Highlights

Abstract

Keywords

1. Introduction

2. Description of theoretical aspects

3. Results of analysis

4. Modification factors

5. Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

References

Abstract

Severe damages of adjacent structures due to structural pounding during earthquakes have emphasized the need to use some seismic retrofit strategy to enhance the structural performance. The purpose of this paper is to study the influence of using linear and nonlinear Fluid Viscous Dampers (FVDs) on the seismic collapse capacities of adjacent structures prone to pounding and proposing modification factors to modify the median collapse capacity of structures considering the effects of pounding. The factors can be used to predict the collapse capacity of structures in pounding condition. A seismic retrofit strategy employs FVDs installed in 3-, 6- and 9-story Special Moment Resisting Frames (SMRFs). The SMRFs were assumed to have different values of separation distance according to the seismic code. To model pounding phenomenon, linear viscoelastic contact elements were used in the OpenSees software. Furthermore, to determine the seismic collapse capacities of each structure, the proposed algorithm was applied to remove the collapsed structure during the incremental dynamic analysis. The results of the analyses illustrate that the existence of FVDs can substantially improve the seismic behavior of structures having a significant influence on the collapse capacities of colliding structures. Moreover, considering the adjacent SMRFs in one or two sides of the main structure can significantly affect the median collapse capacity of the main structure itself. Finally, the proposed modification factors can be successfully used to estimate the effects of pounding on the collapse capacities of adjacent structures.

1. Introduction

Different investigations show that adjacent structures with inappropriate separation distance between them may interact one with another during earthquakes, resulting in significant damage [1]. Although nowadays it is a usual procedure to consider a minimum separation distance during the design of concrete or steel structures, many existing structures were constructed with insufficient in-between gap size since it was allowed due to old seismic codes. This insufficient separation distance, which may lead to impacts between adjacent structures during earthquakes, can significantly influence the seismic structural behavior as well as the collapse capacity. Therefore, improving the seismic performance of existing structures is often challenging because such external circumstances can affect the structural behavior. In this respect, there are two strategies: first, increasing the capacity of structural members, for instance using friction devices into beam to column joints, and second, installing passive energy dissipation devices such as dampers or adding dissipative metal shear panels (see for instance [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]). The first approach is more convenient for low-rise buildings, whereas the second one can be more effective and affordable for all types of structures. Furthermore, the strengthening option for structures may interfere with the architectural plans, while the installation of seismic dampers can play a positive role. Although previous studies were mainly focused on the strengthening option, the idea of using linear and nonlinear Fluid Viscous Dampers (FVDs) for retrofitting structures is not new. Among different passive energy dissipation systems, FVDs have some performance advantages for their ability to reduce both displacements and accelerations, which can be very important for the structures with sensitive components.

1- Introduction

2- Optimal damper placement with respect to transfer function amplitude at natural frequency and optimal damper placement under critical double impulse

3- Plastic deformation characteristics of building structures with optimal damper locations for increasing level of critical double impulse

4- Conclusions

References

Abstract

The effectiveness of the optimal viscous damper placement for elastic-plastic multi-degree-of-freedom (MDOF) structures under the critical double impulse is demonstrated through the comparison with the optimal damper placement for elastic MDOF structures designed with respect to transfer function amplitudes at natural frequencies. The method for optimal viscous damper placement was proposed for elastic-plastic MDOF structures subjected to the critical double impulse as a representative of near-fault ground motions in the previous paper [1]. The double impulse is composed of two impulses with opposite directions and the critical interval is determined by using the criterion on the maximum input energy. The critical timing of the second impulse was found to be the timing which requires the vanishing of the sum of the restoring force and the damping force in the first story. Three models with different story stiffness distributions of main structures are treated to demonstrate the effectiveness of the optimal viscous damper placement under the critical double impulse. The double impulse pushover (DIP) procedure proposed in the previous paper for determining the input velocity level of the critical double impulse is examined further in this paper. It is demonstrated that the optimal viscous damper placement for elastic-plastic MDOF structures under the critical double impulse is more effective for pulse-type recorded earthquake ground motions than the optimal damper placement designed with respect to transfer function amplitudes at natural frequencies because several higher modes arising in the elastic-plastic response under pulse-type recorded earthquake ground motions can be well controlled by the design for the double impulse.

Introduction

Viscous, visco-elastic and hysteretic dampers have been used as effective passive dampers installed at interstories. It is commonly understood that hysteretic dampers are effective in general for impulsive ground motions with short duration [2–6] and viscous dampers are useful for long-duration ground motions. Recently, it is reported after devastating earthquakes that near-fault ground motions of impulsive type with short duration are apt to cause large damage to building structures. Although viscous dampers are not necessarily effective for near-fault ground motions, they have advantages to be able to reduce both displacement and acceleration. It seems, therefore, preferable that, when designing viscous dampers, they can reduce the earthquake response to small-to-moderate level ground motions and prevent excessive deformation to large level ground motions, such as long-period ground motions. Many useful researches on optimal damper placement have been accumulated so far (see Refs. [7–9]). Takewaki [10] proposed an optimality criterion-based approach including an incremental inverse-problem formulation in which the transfer function amplitude in terms of the sum of the interstory drifts at the fundamental natural frequency is minimized under the constraint on total damper quantity. This approach has an advantage that the algorithm is simple and the obtained result is independent of input ground motions. Aydin [11] extended this approach to the transfer function amplitude in terms of the base shear at the fundamental natural frequency. Fujita et al. [12] developed an optimization method for minimizing the maximum interstory drift in the transfer function under a constant total damper quantity. Adachi et al.

1- Introduction

2- The development of the EPRRC

3- Design of the VED parameters

4- Design example and verification

5- Conclusion

References

Abstract

The traditional design method of the elastic response reduction curve (ERRC) was used to design the parameters of the viscoelastic damper (VED), where the plastic behavior of the primary structure is neglected. Considering the structural plastic behavior, in this study, a direct design method is proposed to obtain the parameters of VEDs based on the elastic-plastic response reduction curve (EPRRC), which can illustrate the relationship between damper parameters and the response reduction effect in the elastic-plastic stage. First, the ERRC is developed to the EPRRC, and the differences between them are compared using the displacement and acceleration response difference ratios, which demonstrate an overestimated response reduction effect of VEDs in the elastic-plastic stage using the ERRC. Then, the corresponding design procedures are given based on the EPRRC by referring to the direct displacement-based design theory. Finally, a benchmark model is used to illustrate the effectiveness of this proposed design method by conducting a time history analysis. The analysis results indicate that the target story drift of the structure can be satisfied under different earthquake intensities using VEDs. Thus, based on this study, the development of the EPRRC can be considered worthwhile, and the proposed design method of VED parameters is easy to implement and is effective.

Introduction

Passive control systems using energy dissipation devices have been demonstrated to be effective for seismic damage mitigation of structures [1–4]. Four kinds of devices, friction dampers, metallic yield dampers, viscous dampers and viscoelastic dampers (VEDs), are often used. Among these devices [5–10], VEDs provide both supplemental damping and stiffness and show the characteristics of the phase angle difference of the force-displacement relationship, thus, in some ways, they are more difficult to apply in design and practical use than other devices. A typical VED usually consists of flaky viscoelastic materials bonded with steel plates. When relative shear deformation takes place in the viscoelastic materials, the energy caused by dynamic loads is then dissipated. Due to effective energy dissipation capacity from low to high displacement [11], VEDs were used to reduce the structural vibration caused by different kinds of dynamic loads, such as winds, earthquakes and even human activities. The practical projects of VEDs in resisting wind-induced vibration began in the 1960s [12], and the application of VEDs for reducing the seismic responses of civil engineering structures began in the 1990s [13,14]. Recently, VEDs have been recommended to mitigate human-induced vibrations [15]. Additionally, for fire-damaged structures, VEDs are also applied for vibration control [16]. A number of experiments and theories have been improved by scholars such as Shen and Soong [17], Chang et al. [18], Tsai [19] and Xu et al. [20], who have demonstrated that the mechanical properties of VEDs strongly depend on temperature and frequency, thus complicating the analysis and design of structures with the addition of VEDs. Consequently, to effectively simulate the practical application of VEDs, classical rheological models and fractional derivative models are often suggested to describe the VEDs [21–26]. In the frequency domain, Lewandowski and Pawlak [21] combined the widely used response spectrum theory for structures mounted with fractional VEDs. In the time domain, using the generalized Maxwell model and Laguerre polynomial approximation technique, the computation time of dynamic analyses for structures with VEDs is clearly reduced [27]. Therefore, the reasonable model and appropriate theory have been recognized to be effective and feasible for the application of VEDs in practical engineering.

1- Introduction

2- Friction damped braced frames

3- Fragility function

4- A method of estimating design parameters of friction damper-brace assembly

5- Illustrative examples and parametric study

6- Results and discussion

7- Comparison with an existing design method

8- Conclusions

References

Abstract

In this paper, a probability-based design methodology of the friction dampers in multi-story steel frames is proposed. Both the slip force of the device and the stiffness ratio of the system are analyzed as two important parameters, which affect the behavior of the structures equipped with friction devices. The seismic fragility of friction damped braced frames is evaluated and used to identify the optimal ranges of the above-mentioned design parameters so as to minimize the overall damage probability of the structure under the action of strong ground motions. For this purpose, fragility functions of the structural models are derived using nonlinear incremental dynamic analyses. To demonstrate the efficiency of the proposed method, three structural models of steel moment resisting frames with friction damper systems (including chevron braces and damper devices) are considered for the purpose of the seismic performance analysis. The results of the analyses show that the largest damage probability in each structural model corresponds to the case with the higher slip force and the lower stiffness ratio, where the undesirable buckling failure will govern before full activation of friction damper. For the three considered building frames, the optimal range of slip force lies between 40% and 55% of the total weight of structures and the recommended value for stiffness ratio is 2.

Introduction

Since several decades the seismic response control techniques have been used as complementary solutions to the existing seismic force resisting systems and several types of passive energy dissipation devices have been developed. Friction dampers are considered one of the most effective passive control devices for building structures against earthquake actions. Compared to the velocity-dependent devices, such as viscous and viscoelastic dampers, friction dampers provide energydissipation capacity with a sufficient lateral stiffness. The action of these dampers is based on the mechanism of dry friction which develops between two solid bodies, sliding relatively one to another for providing a specified energy dissipation capacity [1]. The friction devices exhibit hysteretic behaviors similar to those achieved by the metallic devices. Recently, considerable progress has been made in the development of this type of devices. Pall friction damper is the most commonly and widely used type of friction dampers, which was originally introduced by Pall et al. [2,3], based upon the automotive brake. It can be located in structures either as a part of braces [4] or, more recently, as beam-tocolumn joints [5,6]. The friction devices basically consist of steel plates tightened together by means of high strength steel bolts, whose either axial or rotational deformation mechanisms lead to a transformation of kinetic energy into thermal energy; so, the maximum amount of friction force can be controlled by adjusting the friction coefficient of the sliding surfaces, as well as the compression force of the tightened bolts, but it is independent of the sliding velocity and the contact area of the sliding surface. Since the 90s of the last Century, Sumitomo Metal Industry in Japan developed a different type of friction dampers [7]. This device consists of copper pads impregnated with graphite in contact with the wedges inside of the steel cylinder. The load on the contact surface is induced by a series of wedges that act under the compressive force of the Belleville washer springs (cup springs). The graphite serves as a lubricant between the contact surfaces and ensures a stable coefficient of friction and silent operation. The experimental study of Aiken and Kelly [7] allows the Sumitomo damper to be used as a structural damper system. They compared the response of moment resisting frames without and with this damper.

1- Introduction

2- Bridge model

3- Test protocol

4- Test results and discussion

5- Numerical modeling and validation

6- Conclusions

References

Abstract

In current transverse seismic design of long span cable-stayed bridges, the conventional transverse fixed system (TFS) is usually adopted. This strategy inevitably increases seismic demands of substructures and towers, leading to high seismic-induced damage risks to the bridges. To address this issue, the authors recently developed a novel Transverse Steel Damper (TSD) and correspondingly proposed an innovative TSD seismic system (TSDSS), in which the TSDs were placed at deck-tower and/or deck-bent connections. To further verify the reliability and seismic isolation efficiency of TSDSS for long span cable-stayed bridges under near- and far-fault ground motions, a series of experiments on a 1/35-scale model of a kilometer-span cable-stayed bridge were conducted on a four-shake-table testing system. Experimental results indicate that (1) compared with the conventional TFS, the TSDSS can reduce transverse displacement and curvature demands along bent/tower columns, meanwhile limiting displacements at deck-bent/tower connections to an acceptable level in engineering practice. (2) The sensitivity of TSDSS to ground motions is obviously lower than that of the conventional TFS. The isolation efficiency of TSDSS is robust regardless under near- or far-fault ground motions; (3) Increasing the yield strength of TSDs can decrease the relative displacements at deck-bent/tower connections. In general, the TSDSS is experimentally validated to be a capable and reliable strategy for the seismic design of long span cable-stayed bridges. Additionally, the shake-table test is simulated using a finite element model, which provides good agreements with the test results.

Introduction

In the last three decades, a large number of cable-stayed bridges have been constructed in China. These bridges usually serve as key joints in national or local transport networks. Considering the importance of these bridges in networks and to ensure the essential postearthquake functionality, the main structural components of cablestayed bridges, including towers, foundations and superstructures, are required to remain elastic under design-level earthquakes with a return period of 2000 years in Chinese seismic design specifications [1,2]. However, severe damages were still detected in cable-stayed bridges during some recent strong earthquakes. For example, in 1999 Chi-Chi, Taiwan earthquake, Chi-Lu Bridge suffered severe damages in the towers [3]. Therefore, how to improve the seismic performances of cable-stayed bridges has drawn increasing attentions from both the engineering and academic community. In the longitudinal direction, a full- or semi-floating system is often applied in practice. In such a system, Fluid Viscous Dampers (FVDs) [4–7] are often installed at decktower connections to reduce shear and bending demands at tower bottoms and foundations, meanwhile limiting relative displacements between deck and towers to a practically acceptable level. In this manner, longitudinal seismic performances of cable-stayed bridges are generally acceptable for practice. In the transverse direction, however, rigid constraints are often adopted at deck-bent/tower connections for providing enough stiffness to carry traffic and wind loads under service conditions [8]. This is the so-called conventional Transverse Fixed System (TFS), which inevitably increases seismic demands at bents (or piers), towers and foundations. To meet the seismic design requirements, these components are always designed, in an uneconomic manner, with sufficient strength capacity to resist design-level earthquakes. In this regard, transversal isolation systems for cable-stayed bridges, especially in high seismic regions, are highly demanded.

1- Introduction

2- The governing equation of motion for the structure equipped with FTMD including SSI effects

3- A Multi-objective cuckoo search algorithm

4- Numerical studies

5- Results and discussion

6- Conclusions

References

Abstract

The use of TMDs is commonly discouraged for structures subjected to short-duration, pulse-like ground motions such as near-field earthquake excitations. Friction tuned mass damper (FTMD) is an innovative device compound of the traditional linear TMD with the idea of a friction damper which is still in the developmental stage for the seismic applications. The present paper investigates the performance of TMD and FTMD for seismic control of tall buildings under near-field earthquakes including soil-structure interaction (SSI) effects. A 40-story structure with a height-to-width ratio of four, a uniform mass distribution and a linear stiffness distribution in its height is considered in this study. Different conditions of the ground state are also considered for numerical studies. A design process based on a multi-objective cuckoo search (MOCS) algorithm is utilized for the optimum design of TMD and FTMD parameters. The simulation results indicate that ignoring the SSI effects may result in an inappropriate and unrealistic estimation of seismic responses and performance of TMD and FTMD in the high-rise structure. In terms of maximum displacement, acceleration, and drift of floors, it is found that the FTMD is capable of mitigating the structural responses better than the TMD. The efficiency of the FTMD is also compared with the TMD from the energy point of view for dissipation of the seismic input energy. The results show the superiority of the FTMD in the reduction of the maximum seismic input, kinetic and strain energies of the main structure, which confirm the capability of the FTMD being more than the TMD for mitigation of the seismic damages in the tall structure during near-field earthquakes. By increasing the soil softness, an increased trend is often achieved in the maximum seismic input and damage energies, thus ignoring the SSI effects may give an unrealistic result of the performance of TMD and FTMD in reducing the damage of seismic-excited tall buildings.

Introduction

TMD is one of the oldest passive control devices to enhance the safety of the tall structures and to improve the performance of the structures subjected to environmental dynamic loads. TMDs dissipate a portion of the seismic input energy and only a part of this energy is transferred to the main system. TMDs have been implemented in a number of high-rise buildings, towers, and bridges. Citigroup Center in New York City, Yokohama Landmark Tower in Yokohama, Burj Al Arab in Dubai, Trump World Tower in New York City, Taipei 101 in Taipei and The TV Tower in Berlin are some examples of the buildings equipped with TMD [1]. A TMD system consists of a mass, a spring, and a viscous damper. Optimal design of TMD parameters for a single degree of freedom (SDOF) system subjected to certain loads such as external harmonic force, harmonic base acceleration and etc. has been taken into account [2–7]. In these researches, it has been attempted to propose some formulations for optimum tuning of TMD for an SDOF system. However, these formulas are not advisable to use in the highrise structures equipped with TMD because of the consideration of a single vibration mode in the optimum design process of TMD parameters. Moreover, they may not be suitable and applicable for seismicexcited structures. In addition, the only main purpose of the design of TMD is the reduction of the maximum displacement of the main system and the reduction of the maximum acceleration has not been considered in these methods. Hence, the evaluation of the performance of TMDs for mitigation of the seismic responses of the multi-story structures is still an important topic of research [8–12]. In recent years, some meta-heuristic based methods have been employed with the aim of overcoming the above-mentioned design problems. The superiority of meta-heuristic algorithms to the other methods in the literature has been reported [8–11,13,14].

The application of performance-based design (PBD) is gaining increasing interest in the wind engineering community. A popular design approach to minimize wind induced vibrations in flexible civil structures is to size structural stiffness and supplemental damping systems in order to restrict the motion to a given threshold for providing safety and comfort, while ensuring that structural components meet strength requirements. In this paper the PBD paradigm is extended to wind excited tall buildings equipped with motion control systems. The objective is to improve the design of damping systems under different wind events while considering maximum acceleration as performance measure. In addition, since the installation of damping devices implies additional costs (e.g., installation and maintenance costs) while it helps decreasing the costs associated with performance failure, a life-cycle analysis (LCA) is integrated in the PBD. In the LCA framework, the percentage of building occupants affected by discomfort and motion sickness caused by excessive wind-induced vibrations is considered to account for the consequences of different target performance levels. The developed PBD is applied to a 39-story building that has documented issues with excessive vibrations under wind events. The wind load is simulated as a multivariate stochastic process, in the time domain. Two passive vibration mitigation strategies are investigated: viscous and friction dampers, both designed to meet the target performance levels. LCA are conducted for the building equipped with each damper type, and benchmarked against the one without dampers. Results show that the PBD leads to a rational and economically effective approach for the design of the damping systems in wind excited tall buildings.

Introduction

Performance-based design (PBD) has shown to be an effective philosophy to create risk consistent designs within the seismic engineering community [1–3]. In the past years, numerous attempts have been made to extend the application of PBD to other hazards such as wind load [4–6], fire [7], vehicle collision [8,9], and seismic pounding [10]. In wind engineering, many of the proposed PBD procedures have been devoted to tall buildings, with particular focus on the serviceability limit states [11–13]. The use of passive supplemental energy damping systems to improve the performance of high-rise structures under wind load is now widely accepted [14,15]. However, a PBD methodology that integrates the design of passive damping systems in a tall building has yet to be developed. In motion engineering, the idea of PBD is to minimize structural vibrations by appropriately sizing supplemental damping systems in order to restrict the motion to a given threshold for providing safety and comfort, while designing the structural stiffness to ensure that structural components meet strength requirements [16]. This requires the development of a PBD procedure for tall buildings that includes the design of the motion control devices. Passive damping systems, such as tuned-mass damping, viscous damping, and base-isolation, are widely used to enhance the structural performance under natural hazards. Some of the attractive features of this technology are the high reliability of the devices, the robustness against possible mechanical failures, and their inherent stability. In the wind hazard case, passive damping systems are generally designed for a predominant wind speed, without considering temporal and spatial variations of the wind load throughout the life span of the structure [17–19].

1- Introduction

2- Problem formulation

3- Optimization procedure

4- Numerical study

5- Conclusion

References

Abstract

In this paper an efficient first-order multi-objective optimization scheme is adopted for the design of linear viscous dampers for the seismic retrofitting of frame buildings. A retrofitting cost function serves as one objective while the expected losses serve as the other objective. These two objectives are well understood by decision makers that may not be engineers. Furthermore, with the Pareto front for these two objectives at hand, the decision maker can make his decisions with the whole picture at hand. To allow achieving the Pareto front with a reasonable computational effort, a first-order multi-objective optimization approach is adopted. The gradients of the expected loss function, required for the optimization, are analytically derived using the very efficient Adjoint Variable method. This considerably improves the computational efficiency of the methodology. The efficacy of the framework is illustrated with a 2D four storey frame and an eight-storey 3D asymmetric building.

Introduction

Recent seismic events have shown that, when it comes to modern countries with modern seismic codes, loss of human lives due to collapse of buildings is considerably smaller compared to that in the past. For example, the 1994 Northridge and the 2011 Christchurch earthquakes caused a total of 57 and 185 deaths, respectively. As saving human lives has been the major task of earthquake engineering, this is indeed an achievement for the structural engineering community. However, with the relatively small number of deaths due to these events, came huge monetary losses. The 1994 Northridge and 2011 Christchurch led to losses estimated at ∼ US $44 billion and NZ $40 billion (corresponds to approximately 20% of the GDP), respectively (direct and indirect). This motivates a design considering expected losses in parallel to reducing loss of human lives [2]. Adopting such a design approach is necessary for new buildings, where new technologies to limit damage could be easily included. One such loss based design approach is the Loss Optimization Seismic Design, commonly referred as LOSD [3,4]. This may highly affect the losses to be expected in the far future. Nonetheless, in the foreseen future, most buildings to experience earthquakes are ones that have already been designed and built based on the sole goal of saving human lives. Thus, new technologies and design approaches for their retrofitting are required to reduce expected losses.

1- Introduction

2- Design criteria of joints

3- Experimental campaign

4- FEM

5- Parametric Fe analysis

6- Conclusive remarks

References

Abstract

Beam-to-column joints equipped with friction dampers are promising solutions to improve the performance of steel moment resisting frames due to the possibility to guarantee large dissipation capacity limiting the structural damage under severe seismic conditions. In this paper, the experimental tests and the numerical simulations of two types of joints are shown and discussed with the aim of developing pre-qualified configurations. The friction dampers are designed to be easily removable from both the lower beam flange and the column face by means of bolted connections. The devices are composed of a stack of steel plates conceived to assure symmetrical friction. The friction surface is set in vertical direction in first case and in horizontal direction in the second type. The experimental tests confirmed the effectiveness of both examined joints and the finite element analyses allowed characterizing their local response, thus providing additional insights to improve the design requirements.

Introduction

The design criteria currently implemented by seismic codes are based on the philosophy of hierarchy of resistances, which aims to guarantee overall ductile and dissipative behavior by enforcing the plastic deformation, namely the damage, into specific ductile zones of the structure. However, economic and social reasons have recently pushed researchers and designers towards systems that can resist severe ground motions with low or without structural damage [1]. Traditional seismic resisting systems widely adopted for steel buildings (e.g. moment resisting frames, concentrically and eccentrically braced frames) entail a dissipation mechanism based on plastic deformations of several structural elements, which may correspond large repairing costs in the aftermath of a seismic event. Therefore, the idea of low or free from damage structures has become very appealing in the last decades [2–4]. The use of friction connections is a viable and promising strategy to achieve this objective for steel structures [5–24]. In the framework of Eurocodes, this type of connections can be classified as partial strength according to EN1993:1-8 [25] because their design resistance should be lower than the strength of the connected members to prevent any damage into the primary structural members. EN1998-1 [26] allows the use of partial strength connections provided that their rotation capacity is properly demonstrated. In the case of conventional partial strength joints the ductility can be designed by imposing local hierarchy rules among the components constituting the joints [27–31] and verified by means of pre-qualification tests [32–34]. Moment resisting friction connections are conceived to develop the dissipation mechanism by means of the relative slip into ad-hoc devices located between the lower beam flange and the outer cap plate connected to the column flange, while the upper flange of the beam is connected to a plate either bolted or welded to the column. The cover plate connecting the upper beam flange may be subjected to some moderate plastic bending deformations to accommodate the joint rotation following the sliding of the device, thus enforcing the formation of an ideal center of rotation that prevents the damage of the slab. To increase the moment capacity, friction devices can be also adopted for the beam web. In addition, the resistance of the joint can be modulated keeping the same assembly but varying the friction resistance that changes with the clamping force used for the bolts. Indeed, the friction device is composed of a stack of steel plates that are clamped together by means of tightened high strength bolts, which are inserted in the slotted holes of the plates to allow the relative sliding.

1- Introduction

2- Structural and seismic dynamic context

3- Optimum TMD parameters and response reduction

4- Additional results on the control of the foundation movement

5- Conclusions

References

Abstract

Tuned Mass Damper (TMD) devices are widely adopted as a valid mechanical solution for the vibration mitigation of structural systems and buildings under dynamic excitation. In the specific challenging context of seismic engineering, TMDs may represent a convenient option for both aseismic structural design and seismic retrofitting. However, the expectable efficiency rate of TMDs in that context is still debated. Besides, potential Soil-Structure Interaction (SSI) effects may become crucial in the mechanical system, and should properly be taken into account for the optimum TMD design, in order to avoid possible de-tuning. This work contributes to this framework, by investigating the effectiveness of an optimum TMD in reducing the linear structural response to strong-motion earthquakes of a given set of Multi-Degree-Of-Freedom (MDOF) low- and high-rise shear-type frame structures, by embedding SSI within the dynamic and TMD optimisation model. The TMD is seismically tuned through a dedicated two-variable optimisation procedure, for each specific case (primary structure, seismic event and soil type), therefore providing the optimum device setting for each given context. Average primary structure response indices are specifically targeted to that purpose, while maximum ones are monitored. A quite considerable range of optimisation cases is considered (eighty instances), to outline rather general considerations and average trends on TMD optimisation and effectiveness within the seismic SSI framework, for both low- and high-rise buildings. Such an investigation shall provide useful guidelines for a comprehensive tuning of TMDs in mechanical systems and specifically in the presence of seismic SSI, to be consulted in view of real-case applications.

Framing on TMD tuning

The present work concerns a methodological optimisation approach of Tuned Mass Damper (TMD) devices towards vibration mitigation and control of structural mechanical systems under dynamic (seismic) excitation. Specifically, the reduction of the linear earthquake response of low- to high-rise frame buildings equipped with a seismic-tuned TMD added on top is sought, by accounting also for Soil-Structure Interaction (SSI) effects that may jeopardise the response of the whole mechanical system and spoil the TMD tuning. The optimum TMD setting is achieved through an original seismic tuning methodology [1], based on an optimisation process apt to provide the optimum TMD selection for a given structure-earthquake case, which is being developed within a wider research mainstream on TMD tuning [1–5]. The seismic tuning process here considers a complete and extensive analysis on TMD optimisation and performance in the presence of seismic SSI, by focusing on the controlled (average) response of the primary structure. Also, the present investigation is conceived in conjunction with companion work [6], where a separate refined Frequency Domain Decomposition modal dynamic identification approach is developed (see also [7–12]), within the same seismic context explicitly considering SSI effects. Indeed, structural identification and TMD tuning may eventually be coupled into a single calibration process [9], on structures with unknown mechanical properties. The need for seismic protection of buildings motivated, during the last decades, the research investigation on different vibration abatement and control devices. In this sense, TMDs appear to be as one of the most validated mechanical solutions for the reduction of unwanted or excessive structural vibrations, with many existing examples of application to high-rise buildings and different type of structures [13]. In this sense, the adoption of TMDs in the realm of earthquake engineering still constitutes a strategic and currently discussed research and engineering topic, with a wider relevance also in more general terms for different mechanical systems within the realm of structural dynamics. The optimum TMD setting may be conveniently achieved through available tuning formulas, which however are strictly valid only for related benchmark ideal excitations.

1. Introduction

2. Vibration, Dynamics, and FE simulation

3. Important factors in fatigue analysis

4. Experimental modal analysis

5. Improving Fatigue Analysis through Experimental and CAE Correlation

6. Case study

7. Conclusions

References

The durability assessment of a component or structure is often assessed using Finite Element Analysis (FEA)-based structural simulations. Fatigue is a progressive failure mode that is related to the stress cycle’s range in a power law such that the fatigue damage increases exponentially as the stress range increases. When the frequency content of the loading gets closer to the natural frequencies of the structure, resonance effects are activated. In this case, the dynamic properties of the structure must be included in the analysis in order to better account for the increased stress response. A dynamic FEA produces therefore more realistic results compared to a static analysis, but requires knowledge of additional characteristics such as mass and damping. In a dynamic FEA, whether transient or steady state frequency response, the damping property governs indeed the magnitude of the dynamic stress response and hence the durability of the component. Unfortunately a default damping value is sometimes erroneously assumed for all modes leading to errors in the stress response, which in turn leads to significant errors in the fatigue life estimates. The modal parameters – including damping ratios – for the structure’s modes of vibration can be extracted using experimental modal analysis techniques. The purpose of this article is first, to explain the critical role that damping plays in fatigue damage; and second, to recommend best practices for determining modal damping experimentally. A simple example of an automotive component will be used to illustrate the importance of using experimental modal analysis to adjust the properties of a FE-based dynamic structural analysis. An experimental modal test followed by a modal analysis will be described. The mode shapes obtained experimentally and analytically will be compared to validate the mass, stiffness and boundary conditions used in the modelling. Then, it will be shown that using realistic values for the damping ratios will help to obtain stress results that correlate with the measured stress. Finally, this example will illustrate the very high sensitivity of the fatigue life estimate to the damping ratios.

Introduction

Fatigue is a progressive failure mode caused by stress cycles. These stress cycles can be driven by dynamics and resonance under some conditions, so prediction of fatigue life requires an understanding of resonance. FEA is often used for the prediction of product durability and can account for dynamics. However, these analytical stress results need to be correlated to real structural behaviour in order to accurately predict fatigue life. Often a damping ratio is assumed when performing the FE structural analysis. This can lead to errors in the fatigue life estimation. Experimental techniques can be used to properly quantify the damping and therefore improve the accuracy of the durability analysis. This paper recommends best practices for improving FEA-based stress and durability analysis via experimental techniques for determining modal damping and correlating analytical results with the real world.

Abstract

In this paper, the concept of a semi active frictional damper called Adjustable Frictional Damper (AFD) is introduced. The clamping force of such damper is secured by hydraulic pressure, which not only reduces the manufacturing costs but also makes it possible to control the seismic response of the structure by changing the clamping force of the dampers.

The hysteretic behavior of AFD is studied by experimental means as well as by numerical model. Experimental process involves tests with various hydraulic pressures (which cause various frictional forces) at nearly static loading as well as dynamic loading with various frequencies. The results show that the proposed damper has significant energy absorption by stable hysteretic loops, which can be used for enhancement of the performance of structures subjected to earthquake loads with various intensities. Force–displacement characteristics of AFD such as slippage load, dissipated energy, effective stiffness and equivalent viscous damping for consecutive cycles of loading is calculated. The system is qualified based on the requirements for displacement-dependent devices according to ASCE/SEI 41-06 specification. Furthermore, the hysteretic behavior of AFD is studied by numerical method and a close agreement between the experimental and numerical results is observed.

Introduction

Seismic response control techniques involve addition of devices to the system in order to dissipate the energy imparted by earthquake motion (for a survey of such techniques see e.g. [1–4]). Frictional based dampers are one class of such devices which dissipate the energy through frictional mechanism caused by two solid bodies sliding relative to each other. A conventional frictional damper compromises a frictional sliding contact surface and a clamping mechanism that produces normal contact force on the surface and heavily relies on coefficient of friction between surfaces. In a passive frictional damper, the clamping force of the damper and consequently the slippage force is a pre-determined constant value selected by design. If the axial force in the damper which is usually placed in a bracing system overcomes the static frictional force, the passive damper starts to slip and a considerable amount of mechanical energy can be transformed to heat energy and dissipated.

Energy dissipation systems are technologies able to improve the performance of structural systems subject to transient excitations. Their purpose is to dissipate or reduce part of the energy generated by the dynamic excitation and transferred to these systems. Thus, it is possible to reduce the deformation demand upon the structural systems considered. In fact, if properly designed, they can reduce specific systems’ responses of interest, such as selected displacements and accelerations. These technologies can be divided into passive systems, active and semi-active systems (Soong and Dargush 1997; Constantinou et al. 1998). In each of these categories, there are a wide variety of different technologies nowadays well developed and widely used. Examples of their applications are: The seismic retrofitting of buildings subject to seismic excitation with viscous dampers (e.g. Takewaki 1997; Lavan and Levy 2005; Kanno 2013) and tuned-mass dampers (e.g. Almazan et al. ´ 2012; Daniel and Lavan 2015); The reduction of wind and earthquake induced vibrations in high-rise buildings (e.g. Yang et al. 2004; Smith and Willford 2007; Infanti et al. 2008); The seismic protection of bridges with viscous dampers (e.g. Infanti et al. 2004; Simon-Talero et al. 2006; Infanti and Castellano 2007); The control of human-induced vibrations (Caetano et al. 2010; Casado et al. 2013); The vibration control of offshore wind turbines (e.g. Brodersen and Høgsberg 2016); The shock absorption for high-speed boat seats (e.g. Klembczyk and Mosher 2000); The structural control of dynamic blast loading (e.g. Miyamoto and Taylor 2000); The design of passive and semi-active suspension systems for cars subject to road excitation (e.g. Georgiou et al. 2007; Suciu et al. 2012). The efficacy of the energy dissipation systems is strongly related to their location in the system that needs to be controlled and to their size. For these reasons, often their design is based on optimization, to address the placement and sizing of these devices. For instance, in Lavan and Amir (2014) the authors distribute and size linear fluid viscous dampers in linear structures subject to seismic excitation with an optimization approach based on Sequential Linear Programming (SLP). This procedure is then further enhanced in Pollini et al. (2016) in order to consider a more complete and realistic objective cost function. In Kanno (2013) the author presents a mixed-integer programming approach for the optimal placement of viscous dampers in shear frames, where the damping coefficients are selected from a discrete set of values. A discrete optimization approach is presented in Dargush and Sant (2005), where different types of passive dissipation systems (i.e. metallic plate dampers, fluid viscous dampers, viscoelastic solid dampers) are sized and placed with genetic algorithms. Also in this case, the properties of the devices are selected from a predefined set of available sizes. In Georgiou et al. (2007) Georgiou, Verros, and Natsiavas present a methodology for the multi-objective optimization of the suspension damping and stiffness parameters of quarter-car models subjected to random road excitation. The authors consider both passive and semi-active suspension systems.