structure interaction

On the other side, with urbanization, city blocks contain clusters of closely spaced buildings. Under such circumstances, the dynamic interaction of adjacent structures should not be ignored. However, available evidences show that in the field of soil-structure interaction, little attention has been payed to adjacent structures. In addition, the vast majority of studies are subjected to consider two-dimensional models with plain strain behavior assumptions. Such simplifying assumptions lead to obtain not so accurate and reliable achievements.
In this paper, the effects of soil structure interaction and adjacent structures interaction on the seismic response of structures through considering three-dimensional models by using OpenSees software were studied. In this regard, structures are divided in to two major groups, fixed base structures (structures resting on the rigid base) and flexible base structures (superstructures resting on the flexible base), whereas flexible base structures contains Soil-Structures and Structure-Soil-Structure systems. As respects, the common dynamic analysis focus on the fixed base assumptions; therefore, in this study, the evaluations and comparisons between results of flexible base structures and fixed base structures analysis have been paid attention to. Due to the modeling of superstructures for analysis and design processes, three reinforced concrete moment resisting frame, 5, 10 and 15 stories, two spans, resting on shallow foundations with different structural neighborhoods are selected in conjunction with a soil type III (according to ground type classification of Iranian building code), based on the direct method, considering of appropriate lateral boundaries and interface elements to simulate frictional contact and probable slip due to seismic excitation. Besides, for the design and analysis of superstructures according to the Iranian concrete code, gravity and lateral loads with considering Iranian national building code part 6 and Iranian seismic code 2800, respectively, is conducted by using ETABS software, then structural sections are designed according to Iranian national building code part 9.
Nonlinear dynamic analysis using OpenSees software under influence of three different earthquake records is conducted. The study of the response of acceleration, drift and shear force in the stories indicates that effects of soil-structure and structure-soil-structure interaction depend on dynamic characteristics of buildings, frequency content of seismic data and the height of adjacent structures. The results show that considering adjacent structures with common distance lead to increase or decrease about tens of percent of dynamic responses.
The results indicate that when a short building (5-story) has two adjacent close tall buildings (15-story), maximum responses of acceleration, drift and shear force increase up to 10, 20 and 40 percent, respectively. Results show that a 10-story building with two adjacent 5-story buildings, up to 24% decreases in acceleration response; while with two adjacent 15-story buildings increase in responses of drift and shear force, 20 and 16 percent, respectively. In tall buildings (15-story in this study) with the same height adjacent structures, the acceleration up to 15% and drift up to 23% decrease with two 10-story adjacent structures and shear force increases up to 35% with two 5-story adjacent structures. On the other hand accepting the results of common structural dynamic analysis lead to unsafely design for tall buildings, and for short structures are not affordable.

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.

During an earthquake, the dynamic response of a structure located on a soil deposit could be very complex compared with the analysis of the same structure on bedrock due to the interactions between the soil and the structure. This phenomenon is technically termed as Soil-Structure Interaction (SSI) effects in literature. Most of what is currently known about soil-structure interaction (SSI) is based on theoretical and mathematical models. Therefore, it is necessary to investigate the structure treatments when they response to the ground strong motions transferred by SSI. In this regard, in an experimental field, the present study investigated the SSI effects on structure, evaluating the natural frequencies and damping of a pile-group-supported pier of Kermanshah’s LRT. The frequencies and damping evaluations were performed through ambient vibration test results and system identification procedures. The purpose of system identification is to evaluate unknown properties of a system, using known inputs and outputs. There are two principal system identification procedures to build mathematical models of dynamical systems from measured data: (a) non-parametric and (b) parametric procedures. Nonparametric procedures evaluate complex-valued transmissibility functions from the input and output recordings without fitting an underlying model. Accordingly, Fourier Transform (FT), response square of transfer function, peak picking and four spectra are considered as non-parametric procedures. On the other hand, parametric procedures develop numerical models of transfer functions. More precisely, in these procedures, a mathematical model with several parameters is defined first. The considered parameters are featured with specific values determined by experimental results. Then, the system’s input-output function is obtained using this described model. The studied pier was fully instrumented with two SARA and a CEM seismometers. The seismometers recorded signals of two horizontal and a vertical components that were digitally recorded at 200 Hz sampling rate. In general, measure of SSI effects was then obtained by comparing the flexible base and fixed-base parameters to calculate the two most important effects of SSI, period lengthening and foundation damping. SAP 2000 was used to create a finite element model of the whole structure and the accuracy of the model was tested using recorded data from ambient vibration at the structure site. In summary, the current study indicates that all the utilized system identification methods are appropriate in determining the dynamic characteristics of the structure in fixed condition. In addition, it was demonstrated that peak picking and four spectral methods did not have appropriate function in investigating the interaction. However, these two procedures have appropriate function in determining the dynamic characteristics of the structure in fixed condition. As Figure (1) shows, the diagram of the period lengthening obtained from parametric method with the ratio of the structure-to-soil stiffness for the pier is approximately consistent with system identification analyses performed for the 57 sites in Stewart et al. [1]. Accordingly, inertial interaction effects were generally observed to be small for 1/σ < 0.1 and for practical purposes could be neglected in such cases.

There are many parameters that influence the dynamic response of structures in contact with fluid medium. One of these parameters is the direct effect of reservoir، especially for high structures during severe earthquakes، so there are an extensive amount of publications about the analysis of fluid-structure interaction systems in different engineering branches. In this research، studies in the field of civil engineering with an emphasis on dams will be considered. The different methods of analysis of the fluid-structure interaction problems are reviewed and then some case studies for structures such as retaining walls، concrete، CFR and embankment dams، are discussed. Due to the development of numerical methods in recent years، the studies done using such methods are considered with more emphasis. However، analytical methods such as Westergard and Chopra cases are presented and discussed in this paper. Studies in the field of concrete dams are presented in two divisions as modeling approaches and boundary condition effects. Then the studies about the seismic interaction of reservoir-CFR dams are presented in the related section. Also analysis of seismic coupled reservoir-structure systems for a concrete dam and a CFR dam، which have been conducted by the authors with ADINA software، are presented and compared with the results of the previous studies. This comparison shows the accuracy of the results of ADINA software in the field of dynamic coupling of water-structure systems for concrete and CFR dams. Finally، the previous studies for the dynamic interaction of embankment dam-reservoir system are presented and the related results are discussed. It is concluded from reviewing the previous study that the amount of hydrodynamic force in the fluid domain is a function of the frequency content of incident seismic wave and the ratio of the natural frequency of dam to reservoir one. Therefore، there is not a certain height restriction for unconsidering the effect of interaction of reservoir-dam. Also، if the frequency content of energy of seismic wave is less than the natural frequency of reservoir، the effect of considering the coupled water-structure system would be less. Consequently، the final results of such system would be less affected by the reservoir. As another result، if the ratio of the natural frequency of reservoir to dam is almost «2»، the interaction of reservoir-structure has no considerable effect on the final seismic response of coupled system. In the special case of CFR and embankment dams، there are fewer researches in comparison with concrete dams. The main reason can be related to the low slope of upstream face of the dam which reduces the hydrodynamic force of the reservoir. But there are not sufficient available researches that improve the accuracy of such arguments in many different conditions of incident wave (magnitude of PGA، the frequency content of seismic wave، spatial ground motion condition)، and variation of some parameters such as per- meability coefficient of shell materials، height of dam، and slope of upstream face. Therefore، it seems that in the future، more studies should be done in the field of embankment dam-reservoir coupled system.