SSI Effects Research Articles

Seismic response of a mid-story isolated stilted structure in mountainous areas

Research on the SSI effect on flat sites has yielded many valuable conclusions. However, current research on the impacts of various special local terrains on structural dynamics remains limited. For mountainous areas, it is common to construct houses in a multi-step, climbing, and laterally staggered architectural form that follows the mountain terrain. Only through the analysis of the combined action of the upper and lower parts can the seismic performance of this type of structural form be better revealed; considering the influence of SSI effects will be closer to the actual seismic effects. Therefore, to identify the damage factors of the mid-story isolated stilted structures under earthquakes and provide optimized design plans for the structures, six models are established considering three slopes and two types of foundations based on the engineering case in Chongqing, China. Through the elastic-plastic time-history analysis under earthquakes in the down and transverse-slope directions, concludes, compared with not considering SSI, the seismic response of the mid-story isolated stilted structures considering SSI in mountainous areas is amplified. With the increase of the mountain slope, the seismic response of the structures considering SSI increases, and the amplification coefficients are between 1–1.8. The amplification coefficients of the structures without SSI are concentrated around 1, which is less influenced by the slope. The damage to the stilted isolated layer is mainly concentrated in the column and the beam end, and the maximum seismic response appears in the short columns. The foundation soil stress increases with the increase of the mountain slope.

Assessing seismic fragility on direct displacement-based designed RC frame considering SSI effect

Assessing seismic fragility on direct displacement-based designed RC frame considering SSI effect

Shaking Tables Test on Seismic Responses of a Long-Span Rigid-Framed Bridge Considering Traveling Wave Effect and Soil–Structure Interaction

The traveling wave effect and soil–structure interaction have significant influence on the seismic response of large-span bridges with complex site conditions. In this paper, a 1/10 scaled-down large-span rigid-framed bridge model was designed and fabricated, and a shaking tables test considering the traveling wave effect and soil–structure interaction was carried out on a large-scale continuous rigid bridge model by a real-time substructure hybrid test technique. Influences of the traveling wave effect and soil–structure interaction on the seismic responses of the rigid-framed bridge specimen were systematically analyzed with experimental data. The test results showed that when the apparent wave speed was small, the traveling wave effect increased the seismic responses of the rigid-framed bridge. With the increase in apparent wave speed, the structural response under traveling wave excitation and uniform excitation was basically the same. The SSI effect lead to a great change in the seismic input peaks and spectral characters at the bottom of the pier, and increased the seismic responses of the rigid-framed bridge. When both traveling wave and the SSI effect were considered, there was a phase difference in the seismic excitation. The dynamic responses of a continuous rigid-framed bridge could not be simply obtained by superposition of the separate traveling wave effect or SSI effect. Meanwhile, the real-time substructure test method in this paper solved the problems that the traditional soil box experiment cannot be applied to the test of a large-scale model, the soil and bridge structure find it difficult to meet the unified similarity ratio, and the boundary conditions are difficult to simulate accurately.

Multi-objective seismic optimization and evaluation of core-damper-frame tall buildings considering SSI effect

To control the seismic response of the core-frame (CF) structure and attenuate the stress level of the core tube, a novel core-damper-frame (CDF) system is proposed by decoupling the perimeter frame and the central core with flexible damper connectors. In this way, the large inertia force of the perimeter frame cannot be fully transmitted to the core, and the stress of the core is mitigated. In the meantime, the damper connectors would absorb a considerable amount of seismic energy, protecting the perimeter frame as the relative displacement between the two subsystems (perimeter frame and core tube) occurs. By incorporating the soil-structure-interaction (SSI) and the filtered Gaussian white-noise excitation model, the "excitation-soil-core-damper-frame" augmented system is established. Combing the augmented system and the genetic algorithm, a stochastic optimization procedure of the CDF considering the SSI effect is proposed. The optimization aims to minimize a series of performance objectives including displacement, acceleration, shear force, and overturning moment of the CDF system. The conflicting relationships between the multiple objectives are discussed; the influence of the SSI effect and the excitation parameters are evaluated. From an illustrative 40-story CDF structural example, the system shows great superiority in controlling the higher mode vibrations, and could significantly reduce the peak core acceleration (by 54.44 %), frame acceleration (by 56.48 %), core drift (by 28.83 %), frame drift (by 39.28 %), core shear (by 83.67 %), and frame moment (by 22.41 %), as compared to its conventional CF counterpart. In special, over 50 % of the input seismic energy is dissipated by the damper connectors in the CDF system.

Nonlinear earthquake response of integral abutment bridges on vertical and batter piles: A practical approach

AbstractThis paper presents a new finite element framework for seismic analysis of IABs (and other similar structures) on vertical and batter piles that takes into account nonlinear soil‐structure interaction. A previously developed macro‐element and linear impedance matrix are implemented in a time and modal domain solution, respectively. Detailed pseudo‐codes describing the implementation strategy for the macro‐element are provided. The solutions are validated using rigorous numerical models constructed in OpenSees MP. The software is further demonstrated by performing an extensive set of incremental dynamic analyses, where the effect of linear and nonlinear SSI, batter angle, pile spacing and asymmetric configuration is addressed and discussed.

Research on irregular plane mid story isolation structures in castor earthquake prone areas considering SSI effect

The rock composition of karst area is mainly limestone, which is easy to be eroded and dissolved by water, forming a complex and fragile tectonics. Under the action of earthquake, the instability of stratum is aggravated. The mid story isolation structure is a new type of isolation structure developed from the basic isolation structure, and irregular mid story isolation buildings are highly susceptible to adverse seismic conditions. Previous studies have mostly adopted the assumption of rigid foundations, neglecting the influence of soil on irregular structures. In order to study the seismic response of isolated structures in irregular planes in karst earthquake prone areas considering soil structure interaction, a finite element model of isolated structures in irregular planes considering soil component interaction was established, and the seismic response of the structure under rigid foundation was calculated. Soft and hard soil foundations were studied, and compared with structures without considering SSI effect. Research shows that when considering the SSI effect, the seismic response of the structure increases and the damage to the structure intensifies; When considering SSI, the seismic response of soft soil is greater than that of hard soil, and the softer the soil, the more obvious it becomes; After considering the SSI effect, the torsional effect caused by the irregularity of the plane increases; The design of building structures should fully consider the adverse effects of soil layers on the seismic response of structures under plane interlayer seismic irregularities. In the design of building structures, it is necessary to fully consider the adverse effects of irregular planes on the seismic response of the structure. This consideration is particularly important in the design of soft soil high-rise buildings in karst earthquake areas to prevent unsafe designs and consider the SSI effect in actual seismic resistance.

Seismic performance and SSI effects of a two-storey RC school building during the 2014 Cephalonia, Greece, earthquake sequence

The response of a two-storey RC school building in the town of Argostoli, Cephalonia Island, Greece, during the seismic sequence of January and February 2014, is examined. The structure was built following an older generation of seismic codes dating from the 1950s, which provide limited strength and ductility against lateral loads. Despite the severity of ground shaking and the two successive events, the building suffered relatively minor damage, like most of the RC buildings in the town. Following a short presentation of some basic seismological, structural and geotechnical aspects of the seismic sequence, the paper focuses on the seismic performance of the structure at hand. To this end, a series of detailed non-linear static and time-history dynamic analyses are reported, which highlight the interplay of soil, foundation and superstructure in modifying the seismic demand. It is demonstrated that SSI had an unexpectedly important (detrimental) role in the behaviour of the structure, increasing its natural period by about 25% and aggravating ductility demand in almost all columns, despite the moderately soft soil conditions (average VS = 180 m/s). The results shed light on the seismic performance of the building and help drawing conclusions on the engineering effects of the 2014 Cephalonia earthquake sequence.

The optimum design of particle tuned mass damper for structural seismic control considering soil-structure interaction

Particle tuned mass damper has received considerable attention on mitigating structural seismic responses recently. However, soil-structure interaction effects have not been considered for the optimization design of particle tuned mass damper in the previous studies yet, which may play significant roles in particle tuned mass damper’s effectiveness. This paper investigates the seismic control performance of particle tuned mass damper for tall buildings considering SSI effects. A 40-story benchmark structure is adopted as the primary structure, and different soil conditions including soft, medium, dense soil, and the fixed-base case are considered. Based on the Cuckoo Search algorithm, an optimum design approach for the particle tuned mass damper implemented in SSI system under earthquake excitations is proposed, and the optimum parameters of particle tuned mass damper are obtained from the optimum design process. The optimum results indicate that the optimized particle tuned mass damper can effectively decrease the displacement response without the increase of peak acceleration, and the reduction rates could exceed 30% in some cases. Compared with other soil type cases, under soft soil condition, the optimum parameters of particle tuned mass damper are especially different, and the mitigation effects of the optimized particle tuned mass damper on maximum displacement weaken. Hence, it is necessary to consider the SSI effects for particle tuned mass damper’s seismic design. Furthermore, the interaction effects between the primary structure and particle tuned mass damper in SSI system are also evaluated through energy analysis. The results show that considering SSI effects, the optimized particle tuned mass damper device dissipates the overwhelming majority of the input energy, and it greatly decreases the structural energy, displaying excellent energy dissipation performance on seismic vibration control of structure.

Numerical Analysis of Seismic Pounding between Adjacent Buildings Accounting for SSI

The structural pounding caused by an earthquake may damage structures and lead to their collapse. This study is focused on the pounding between two adjacent asymmetric structures with different dynamic properties resting on the surface of an elastic half-space. An exploration of the relationship between the effects of the seismic analysis with the impact response to the torsional pounding between adjacent buildings under different SSI effects has been presented. In this paper, the authors have proposed a procedure for analyzing the response for adjacent buildings subjected to the pounding effects, considering systems with multiple degrees of freedom and modal equations of motion with four types of soil. All the calculations have been performed based on the fourth-order Runge–Kutta method. The novelty of the present study is related to the fact that the rigorous and approximate methods are used to examine the effects of pounding and SSI simultaneously. As a result, these two methods have been thoroughly investigated for both effects and the results have been compared. The results show that the approximate method produces results that are slightly different from those obtained by the rigorous direct integration method in the case of small SSI effects due to an increase in the pounding force. The efficiency of the method is also validated using numerical examples.

Adopting a fiber macroelement for accounting the SSI effect in nonlinear seismic analysis

The main purpose of this paper is to propose a practical approach for considering the soil - shallow foundation - superstructure interaction effect in nonlinear seismic analysis. Soil structure interaction is a complicated task dealing with various engineering problems. Taking into account the soil structure interaction extends beyond the numerical analysis, geotechnical investigation, or structural design fields. The artistry of considering the soil structure interaction effect lies in the balance between incorporating soil conditions and the superstructure into one collective model. Following this methodology alongside classical soil mechanics principles provides additional confidence to the structural engineering society in adopting a complete practical concept. The herein presented study introduces a fiber macroelement that represents the soil beneath shallow foundations. This allows for the direct utilization of the fiber macroelement in nonlinear pushover and dynamic analysis in commercial software, as well as its simple implementation in open software systems. The introduction of a procedure for forming the stiffness matrix of the element and a hysteresis soil model is also a key component. Special attention is devoted to obtaining the material backbone curve through experimental results and Eurocode 8?s procedures. Furthermore, this paper demonstrates the implementation of a macroelement for considering the soil-structure interaction effect in nonlinear seismic analysis within commercial software. The primary aim is to provide structural designers with a streamlined approach for incorporating soil conditions into superstructure analysis. Numerical analysis results from an example building with a reinforced concrete structure in commercial software highlight the significance of not underestimating soil conditions. This work emphasizes that relying solely on fixed models in everyday structural design could lead to dangerous and unexpected consequences.

Analysis of an Isolated Footing taking into account Soil Structure Interaction

In the proposed study an attempt is made to understand the effect of soil interaction on the performance of building frames resting on isolated footing. The purpose of this study is to describe and investigate different approaches of considering the linear interaction analysis (SSI- Soil-Structure Interaction) with regard to the response in the superstructure. The present study is focused on SSI analysis of symmetrical space frame of 3 bays in both x and y direction, 2 storey (3X3X2), 3 bays in direction, 4 storey (3X3X4) and 3 bays in both direction, 6 storey (3X3X6) resting on isolated footing with fixed base. Medium hard clay type of soil is used for this SSI study. The interaction approach is incorporated in the analysis using Conventional analysis (NIA model - Non interaction analysis) and Soil-structure interaction analysis (LIA model - linear interaction analysis). ANSYS is used for developing these models. The effect of SSI on various structural parameters i.e. natural time period, base shear, roof displacement, beam moment and column moment and axial force are studied and discussed. The comparison is made between the approaches of conventional and liner interaction modeling. The study reveals that the SSI significantly affects the response of the structure.

Salp swarm optimization of active tuned mass dampers in asymmetric plan buildings including SSI effects

Salp swarm optimization of active tuned mass dampers in asymmetric plan buildings including SSI effects

Torsion in symmetric buildings due to soil‐structure interaction: A discussion on code values of accidental eccentricity factor in seismic design

AbstractSoil‐structure interaction (SSI) is often neglected in seismic design because of its inherent complexity. Therefore, SSI‐related research is crucial for ensuring that more significant issues reach engineering practice. This study focused on torsional motion to investigate the accidental eccentricity caused by SSI effect by comparing the proportional relationship between the torsional motion of an SSI system and the translational motion of a fixed‐base oscillator. The SSI model was created using a three‐dimensional rectangular foundation embedded in a layered half‐space with an oscillator mounted on the top. We employed an indirect boundary element method (IBEM) combined with non‐singular Green's functions of distributed loads to calculate the system responses of the SSI. Parametric analysis‐based results in the frequency domain revealed that the accidental eccentricity factor owing to SSI was typically in the range of 0.03–0.15. Additionally, a case study of four buildings subjected to 42 earthquakes revealed that the accidental eccentricity factor exceeded 0.05 in several cases, and 0.2 in some cases. Furthermore, it depended on the excitation spectral shape and dynamic characteristics of the SSI system. Therefore, the coded accidental eccentricity factor value of 0.05 is insufficient for ensuring the safety of buildings from the torsional effect of SSI, and a value of 0.1 is more reasonable.

Fragility curves for existing reinforced concrete buildings, including soil–structure interaction and site amplification effects

SSI and site amplification effects are investigated as influences on the seismic fragility of existing reinforced concrete moment-resisting frame and dual frame-wall system buildings supported on shallow foundations without interconnecting beams. We build upon a holistic methodology that accounts for site amplification and soil–foundation–structure interaction effects using a modular approach. We calculate fragility curves based on nonlinear dynamic analyses for various building structural typologies and geometries, infill conditions, code provisions, and soil profile materials and dynamic characteristics. We demonstrate that site amplification during earthquakes may significantly increase the fragility of the soil–foundation–structure system, which is reflected in its vulnerability. Moreover, SSI is especially prevalent for buildings on soft soil profiles and might modify their fragility. We propose a modular method to include site amplification and/or soil–structure interaction effects in a large-scale earthquake vulnerability assessment using fragility modifiers, which we express using an easy-to-code equation form.

Experimental research on seismic response of split‐type prefabricated utility tunnels through shaking table tests

AbstractDue to the advantages of prefabricated structure, its applications in utility tunnels are increasing. However, more connection joints have raised concerns about the vulnerability of prefabricated utility tunnels in earthquake disasters. The main objectives of this research are to analyze the seismic response, vulnerable parts and failure modes of a prefabricated utility tunnel. A pair of twin models of prefabricated utility tunnels with two‐compartment layouts were designed, and then set up to load ground motions synchronously through a large‐scale shaking table of 6 m × 6 m under two different boundary conditions: one buried inside a soil box and its twin laid outside the soil as a comparison. The results show that: the damage of the split‐type prefabricated utility tunnel is mainly concentrated in the connection joints, and the connection joint at the top corner of the compartment with larger cross section is the vulnerable part that needs special attention; the SSI effect has a great influence on the seismic response of the prefabricated utility tunnel, and the model buried inside the soil box shows stronger seismic response than its twin laid outside the soil. The experimental results and analysis in this paper can provide a helpful reference for risk assessment and prevention, seismic design, and structural optimization of prefabricated utility tunnels.

Effect of CPRF on nonlinear seismic response of an NPP structure considering raft-pile-soil-structure-interaction

A number of Nuclear Power Plants (NPPs) in India are coming up in soft soil condition particularly on alluvium in active seismic zone on Combined Piled-Raft Foundation (CPRF). So far, no study has been reported considering nonlinearity of soil for the NPP structures founded on CPRF considering earthquake loads.In the present study, the dynamic responses of raft, pile group and CPRF of a NPP are investigated by simulating the nonlinear finite element model. The effects of nonreflecting boundary condition, pile-soil-interaction by the embedded constraint, raft-pile interaction by the tie constraint and raft-soil-interaction by the penalty constraint, were considered. The material nonlinearity of soil was taken into account using an advanced constitutive model Drucker-Prager with cap plasticity. The geometrical nonlinearity is also considered using the concept of the contact surface. Thus, a complete 3D finite element numerical model of a nuclear structure considering raft-pile-soil-structure interaction (RPSSI) for nonlinear seismic analysis is developed. The validation is carried out by comparing the results from the present study with experimental and numerical results available in the literature. Nonlinear analyses of raft, pile group and CPRF in the time domain finite element formulation carried out and results are compared with that using the linear analysis.A comparison of two analyses shows that linear analysis underestimates the response of soil, piles and structure. The effect of type of boundary used in the analyses is significant. It was also observed that the response of NPP structure is significantly affected by the type of foundation used under the superstructure. The effect of SSI is examined with two major novelty i.e. (a) Effect of CPRF (b) Consideration of Nonlinearity of Soil. Effect of frequency of excitation and PGA of earthquake motion is examined. This study indicates the importance of CPRF over the raft foundation and pile group foundation for the design of nuclear power plants. Also, it examines the effects of nonlinearity on the response.

Seismic behavior of existing elevated water tanks resting on different type of foundations

It has been well recognized that heavy structures built on soft soils, experience significant impact on seismic response, because of considerable foundation movements and hence, the effect of Soil-Structure-Interaction (SSI) can be a major factor while analyzing important structures like; nuclear power plants, elevated water tanks, chimneys, etc. against seismic forces. In general, elevated water tanks that were constructed on different type of soils in seismic regions, prior to the revision of seismic codes, must be checked on behavior and safety of such structures against earthquake loads as per present codes.In this paper, two similar elevated R.C. circular water tanks, each of 4.0 lakh liters capacity supported by two types of foundations; one with circular ring raft below the columns of tank resting on sandy soils, and the other with circular ring raft supported by piles embedded in black cotton soils, are chosen for carrying out seismic analysis. In the analysis, the water tank is idealized as a system with three degrees of freedom, treating the staging of tank as one spring attached to the mass of the water tank and the soil is treated with two springs; one in horizontal and the other in rocking mode, both attached to the soil mass and the response parameters like; time periods, base shears and displacements are worked out against earthquake loads. Results indicate that, irrespective of type of foundation, the soil play important role on seismic response of elevated water tanks by virtue of their configuration with top heavy mass and slender staging; particularly displacements are found high when they rest on loose soils. According to the new guidelines provided in the revised IS 1893 (Part-1) 2016 code, SSI effect should be considered in seismic analysis when the structure rests on loose or soft soils.

SSI effects on seismic settlements of shallow foundations on sand

The gradual extension of performance-based design in geotechnical engineering has focused the attention on the seismic performance of foundations, typically, in terms of settlement, tilt and bearing capacity degradation. The aim of the present paper is to explore the role of superstructure inertia on the performance of shallow foundations through a series of non-linear numerical analyses of structure-foundation-soil systems, performed with the Finite Difference Code FLAC3D. The NTUA-Sand constitutive model is used to simulate the nonlinear cyclic soil response. The parametric analyses focus upon the effect of key soil-structure interaction parameters, such as the frequency characteristics of the structure – foundation (SFS) system and that of the free-field soil profile, as well as, the structural and excitation properties. Initially, the nonlinear fundamental period of vibration of the SFS system is correlated to the above key parameters of the system and the seismic excitation. In the sequel, to isolate the SSI effects, settlements of SFS systems are compared with the respective settlements of equivalent foundation-soil systems (FS). Simplified relations are finally developed for the analytical calculation of SSI effects on foundation settlements, based on a multi-variable statistical analysis of the available numerical predictions.

RC Medium-Rise Building Damage Sensitivity with SSI Effect.

Global vibration-based methods in the field of structural health monitoring are intended to capture structural stiffness changes of buildings or other civil engineering structures. Natural frequencies of buildings or bridges are commonly used parameters to monitor these stiffness changes. Therefore, it is essential to clarify the limit at which this method is no longer sensitive enough to be useful for structural health monitoring purposes. This paper numerically investigates the effect of structural damage and soil–structure interaction on cellular-type reinforced concrete buildings’ natural frequencies. These buildings are a common housing stock of Eastern Europe but are rarely investigated in this context. Comparisons with a reinforced concrete frame and infill structure building are made. Finite element models representing three structural system types of nine-story reinforced concrete buildings were used for the numerical simulations. Furthermore, a five-story finite element model was used for a damage sensitivity comparison. It is established that, for cellular-type structure buildings to detect damage comparable to that investigated in the paper, structural health (fixed base model frequency) should be monitored directly. Then, a statistical significance level for frequency changes of no more than 0.1% should be adopted. Conversely, the rocking frequency is a very sensitive parameter to monitor building base condition changes. These changes are often a cause of the cracking of building elements.

Effect of Embedded Basement Stories on Seismic Response of Low-Rise Building Frames Considering SSI via Small Shaking Table Tests

The dynamic soil-structure interaction is a combination of phenomena caused by the flexibility of soil foundation in structure response. The structure response may be changed by embedded basement stories. Thus, this study seeks to assess the dynamic response of seven-story concrete frame type buildings without a basement, one basement story, and two basement stories, considering fixed and flexible bases. For this purpose, the experimental tests on the small shaking table were executed with a small scaling coefficient of 1:50. Consequently, three scaled models of steel skeleton structures with variable embedded depths have been constructed with fixed and flexible bases. These models are exposed to three seismic input motions: Northridge (1994), Kobe (1995), and Chi-Chi (1999) at the base of the structure as a fixed base and the bedrock level in the soil structure system as a flexible base. The finite element technique is carried out for scaled and real models. Both the scaled and real numerical models are in good agreement with the obtained experimental observations with reasonable accuracy. It is concluded that the lateral deflections are overestimated by excluding embedded depths of structural elements. In the flexible prototype model, the lateral deflections of the superstructure with embedded depths (3 m, 6 m) decrease compared with no embedded depth, in which the maximum reduction percentages of 7-story with embedded depths 3 m and 6 m at the roof floor level are 21% and 42% compared with no embedded depth, respectively, under Northridge earthquake. Otherwise, ignoring the SSI effects (fixed base case), the lateral displacements are underestimated compared with the flexible base. The maximum amplification percentages at the roof floor level between flexible and fixed bases models with variable embedded depths are 35%, 37%, and 65% under Northridge, Kobe, and Chi-Chi earthquakes, respectively. The amplification and reduction percentages may be high or low, mainly depending on soil condition (fixed, flexible), variable embedded depths, characteristics of seismic motion, travel pass, and source of seismic motion. These items are summarized as the frequency domain of the coupled system compared with the frequency domain of the earthquake motion.