مجله علوم و مهندسی زلزله
سال سوم شماره 3 (پیاپی 8، پاییز 1395)

Knowledge of the dynamic behavior curves of soils is important for designing various geotechnical structures subjected to different kinds of vibrations, including earthquakes. Reviewing the previous studies on soil dynamic behavior shows that most of the studies have focused on the siliceous soils. Geological studies suggest that vast areas of the earth in the tropical regions are covered by calcareous deposits. Large areas of the southern Iran, which are seismic-prone based on the recent earthquake activities, are also covered by calcareous soils. Foundation problems associated with carbonate soil deposits, particularly as experienced by the offshore hydrocarbon industry have led to significant research focused on understanding the behavior of these soils. Origination of calcareous soils from processes and minerals different than those known for the silicate soils clarifies the necessity for comprehensive research on geotechnical properties of such soils. The wide variation of the origin of calcareous soils, due to their offshore locations and the related fauna that make their formation, merit more research into the behavior of these soils from various regions.
In the current research, shear modulus and damping ratio of Hormuz calcareous sand and Babolsar siliceous sand were measured and compared. An identical grains size distribution curve was synthetically attained for the tested sands. The soils are classified as poorly graded sand (SP) according to the USCS (ASTM D2487). The shear modulus and damping ratio values were calculated at small and large shear strains using resonant column and cyclic triaxial tests. The calcareous and siliceous soil specimens were tested by a fixed-free type of resonant column apparatus (SEIKEN model). By using the resonant column apparatus, shear modulus and damping ratio of the calcareous and siliceous sands were measured for the shear strain amplitude ranging from about 10-4 % to 10-2 %. The cyclic triaxial tests were conducted using a fully automated GDS triaxial testing apparatus. The cyclic tests were done on the samples with shear strain amplitudes ranging from about 10-2 % to 1 %. The procedure used to perform the resonant column and cyclic triaxial tests was the multi-stage strain-controlled loading under undrained condition.
The results for both calcareous and siliceous sands indicate that the shear modulus (G) decreases and damping ratio (D) increases with an increase in shear strain amplitude (γ), as expected. Moreover, the shear modulus of tested soils increases with the increase of effective confining pressure (σ'0). Further, the tests results indicate that the increased amount of effective confining pressure leads to the smaller damping ratio for the Hormuz and Babolsar sands. The shear modulus of Hormuz calcareous sand is higher than that of the Babolsar siliceous sand while grains size distribution, effective confining pressure, and void ratio of both soils were identical. It is evident from the resonant column and cyclic triaxial tests that the Hormuz calcareous sand has lower values of damping ratio (D) in comparison to the Babolsar siliceous sand. Finally, dynamic behavior curves of the studied sands are compared with the available relationships. These comparisons indicate that the available relationships are unable to accurately assess behavior of the tested calcareous sand.

There are proven reasons indicating that during seismic excitation, considering substrate flexibility, i.e. Soil-Structure Interaction (SSI) may intensify displacement, change in internal members’ forces and damage and even lead to the collapse of the construction. Conventional structural design methods neglect the SSI effects. The lesson learnt from past earthquakes revealed a significant effect of SSI phenomena on the dynamic response of tall, bulky and heavy structures resting on relatively soft soils, for example, nuclear power plants, high-rise buildings, and elevated RC water tanks on soft soil. Neglecting SSI is reasonable for light weight structures in relatively stiff soil such as low-rise buildings.
The tunnel form buildings are one of these heavy and stiff systems that considering SSI may be important in modeling for seismic loading. To introduce, this system is a modern constructing technique that is recently used in mass construction projects This system lacks structural beams and columns in which only the elements of slab and wall as vertical and lateral load-bearing elements are used. The wall and upper slab are concreted at the same time. It seems that considering SSI phenomenon for such buildings concerning high lateral stiffness and weight, especially under strong earthquake or soft soil ground is of great importance. Despite widespread usage, unfortunately in the current design codes, the system is not considered as an independent structural system. Although there are valuable researches carried out on tunnel form buildings, they are still limited in a literature survey.
A literature survey shows that the experimental and numerical study to evaluate the seismic behavior of tunnel form buildings considering SSI effect is very limited. Now, in many densely populated cities with a relatively high risk of occurring earthquakes, this system is used as mass housing projects. Since the rigidity of soil bed below the foundation in analysis and design of these structures is a common assumption among designers, this study attempts to assess this presumption in a structural reliability framework. Therefore, in this study, these structures are examined through the considering SSI in analytical modeling and its influences on their seismic response and behavior. First 5 and 10-story regular buildings were designed with and without SSI modeling based on the current revision of Iranian seismic code. After controlling performance levels and responses based on probabilistic approach, the overturning and sliding factors were examined under seismic intensity levels of 0.35 and 0.65 corresponding to seismic hazard level of DBE (Design Base Earthquake) and MCE (Maximum Considered Earthquake).
It is to be noted that in this study, the elastic behavior of soil was the basic premise assumption. The results show that, with increasing building height and intensity of earthquakes, the SSI phenomenon influenced the structural responses including shear and inter-story drifts and also commence of starting position of damages. The research results indicated that the first damage level probability could be increased up to 30% and the sliding and overturning
probability increased at least 10 percent considering SSI respect to non-SSI assumption. It may be concluded that, especially in areas with high seismicity and soft soils considering SSI can reduce the reliability of this type of structures to attain predetermined performance. Thus, in the case of areas with high seismicity, soft soil and for taller buildings, special attention to the phenomenon of SSI in the seismic assessment of this structural system is necessary.

Soil-Structure Interaction (SSI) problems are usually broken down into four fundamental parts. The first step is the estimation of free field motion (FFM). FFM is representative of the field motion in the absence of any activity relating to the building construction procedure. The second step is the calculation of excavated field motion (EFM), which translates the effect of including void on alternation in FFM. This later motion usually is defined in the perimeter of the cavity. The third step accounts for kinematic aspects of SSI. In this part, foundation deformations due to EFM are estimated. In the fourth and final step, the previously calculated deformations are used to impose the acceleration history on structural mass. In current practice, it is known that the third step has the highest influence on underground structures and hence it dictates the design criteria for such systems.
To implement the above-mentioned analysis plan, FFM is usually considered as shear waves with upward propagation direction. Such assumption has formed the popular simplified seismic design method for underground structures. Though, this common assumption may not be valid for topographic urban areas were wave fields reach the surface through different incident angles. Such inclination would lead to various states of confrontation between embedded structures and wave fields. The state variety, in turn, causes underground constructions to experience different stress fields and hence dissimilar lining deformations. Therefore, there is a serious need to uncover the role of face-off orientation of wave field and the structure on lining strain demands.
Here, the effect of face-off angle between shear wave field and rectangular underground structures, on lining strains is investigated. For this purpose, a 2D isotropic soil model including homogeneity is included under statically simulated seismic shear deformations. The analysis was performed through finite element method regarding different aspect ratios for underground structure and subsequently lining strains were reported. To drive strain demands, in the analyzed model, first, the axial, shear and moment demands are estimated. Then the results are normalized to proper parameters that lead to relative strains. Beside these three types of strain, with a combination of strains resulted from axial and moment forces, the total axial strain is also extracted. This parameter is commonly used in any structural member design. The analysis was repeated for three aspect ratios of 1, 2 and 4. Besides, four face-off angles of 0, -15, -30 and -45 degrees were considered while the flexibility ratio was set to 10. The outcomes were reported in contour format. There in each graph, the variation of strains was illustrated by changing incident angle in one axis against different sections along the target element in the other axis. The first part of this research examines the performance of perfect rectangular structures against different incident angles. The second part deals with samples of semi-rectangular sections. The selected cases, which belong to different metro stations in Kobe metropolitan, possess rectangular sub-parts.
With an overview of results for perfect rectangles, it can be figured out that the total axial strain is notably governed by moment induced strain rather than pure axial strain. In that case, the maximum strain at corners belongs to zero incident angle while in the middle part of the element, the confrontation angle of -45 degree takes the highest strain values. For shear strains, zero face-off angle caps all results for both corner and middle parts. For the case of semi-rectangular sections, the effect of variation in incident angle on demands becomes highlighted. From the results, it can be seen that for more than 50 percent of elements, face-off angles other than zero dominate the results. It is worth mentioning that to reach a comprehensive influence map on the critical face-off angle, further investigation is required.

Seismometers and accelerometers pick up velocity and acceleration of seismic events and have different applications, which are mostly used by seismologists and engineers, respectively. Therefore, they are not regularly installed in the same location. It is believed that acceleration and velocity records can be calculated by each other, regarding that acceleration is the derivative of velocity. However, this belief has not been confirmed yet. For example, calculated acceleration, through the derivative of the seismometer record, has not been validated by comparing with the recorded accelerograms. This paper tries to find an answer for this challenge by having a comparison between the recorded accelerograms and the one calculated from seismometer records. For this, an accelerometer and a seismometer, both from the same factory, were installed in Mormori after the main earthquake of this city on August 18, 2014, to record aftershocks. Four events with considerable accelerations were selected among the events and recorded by these two instruments. All recorded data were corrected, and then the recorded accelerograms are compared with those obtained from differentiating of the corresponding seismograms. Some quantitative parameters, as well as response spectrum and Fourier amplitude spectrum have been used for the comparison. The parameters indicate that there are some ignorable differences between the records; the average of the parameters, Cave, is greater than 9.0 for all records; therefore, there is an excellent fit between the compared records.
Response spectrums of the recorded accelerograms and the one obtained by differentiating respective seismograms are almost consistent for Tn

There is a large number of situations which expose structures to the fire. Most of the studies have focused on the safety of structures after fire, or the effect of fire that would take place after earthquakes. Observations show that there have been structures that experienced fire and still remained stable, and only by modifying the appearance, have continued to serviceability without structural retrofitting. Due to the risk of earthquakes, it is necessary to determine whether the seismic resistance of the structure to tackle against lateral load is enough or not. In this study, therefore, the seismic behaviour of three steel moment frames after fire were assessed and compared with a building which have not experienced fire. For this purpose, these models with different stories have been selected and designed. The structural models have been designed according to the Iranian seismic code 2800 (4th edition). For performing the nonlinear static analysis, ABAQUS software was implemented. Because most of the existing steel structures are not insulated against the fire with fire-retardant cover, it is assumed that the investigated frames do not equipped with any fire-resistant substances such as fire-fighting foams or fire-retardant gels. For these frames, two symmetrical fire scenarios were considered: 1) each corner spans in the ground floor were exposed to fire; 2) the middle spans in the ground floor were exposed to fire. The process of fire consists of two stages. The first stage illustrates the heating process causing by ignition, and the second stage illustrates the effects of cooling process in fire extinguishing duration. In structural analysis, according to the Eurocode, the parametric curve has been used to apply the first stage, and a linear curve has been used for cooling stage. Having been heated frames according to parametric time-temperature curve of the fire, in order to obtain the maximum time and the corresponding temperature, a failure criteria was determined. By determining these parameters and the fire compartment’s properties, the second stage of the fire behaviour (the cooling curve) was obtained. The seismic behavior of frames, by imposing the complete curve (fire-cooling) to the structure, through a nonlinear static analysis were examined. In the process of the analysis, the maximum relative displacement specified by the Iranian seismic code 2800 has been applied to each story, then the response of the structure has been evaluated. The illustrated results of this research show that in a similar base shear, the nodal displacements of the structure exposing to fire and cooling are more than that of a structure which have never experienced fire. In addition, in the structures that were exposed to fire, the limited drift was not satisfied above the story where the fire was applied. Besides, the maximum drift in the first scenario in which the fire was happened in the corner spans, was significantly higher than that of the one in the second scenario (fire in the middle spans).

In tall buildings, usage of efficient structural systems technically and economically is one of the most essential issues. One of these systems can be formed by a central core embedded by framed tubes which contain outriggers and belts truss. This system caused to reduce lateral displacement of structure and moments of the central core. Tall buildings with a central core, belt truss, and outrigger braced system, are usually designed and implemented by steel braces. Now, in this study, RC is replaced instead of steel where coupled shear walls are used within the mentioned system to investigate their performances and behaviors as a new method. The behavior of RC structures with 40, 80 and 120 stories, using outrigger and belt truss system is evaluated. Outriggers and belt truss for efficient performance are located in height of H/2 for 40 story, H/3 and 2H/3 for 80 story and H/4, H/2, 3H/4 for 120 story structures. The simulation results show that the belt truss within the RC structure is the main factor to reduce the roof displacement. It also shows that the effect of belt truss to absorb the shear force is more apparent than the outrigger braced system. In this investigation, drift, maximum roof displacement, absorption percentage of shear force, negative shear force and optimum height for removing central core are evaluated. By adding outriggers and belt truss systems negative shear create in some elevation levels and from this points to the top, core shear wall have negative performance and produce a force in earthquake direction. It is recommended that the optimum height to remove the central core within 40, 80 and 120 story structures is 0.67H, 0.78H, and 0.84H, respectively, where H is the total height of the building.