• Smart Structures and Systems
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• v.31 no.5
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• pp.531-543
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• 2023

Base isolation is one of the most widely implemented and well-known technique to reduce structural vibration and damages during an earthquake. However, while the base-isolated structure reduces storey drift significantly, it also increases the base drifts causing many practical problems. This study proposes the use of Shape Memory Alloys (SMA) wires for the reduction in base drift while controlling the overall structure vibrations. A multi-degree-of-freedom (MDOF) structure along with base isolators and Shape-Memory-Alloys (SMA) wires in diagonal is tested experimentally and analytically. The isolation bearing considered in this study consists of laminates of steel and silicon rubber. The performance of the proposed structure is evaluated and studied under different loadings including harmonic loading and seismic excitation. To assess the seismic performance of the proposed structure, shake table tests are conducted on base-isolated MDOF frame structure incorporating SMA wires, which is subjected to incremental harmonic and historic seismic loadings. Root mean square acceleration, displacement and drift are analyzed and discussed in detail for each story. To better understand the structure response, the percentage reduction of displacement is also determined for each story. The result shows that the reduction in the response of the proposed structure is much better than conventional base-isolated structure.

• Advances in aircraft and spacecraft science
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• v.3 no.2
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• pp.197-209
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• 2016

The Spacecraft (S/C) numerical sine test-predictions are usually performed through Finite Element Method (FEM) Frequency Response Analysis (FRA), that is the hypothesis of steady-state responses to harmonic excitation to the S/C base is made. In the test practice, the responses are transient and may be significantly different from those predicted through FRA. One of the most significant causes of discrepancy between prediction and test consists in the beating phenomena. After a brief overview of the topic, the typical causes of beating are described in the first part of the paper. Subsequently, focus is made on the sine sweep rate effect, which often leads to have beatings after the resonance of weakly damped modes. In this work, the approach illustrated in the literature for calculating the sine sweep rate effect in the case of Single-Degree-Of-Freedom (SDOF) oscillators is extended to Multi-Degrees-Of-Freedom (MDOF) systems, with the aim of increasing the accuracy of the numerical sine test-predictions. Assumptions and limitations of the proposed methodology are detailed along the paper. Several assessments with test results are discussed and commented.

• Earthquakes and Structures
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• v.3 no.3_4
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• pp.413-434
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• 2012

This paper presents a displacement-based seismic design method with damage control, in which the targets for the considered performance level are set as displacements and a damage distribution is proposed by the designer. The method is based on concepts of basic structural dynamics and of a reference single degree of freedom system associated to the fundamental mode with a bilinear behaviour. Based on the characteristics of this behaviour curve and on the requirements of modal spectral analysis, the stiffness and strength of the structural elements of the structure satisfying the target design displacement are calculated. The formulation of this method is presented together with the formulations of two other existing methods currently considered of practical interest. To illustrate the application of the proposed method, 5 reinforced concrete plane frames: 8, 17 and 25 storey regular, and 8 and 12 storey irregular in elevation. All frames are designed for a seismic demand defined by single earthquake record in order to compare the performances and damage distributions used as design targets with the corresponding results of the nonlinear step by step analyses of the designed structures subjected to the same seismic demand. The performances and damage distributions calculated with these analyses show a good agreement with those postulated as targets.

• Computers and Concrete
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• v.32 no.5
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• pp.527-541
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• 2023

This paper emphasizes the use of CFT columns in frame structures subjected to strong horizontal forces and shows that the efficiency of using CFT columns is increased when the plastic design approach is adopted. Because the plastic design approach is based on redistribution of the force of the internal member, a double node for the rotational degrees of freedom, where the adjacent two rotational degrees of freedom can be connected by a non-dimensional spring element, is designed and implemented into the formulation. In addition, an accompanying criterion is considered in order to make it possible to describe the continuous moment redistribution in members connected to a nodal point up to a complete plastic state. The efficiency of CFT columns is reviewed in comparison with RC columns in terms of the cost and the resistance capacity, as defined by a P-M interaction diagram. Three representative frame structures are considered and the obtained results show that the most efficient and economical design can be expected when the use of CFT columns is considered on the basis of the plastic design, especially when a frame structure is subjected to significant horizontal forces, as in a high-rise building.

• Earthquakes and Structures
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• v.14 no.2
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• pp.155-165
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• 2018

Methods based on nonlinear static analysis as simple tools could be used for the seismic analysis and assessment of structures. In the present study, capability of the N2 method as a well-known nonlinear analysis procedure examines for the estimation of the damage index of multi-storey reinforced concrete frames. In the implemented framework, equivalent single-degree-of-freedom (SDOF) models are utilized for the global damage estimation of multi-degree-of-freedom (MDOF) systems. This method does not require high computational analysis and subsequently decreases the required time of seismic design and assessment process. To develop the methodology, RC frames with period range from 0.4 to 2.0 s under 40 records are studied. The effectiveness of proposed technique is evaluated through numerical study under near- and far-field earthquake ground motions. Finally, the results of developed models are compared with two other simplified schemes along with nonlinear time history analysis results of multi-storey frames. To improve the accuracy of damage estimation, a modified relation is presented based on the N2 method results for near- and far-field earthquakes.

• Smart Structures and Systems
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• v.3 no.4
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• pp.423-437
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• 2007

The empirical mode decomposition (EMD) method is well-known for its ability to decompose a multi-component signal into a set of intrinsic mode functions (IMFs). The method uses a sifting process in which local extrema of a signal are identified and followed by a spline fitting approximation for decomposition. This method provides an effective and robust approach for decomposing nonlinear and non-stationary signals. On the other hand, the IMF components do not automatically guarantee a well-defined physical meaning hence it is necessary to validate the IMF components carefully prior to any further processing and interpretation. In this paper, an attempt to use the EMD method to identify properties of nonlinear elastic multi-degree-of-freedom structures is explored. It is first shown that the IMF components of the displacement and velocity responses of a nonlinear elastic structure are numerically close to the nonlinear normal mode (NNM) responses obtained from two-dimensional invariant manifolds. The IMF components can then be used in the context of the NNM method to estimate the properties of the nonlinear elastic structure. A two-degree-of-freedom shear-beam building model is used as an example to illustrate the proposed technique. Numerical results show that combining the EMD and the NNM method provides a possible means for obtaining nonlinear properties in a structure.

• Structural Engineering and Mechanics
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• v.28 no.4
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• pp.387-410
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• 2008

The fatigue damage accumulation rates of horizontally curved thin walled box-girder bridge have been estimated from vehicle-induced dynamic stress history using rain flow cycle counting method in the time domain approach. The curved box-girder bridge has been numerically modeled using computationally efficient thin walled box-beam finite elements, which take into account the important structural actions like torsional warping, distortion and distortional warping in addition to the conventional displacement and rotational degrees of freedom. Vehicle model includes heave-pitch-roll degrees of freedom with longitudinal and transverse input to the wheels. The bridge deck unevenness, which is taken as inputs to the vehicle wheels, has been assumed to be a realization of homogeneous random process specified by a power spectral density (PSD) function. The linear damage accumulation theory has been applied to calculate fatigue life. The fatigue life estimated by cycle counting method in time domain has been compared with those found by estimating the PSD of response in frequency domain. The frequency domain method uses an analytical expression involving spectral moment characteristics of stress process. The effects of some of the important parameters on fatigue life of the curved box bridge have been studied.

• Wind and Structures
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• v.1 no.1
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• pp.43-57
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• 1998

A three-degree-of-freedom base hinged assembly (BHA) for aeroelastic model tests of tall building was developed. The integral parts of a BHA, which consists of two perpendicular plane frames and a flexural pivot, enable this modeling technique to independently simulate building translational and torsional degree-of-freedom. A program of wind tunnel aeroelastic model tests of the CAARC standard tall building was conducted with emphasis on the effect of (a) torsional motion, (b) cross-wind/torsional frequency ratio and (c) the presence of an eccentricity between center of mass and center of stiffness on wind-induced response characteristics. The experimental results highlight the significant effect of coupled translational-torsional motion and the effect of eccentricity between center of mass and center of stiffness on the resultant rms acceleration responses in both along-wind and cross-wind directions especially at operating reduced wind velocities close to a critical value of 10. In addition, it was sound that the vortex shedding process remains the main excitation mechanism in cross-wind direction even in case of tall buildings with coupled translational-torsional motion and with eccentricity.

• Smart Structures and Systems
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• v.13 no.1
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• pp.137-154
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• 2014

Servohydraulic shaking tables are being increasingly used in the field of earthquake engineering. They play a critical role in the advancement of the research state and remain one of the valuable tools for seismic testing. Recently, the National Earthquake Engineering Research Center, CGS, has acquired a 6.1m x 6.1 m shaking table system which has a six degree-of-freedom testing capability. The maximum specimen mass that can be tested on the shaking table is 60 t. This facility is designed specially for testing a complete civil engineering structures, substructures and structural elements up to collapse or ultimate limit states. It can also be used for qualification testing of industrial equipments. The current paper presents the main findings of the experimental shake-down characterization testing of the CGS shaking table. The test program carried out in this study included random white noise and harmonic tests. These tests were performed along each of the six degrees of freedom, three translations and three rotations. This investigation provides fundamental parameters that are required and essential while elaborating a realistic model of the CGS shaking table. Also presented in this paper, is the numerical model of the shaking table that was established and validated.

• Smart Structures and Systems
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• v.6 no.8
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• pp.921-938
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• 2010

Most of studies on control of beams containing piezoelectric sensors and actuators have been based on linear quadratic regulator (LQR) with state feedback or output feedback law. The aim of this study is to develop velocity-acceleration feedback law in the optimal control of smart piezoelectric beams. A new controller which is an optimal control system with velocity-acceleration feedback is presented. In finite element modeling of the beam, the variation of mechanical displacement through the thickness is modeled by a sinus model that ensures inter-laminar continuity of shear stress at the layer interfaces as well as the boundary conditions on the upper and lower surfaces of the beam. In addition to mechanical degrees of freedom, one electric potential degree of freedom is considered for each piezoelectric element layer. The efficiency of this control strategy is evaluated by applying to an aluminum cantilever beam under different loading conditions. Numerical simulations show that this new control scheme is almost as efficient as an optimal control system with state feedback. However, inclusion of the acceleration in the control algorithm increases practical value of a system due to easier and more accurate measurement of accelerations.