Strip Dampers Research Articles

Acoustic source localization in an isotropic plate: Damper’s coverage length optimization based on response surface method (RSM)

There are two common approaches for avoiding the effect of reflected waves on sound source localization. The first approach uses high Sampling Rate Data (SRD) values, while the second one is based on conducting experiments with a distance far from the plate edges. Instead of the current approaches, this paper proposes installing Silicon Strip Dampers (SSDs) on the plate to reduce reflected waves. This study discusses how geometrical features of SSDs affect acoustic emission in an isotropic Plexiglas plate using low sampling rates. The target function is modeled by using the experimental data, the coverage length of the SSD, and the energy of the recorded signal. Then, this model is simulated by Response Surface Method (RSM) in the Design Expert software. The results show that the reduction of reflected waves strongly depends on the sensors, the impact locations, and the coverage length as well as the position of the SSD.

Seismic performance assessment of steel strip dampers equipped in high-rise steel frame

This paper aims to utilize a novel steel strip damper to enhance seismic resilience of high-rise steel frame in high seismic intensity regions. The developed damper is composed of external casing, internal casing and steel strip fuses dissipating seismic energy via bending-dominated deformation throughout the entire section, which is convenient for post-earthquake replacement. A series of cyclic loading tests have shown the damper's good energy dissipation capacity with features of stable and full hysteretic loops. A numerical model for the damper is proposed and validated by the experimental results. Then a 20-story steel frame is adopted as the representative building and retrofitted using a design procedure based on the stiffness and displacement demands. Nonlinear dynamic analysis considering damper failure is performed to evaluate seismic responses of the steel frames. And incremental dynamic analysis is employed to discuss the seismic fragility. Finally, seismic losses of the steel frames are assessed through the story-based loss estimation method in which the demolition-related economic losses are included. The analyses indicate that the presented damper can obviously mitigate the roof displacement, inter-story drift, plastic damage and soft story deformation mechanism; the damper-fused frame has much lower probability to collapse; the expected total losses, expected annual losses and expected life-cycle losses are significantly decreased by retrofitting with the novel dampers.

A Test Rig for the Experimental Investigation on the Nonlinear Dynamics in the Presence of Large Contact Interfaces and Numerical Models Validation

Abstract The simulation of the coupling between components modeled by finite elements (FEs) plays an important role for the prediction of the forced response of the assembly in terms of resonant frequencies, vibration amplitudes, and damping. This is particularly critical when the time-varying stress distribution must be limited for vibrating components with thin thickness coupled with large contacts. Typical examples can be found in aeronautical structures (plates, panels, and bladed disk components) assembled with bolted flanges, riveted lap joints, or joints without hole discontinuities like rail-hook joints, lace wire sealings, and strip dampers. In this paper, a new test rig is introduced for the experimental validation of a reduced-order model (ROM) based on the Gram–Schmidt Interface (GSI) modes applied to a friction contact whose dimensions are not negligible with respect to the size of the substructures. In this case, classical approaches like Craig–Bampton technique might be not effective in reducing the size of the problem when many contact nodes subjected to nonlinear contact loads cannot be omitted. The technique is implemented in a solution scheme in the frequency domain using penalty contact elements and the harmonic balance method. The preload on the joint is produced by permanent magnets to enhance the friction contact without introducing uncertainties due to bolting. Measurements are compared with the ROM simulations and with standard time-domain integration of the full FE model. The advantage of using the GSI technique is shown in terms of time computation and accuracy of the simulation.

Cyclic Loading Performance of Radius-Cut Double Coke-Shaped Strip Dampers.

Conventional slit dampers are widely used for the purpose of seismic retrofitting, however, the structure of these dampers is susceptible to fractures, due to stress concentration at the ends of the strips in the event of large earthquakes. To address this issue, a novel radius-cut coke-shaped strip damper featuring improved ductility is proposed herein. This damper was developed based on the moment distribution over the strip when both its ends were constrained. The height-to-width ratio of the strip was increased to induce bending rather than shear deformation, and the reduced beam section method was employed. A radius-cut section was used to intentionally focus the stress to induce the plastic hinge. This reduced the fracture fragility of the specimen, resulting in an increased inelastic deformation capacity. Cyclic loading tests were conducted to verify damping performance against earthquakes. Experiments and finite element analyses proved that the coke-shaped damper exhibits improved ductility. The final fracture occurred in the radius-cut section after sufficient energy dissipation during cyclic loading. The results also indicated further improvements in strength due to the membrane effect under cyclic loading, caused by the tensile resistance of the strip due to its constrained ends.

Mitigating Inter-Story Drift Concentration of Concentrically Braced Steel Frames Using Energy-Dissipative Columns

ABSTRACT Concentrically braced steel frames (CBSF) always suffer from weak-story failure with a large inter-story drift concentrated on the weakest story. A novel system of Energy-Dissipative Column (EDC) is proposed in this paper to improve the seismic behavior of low and mid-rise CBSFs. The EDC system consists of dual hinge supported steel boundary branches and replaceable steel strip dampers between them. Nonlinear dynamic analyses are conducted on 3-story and 6-story benchmark CBSF buildings with and without EDCs. The effectiveness of EDCs with different stiffness ratios on mitigating the seismic responses of CBSFs under design based and maximum considered earthquakes are investigated. The results show that the presence of EDCs in an existing multi-story CBSF can not only dissipate seismic energy to reduce the maximum inter-story drift, but facilitate more uniform inter-story drift distribution by redistributing the seismic forces along the structural height. It is found that the maximum inter-story drift and drift concentration can be reduced by half when the lateral stiffness ratio of EDC to CBSF reaches 0.2.

Experimental study on the efficiency of tapered strip dampers for the seismic retrofitting of damaged non-ductile RC frames

Many non-ductile reinforced concrete (RC) frames were significantly damaged in past earthquakes. Although demolishing of such damaged structures has become common practice, it results in a significant amount of economic loss. The efficiency of using tapered strip damper (TSD) for seismic retrofitting of damaged RC frames is investigated in this study. A full-scale one-bay single-story non-ductile RC frame was constructed and subjected to quasi-static cyclic loading until it reached its ultimate load. A specially designed TSD was used in retrofitting the damaged frame. The retrofitted frame was subjected to a similar cyclic loading, and its response was recorded. The experimental results are very similar to the analytically calculated values for the lateral stiffness and ultimate strength of the retrofitted frame. Besides, the retrofitted frame exhibited higher lateral strength and displacement capacity compared with the reference RC frame. Compared with the reference frame, the stiffness degradation of the retrofitted frame was slower, and its energy dissipation was superior. It was concluded that the employed TSD was effective in retrofitting the damaged non-ductile RC frame.

Seismic performance improvement of tension-only-braced frames with Energy-Dissipative Rocking Columns

Steel braced frames consisting of strong beams, weak columns, and tension-only braces (BTFs) have been extensively used in low-rise steel buildings, since it is prefabricated rapidly and is cost-efficient. However, BTFs are vulnerable to soft story failure due to the negligible post-yielding stiffness of tension-only braces. A novel device of Energy-Dissipative Rocking Column (EDRC) is proposed to improve the seismic performance of BTFs. An EDRC consists of dual hinge supported steel column branches and replaceable steel strip dampers between them. Firstly, the effectiveness of EDRCs were verified through nonlinear time history analyses on benchmark multi-story BTFs using a suite of far-field ground motion records. Secondly, systematic parametric studies are conducted to quantify the effect of key influencing factors such as stiffness ratio of EDRC to BTF and bending stiffness of column branches on the seismic performance improvement of BTF-EDRC systems. The results show that the maximum inter-story drift and inter-story drift concentration under earthquakes as well as the residual drift after earthquakes can be significantly mitigated by coupling with EDRC. It is found that the maximum inter-story drift ratio can be reduced by 50% when the stiffness ratio of EDRC to BTF reached 1.0. The most effective stiffness ratio of EDRC to BTF for mitigating the inter-story drift concentration and residual drift ratio falls in a range of no more than 0.2 and 0.4, respectively.

Use of energy-dissipative rocking columns to enhance seismic performance of buckling-restrained braced frames

Buckling-Restrained Braced steel frames (BRBFs) exhibit well-controlled seismic behavior due to their excellent lateral load resisting capacity and energy dissipation capacity. However, they always suffer from concentration of inter-storey drifts and excessive residual deformations in weak/soft stories under strong earthquakes. A novel system of Energy-Dissipative Rocking Column (EDRC) is proposed in this paper to improve the seismic performance of multi-storey BRBFs. It consists of dual hinge supported steel column branches and replaceable steel strip dampers between them. Nonlinear time historey analyses are conducted on a six-storey prototype BRBFs with EDRCs to investigate the effectiveness of EDRCs on improving the seismic performance of BRBFs under different levels of earthquakes. It is found that the EDRC can not only mitigate the maximum and residual inter-storey drifts, but facilitate more uniform distribution of inter-storey drifts. A rational range of EDRC stiffness ratio less than 0.2 is recommended for the seismic design of BRBFs with EDRCs.

Experimental Verification of the Dynamic Model of Turbine Blades Coupled by a Sealing Strip

This paper presents the experimental-numerical comparison of a bladed system with flexible strip dampers. The experimental results constitute, to the authors’ knowledge, the first published experimental evidence on strip dampers. They explore the in-phase and out-of-phase mode of vibration of two blades coupled by a strip damper. The great influence the mode of vibration has on the strip behaviour and consequent effect on the blades’ response is highlighted, analyzed and successfully simulated numerically. Experiments are made possible by a purposely developed loading system based on compressed air. This non-contact system enables the experimenter to apply a realistic value of contact pressure on the strip without adding spurious stiffness to the system or modifying the contact conditions. The availability of experimental data obtained by imposing realistic contact conditions constitutes a true added value. It is here shown how the full stick linear FRFs (Frequency Response Functions), typically used to predict the response in case of limited excitation on blades coupled by rigid dampers, do not offer significant results in the case of flexible strip dampers.

Mitigating seismic response of RC moment resisting frames using steel energy-dissipative columns

A novel system of steel Energy-Dissipative Columns (EDCs) is proposed to mitigate seismic responses of reinforced concrete Moment Resisting Frames (MRFs). The EDC-MF dual system aims to not only dissipate seismic energy by replaceable steel strip dampers in EDCs but mitigate inter-story drift concentration to avoid soft/weak-story failure of conventional MRFs. A simplified numerical model of EDC-MF dual system is established by using lumped mass shear model for MRFs and finite element model for EDCs. Parametric studies are conducted to investigate the effect of storey number, stiffness ratio, soft story factor and seismic intensity factor on the mitigation efficiency of EDCs. The results show that the lateral stiffness ratio of EDC to MRF, the story irregularity factor of MRFs, and the ratio of story shear capacity of EDC-MF systems to seismic base shear are the most important parameters. It is found that both the maximum inter-story drift and the inter-story drift concentration can be effectively mitigated due to the presence of EDCs, compared to traditional MRFs. The mitigation efficiency of EDCs increases with increasing lateral stiffness ratio and story irregularity factor under various ground motion intensities. A rational range of lateral stiffness ratios of 0.02–0.2 is recommended for the seismic design of EDC-MF dual systems.

Modeling and Testing Friction Flexible Dampers: Challenges and Peculiarities

This paper deals with the dynamic of blades with strip dampers. The purpose is 1) to present the results of the dynamic numerical calculation, 2) to demonstrate the need for the experimental data on the blade-strip contact to be used as input to the calculation, 3) to propose a new test rig design to obtain them and 4) to test the key components of the new test rig. The forced responses of two blades coupled by a strip damper are calculated at different excitation and centrifugal force values. The dependence of the numerical results on the contact parameter values is confirmed in this significant reference case. The design of a new test rig is then proposed: both the blade frequency response function and the contact hysteresis cycles at the blade-strip contact are measured. It is shown how contact parameters can then be derived from experimental data. The main novelty of the test rig here proposed is the strip loading system, which simulates the uniform pressure distribution provided by the centrifugal force in real operating conditions. This loading system is non-contact and uses compressed air. Classical loading systems which see dead weights directly connected to the strip are assessed and their expected inadequacy is confirmed. The compressed air system is tested by measuring the pressure produced between strip and blades: pressure is uniform across the contact patch, constant in time and its mean value corresponds to realistic pressure values actually experienced by strip dampers during service.

Numerical analysis of multiple friction contacts in bladed disks

The damping potential of multiple friction contacts in a bladed disk is investigated. Friction contacts at tip shrouds and strip dampers are considered. It is shown that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single kind of friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations. A highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.

Modeling of Behavior of Continuous Energy-Dissipative Steel Columns Under Cyclic Loads

ABSTRACTThis paper presents analytical and numerical models of cyclic behavior of a novel continuous energy-dissipative column (CEDC) system. It consists of two steel columns connected by replaceable steel strip dampers (RSSDs). The CEDC system is designed to realize the function of energy dissipation, lateral resistance, and mitigating inter-story drift concentration. The formulas to determine the elastic stiffness, yield, and ultimate capacity of the CEDC system are derived based on continuous medium method. A simplified model of hysteretic behavior of the CEDC system is proposed and validated with experimental results.

Forced Response Prediction of Turbine Blades with Flexible Dampers: The Impact of Engineering Modelling Choices

This paper focuses on flexible friction dampers (or “strips”) mounted on the underside of adjacent turbine blade platforms for sealing and damping purposes. A key parameter to ensure a robust and trustworthy design is the correct prediction of the maximum frequency shift induced by the strip damper coupling adjacent blades. While this topic has been extensively addressed on rigid friction dampers, both experimentally and numerically, no such investigation is available as far as flexible dampers are concerned. This paper builds on the authors’ prior experience with rigid dampers to investigate the peculiarities and challenges of a robust dynamic model of blade-strips systems. The starting point is a numerical tool implementing state-of-the-art techniques for the efficient solution of the nonlinear equations, e.g., multi-harmonic balance method with coupled static solution and state-of-the-art contact elements. The full step-by-step modelling process is here retraced and upgraded to take into account the damper flexibility: for each step, key modelling choices (e.g., mesh size, master nodes selection, contact parameters) which may affect the predicted response are addressed. The outcome is a series of guidelines which will help the designer assign numerical predictions the proper level of trust and outline a much-needed experimental campaign.

11.16: Seismic performance evaluation of MRFS with dissipative continuous columns

ABSTRACTThe Dissipative Continuous Column (DCC) concept is proposed as a gravity load and seismic energy dissipating dual‐functional structural component. The proposed DCC is made up of two closely spaced continuous columns which are interconnected with replaceable steel strip dampers (RSSDs) over the height. It works in a way similar to pin supported coupled shear walls, but minimizes the construction invasive and ensure rapid return to occupancy after severe earthquakes. The innovative moment resisting frame (MRF) system with DCCs can also mitigate the inter‐storey drift concentration and reduce the soft storey failure risk significantly due to the contribution of large flexure stiffness of the DCCs. Based on the experimental test results, a simplified analysis model of the DCCs is developed for the design and seismic performance evaluation of the MRFs system with DCCs. The simplified analysis model consists of dual column and multiple zerolength equivalent elasto‐plastic springs with rigid end. Numerical models of a 4‐story conventional steel MRF and a 4‐story steel MRF with DCCs were set up using the proposed simplified model. Nonlinear Dynamic Time History Analysis were undertaken, and the results showed that the MRFs with DCCs can reduce the economic loss by limiting the structural damage in the replaceable steel strip dampers and mitigating both the maximum inter‐story drift ratio and drift concentrations.

Hourglass-shaped strip damper subjected to monotonic and cyclic loadings

An hourglass-shaped strip damper (HSD) was proposed to improve on the conventional slit damper. The damper has non-uniform strips which have a smaller cross-sectional area close to the middle height. To find the structural capacities of HSD subjected to monotonic and cyclic loadings, experimental tests were carried out in this study. Test parameters were loading rate, material strength, and the number of damper plates. The results showed substantial load–resistance capacity under monotonic loadings, and excellent ductility and energy dissipation were exhibited under cyclic loadings, with even distribution of damage over the entire height of strips. Based on the test results, a simple hysteretic model using a combined isotropic–kinematic hardening rule was also proposed. The comparison demonstrated that it represents the tested cyclic load–displacement hysteresis well. It is expected that the proposed model can be successfully used to predict the behavior of HSD in real-world applications.

Numerical and experimental analysis of combined behavior of shear-type friction damper and non-uniform strip damper for multi-level seismic protection

A new hybrid damper which combines a friction damper and steel strip damper is proposed for improving the seismic performance of structures at multiple levels of ground motion. In order to investigate the combined behavior of the proposed damper, quasi-static cyclic tests were carried out on ten specimens. Experimental results demonstrated that hysteretic response was stable, and multi-phased behavior (i.e., activation of two different kinds of dampers) functioned as intended. However, depending on the type of strip damper applied, the behavior and failure modes showed distinct differences due to rotational motion induced during combined behavior; deformation and energy dissipation capacities were enhanced when a strip damper with adequate out-of-plane stiffness was applied. Furthermore, numerical analysis based on both material strength and expected strength well represented behavioral characteristics of the damper, and dissipated energy was reliably predicted. It is expected that the proposed analytical model can be practically applied to predict the performance of structures strengthened by the hybrid damper.

Non-uniform steel strip dampers subjected to cyclic loadings

This paper describes the cyclic performance of non-uniform steel strip dampers. The shapes proposed are (a) a dumbbell-shaped strip, (b) a tapered strip, and (c) an hourglass-shaped strip. Each of these strip shapes was designed to reduce stress concentration when subjected to cyclic loadings. In order to evaluate the performance of the dampers, six specimens were tested cyclically. Cumulative damage caused by hysteretic motion was effectively distributed throughout the entire height of the strips, and SEM microstructures of the fracture surfaces represented a typical ductile failure mode. The experimental results showed that the proposed strip dampers indicate excellent seismic performance compared to conventional prismatic slit dampers. Furthermore, it was verified that structural performance can be accurately estimated using the design equations presented in the paper.

Modified Fatigue Model for Hourglass-Shaped Steel Strip Damper Subjected to Cyclic Loadings

AbstractMetallic dampers have been used to dissipate large amounts of seismic energy through inelastic deformation in order to minimize damages to the main structural components. However, cumulative inelastic deformation of the metallic dampers results in fatigue cracks, so the life of the device may be reduced significantly. In this paper, an experimental study was carried out to investigate the low-cycle fatigue damage for the proposed hourglass-shaped strip damper (HSD). Four different types of cyclic loadings were applied in this test. The microstructures of the fracture surfaces and the crack distribution patterns were also observed. From the results, it was found that the shape of HSD was well designed for excellent fatigue performance, and the low-cycle fatigue characteristics of the steel strip damper could be well defined by the Manson-Coffin relationship. To consider the mean deformation and the loading sequences, a modified fatigue damage prediction model was proposed that incorporates the effe...

Explicit Finite Element Models of Friction Dampers in Forced Response Analysis of Bladed Disks

A generic method for analysis of nonlinear forced response for bladed disks with friction dampers of different designs has been developed. The method uses explicit finite element modeling of dampers, which allows accurate description of flexibility and, for the first time, dynamic properties of dampers of different designs in multiharmonic analysis of bladed disks. Large-scale finite element damper and bladed disk models containing 104−106 degrees of freedom can be used. These models, together with detailed description of contact interactions over contact interface areas, allow for any level of refinement required for modeling of elastic damper bodies and for modeling of friction contact interactions. Numerical studies of realistic bladed disks have been performed with three different types of underplatform dampers: (i) a “cottage-roof” (also called “wedge”) damper, (ii) seal wire damper, and (iii) a strip damper. Effects of contact interface parameters and excitation levels on damping properties of the dampers and forced response are extensively explored.