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CONTENTS
Volume 39, Number 5, September10 2011
 

Abstract
Passive tuned mass dampers (TMDs) efficiently suppress vibrations induced by quasistationary dynamic inputs, such as winds, sea waves or traffic loads, but may prove of little use against pulse-like excitations, such as near-field (NF) ground motions. The extent of such impairment is however controversial, partly due to the different evaluation criteria adopted within the literature, partly to the limited number of seismic records used in most investigations. In this study, three classical techniques and two new variants for designing a TMD on an SDOF structure are tested under 338 NF records from the PEER NGA database, including 156 records with forward-directivity features. Percentile response reduction spectra are introduced to statistically assess TMD performance, and TMD robustness is verified through Monte Carlo simulations. The methodology is extended to a variety of MDOF bending-type and shear-type frames, and simulated on a case study building structure recently constructed in Central Italy. Results offer an interesting insight into the performance of TMDs against NF earthquakes, ultimately showing that, if properly designed and sufficiently massive, TMDs are effective and robust even in the face of pulse-like ground motions. The two newly proposed design techniques are shown to generally outperform the classical ones.

Key Words
structural control; tuned mass damper; near-field earthquake; PEER NGA database; robustness; optimal design

Address
E. Matta: Structural and Geotechnical Engineering Department, Turin Politechnic, Turin, Italy

Abstract
The design of box girders requires the determinations the buckling stress of the flange and the webs. Existing design equations available in codes of practice ignore the interactions between the box girder components. The paper illustrates the influence of the geometric interaction on the buckling stress of box girders. Generalized equations are first derived in terms of the web the flange geometric properties. Industrial examples are then presented showing the variation of the flange buckling stress for various stiffening configurations. The influence of the flange/web proportions on the buckling stress of box girder components is also highlighted. It is shown that buckling strength of the flange is largely affected by the restraints imposed by the webs or attached diaphragms. Graphs are presented showing various limiting states of box girders. These graphs are useful to use in practice in order to achieve economical and efficient design of box girders and rationally predict local buckling stress.

Key Words
steel design; structural analysis; industrial facilities

Address
Osama Bedair: Technip Corporation, 116 Mt. Aberdeen Manor SE, Calgary, Alberta, T2Z 3N8, Canada

Abstract
In the famous equivalent elasticity modulus method proposed by Ernst for the geometrical nonlinear analysis of stay cables, the cable shape was assumed as a parabolic curve, and only a part of the gravity load normal to the chord was taken into account with the other part of gravity load parallel to the chord being ignored. Using the actual catenary curve and considering the entire gravity load of stay cables, the present study has derived the equivalent stiffness method to analyze the sag effect of stay cables in cable-stayed bridges. The derived equivalent stiffness can be degenerated into Ernst\'s equivalent elasticity modulus method with some approximations. Therefore, the Ernst

Key Words
equivalent stiffness method; equivalent elasticity modulus method; catenary curve; stay cable; geometrical nonlinearity; sag effect

Address
G.Y. Xia: School of Civil Engineering and Architecture, Changsha University of Science and Technology, Changsha, 410004, China
C.S. Cai: School of Civil Engineering and Architecture, Changsha University of Science and Technology, Changsha, 410004, China; Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, USA

Abstract
A Chebyshev spectral method (CSM) for the dynamic analysis of non-uniform Timoshenko beams under various boundary conditions and concentrated masses at their ends is proposed. The matrixbased Chebyshev spectral approach was used to construct the spectral differentiation matrix of the governing differential operator and its boundary conditions. A matrix condensation approach is crucially presented to impose boundary conditions involving the homogeneous Cauchy conditions and boundary conditions containing eigenvalues. By taking advantage of the standard powerful algorithms for solving matrix eigenvalue and generalized eigenvalue problems that are embodied in the MATLAB commands, chebfun and eigs, the modal parameters of non-uniform Timoshenko beams under various boundary conditions can be obtained from the eigensolutions of the corresponding linear differential operators. Some numerical examples are presented to compare the results herein with those obtained elsewhere, and to illustrate the accuracy and effectiveness of this method.

Key Words
Chebyshev spectral method; modal analysis; spectral differentiation matrix; chebfun; Timoshenko beam

Address
W.Y. Wang: Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan
J.Y. Liao: System Development Center, Chung-Sun Institute of Science and Technology, Taoyuan, Taiwan
L.W. Hourng: Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan

Abstract
In this study, inelastic displacement ratios and ductility demands are investigated for SDOF systems with period range of 0.1-3.0 s. with elastoplastic behavior considering soil structure interaction. Earthquake motions recorded on different site conditions such as rock, stiff soil, soft soil and very soft soil are used in analyses. Soil structure interacting systems are modeled with effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. Results are compared with those calculated for fixed-base case. A new equation is proposed for inelastic displacement ratio of interacting system (CR) as a function of structural period of interacting system (T), strength reduction factor (R) and period lengthening ratio (T/T). The proposed equation for which takes the soil-structure interaction into account should be useful in estimating the inelastic deformation of existing structures with known lateral strength.

Key Words
soil-structure interaction; inelastic displacement ratio; ductility demand; lateral strength; seismic analysis

Address
Muberra Eser and Cem Aydemir: Department of Civil Engineering, Yildiz Technical University, Istanbul, Turkey

Abstract
The accurate dynamic analysis of structures is usually performed by a fine finite element discretization with very large number of degrees of freedom. Apart from modal analysis, one can reduce the number of final equations by assuming the deformed shape of the structure as a linear combination of independent Ritz vectors. The efficiency of this method relies heavily on the vectors selected. In this paper, a new set of Ritz vectors is proposed. It is primarily proved that these vectors are linearly independent. Subsequently, various two and three-dimensional examples are analyzed based on the proposed method. In each case, the results are compared with the ones obtained based on usual Ritz and modal analysis methods. It is finally concluded that the proposed method is very effective and efficient method for dynamic analysis of structures in frequency domain.

Key Words
Ritz vectors; structural dynamics; frequency domain analysis; DOF reduction methods; linearly independent vectors; convergence of Ritz method

Address
S. Aliasghar Arjmandi and Vahid Lotfi: Civil Engineering Department, Amirkabir University of Technology, Tehran, Iran

Abstract
Nonlinear dynamic analysis and evaluation of eccentric braced steel frames (EBF) equipped with friction damper (FD) is studied in this research. Previous studies about assessment of seismic performance of steel braced frame with FD have been generally limited to installing this device in confluence of cross in concentrically braced frame such chevron and x-bracing. Investigation is carried out with three types of steel frames namely 5, 10 and 15 storeys, representing the short, medium and high structures respectively in series of nonlinear dynamic analysis and 10 slip force values subjected to three different earthquake records. The proper place of FD, rather than providing them at all level is also studied in 15 storey frame. Four dimensionless indices namely roof displacement, base shear, dissipated energy and relative performance index (RPI) are determined in about 100 nonlinear dynamic analyses. Then average values of maximum roof displacement, base shear, energy dissipated and storey drift under three records for both EBF and EBF equipped with friction damper are obtained. The result indicates that FD reduces the response compared to EBF and is more efficient than EBF for taller storey frames.

Key Words
friction damper (FD); slip load; nonlinear dynamic analysis; performance indices

Address
J. Vaseghi Amiri, B. Navayinia and S. Navaei: Department of Civil Engineering, Babol University of Technology, Shariati Avenue, P.O. Box 47144-484 Babol, Iran

Abstract
Recently, identification of modal parameters using the response only data has attracted considerable attention particularly where the classic modal testing methods is difficult to conduct. One drawback of the response only data, also known as Operational Modal Analysis (OMA), is that only the unscaled mode shapes can be obtained which restricts the applications of OMA. The Mass change method is a usual way to scale the operational mode shapes. In this article a new method is proposed to optimize the additive mass for scaling of the unscaled mode shapes from OMA for which a priori knowledge of the Finite Element model of structure is required. It is shown that the total error of the scaled mode shapes is minimized using the proposed method. The method is validated using a numerical case study of a beam. Moreover, the experimental results of a clamped-clamped beam demonstrate the applicability of the method.

Key Words
operational modal analysis; scaling; mode shapes; mass-change; sensitivity analysis

Address
M.M. Khatibi, M.R. Ashory and A.R. Albooyeh: Modal Analysis Lab., School of Mechanical Engineering, Semnan University, Semnan 35195-363, Iran


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