پژوهشنامه زلزله شناسی و مهندسی زلزله
سال هفدهم شماره 3 (پیاپی 66، 1393)

In recent years, considerable research has been devoted to the study of seismic data carrying information about the complex events that lead to earthquakes. Much of the recent geophysical research associated with earthquakes has centered on investigating spatial and temporal patterns of aftershocks in local and regional seismicity data (Kanamori, 1981). Notable examples include characteristics of earthquakes and temporal clustering (Frohlich, 1987; Press and Allen, 1995; Dodge et al., 1996). The principal properties of aftershock sequences are described empirically (e.g. Utsu, 1961; Omari, 1894). Although much of this work represents important attempts to describe these characteristic patterns using empirical probability density functions, none of these observations or methodologies systematically identifies all possible seismicity patterns. The quantification of all possible space-time patterns would seem to be a necessary first step in the process of identifying which patterns are precursory to large events, leading to the possible development of new approaches in forecast methodology. It is difficult for most scientists to understand why events with large magnitude such as Bandar-Abbas earthquake are not preceded by at least some causal process. This paper presents an investigation on spatial and temporal fluctuations of earthquake form time series, using the inverse statistical analysis, a concept borrowed from turbulence. Inverse statistics propose to invert the structure function equation, and instead consider averaged moments of distance between two points, giving a difference value between those points. In the inverse statistics method is used another well-known method that called Level Crossing method (LC). In level crossing method, no scaling feature is explicitly required and this is the main advantage of this method in estimating the statistical information of the series. Level crossing is based on stochastic processes that grasp the scale dependence of the time series. The analysis is also performed for the shuffled time series of earthquakes. Since the application of shuffling data destroys the correlation and demonstrates whether there are actually correlations among time (distance) series. We report some significant differences between the results of these two sets of data.

Liquid sloshing is one of the major considerations in design of liquid storage tanks. In the case of a fixed roof tank, the free surface motion caused by earthquake may collide to the tank roof and induce an extensive hydrodynamic force on it. On the other hand, for the case of a single deck floating roof storage tank in which the tank is covered with a floating roof to suppress the escape of the oil vapor from the free surface, the seismic loads can induce a large displacement of the floating roof due to the liquid sloshing motion. This motion possibly leads to severe structural damage and catastrophic fire accident as experienced in some past earthquakes and overviewed in this paper. Seismic behavior of a single deck floating roof excited by long period ground motion generated by earthquakes is characterized by several important features. One of these features is the nonlinear effect of free surface motion which causes extra stresses in single deck floating roofs. Neglecting the nonlinear effects of the large amplitude oscillating of a single deck floating roof due to the resonance conditions significantly underestimates the stresses in the single deck floating roof. The nonlinear effects of the first sloshing mode are much higher than the other sloshing modes. Therefore, liquid-roof interaction caused by first mode oscillation imposes a complicated distribution of out-of-plane deformation in a single deck floating roof which is the main source of high seismic stress. The second sloshing mode is also susceptible to be excited by low-frequency components of earthquake ground motions. Second sloshing mode induces the complicated vertical out of-plane deformation of the deck plate in a single deck floating roof tank. This vertical displacement of the deck leads to the contraction of the pontoon plate which is another source of seismic stress. In In this paper, the seismic effects of these two important sloshing modes are quantitatively discussed. Moreover, a seismic design procedure for evaluating the seismic stress of a single deck floating roof is proposed. In this method, the relationship between vertical deformation of the deck plate and the radial shrinkage of the pontoon, as well as the estimation of the free surface nonlinear behavior are considered. Moreover, the attenuation of the sloshing wave height due to the presence of a single deck floating roof is proposed based on an analytical evaluation. A design flowchart according to the new method is suggested. This method emphasizes on the nonlinear effects of large amplitude wave for the smaller capacity tanks, and the seismic stress caused by the second sloshing mode for broad tanks.

One of the concerns of assessing structural collapse performance is the appropriate selection of ground motions for use in the nonlinear dynamic collapse simulation. For this purpose, the selected earthquakes must be stronger than the earthquake which used in the initial design of structure. For modern buildings which have the ductile behavior against earthquake, ground motions that cause collapse are expected to be rare high-intensity motions associated with a large magnitude earthquake. The researches have shown that rare high-intensity ground motions have a peaked spectral shape that should be considered in groundmotion selection and scaling. They have shown that spectral shape, in addition to ground-motion intensity, is a key characteristic of ground motions affecting structural response. One method to account for this spectral shape effect is through the selection of a set of ground motions that is specific to the building’s fundamental period and the site hazard characteristics. The using of this method for each building faces challenges as need to selecting specific sets of ground motions. The selection of a specific ground motion set for each building is not practical; nor is it desirable because the goal is to generalize the collapse assessment results across seismic design categories. Thus, another method was presented in which uses a general ground-motion set, selected without regard to ε values, and then corrects the calculated structural response distribution to account for the target epsilon expected for the specific site and hazard level. To provide a more practical method for adjusting the collapse capacity, a simplified version of the second method presented that can be used to determine the appropriate adjustment factors for the collapse capacity distribution without requiring the computation of the epsilon values for each record and then performing a regression analysis. The development of simplified method causes that reduced computation. The collapse capacity results of three different methods were compared by several examples and the results of past research. The results show that the first two methods produce nearly identical results, with the predictions of the mean collapse capacity differing by only. Comparison of the Simplified Method and Method 2 indicates that the results are very close to each other, so that the full regression analysis results yield 0.257, which agrees very well with the simplified value of 0.254. The corresponding mean collapse capacity from Method 2 is 2.63 g as compared to the simplified value of 2.83 g. This difference of about 8% is reasonable for most applications, particularly in contrast to the alternative of neglecting the spectral shape effects. The calculated dispersion from Method 2 is 0.35, which is about 10% lower than the slightly conservative value of 0.40 used in the simplified method. This paper indicates all methods that modified the resulting collapse capacity by the spectral shape effects. Also, these methods have been compared and introduced the best method by the examples provided.

Double-I Built-Up Column is a type of columns used extensively in buildings in Iran. When we use these columns in moment resistant connections, the beams are connected directly to column's cover plate. Since the cover plates are only welded to I-shaped sections of column along their longitudinal edges, and we can’t use the continuity plates to connect the cover plates to each other, they behave very flexible under force of beam flanges. This deflection takes place during the load-transferring process, it results in significant beam-to-column rotation, and semi-rigid behavior of connection. This phenomenon causes also the important stress concentrations at groove welds between beam flanges and column cover plate and fillet welds between column’s cover plates and I-shapes members of the column. In this paper, the behavior of the conventional welded I-beams to double-I built-up columns is investigated to identify the unfit behavior of these connections. Finite element method, as a numerical tool, provides a powerful mean for precise analytical prediction of many structural systems. This technique is used in this study for assessing the behavior of connection under study. ABAQUS software is selected for this purpose according to its special capability in analysis of large deformation problems. In this paper, we have used the XFEM method for modeling the failure in finite element models. The Extended Finite Element Method (XFEM) capabilities of ABAQUS v6.10 is a powerful technique for modeling of fracture and modeling the failure of any materials. A series of two non-linear threedimensional finite element models were developed using ABAQUS software to study the behavior of the conventional connection. Finite element models have been performed using 8-node C3D R elements. The outcomes of numerical investigations indicate that these type of connections have not sufficient strength and rigidity and it may cause irreparable damages in structures. The tensile failures occurred in edges of flange cover plates under force of beam flanges and this brittle failure caused by the localized stress concentrations. According to AISC Specifications for Structural Steel Buildings the conventional connections are partially restrained and the stiffness, strength and ductility of the connections must be considered in the design. The results of numerical investigations show that the large deformation takes place in column cover plate under tensile force of beam flange. This event leads to physical deterioration in columns. The results of numerical investigations indicate that using these conventional connections in Moment-Frame systems or in combined systems may cause irreparable damages in structures, and structures with these connections without retrofitting must be considered as simple connections.

In August 11th 2012, at 16:53 and 17:04 two earthquakes with magnitudes of 6.3 and 6.4 occurred in East-Azerbaijan province, North-West Iran. The epicenter of the event was located about 10 km south of Varzaghan city. The peak ground acceleration for the first and second events were recorded as 427 and 532 cm/sec2, respectively. The Responsible author had the opportunity to visit the area, immediately after the events as a member of IIEES reconnaissance team. The villages around Ahar and Varzaghan as well as both cities suffered severe damage. Light structural damage was observed in transportation structures while a severe damage occurred in the buildings. The office buildings and educational facilities were among the damaged structures. The Azad Islamic University of Varzaghan is a 4 storey building with steel structure. The facility is located at the south of the City. The structure is equipped by bracings and moment frames as lateral bearing system. The field observation revealed the severe damage to non-structural components of the building. Cracks occurred in infill walls. Toppling and falling of equipment and objects was observed in different floors. Damage to ceilings was observed in all levels of the building. Both out of frame and in frame failure of masonry infill walls were observed in the building. A Numerical Study was carried out to estimate the performance of the building during the earthquake and find out the location and intensity of probable nonlinear behavior if the elements and connections.Push over analysis was performed to evaluate the behavior of the structure in extensive seismic displacement demands. Target displacement was calculated according to the FEMA 356. The analysis was performed by SAP2000, a finite-element analysis software, which is capable of modeling nonlinearbehavior of structural frames through concentrated hinge models. The analysis resulted in estimation of sequential nonlinear behavior of the building and sequence of appearance of plastic hinges in the frame elements. The strong motions of both events were recorded by the Iran Strong Motion Network. They could make the time history analysis of the structure possible. Time history of acceleration and displacement of the building in different floors were calculated. The analysis could reveal that although the floors of the building experienced considerable acceleration during the strong motion, the relative displacement of the building was limited. The calculated floor relative displacement values are smaller than the '0.025Ífloor height' as the code criteria. This could result in a limited response of the structural elements and prohibit the occurrence of nonlinear behavior and seismic structural damage to the building.

From the various intensity measures that may be applied to evaluation of the seismic risk of structures, the acceleration response spectrum, Sa(T), is the most famous. As a key assumption in usual risk assessment procedures, like as PEER methodology, the structural response depends only upon the applied intensity measures, and not on any other properties of the ground motion. This required condition has termed “sufficiency” of used intensity measure. The limited “sufficiency” of Sa(T) has been emphasized in the recent researches and as a result, different methods have been proposed to modify the structural response analysis. In this paper, the problem has been re-defined and then the recent studies have been surveyed. This paper is mainly focused on the spectral shape concern. It has been discussed how the spectral shape of a ground motion affects the structural nonlinear response. Epsilon, as a well known seismological parameter is introduced as a convenient indicator of spectral shape. Epsilon as an indicator on the spectral shape has significant influence in the structural collapse risk assessment. The epsilon is defined as a measure of the difference between the spectral acceleration of a record and the mean value obtained from a ground motion attenuation model for a given period. As a direct approach for the consideration of the spectral shape in the record selection, a target ε value, associated with a selected hazard level, is first obtained from the hazard disaggregation procedure, and then records with a closer epsilon value to the target value can be chosen. The major challenge in considering the spectral shape for the selection of records lies in the finding of different sets of ground motion records for each level of hazard for calculation of the MAF of a limit-state for a given structure. Due to the dependence of epsilon on period, it may not be practical to select different specific ground motion sets for any specified period (T1) corresponding to a given site with a particular hazard level. A simple alternative has been proposed in the ATC63 project which could be used instead of the direct selection approach. In this approach, a general set of ground motion records could be used for assessment of the collapse fragility of any structure, without considering the spectral shape of the records. Theresulting mean collapse capacity can then be adjusted to meet the hazard-related target epsilon value. The major challenge of this method is that the objective structure shall be necessarily analyzed via a huge number of ground motions which is unfavorably time consuming task. Another more straight-forward approach is defined in this paper for the mentioned issue. In this method, an efficient range of epsilons is proposed to select a limited number of ground motions for collapse risk assessment of the objective structure. The proposed efficient range depends on the structural major parameters; period and ductility as well as on the seismicity level of the site. The initial results confirm the validity of this simple approach.