مجله علوم و مهندسی زلزله
سال دهم شماره 2 (پیاپی 35، تابستان 1402)

Historical background attention to seismic events in the near-field ground motion goes back to the Parkfield (1966) and San Fernando (1971) earthquakes. In fact, the occurrence of earthquakes and the concentrated presence of population centers in earthquake- stricken areas have further clarified the destructive nature of seismic events in near-fault area for structural and earthquake engineers. Recognition and collection of recorded data from earthquakes in the near-fault area that have different effects can be effective in reducing damage to structures and can also be used in seismological studies and engineering research; therefore, forming a Database of  the near-field ground motions is necessary and vital. The most important effects of strong ground motion in the near-field are   long-period pulses due to the rupture directivity effect in the vicinity of the earthquake source, which appears   mainly in the direction perpendicular to the fault and in a short time, apply a lot of energy to the structure.In this study, the near-field records of Iranian earthquakes to evaluate the status of pulses long-period have been investigated. The studied seismic database includes 450 events of recorded earthquakes in Iran, which have been recorded and stored since the establishment of the Iran strong motion network until 2014.All of the selected earthquakes have characteristics of magnitude, epicentral distance, geological characteristics (soil type), location of accelerometer, date and time of event, focal depth, maximum ground surface acceleration, maximum ground surface velocity, and maximum ground surface displacement. It should be noted that the moment magnitude of all selected events is equal to a value between 5 and 7, and their epicentral distance is less than 30 km from the event location.Near-field records have been separated from other data due to their Forward Directivity and pulses long-period, and a total of 77 records of near-fault strong ground motion were identified with Forward Directivity.The selected data have the special characteristics of near-fault records that have pulses in the acceleration or velocity time history record, and their characteristics include a short period of strong ground motion, rupture directivity effects, and low-frequency pulses in velocity time history. The selected data in this study have the characteristics and values that have been mentioned, so it is possible that there are other data that have the characteristics of near-fault records and have long-period pulses in the earthquake record and at a distance of more than 30 km or outside from the moment magnitude range of 5-7.In general, the result of this research has led to the preparation of the database of the near-fault with the country's pulse-like records from the beginning of its establishment (in 1973 using the SMA1 analog accelerometer) until 2014 of the Iran strong motion network of the research center of the Ministry of Roads and Urban Development. The database collected in 60 stations (77 records) has provided comprehensive and reliable information about parameters such as moment magnitude, epicenter distance, focal depth, soil type, Predominant Period (Tp), Endurance Time. This database can be used as a reference to be used in projects, reports and researches of students at different levels of education, technical and engineering offices and companies, seismic hazard analysis studies, earthquake engineering projects and researches that require the use of near-field ground motion in Iran. The use of this refined database for the design of important structures in addition to the development of earthquake engineering knowledge is very important

Calculating the input motion to the foundation as a result of kinematic interaction is one of the challenging issues in the topic of soil-structure interaction, which is accompanied by computational difficulties. In order to overcome these difficulties, approximate approaches have been presented in the past, which are based on the relations of design and evaluation regulations. The averaging method is one of the conventional approximate methods for calculating the horizontal and rotational displacement component of the foundation, which is used by structural design engineers. In the past, the acceptable approximation of this method in estimating the input motions imposed on foundations, having full contact with the surrounding environment, has been approved by researchers. However, the accuracy of this approach in the case of incomplete contact of the foundation with the surrounding soil has not been investigated so far. However, for many approaches in the construction sequences of structures, the lack of contact between the foundation walls and the surrounding environment is inevitable. In this research, the capability of the averaging method to estimate the input motion to the foundations, assuming the possibility of incomplete contact between the foundation and the surrounding environment in three-dimensional mode, has been investigated. According to the results, in the case that the contact of the walls with the soil is complete, the maximum estimation error in the horizontal component reaches 30% and in the rotation component, this value is about 20%. Nevertheless, the averaging method has been able to correctly predict the trend of the graphs in the same case and resulted in a relatively acceptable approximation. When the contact state is reduced from the full state to 75% of the wall surface, the averaging method still provides reasonable results for both components. In the rotational component, the maximum estimated error is 15%, which is a lower error than the full contact state, but the frequency at which the maximum rotation occures has not been detected correctly. For the horizontal component in this case, the error is 30%. Further, by reducing the contact surface and reaching a regular incomplete contact state of 50%, the values of the horizontal component are almost predicted correctly both in ordinate and trend. However the differences have become more significant in the estimation of the rotational component. In the contact state of 25%, the horizontal displacement component is well estimated by the averaging method, but in the rotational component, the values are close to zero, which show a significant difference with the actual results. The weakness of the averaging method intensifies in the case of zero wall contact and the rotational component about the y axis is completely vanished. The inability of the averaging method to detect the existence of a component, maybe more critical for imperfect irregular contact modes and may occur many times in an uncontrolled manner. Examples of such wrong approximations are presented in this research. Finally, it can be stated that using the averaging method in estimating the input motion to foundations with incomplete contact with the surrounding environment can be associated with large errors and can unconservatively overshadow the seismic design and evaluation procedures.

In geotechnical engineering, stability analyses are used to predict the maximum load, which can be supported by a geostructure without inducing the failure. Limit equilibrium, limit analysis, slip-line methods, and displacement finite element method are among the main methods used for performing stability analysis in geotechnical problems. Among the mentioned methods, limit analysis is based on plastic bounding theorems developed, and it assumes small deformations, a perfectly plastic material, and an associated flow rule. Despite the limitations arising from the assumption of a simple purely plastic material model, the ability of limit theorems to provide the bounds on the collapse load is one of their great advantages. This is an important advantage for complex practical problems where the failure load is difficult to estimate by other methods and the maximum error in the solution can be precisely bounded. So far, several studies have been conducted to investigate the stability of earth slopes under different loading conditions using the upper bound limit analysis approach, assuming a failure mechanism consisting of one or several rigid block(s). As the safety factor of the slope stability obtained by the upper bound method is greater than the actual one, the more appropriate failure mechanism led to the lower and closer to actual safety factor. The present study has used the failure mechanism consisted of plastic blocks of which the behavior is closer to reality than rigid blocks; hence, the results have been more appropriate. In this paper, using plastic blocks to determine the safety factor of the slope stability under earthquake loads has been made possible by applying some changes to the Sloan-Kleeman formulation, and a suitable method has been developed to determine the optimal dimensions of plastic blocks using mathematical techniques. Researchers believe that such parameters as the slope height/angle, the slope material strength characteristics and the horizontal acceleration coefficient of the earthquake force can be effectively used to evaluate the slope stability. They have varied the values of these parameters to determine the related safety factor. This paper, too, calculated the mentioned safety factor for various values of the mentioned parameters and compared its results with those of other researchers’ upper and lower boundary methods to evaluate the capability of the proposed method. Since the real solution lies between the lower and upper boundary solutions, comparisons are more valid in calculations where the latter are close to each other. Therefore, this paper not only compared its findings with the results of other studies conducted by the OptumG2 software prepared by the related researchers (a version of this software was provided to the authors), but also made more calculations and compared the results with those of the proposed method to allow for better evaluations. All the mentioned comparisons revealed that the accuracy of the proposed method was acceptable in applied problems (differing, generally, by less than 7% from the average results of the upper and lower boundaries). Increased slope angle and the horizontal acceleration coefficient of the earthquake force made the answers obtained from the proposed method closer to reality.

The huge number of failures and collapsed structures in the 1994 Northridge earthquake is always referred to as a turning point in the design of moment-resisting connections because it influenced the design of this type of beam-to-column connections. The occurrence of brittle failures in the welding of the joints, which led to the rupture of the connection plate, beam or column flanges, was one of the major damages that occurred in this type of connection in the mentioned enormous ground motion. After the Northridge earthquake, the attitude of researchers about designing moment-resisting connections completely changed. Many researchers tried to propose novel methods and ideas to improve the performance of rigid beam-to-column connections. Some of these methods are focused on different areas of the connections such as preventing the creation of three-dimensional stresses at the location of beam-to-column connection, reducing the shear deformation of the panel zone of connections, and removing the double or continuing plates of them. Furthermore, the proper seismic performance of each method was assessed based on numerous experimental and analytical studies. One of the most applicable rigid connections that we can refer to in this case is the connection with Welded Flange Plates (WFP). This connection is designed in such a way that the plastic hinge occurs at the end of the connecting plates. This type of connection is often used with hollow box columns. In this connection, due to the creation of concentrated forces in the column, which are created in the alignment of the connection plates, the flange or the web of the column may be damaged. For this reason, connection plates are used inside the column (continuing plates), which are generally installed by going through multiple steps and making additional cuts in the column. On the other hand, the occurrence of significant shear forces in the panel zone of connection may cause damage to it and require the implementation of double plates. The implementation of these types of steel plates can also cause difficulties in implementing the whole connection. For this reason, finding a solution that makes the implementation of rigid connections with welded flange plates unnecessary from continuing plates or double plates can be an important solution to facilitate the implementation of such connections. Rigid connections such as connection with the side plate and connection with the diaphragm plate (external stiffener) are options that have provided modifications to remove the connection continuing and double plates. The side plates prevent three-dimensional stresses and assist in strengthening the panel zone of connection. On the other hand, by removing the connection plates and preventing the discontinuity of the column, the diaphragm plate makes the panel zone stiffer and improves its seismic performance. In this article, a novel idea for rigid connection is proposed, which is a combination of connection with side plate, connection with diaphragm plate and rigid connection with welded flange plates, prequalified in steel structures standard of Iran. The new method has been tried to use the advantages of three rigid connection methods (WFP, connection with side and diaphragm plate) in hollow square (box) columns. In other words, the main purpose of this study is to integrate the advantages of the three mentioned methods and help to improve the seismic performance of the rigid connection. To assess the seismic performance of this connection, its 3D model has been created in finite element software and then analyzed with a nonlinear method. To perform the analysis, the models are analyzed under dynamic loading and their results, including the distribution of plastic strains, the location of failures, and the moment-rotation and shear force-deformation diagrams of the connection panel zone are calculated and evaluated. To show the damage in the components of this steel connection, the Birth and Death method is used in the analysis.The results show that the proposed connection has significant advantages. In addition to removing the internal connection plates, this connection has significantly strengthened the panel zone of the connection, and by removing the direct connection of the beam to the column, it has reduced the deformation of the column flange and, as a result, eliminated the three-dimensional stresses at this location.

Seismic assessment of concrete gravity dams is mainly conducted using capacity estimation of limit states or determination of damage indexes. The main purpose of this study is to estimate the capacity of limit states and damage levels of concrete gravity dams based on energy. Therefore, by selecting Pine Flat gravity dam as a case study, incremental dynamic analysis has been performed using hunt & fill algorithm on the dam-reservoir-rigid foundation system, under near-fault records with forward directivity effects. In this research, Arias intensity, which represents accelerogram cumulative energy has been considered as intensity measure and three parameters of dissipated energy due to fracture (DFE), dissipated energy due to fracture in crest motion into down-stream direction (DFE D/S) and dissipated energy due to fracture in crest motion into up-stream direction (DFE U/S) have been selected as damage measures. In the field of scaling records, previous studies that have used the IDA method to analyse concrete gravity dams have used the stepping algorithm. This algorithm has two main weaknesses. One is inefficiency, because its quality depends very much on the choice of IM step. Another is the implicit coupling of demand and capacity estimation, as the demand and the capacity resolutions are effectively the same in this method and equal to the step size. In the present study, the advanced hunt & fill algorithm was used to scale the records to eliminate the above weaknesses. In this research, using IDA curves and performing a series of additional statistical analyses, limit states, damage levels and global dynamic capacity of the structure were determined based on energy-based parameters. By investigation of the incremental dynamic analysis curves related to DFE D/S, DFE U/S and DFE, the following results were obtained:The study of all IDA curves showed that DFE-IA curves generally consist of two branches. The first branch consists of two parts. The initial part is linear (with a constant slope) that is observed at the beginning of all curves and represents the elastic part of the diagram. The second part of this branch is segment of the inelastic region of the curve. In this part, there are slight changes in the slope of the curve. In general, in the first branch, the structure exhibits hardness behavior. The second branch is the continuation of the inelastic region of the curve. In general, in this branch the slope of the curve is significantly reduced. Relatively severe softening is observed in this branch.In all records up to the intensity level that DFE in U/S direction is equal to zero, the dominant energy response of the dam is the response due to motions into D/S direction and base level cracking. While after the mentioned intensity level, at each intensity level, the overall response of the structure is equal to the sum of the responses in motion into D/S and U/S directions. The results showed that the representation of structural behavior based on the overall energy response seems more comprehensive and accurate.In the case of dissipated energy due to fracture in crest motion into down-stream direction (DFE D/S), points (0.315 KJ, 0.005 m/s), (5.378 KJ, 0.105 m/s), (16.944 KJ, 0.417 m/s) and (19.731 KJ, 0.538 m/s) were extracted corresponding to the BLci, Yielding, NZci and CP limit states, respectively.In the case of dissipated energy due to fracture in crest motion into up-stream direction (DFE U/S), points (0.157 KJ, 0.417 m/s) and (10.454 KJ, 0.538 m/s) were extracted corresponding to the NZci and CP limit states, respectively.In the case of dissipated energy due to fracture (DFE), points (0.315 KJ, 0.005 m/s), (5.378 KJ, 0.105 m/s), (17.26 KJ, 0.417 m/s) and (32.047 KJ, 0.538 m/s) were extracted corresponding to the BLci, Yielding, NZci and CP limit states, respectively.

Iran's Standard No. 2800 provides a code for the design of structures against earthquake loads. Due to the fact that mostly, far-field records have been used to prepare the seismic design spectra, in order to consider the destructive effects of near-field earthquakes in the 4th edition of Standard 2800, the incremental spectral correction coefficient (N) was introduced. In this paper, the accuracy and estimation of the value of this coefficient for five structures of special reinforced concrete moment-resisting frame with the number of floors from 3 to 15 are evaluated. Due to the fact that the increase in seismic requirements under the pulses of near-fault earthquakes is not the same for all seismic response parameters, so different correction coefficients can be used to estimate displacement response quantities and force quantities. For this purpose, first, the structures are statically analyzed according to the criteria of Iranian Standard 2800 and are designed according to the criteria of Article 9 of the National Regulations of Iran. Then the dimensions of beams, columns and rebars required by the structures are determined. After that the response spectrum of single-degree of freedom system to a set of records (including seven far-field records, and 22 near-field records) is calculated, and then by using incremental dynamic analysis, the seismic response of structures at different seismic intensities is calculated. By calculating the response ratio of structures under near-field records to far-field records, the value of the N-coefficient is calculated. Based on the results, the value of the N-coefficient of the standard spectrum of Standard 2800 is suitable for estimating the base shear demand of structures, but this coefficient is not accurate enough to estimate the need for lateral drift of structures. In general, the coefficients obtained from elastic and inelastic analyzes for the need for displacement in reinforced concrete flexural frame structures are higher than the values provided by the Standard 2800. This difference has reached 58% in some structures. It was also observed that there is no regular relationship between the 1st period of the structures and the magnitude of the spectral correction coefficient and the magnitude of the spectrum correction decreases with increasing seismic intensity.

In 1994, Northridge, California, an earthquake created brittle fractures and cracks near the beam-to-column connection welds in more than 170 steel MRF buildings. In many cases, these fractures and cracks were observed in the beam without any signs of plastic deformation. Since connections are one of the most important components of a building frame, a good understanding of the structural behavior of connections and sufficient knowledge of how it is transmitted is a prerequisite for designing a safe connection. Steel moment frame structures to resist the earthquakes are designed based on the assumption that they can resist yield or plastic deformation without reducing strength. The plastic deformation involves the plastic rotations of the beams at their connection to the columns and theoretically resulting in the loss of seismic energy applied to the structure. All the modifications proposed in recent years for the moment connections are aimed at shifting the maximum plastic capacity or ductility demand from the end of the beam or connection zone and transferring the ductility demand to an area within the beam and away from the column. On the other hand, the details of seismic moment connections provided in FEMA351 can be classified into two groups: a- Connection zone strengthening methods; b- Reduced beam methods. There are different types of the connections presented with the idea of ​​strengthening the connection zone; for example, we can refer to the connection with welded cover plates to the flange, the connection with the side plate, the connection with the screwed seat, the connection with the welding muscle, etc. However, the connections provided and used with the idea of ​​reducing the beam are more limited, such as RBS or reduced section beam connection, free-flange beam connection, sheared beam-to-web connection, and reduced beam-to-web connection. RBS connections form a plastic hinge outside the connections site and reduce force and moment at the connection, and is one of the pre-confirmed rigid connections after the 1994 Northridge earthquake. One method of building RBS connections is to use a connection of beam with a reduced section by replaceable links. The use of these replaceable links is one of the methods to increase the ductility and transfer the plastic hinge into the reduced area of ​​the beam. In these connections, the goal is to displace the plastic hinge into the reduced area of ​​the beam at a certain distance from the column, so in these connections, moving the plastic hinge away from the column reduces the concentration of strain in the welded zone. As a result, it reduces the weld cracking rate and thus reduces brittle failure in the connections.Connection of the reduced beam section was proposed after damage to structures in the 1994 Northridge earthquake. This connection reduces the damage caused to the panel zone by forming a plastic hinge outside the beam-to-column connection area. In this research, we investigate in a laboratory a new type of beam section connection, which in recent years has been named as replaceable links. Among the links designed in the first laboratory sample, the semicircular cutting method on the flange of the replaceable beam was used to reduce the finding, and in the second sample, circular holes in the flange of the sample were used to reduce the beam section. Studies have shown that high stress concentration occurs in the reduced area and other parts of the beam and column elements will remain without significant stress, which makes the beam replaceable after an earthquake. Analytical results show that the link with reduction by creating a hole in the flange has more energy absorption and ductility than the link with reduction by cutting a semicircle in the flange of the link beam. Also, according to the maximum amount of time recorded in the samples and referring to the existing regulations in the design of structures, it can be stated that both samples are among the allowable connections in a special moment frame.

Earthquakes have long been associated with change and destruction. Human has always been looking for a solution to identify and predict the occurrence of earthquakes. In order to identify the occurrence of an earthquake, other events that can be a sign of the conditions of the earthquake should be examined. In order to study these conditions, the status of climatic variables in hourly, weekly and monthly time periods was studied. Even theories about the relationship between weather and earthquakes have long been proposed by the ancient Greek philosopher (Aristotle) called "earthquake weather". He proposed the theory that the weather would be warm and calm before the earthquake, and the next theory he stated was that earthquakes occur in calm and cloudy conditions. The U.S. military has since examined the relationship between the weather and its relationship to earthquakes.Identifying the relationship between weather and earthquakes can help to predict and reduce financial and life losses, so in this research, the weather conditions at the time of the earthquake in some of the major earthquakes of Iran have been investigated.  In this article, using the hourly, weekly and monthly data, the observed meteorological variables at the time of the earthquake were investigated. These variables include: Temperature (in degrees Fahrenheit), Dew point (in degrees Fahrenheit), Relative Humidity (in percent), Wind direction and wind speed (in meters per hour), Gusty wind (in meters per hour), Pressure (in inches), Precipitation (in inches), Cloud Cover, Solar Altitude, Lunar Altitude, General Weather Conditions on an hourly, daily, weekly and monthly basis can be named boards that were examined. The studied variables from the time before the earthquake to after the specified time intervals were examined. Due to the large number of these charts, data and maps, only some of them were presented as examples in this article. Based on the study, these two theories are rejected due to the impossibility of generalization to all earthquakes, and it is suggested that it is due to the occurrence of a series of simultaneous changes in the Earth's system, including the biosphere, lithosphere, hydrosphere and atmosphere.Based on the observed data, changes can be seen in most of the weather factors at the time of the earthquake. Air temperature, dew point, lunar altitude, solar altitude, relative humidity increased and then decreased, pressure decreased and then increased. The direction of the wind changed at the moment of the earthquake and as it can be seen, the direction of the wind was not observed at the time of the earthquake in any of the hours of that day. The wind speed has decreased at the moment of the earthquake. These changes in a system that includes cryosphere, biosphere and atmosphere.It happens in a moment and this change can be clearly seen in most weather factors. It is suggested that these changes be known as the "Earth Systemic Evolution Theory".These changes can be known as the "Earth System Evolution Theory". This theory is presented for the first time in this article.