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This study presents a finite element model for a hybrid self-centering damper considering the rate and temperature effects and explores the effects of different design parameters on the damper response. The damper, called as superelastic friction damper (SFD), consists of superelastic shape memory alloy (SMA) cables and a frictional energy dissipation mechanism. The experimental response of the SMA cables, frictional unit and overall damper at different loading frequencies and temperature are used to develop numerical model of the damper. Once a validated numerical model is obtained, parametric studies are carried out to evaluate force-displacement response of the damper when the design parameters are altered. The effects of damper design parameters on the equivalent stiffness, dissipated energy, equivalent viscous damping and self-centering capabilities of the damper are analyzed. Based on the findings, the recommendations for the design of the damper are presented. [ABSTRACT FROM AUTHOR]

In light of the significant damage observed after earthquakes in Japan and New Zealand, enhanced performing seismic force‐resisting systems and energy dissipation devices are increasingly being utilized in buildings. Numerical models are needed to estimate the seismic response of these systems for seismic design or assessment. While there have been studies on modeling uncertainty, selecting the model features most important to response can remain ambiguous, especially if the structure employs less well‐established lateral force‐resisting systems and components. Herein, a global sensitivity analysis was used to address modeling uncertainty in specimens with elastic spines and force‐limiting connections (FLCs) physically tested at full‐scale at the E‐Defense shake table in Japan. Modeling uncertainty was addressed for both model class and model parameter uncertainty by varying primary models to develop several secondary models according to pre‐established uncertainty groups. Numerical estimates of peak story drift ratio and floor acceleration were compared to the results from the experimental testing program using confidence intervals and root‐mean‐square error. Metrics such as the coefficient of variation, variance, linear Pearson correlation coefficient, and Sobol index were used to gain intuition about each model feature's contribution to the dispersion in estimates of the engineering demands. Peak floor acceleration was found to be more sensitive to modeling uncertainty compared to story drift ratio. Assumptions for the spine‐to‐frame connection significantly impacted estimates of peak floor accelerations, which could influence future design methods for spines and FLC in enhanced lateral‐force resisting systems. [ABSTRACT FROM AUTHOR]

The motion of cladding systems can be leveraged to mitigate natural and man-made hazards. The literature counts various examples of connections enhanced with passive energy dissipation capabilities at connections. However, because such devices are passive, their mitigation performance is typically limited to specific excitations. The authors have recently proposed a novel variable friction cladding connection capable of mitigating hazards semi-actively. The variable friction cladding connection is engineered to transfer lateral forces from the cladding element to the structural system. Its variation in friction force is generated by a toggle-actuated variable normal force applied onto sliding friction plates. In this study, a multiobjective motion-based design methodology integrating results from the previous work is proposed to leverage the variable friction cladding connection for nonsimultaneous wind, seismic, and blast hazard mitigation. The procedure starts with the quantification of each hazard and performance objectives. It is followed by the selection of dynamic parameters enabling prescribed performance under wind and seismic loads, after which an impact rubber bumper is designed to satisfy motion requirements under blast. Last, the peak building responses are computed and iterations conducted on the design parameters on the satisfaction of the motion objectives. The motion-based design procedure is verified through numerical simulations on two example buildings subjected to the three nonsimultaneous hazards. The performance of the variable friction cladding connection is also assessed and compared against different control cases. Results show that the motionbased design procedure yields a conservative design approach in meeting all of the motion requirements and that the variable friction cladding connection performs significantly well at mitigating vibrations. [ABSTRACT FROM AUTHOR]

This paper presents a low-damage solution for self-centering steel and composite beam-to-column connections, with the issue of beam-growth being particularly addressed by permitting the connections to rotate only about the top flange of the beam. Self-centering devices incorporating novel shape memory alloy (SMA) ring springs are the kernel components for the proposed connections. The fundamental working principle of the connections is depicted, followed by an experimental study on four proof-of-concept specimens, including three bare steel and one composite connections. The testing parameters are preload of web bolts, training of the SMA ring springs, and influence of the slab. Among other findings, it is shown that the damage to the concrete slab and the reinforcement are minimal for the composite connection. Stable flag-shaped hysteretic responses are typically exhibited, and good ductility, and energy dissipation performance are confirmed. The specimens can be generally classified as rigid and partial-strength connections. Slip-critical HS web bolts could provide an extra source of energy dissipation through friction. Applying training to the SMA outer rings further stabilizes the hysteretic behavior and mitigates the possible degradation. Based on the experimental observations, preliminary design recommendations are proposed, with the focus on the design of the SMA ring springs, web plate, cover plate, and other components of the device. Empirical design equations are also proposed for predicting the moment resistance of the connections, and the predicted values are shown to agree well with test results. [ABSTRACT FROM AUTHOR]

Summary: This paper presents a comprehensive study on self‐centering dampers equipped with friction springs. The basic mechanical behavior of individual friction springs is first understood via analytical descriptions that are verified by a preliminary experimental study. The working principle, fabrication process, and mechanical performance of the proposed dampers are subsequently described, where the design details related to seismic applications are highlighted. This is followed by physical tests on three damper specimens, where the influence of the treatment of the taper surfaces of the friction springs on the overall damper behavior is examined, and the reliability of the dampers under repeated rounds of cyclic loading is evaluated. The damper specimens show reliable flag‐shaped load‐deformation hysteretic curves with excellent self‐centering capability. Satisfactory energy dissipation with an equivalent viscous damping (EVD) of up to 20% is shown, and the EVD is stabilized throughout the entire loading process. The dampers are capable of withstanding multiple rounds of cyclic loading with stable hysteretic behavior, indicating that they are fully reusable after multiple strong earthquakes. A larger friction coefficient on the taper surfaces leads to increased yield load and energy dissipation. Following the experimental study, some practical design recommendations are provided. Based on the available analytical model, an additional parametric study is performed to highlight the influences of precompression and friction condition of the friction springs on the damper behavior. Finally, the seismic performance of a building which adopts the friction spring dampers is evaluated via numerical simulation. [ABSTRACT FROM AUTHOR]

Summary: This paper presents the authors' response to the discussion by Dean J. Maxam and Kumar K. Tamma of the paper titled "Development of a family of unconditionally stable explicit direct integration algorithms with controllable numerical energy dissipation." [ABSTRACT FROM AUTHOR]

The behavior of a self-centering moment-resisting frame (SC-MRF) is characterized by the connection gap opening and closing at beam-column interfaces under earthquake loading. A SC-MRF uses high-strength posttensioning strands to precompress the beams to the columns and to close the gaps that develop under earthquake loading, returning the frame to its initial position (i.e., the frame is self-centering). In this study, a beam web friction device is included in each beam-column connection to dissipate energy under seismic loading. Unlike a special steel moment-resisting frame with welded connections (W-SMRF), a SC-MRF can be designed to survive the design basis earthquake (DBE) without structural damage, leading to the potential for immediate occupancy performance, and to suffer only modest damage under the maximum considered earthquake, leading to collapse prevention performance. A 7-bay, 4-story SC-MRF prototype building was designed for a location in the Los Angeles area. A 0.6-scale model of two bays of the SC-MRF building was constructed in the laboratory and subjected to simulated earthquakes using the hybrid simulation method. This paper focuses on the DBE-level performance of this SC-MRF. DBE-level simulation results and performance evaluations are presented. The behavior of the SC-MRF is compared with that of a W-SMRF using nonlinear pushover analysis results. [ABSTRACT FROM AUTHOR]

A new beam-to-column connection for earthquake-resistant moment-resisting frames is introduced. The connection has a beam bottom flange friction device (BFFD) and posttensioned (PT) high-strength steel strands running parallel to the beam. The BFFD provides energy dissipation to the connection and avoids interference with the floor slab. The PT strands produce self-centering connection behavior. The connection behavior requires minimal inelastic deformation of the connection components and the beams and columns, and requires no field welding. A series of seven large-scale tests were performed to investigate the effect of the BFFD friction force, connection details, and the loading history on the performance of the connection under cyclic loading. The test results indicate that the BFFD provides reliable energy dissipation, and that the connection remains damage-free under the design earthquake. [ABSTRACT FROM AUTHOR]

As buckling-restrained braced frames (BRBFs) have been used increasingly in the United States, the need for knowledge about BRBF behavior has grown. In particular, large-scale experimental evaluations of BRBFs are necessary to demonstrate the seismic performance of the system. Although tests of buckling-restrained braces (BRBs) have demonstrated their ability to withstand significant ductility demands, large-scale BRBF tests have exhibited poor performance at story drifts between 0.02 and 0.025 rad. These tests indicate that the large stiffness of the typical beam-column-brace connection detail leads to large flexural demands that cause undesirable failure modes. As part of a research program composed of numerical and experimental simulations, a large-scale BRBF with improved connection details was tested at the ATLSS Center, Lehigh University. During multiple earthquake simulations, which were conducted using a hybrid pseudodynamic testing method, the test frame sustained story drifts of close to 0.05 rad and BRB maximum ductility demands of over 25 with minimal damage and no stiffness or strength degradation. The testing program demonstrated that a properly detailed BRBF can withstand severe seismic input and maintain its full load-carrying capacity. [ABSTRACT FROM AUTHOR]

As the use of buckling-restrained braced frames (BRBFs) has increased in the United States, the need has grown for knowledge about member and system behavior under seismic loads and for implementing this knowledge into design provisions. In particular, methods for designing BRBFs and predicting seismic response require validation. To address this need, along with the need for experiments demonstrating system-level BRBF performance, a research program composed of numerical and large-scale experimental simulations was initiated at the ATLSS Center, Lehigh University. This paper describes the nonlinear dynamic analyses that were conducted as part of this research program. Numerical simulations of BRBF response were conducted using ground motion records scaled to two seismic hazard levels. The performance of the prototype BRBF was acceptable and performance objectives were met in terms of structural damage. It is shown that the currently accepted deflection amplification factor underestimates mean inelastic lateral displacements under design-level earthquakes and the system overstrength factor may be unconservative. The current method for predicting BRB maximum ductility demands is also shown to be unconservative and a more rigorous method for predicting BRB maximum ductility demands is provided. [ABSTRACT FROM AUTHOR]

Six full-scale interior connection subassemblies of posttensioned wide flange beam-to-column moment connections were subjected to inelastic cyclic loading up to 4% story drift to simulate earthquake loading effects. Bolted top and seat angles are used in the connection, along with posttensioned high strength strands that run parallel to the beam. These strands compress the beam flanges against the column flange to develop the resisting moment to service loading and to provide a restoring force that returns the structure to its initial position following an earthquake. The parameters studied in these experiments were the initial posttensioning force, the number of posttensioning strands, and the length of the reinforcing plates. The experimental results demonstrate that the posttensioned connection possesses good energy dissipation and ductility. Under drift levels of 4%, the beams and columns remain elastic, while only the top and seat angles are damaged and dissipate energy. The lack of damage to the beams, columns, and the posttensioning enable the system to return to its plumb position (i.e., it self-centers). Closed-form expressions are presented to predict the connection response and the results from these expressions compare well with the experimental results. [ABSTRACT FROM AUTHOR]

The behavior of high strength square concrete filled steel tube (CFT) beam columns subjected to constant axial load and cyclically varying flexural loading was investigated experimentally. The effects of the width-to-thickness (b/t) ratio, the yield stress (σ[sub y]) of the steel tube, and the level of axial load on the behavior (stiffness, strength, ductility, and energy dissipation) of high strength square CFT beam columns were studied. Eight cyclic beam-column specimens were tested. Crushing of concrete and cyclic local buckling in the steel tube affected the cyclic strength and stiffness of the CFT specimens. The elastic section flexural stiffness under cyclic loading decreases rapidly due to tension cracking of the concrete infill and local buckling of the steel tube. The total energy dissipated by the cyclic specimens is dominated by the flexural energy dissipated in the failure segment (plastic hinge region). Both the current American Concrete Institute code provisions and the modified Architectural Institute of Japan method predict the moment capacities of the high strength square CFT specimens accurately. The cyclic curvature ductility (μ[sub [lowercase_phi_synonym]-c]) decreases significantly with an increase in the axial load level. At higher axial load levels, the steel tube b/t ratio and σ[sub y] have a small influence on μ[sub [lowercase_phi_synonym]-c]. However, at lower axial load levels, increasing the steel tube b/t ratio or σ[sub y] reduces μ[sub [lowercase_phi_synonym]-c]. [ABSTRACT FROM AUTHOR]

Summary: Cladding systems are conventionally designed to serve architectural purposes and protect occupants from the environment. Some research has been conducted in altering the cladding system in order to provide additional protection against natural and man‐made hazards. The vast majority of these solutions are passive energy dissipators, applicable to the mitigation of single types of hazards. In this paper, we propose a novel semiactive variable friction device that could act as a connector linking a cladding panel to the structural system. Because of its semiactive capabilities, the device, here termed variable friction cladding connection (VFCC), could be utilized to mitigate different hazards, either considered individually or combined, also known as multihazards. The VFCC consists of two sets of sliding friction plates, onto which a variable normal force can be applied through an actuated toggle system. A static model is derived to relate the device's Coulomb friction force to the actuator stroke. This model is integrated into a dynamic friction model to characterize the device's dynamic behavior. A prototype of the VFCC is constructed using 3D printing. The prototype is tested under harmonic excitations to identify the model parameters and characterized on a set of nonstationary excitations under different actuator stroke lengths. Results show good agreement between the model and experimental data, demonstrating that the device functions as‐designed. [ABSTRACT FROM AUTHOR]

In implementing a substructure shake table testing (SSTT), the complete structure is divided into the experimental and the analytical substructures, which are driven by shake table and numerically simulated in a computer, respectively. A number of unconditionally stable and explicit integration algorithms referred to as model-based algorithms with highly computational efficiency have been developed to meet the requirement of real-time in conducting SSTT. This study aims to comprehensively investigate the stability of the SSTT system using two recently proposed model-based integration algorithms with controllable numerical energy dissipation, i.e., Kolay-Ricles-α (KR-α) algorithms and modified KR-α (MKR-α) algorithms. The SSTT system for the 2-degree-of-freedom (2-DOF) structures are firstly formulated. In order to take the contribution of the experimental substructure into consideration, the dynamic condensation is adopted to calculate the integration parameters for the analytical substructure. The discrete transfer function of the 2-DOF-SSTT system are derived. The influences of the mass ratio, the frequency ratio and the time step on the stability of the SSTT system are comprehensively investigated by using the discrete control theory. The results show that for larger values of the mass ratio, the frequency ratio and the time step have negative impact on the stability of the SSTT system. In addition, a subfamily of the MKR-α algorithms along with the dynamic condensation can always ensure the SSTT system stable. • The 2-DOF-SSTT systems using two model-based algorithms are formulated. • Dynamic condensation is adopted to calculate the integration parameters for the analytical substructure. • The discrete transfer function of the 2-DOF-SSTT systems are derived. • The influences of mass ratio, frequency ratio and time step on the SSTT systems are investigated. [ABSTRACT FROM AUTHOR]

Cladding systems are conventionally designed to provide buildings with environmental protection against wind, temperature, humidity, moisture, etc. Recently, researchers have proposed to leverage these systems to provide additional protection against manmade (e.g., blast) and natural (e.g., earthquakes, hurricanes) hazards. This can be achieved, for example, by redesigning the connection between the cladding and the structural system to provide energy dissipation via friction. While promising, the use of flexible cladding connection has only been considered for singular hazards. In this study, the authors propose a novel semi-active damping system to connect the cladding to the structure via a variable friction mechanism. By varying the normal force applied on friction plates through a system of adjustable toggles, it is possible to mitigate vibrations over a wide frequency range, therefore enabling mitigation of different types of hazards (i.e. to achieve multi-hazard resistance). In its passive in-situ mode, the device is designed to provide very high stiffness and friction resistance to mitigate the effects of blast. The objective of this paper is to enable a holistic integration of said device within the structural design process by developing a performance-based design procedure. The study will focus on the passive in-situ mode of the device, which will provide a stepping stone for the development of performance-based design procedures for its semi-active (i.e. actuated) capabilities. The proposed performance-based design procedure consists of the following: 1) determine the design performance criteria, including the blast properties and allowable connection gap between the cladding and structure; 2) select design properties for the cladding connection, including stiffness and damping; and 3) design a rubber impact bumper located between the structure and the cladding in order to mitigate slamming of the cladding into the structure for very high blast loads. [ABSTRACT FROM AUTHOR]

This paper presents superelastic shape memory alloy (SMA) ring spring systems for seismic applications. The study commences with an experimental investigation looking into the mechanical performance of individual SMA ring springs under cyclic loading. The strength, self-centering ability, and energy dissipation capacity are shown to be dependent on the ring size as well as the treatment of the contacting taper face. Subsequently, a proof-of-concept beam-to-column self-centering connection is physically tested, shedding further light on the practical application of the ring spring systems for high-performance seismic resisting steel frames. The connection shows favorable cyclic behavior, with the majority of the deformation demand resisted by the SMA ring spring systems. The remaining structural components generally stay undamaged. Driven by the superelastic behavior of SMA, excellent self-centering ability with satisfactory energy dissipation is exhibited. A numerical investigation is then conducted to gain a further understanding of the load resistance mechanism of the proposed connection. The numerical model is validated against the experimental results with good agreement observed, enabling a further parametric study to be conducted taking a more in-depth look into the influence of ring pre-compression, taper face friction, ring size, and shear tab bolt preload on the overall performance of the connection. Based on the test and numerical data, some design comments are also made. [ABSTRACT FROM AUTHOR]

• A hybrid self-centering and energy dissipating damper was developed. • The response of the damper was characterized considering rate effects. • The effects of temperature on the components of the damper and overall response were evaluated. • The damper provided a minimum of 12 % equivalent viscous damping and 89 % recovery of the peak displacement. A new hybrid shape memory alloy (SMA)-based damper was developed to address two major challenges for SMA-based seismic control devices: (i) insufficient force capacity for real-world application and (ii) relatively low energy dissipation capacity. The hybrid damper, named Superelastic Friction Damper (SFD), leverages the high tensile resistance and excellent self-centering capability of SMA cables and non-sacrificial energy dissipation of a frictional damping mechanism. In this paper, the components and basic working principle of the proposed damper are first described, and the advantages of the proposed damper compared to the existing SMA-based hybrid dampers are highlighted. Then, the fabrication of a prototype damper and its experimental testing are discussed. The mechanical response of the damper under repeated cyclic loading at various displacement amplitudes and loading rates is revealed. The effects of ambient temperature on the hysteretic behavior of the developed damper are explored. Results reveal that the proposed damper exhibits stable hysteretic behavior with negligible sensitivity to the loading rate and temperature. The damper provides an equivalent viscous damping of 12 %, accompanied by 89 % recovery of the peak damper displacement. [ABSTRACT FROM AUTHOR]