resisting steel frames. For this purpose, a series of nonlinear time history analyses were carried out on 3, 6, 9, 12, 15 and 20 moment resisting steel frame using OpenSEES. 107 records pair included 100 far-field records and 114 near-field records were selected from FEMA P695 as well as Pacific Earthquake Engineering Research (PEER) databases. 26 seismic parameters were extracted from these records, i.e. 5564 earthquake parameters were considered. Two damage indices including maximum inter-story drift (MISD) and the amount of absorbed energy were calculated in studied frames. To assess the relationship between seismic parameters and structural damage Pearson correlation coefficient was determined. The results show that Corvoda, Sa(T1), PGV, velocity spectral intensity (VSI), Housner intensity have the best correlation with response of structure in MISD and structure absorb energy in both far-field and near-field earthquakes. Also, the correlation coefficient of displacement dependent parameters is enhanced by increasing the number of stories. Moreover, the results indicate a weak correlation between PGA and response of structures for both MISD and absorbed energy as reported by previous researches. It should be noted that due to the rich frequency content of near-field earthquakes, the correlation of their parameters with seismic response of the studied frames was sensitive to the frames height. Noted that the short period structures (short height) are named acceleration sensitive and the medium and high rise building are recognized as velocity and displacement sensitive, respectively. Therefore, because of rich frequency content of near-field earthquakes, the correlation between acceleration based parameters such as PGA and displacement response of the structures are reduced by increasing the frame height. In a contrary manner, the correlation between damage indices of the structures with displacement based parameters is increased by increasing the frame height. Meanwhile, the correlation between damage indices of the structures with velocity based parameters such as PGV is the most for mid-rise structures. According to the result it can be concluded that the parameters depended on velocity like Housner intensity and velocity spectral intensity have the best correlation with MISD and absorbed energy. Also, the parameters depended on earthquake duration like Significant Duration and Effective Duration have weak correlation with the response of structures. It is shown that one cannot expect the longer duration earthquake result in more damage i.e. maximum story drift in structures.

Research aim: Cold-formed steel (CFS) members form the main structure of lightweight steel frames (LSF). Structures made with these members, due to their lightweight compared to structures made with other building materials, can be considered as an appropriate solution to prevent and reduce earthquake vulnerability in seismic zones. On the other hand, geology evidence shows that due to the high density of active faults in Iran, this zone has a high seismic potential; as a result, it can be found that a large part of the country's population lives in the fault zone. In recent years, regarding the use of lightweight steel structures, providing lateral bearing capacity as a challenging subject has led to various studies in this field. These studies show that the study of the performance of CFS shear walls under near-fault field earthquakes is one of the subjects that has less addressed. In the present paper, the lateral performance of CFS shear walls with flat steel sheet sheathing has been studied based on non-linear dynamic analysis in the near-fault field. Research methodology: Whereas today, the use of modeling software in the field of lightweight steel structures consider an appropriate alternative to some time-consuming and costly studies with experimental tests. In this research, the OpenSees finite element software has been used for studies. Considering that previous studies in the field of CFS shear walls more focus on static loading (monotonic and cyclic); first, with the help of two presented numerical models, the existing models have been developed for the appropriate ‎prediction of CFS shear wall responses under non-linear dynamic loading. It should be noted that in this research, the use of OpenSees has been preferred over other software for two main reasons: 1) Considering that CFS shear walls with steel sheet sheathing due to the early onset of shear buckling in the sheathing, experience significant pinching of the strength ‎versus displacement response; the presence of an extensive ‎library of materials and elements in the OpenSees, makes it possible to study lateral performance of CFS shear walls with pinching effects without the need for subroutine ‎coding; and 2) The high speed of the OpenSees in modeling and analysis, provides the possibility of numerous calculations and thus the study of the seismic performance of shear walls under multiple earthquakes. Also, considering that often the models presented in OpenSees for modeling CFS shear walls, contain data dependent to experimental tests; one of the most important features of the numerical model developed in this research is the reduce dependence from the conditions and results of experimental tests that provides ‎the modeling of CSF shear walls with new configurations in a simpler way. It can be found that the improvement of numerical models that in defining their data is not so much required time-consuming and costly experimental tests, to study the performance of this type of lateral bearing systems is efficient. Due to this issue in the following, 18 specimens ‎of single-storey CFS shear wall have been modeled with ‎different configurations and have ‎been analyzed and studied in the near-fault field under a set of no pulse-like and pulse-like earthquakes.‎ Findings: The results showed that the developed numerical model is able to predict CFS shear wall responses under non-linear dynamic loading and similar to previous studies, in order to appropriate predict the seismic performance of shear walls, it is necessary to consider more details in the numerical model. In order to better evaluating the performance of shear walls, the measurement of the used steels is necessary and a definite result only based on the nominal specifications cannot be reached. The spacing of the fasteners affects the response of the shear wall specimens and in addition, long spaces of fasteners, especially in pulse-like earthquakes, cause undesirable shear wall performance.

The present paper focuses on the development of fragility curves and on the seismic vulnerability assessment of existing steel structural systems with a certain type of semi-rigid connection known as Khorjini (Saddle) connections. A significant number of such building type suffered from extensive damage to total collapse in 1990 Manjil (Northern Iran) M 7.4 earthquake. In this study, several variants of two-dimensional structural models with such connections including concentrical braced frames only, infilled frames only, and a combination of both, have been investigated. Three- and five-story models have been considered as they are the most common structures with Saddle connections in Iran. Development of fragility curves have been completed based on HAZUS methodology. The methodology benefits from a damage estimation approach based on the capacity spectrum method. Previous studies have proved that the methodology adopted in HAZUS documents is applicable in risk assessment studies. The methodology requires an inelastic demand spectrum as well as the capacity curve of the structure presented in Acceleration-Displacement Response Spectrum (ADRS) format. The former is derived by reducing elastic design spectral ordinates to account for hysteretic damping, while the latter is obtained from nonlinear static (pushover) analysis of the nonlinear model of the structural system. For the design spectrum, the NEHRP acceleration spectra at two levels, 2% probability of exceedance in 50 years (level 1) and 10% probability of exceedance in 50 years (level 2) were utilized. Nonlinear models of the buildings are constructed in OpenSees platform utilizing nonlinear beam-column elements with their plasticity behavior concentrated at both ends of the elements (zero length nonlinear rotational springs). Saddle connections are modeled using two torsional springs to model their rotational constraining effects at beam-columns joints. Moment-rotation responses of such springs were calibrated to match available experimental results with similar connection details considered in this study and in relevant literatures. Moreover, all infill panels are modeled using equivalent compression struts based on the current acceptable approaches found in reference literature. Performance points of the structures are found by intersecting the demand spectrum with the capacity curve of the structure. The final output of the HAZUS methodology is the fragility curve of the structure, which depicts the probability of exceedance of certain levels of drift-based performance levels according to peak ground acceleration (PGA) values. Nevertheless, uncertainties are incorporated in the fragility curves development process. Results show that the probability of exceedance of Immediate Occupancy (IO), Life Safety (LS) and Collapse Prevention (CP) performance levels in 3-story models subjected to the level 1 earthquake (475 return period) are, respectively, 50%, 23% and 12%; while the corresponding values for the level 2 earthquake (2475 values) are 87%, 70% and 56%, respectively. For the 5-story models, the probabilities are 64%, 29% and 16% for the level 1 earthquake and 96%, 81% and 67% for level 2 earthquake. Due to such high levels of vulnerabilities, seismic retrofitting of such existing buildings in the city of Tehran is absolutely essential.

This is a study of the seismic behavior of earth slopes exposed to vertical Ricker shear waves. Two-dimensional FLAC2D software based on the finite difference method was used for modeling. Layering in earth fill exists in natural situation, and the seismic response of the earth fills is mostly dependent on these different soil layers. When the situation of the weak layer changes in height of the earth fills, we expect to see different responses in seismic behavior of the system. The Mohr-Coulomb criterion is used to investigate the nonlinear soil behavior. To investigate the effects of the input wave frequency on the horizontal acceleration amplification coefficient (considered in this study as the response), four different frequencies, to investigate the effects of the input movement level on the response, four acceleration amplitudes, to investigate the effects of angle, three slopes with various angles and five layering modes are considered for the layering effects. This study presents the results of a numerical study on the seismic behavior of two-dimensional semi-sine shaped hills that were subjected to vertically propagating incident SV wave of the Ricker wavelet. The finite difference software is used to model and analyze the different sizes of the hills. Concentration is on topographic effects, so parameters such as shape factor (the ratio of the height to half width of the hill) and the type of ground took into account in this research. Another variable is the soil type that the difference being in ρ and vs. Therefore, according to the by Code 2800, three soil types have been investigated, each of which has been affected by the Ricker wave. The results show that the horizontal acceleration amplification coefficient overlay decreases with increasing movement and angle. Four different frequencies 1, 3, 5 and 10 Hz are studied in input motion Ricker wave. These different input motion frequencies are selected to evaluate the response of system to these frequencies when natural frequencies of soil layers differ. The effects of the frequency and the layering of the soil are also correlated so that if the input wave frequency is close to the natural frequency of the soil, in each layering mode, that layering mode reports the maximum value for the horizontal acceleration amplification coefficient. Generally, in the presence of a relatively weak layer (with Type III material), the response decreases. However, the high frequency does not follow this trend and offers significant responses. The highest amplification in horizontal acceleration happens in high frequency of soil layers or frequencies like them. The obtained responses are sharply dependent on input motion frequencies and also natural frequencies of soil layers. As it can be seen, when natural frequencies of soil layers and input motion frequencies are near each other, the highest responses are evaluated, which we can say that the resonance is happened. In this case, the highest horizontal acceleration in response results is evaluated. When weak layer exists in different soil layers, the de-amplification happens. The results show that the most de-amplification happens due to the weak soil layer. Also, results show that this high de-amplification happens in weak soil layer, and refers to the horizontal acceleration response.