BULLETIN OF EARTHQUAKE SCIENCE AND ENGINEERING
Year:2021 | Volume:8 | Issue:3|Papers Count:8

The earthquake phenomenon has been explained by power-law relations with respect to magnitude, time and space. Fractal is one such power-law relation, which is a two-point spatial correlation function for earthquake epicenters [1-2]. It reflects the heterogeneity of seismic activity in a fault system. Another power-law relation is bvalue, which is a frequency-magnitude relation defined by Gutenberg-Richter [3]. The b-value of a region reflects the frequency-magnitude characteristics of seismogenic structures, stress distribution in space and depth [4-8]....

The need to construct structures on soft and unstable soils due to the appropriate technical and economic conditions has led to the development of various soil remediation methods. Moreover, the experience obtained from recent earthquakes has indicated the influence of sites’ stiffness on the surface seismic ground response. One of the ways to increase the stiffness to improve the soil, especially in soft soils, is to employ inclusion piles. These types of piles can be used at the bridge's piers to reduce the seismic response of the aboveground structures. In this regard, the role of the geometry characteristics of the inclusion piles can be significant. This paper investigates the effect of changes in the geometric parameters of inclusion piles such as diameter, length, the distance between them, and surcharge on the ground seismic response based on the offshore Turkish Izmit Bridge as a case study and base model. The effective depth was obtained by comparing the ground response spectrum of the two-dimensional model with inclusion piles using FLAC2D software based on the nonlinear hysteresis model, with the depth equivalent to the acceleration response spectrum of the free-field model. The geotechnical subsurface conditions at the North Tower Izmir bay bridge consist of 10 meters of loose to medium dense sand layers with silt, underlain by 127 meters of dense sand and hard sand clay. Bedrock lies approximately 144 meters below the mudline datum. The 1D responses obtained from the FLAC 2D and DEEPSOIL 1D software have been compared using the nonlinear soil behavior to verify the numerical modeling results. Then, with the calibration of soil parameters and lateral and bottom boundaries, inclusion piles have been added to the validated free-field model in FLAC2D software. In this study, the 2D modeling process includes introducing soil layers’ characteristics and determining the lateral free-field boundaries and the quiet boundary as the bottom boundary subjected to the seven earthquake excitations is performed. The inclusion pile was modeled using the beam and cable combine elements in the FLAC2D. Besides, inclusion piles are two-dimensional elements with 3 degrees of freedom (two displacements and one rotation) at each end node. Piles interact with the FLAC grid via shear and normal coupling springs. The obtained results indicated that by increasing the ratio of distance to the diameter of inclusion piles (S/D), the effective depth decreases due to reducing the stiffness of the inclusion pile system, and after reaching a ratio of 5, it has reached a constant value. In other words, with increasing stiffness of the soil-pile system, the effect of kinematic interaction on the soil-pile system increases. Moreover, by increasing the length to diameter ratio of inclusion piles (L/D), the effective depth will first increase and then reach a constant value, in which the optimal range for the length to diameter ratio of piles is 15 to 30. Also, the effective depth increases linearly with an increasing surcharge ratio above the inclusion piles ( q ). Finally, it should be noted that the soil improvement using inclusion piles due to the kinematic interaction can apply a new foundation input motion altered from the free-field ground response. This interaction increases the effective depth of the equivalent free-field model, which can reduce responses of the aboveground structures (e. g., buildings or bridges, etc. ). Therefore, the use of this type of piles due to having more stiffness than traditional soil improvement approaches such as stone columns or deep soil mixing, etc., can be effective in order to optimally design structures located on loose or soft saturated soils.

Moment resisting steel frames (MRSFs) are widely used as a lateral load resisting system in steel structures in very high seismic regions. The seismic performance of this system depends mainly on the behavior of beam-tocolumn moment connections such that trivial damages in the connections may lead to the collapse of the whole structure or at least post-earthquake demolition of the structure. In the 1994 Northridge earthquake, beam-to-column connections in the MRSFs damaged noticeably and unexpectedly. Several researchers then proposed various suggestions to improve the seismic performance of beam to column connections. One of these suggestions was using a column-tree system to avoid the low-quality field-welded moment connections. These days, column-tree connections are widely used in the special moment resisting frames (SMRFs) buildings due to their well-known ease of installation and inspection of welding zone, especially in the high seismic areas. However, the prequalification and seismic behavior of these connections had not been evaluated well prior to this research; hence the structural designers usually consider these connections as the prequalified connection for using in the SMRFs without following a robust and validated approach. Therefore, in this study, the effect bolted splice design method on the prequalification and cyclic response of the column-tree connections were investigated using experimentally validated finite element analysis in ABAQUS FEA software. The column-tree connection must be designed such that the ductile failure modes occur prior to the brittle failure modes. This may be achieved through an appropriate design approach. Based on the bolted joint type in the AISC specification (i. e., pre-tensioned joint and slip-critical joint) and removing plastic hinge from the column edge (weakened bolted splice), there are three bolted splice design methods available. These are bolted splice design methods based on the slip critical joint, pre-tensioned joint, and weak splice plates. This research studied three samples to evaluate the effects of the bolted splice design methods on the prequalification and seismic behavior of the connection. The results show that the column-tree connection prequalification depends on the bolted splice design methods; moreover, the bolted splice design method influences the monotonic and cyclic behavior, strength, stiffness, fracture tendency, ductility, and energy dissipation characteristics of the connection. Also, it is observed that based on the moment strength and rotational stiffness of the bolted splice, the column-tree connection is classified as a rigid or semi-rigid moment connection. This is a significant point that needs to be taken into account in the column-tree moment frame design. The structural designers should consider these effects in their design approach for the column-tree connections. The column-tree connection with its bolted splice designed based on the pre-tensioned joint exhibits a reduction in the fracture tendency and increase in the ductility of the connection and also a smaller number of required bolts. For these reasons, it is recommended that the pre-tensioned joint method be implemented in designing a bolted splice instead of the slip-critical joint method.

One of the main targets in designing structures is achieving adequate level of inter-story drift and ductility for controlling damage level. When a structure is subjected to sequential earthquakes, damage control becomes more critical. This seismic sequence as a result of the occurrence of strong and moderate earthquake ground motion in a short time strongly affects the seismic response of structures. Studies revealed that variation in the seismic responses such as displacement, ductility, absorbed energy etc. cause modifications in structural damage states. A damage index can numerically quantify the structural damage of buildings. Several damage indices have been developed for evaluating the seismic performance of structures. The aim of this paper is to investigate the effect of seismic sequence of earthquake ground motion on the damage indexes variation and the seismic performance of steel moment frame structures considering soil-structure interaction (SSI). For the present work, the drift damage index is selected for measuring the story damage due to the inter-story drift. This index has been calculated by performing the nonlinear time-history analysis on a set of selected group of steel frames subjected to selected seismic excitations and sequential earthquakes. 2. Methodology To this end, four steel moment frames with 8 and 16 stories are considered each of which three and six bays have. Soil-structure-interaction is considered by employing dampers and springs having specified damping and stiffness through cone method. For modeling SSI effects, three types of soil based on Standard 2800 are considered. For each soil-structure model, different responses are measured under different scenarios of sequential earthquakes. 3. Results and Discussion The results of time-history analysis show that the relative displacement of the structure and inter-story drift are increased when the structure is subjected to the seismic sequence. As a result, the drift damage index induced to the structure. The study findings demonstrate as soil becomes softer, the structural demands increases and consequently the damage index rises and SSI amplified the structural damage level. Therefore, frames built on soft soil subjected to sequential earthquakes experienced more damages. 4. Conclusions The obtained responses show that a special attention should be paid for designing of steel structures due to the preventing extensive structural damage in high seismic zone when they are subjected to seismic sequence excitation. Furthermore, soil-structure-interaction is an important parameter that should be considered for such conditions. Therefore, it is recommended that the effects of seismic sequence and soil-structure-interaction should be accounted for better estimation and evaluated accurate responses of steel shear frame.

In recent years, structural systems with diagrid skeletons comprising modular configurations have attracted lots of attention due to having structurally efficient system and architecturally aesthetic advantages. In this study, the seismic performance parameters of steel diagrid systems have been evaluated through conducting nonlinear time history analyses subjected to near-field earthquakes. The main subject of this study is to assess the effect of incidence angle of near-fault records containing forward directivity on the seismic behavior of steel diagrid systems. Following this purpose, three 20-story diagrid studied structures with different diagonal angles of 56° , 64° and 76° , having the same sizes and skeletal configurations were designed. Furthermore, a set of notified incidence angles with respect to the main axis including 0, 15, 30 and 45 degrees have also been applied to all the studied structures. It should be noted that the significant characteristics of strong ground motions are the frequency content, effective duration of strong motions and peak ground motion parameters, i. e. PGV, PGA and PGD as well as corresponding response spectra. According to the aforementioned characteristics, an ensemble of strong earthquake records was selected. The selected ground motions include the main shock of the 1978 Tabas earthquake (Iran, Mw=7. 3), the Bam record due to the 2003 Bam earthquake (Iran, Mw = 6. 6) as well as two other powerful records entitled Rinaldi Receiving Station (RRS), and Sylmar Olive View (SYL) accelerograms related to the 1994 Northridge earthquake (Mw = 6. 7) in California. It is worth mentioning that all the selected ground motions are categorized as strong near-fault records containing forward directivity effects. The specific characteristics of near-fault records containing forward directivity effects are the existence of long-period pulses. These pulses are the evident result of the effects that manifest due to the occurrence of forward directivity process. The forward directivity process causes the induced fault rupture propagation to move toward the site. It usually happens when a velocity of rupture propagation is appropriately close to the earthquake shear wave velocity. In this research, a number of engineering demand parameters (EDP) i. e. drift ratio, rotation of joints and plastic hinges nonlinear domain have been computed and assessed through conducting nonlinear time history analyses (NTHA). It is demonstrated that maximum seismic response parameters for all the studied structures do not occur necessarily along the main skeletal axes. Furthermore, it is extremely difficult to define the specific structural behavior trends subjected to a nominated critical incidence angle which can be imposed to earthquake records. It is noticeable that the numerical trends in the structural response parameters would change remarkably under the influence of the different frequency content of earthquake records, as well as the probable non-uniform variation for the EDPs and the diagrid skeletal patterns. The results of this research indicate that the influences of incidence angle of ground motions on the structural seismic performance have different trends which evidently include mingle effects on the nonlinear rotation of joints as well as a complicated distribution for the lateral drift parameter. Moreover, it was obtained that the embossed structural skin containing the triangular modules with the diagonal angle of 56 degrees has relatively the lowest seismic demands subjected to strong near-field earthquake records.

Young's modulus (E) is one of the important parameters in soil mechanics that is commonly used in geotechnical projects. This parameter is very important because it directly indicates the hardness of the material. Elastic constants (Young's modulus and Poisson's ratio (ν )) are used as the main parameters of soil mechanical properties for the analysis and design in construction projects. Two common methods for extracting the Young's modulus of soil materials are the static method and the dynamic method. In the static method, the Young's modulus (E50), which is usually obtained from the stress-strain curve in element tests such as the triaxial test. The dynamic elastic modulus (Ed) can be obtained by measuring the compressional wave velocity (Vp) in situ wave propagation experiments or by using bender element in element tests. In this study, using a triaxial device, Young's modulus over a wide range of strains have been studied. The effect of grading characteristics such as gravel content, anisotropic consolidation, sample preparation method, confining stress and relative density on the Young's modulus of granular soil has been evaluated. Using local strain measurements, the Young's modulus of granular materials is investigated. In conventional triaxial machines, load plate errors and system capability effects usually limit shear strain measurements to values greater than 0. 01%, although local strain measurements can be used to make accurate measurements. The materials used in the present study were 161 Firoozkooh silica sand and Mesutak gravel. Firoozkooh sand 161 has a golden yellow color and has a uniform granulation. The reconstructed specimens in this study are mainly based on the wet tamping (WT) method. It is noteworthy that in order to evaluate the fabrication method in the last stage, two samples with water deposition (WP) method and air deposition or dry precipitation (AP) method have also been studied. The results show that the Young's modulus increases at small strain levels with the addition of gravel to the host sand at a certain stress level and relative density, but its effect at large strain levels is almost negligible. On the other hand, as the confining stress increases, the effect of gravel content on the Young's modulus decreases. The results show that the Young's modulus has an increasing trend with increasing confining stress or relative density. At small strain levels, sand mixed with 50% gravel has the highest Young's modulus. The results related to the studied sand show that in this particular type of sand, sand mixed with 50% gravel has the highest degradation in hardness. This indicates that in this amount of sand, the combination of sand and gravel is such that the most slippage or rolling occurs between the grains, which further reduces the hardness. The reason for this is due to the special arrangement of sand grains in samples containing 50% gravel. In fact, in 50% gravel samples, the grains of sand are placed between the grains of sand like tiny balls, and the application of force causes the grains of sand to move and slip. Also, the results of anisotropic tests show that in the consolidated specimens under anisotropic consolidation, the Young's modulus increases with increasing the initial deviatoric stress compared to the consolidated specimens in the isotropic condition. The results show that different methods of samples preparation have a significant effect on Young's modulus.

Floor isolation technique is considered as a new approach in seismic design of structures in which the mass of building at each floor is isolated from the main structural system via seismic isolators. In this case, during earthquake actions, large relative movement between isolated floors and the main structural system at each floor level would be expected. In the current work, numerical studies have been carried out on a typical floor isolated building to investigate the role of displacement constraints (Stoppers) in limiting the gap between the floors and the structural system during seismic actions. A ten-story steel frame structural system located on stiff soil subjected to far-field earthquakes was selected for parametric study in this work. The structure has been proportioned in three different configurations, not isolated, floor isolated and floor isolated equipped with Stoppers. Since using floor isolation improves the structural performances of the system, a fourth configuration for the same structural system with 18% reduction in steel consumption is also taken into consideration. This structure was deliberately proportioned to provide the state of comparable seismic performances between the isolated structure and the nonisolated one. Direct time integration analyses using seven scaled bi-directional earthquake records have been carried out on the same structural system for all its configurations. According to the results of this study, floor isolation is quite effective in improving structural performances of the system. In fact, on average, floor isolation causes significant reduction on lateral displacement of the structural system (more than 50%) if it compares with the nonisolated one. The results also show the benefit of using floor isolation technique in design of structural system by decreasing the construction cost of the building (18% reduction in the weight of structural material) if a comparable seismic performance with the non-isolated structural system is required. It should be noted that, improving in seismic performances of the building or reduction in its construction cost comes in the expenses of large relative movement of isolated floors with respect to the main structural system. In the example used in this study such relative movement (average of seven earthquakes) reached to the level of 0. 1 meter. However, in one of those earthquakes (Imperial Valley, 1979) this relative movement has soared up to 0. 19 meters. Such large relative movement needs complicated non-structural detailing for the building assembly and expensive seismic isolators. To deal with this problem, displacement constraints have been provided for the floor movement using elastic Stoppers. The gap between Stoppers and the floor system is chosen at 0. 1 meter to limit the Stopper’ s functionality in structure only to the case when the system is subjected to large earthquakes. The results of parametric studies on the system with reduced structural weight (configuration fourth) shows a reasonable reduction in relative displacement between floors and the main structure in case of using Stoppers. These results were obtained using nonlinear time integration analyses on the structural system subjected to Imperial Valley earthquake record. According to these results, while Stoppers can reduce the relative movement between floors and the structural system to about 35%, they considerably add to the acceleration of the floor system (up to twice for the isolated structure at the roof level). In addition, the results also show using Stoppers may add to the inter story drift limit of the structure (up to 20%). These shortcomings in using Stoppers in this work can possibly be reduced using Stoppers with different arrangement and sophisticated characteristics.

The wave propagation problem is one of the most important topics studied by numerous researchers. Therefore, in this paper, the background of the researches on the propagation of anti-plane SH-waves in a non-homogeneous linear elastic orthotropic medium is presented based on the topographic features as a case study. In this regard, a brief review is illustrated on the theoretical expression of the elasticity of non-homogeneous materials, scalar wave equation, and the technical literature of the obtained Green's functions to solve the mentioned problems. The researchers have proposed various approaches for seismic analysis of topographic features where their studies are categorized according to the development. In general, these methods can be divided into analytical, semi-analytical, and numerical methods. Despite the high accuracy of analytical methods, their lack of flexibility in modeling and analyzing the complex features in accordance with real paradigms in nature, has forced the researchers to use alternative approaches such as numerical methods. In recent decades, increasing the power of computers besides the development of numerical approaches has made researchers eager to use them for analyzing wave propagation problems as well as predicting the real responses of topographic features more than ever. Based on the formulation, the numerical methods can be usually divided in two general categories known as the domain and boundary methods. The common domain methods are including the Finite Element Method (FEM) and Finite Difference Method (FDM), which require discretization of the whole body including internal parts of the model and its boundaries. Although the simplicity of domain methods makes them favorable for seismic analysis of finite media, the models are complicated because of discretizing the whole body and its boundaries at a considerable distance from the desired zone. In boundary methods that are mostly known today as the Boundary Element Method (BEM), due to the concentration of meshes only around the boundary of the desired features, automatic satisfaction of wave radiation conditions at infinity, reducing the volume of input data and analysis time is remarkably achieved as well. On the other hand, because of the large contribution of analytical processes in solving various problems by BEM, the high accuracy of the obtained results is guaranteed. Therefore, the BEM provides a better manner for analyzing the infinite/semi-infinite problems. The BEM formulation can be formed in two categories, full and halfplane. In full-plane BEM, in addition to truncate the model from a full-space, it is required to discretize all the boundaries of the problem including the interfaces, smooth ground surface, and enclosing boundaries. This leads to approximate the satisfaction of stress-free conditions on the ground surface and makes its results less accurate in some cases. In the half-plane BEM approach, the discretization of smooth surface and definition of fictitious elements for enclosing boundaries are ignored, and the stress-free boundary condition of the surface is satisfied in an exact process. Despite difficult implementation and creating large equations in the half-plane BEM compared to the full-plane case, the mentioned advantages help to make the simple models. According to the appropriateness of the BEM in the analysis of wave propagation problems, especially in the presence of topographic features, this method is expanded in two mediums of isotropic and orthotropic. This paper is recommended as a starting point for all researchers who are interested in the field of seismic analysis of homogeneous and non-homogeneous sites.