The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Saiidi, Mehdi S. - One of the best experts on this subject based on the ideXlab platform.
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Effects of Hinge Restrainers on the Response of the San Gregorio Bridge during the Loma Prieta Earthquake
9999Co-Authors: Maragakis, Emmanuel M., Saiidi, Mehdi S., Feng Shiping, Flournoy LindaAbstract:Report No. CCEER-93-5The primary objective of this study is to study the effects of cable restrainers on the nonlinear seismic response of the San Gregorio Bridge. The computer program NEABS-86 was used in the nonlinear analyses. The earthquake analyses focus on the relative displacements at the Hinges, the restrainer stresses, and the abutment forces in both the longitudinal and the transverse directions of the bridge. The peak ground acceleration of the earthquake input, the seismic retrofit, the restrainer gap, and the Hinge gap were varied. It was found that, in general, the cable restrainer system reduces the Hinge responses in both the longitudinal and the transverse directions. Also found was that, when a zero restrainer gap is assumed, the Hinge responses change significantly. It is recommended that the restrainer design be based on cases with both zero and non-zero restrainer gaps, to encompass all the critical forces, stresses and displacements. Finally, some design methods for the longitudinal restrainer cable at intermediate Hinges of the bridge were evaluated. A modified "Dynamic-Caltrans" design procedure was tested preliminarily, and showed some promising results (Abstract by authors)
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An Evaluation of the Current Caltrans Seismic Restrainer Design Method
9999Co-Authors: Saiidi, Mehdi S., Maragakis, Emmanuel M., Feng ShipingAbstract:Report No. CCEER-92-8The primary objective of this study was to develop an understanding of the implications of the current Caltrans Hinge restrainer design procedure. Two aspects of the problem were studied. One was the effects of changing (a) the cross sectional area of restrainers and (b) the restrainer gap on the nonlinear response of a bridge with several Hinges. The other was the sensitivity of the number of required restrainers to changes in some of the simplifying assumptions which are made in the current Caltrans restrainer design method. Computer program NEABS-86 was used in the nonlinear analyses [3]. The focus of this part of the study was the relative displacements at the joints, restrainer forces, and restrainer stresses. Three earthquake records, the El Centro 1940, Eureka 1954, and Saratoga 1989. In addition to input earthquakes, the number of restrainers at each Hinge, the restrainer gaps, and the Hinge gaps were varied. It was found that when a restrainer gap of 0.75 in. is assumed, the number of restrainers does not affect the response significantly. It is recommended that the design should be based on cases with and without restrainer gaps to encompass all the critical forces, stresses, and displacements. To study the effects of design assumptions on the required number of cables, several manual calculations of the example in the Caltrans restrainer design guidelines were carried out. The deviations from the method included the treatment of mass and stiffness of bridge segments as different Hinges closed. Another variable was the simultaneous reduction in the restrainer gap and increase in the Hinge gap. The results indicated that slight variation in some of the assumptions can change the number of restrainers significantly. A more streamlined design method that incorporates the nonlinear response of bridge components needs to be developed (Abstract by authors)
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Response of Bridge Hinge Restrainers during Earthquakes -Field Performance, Analysis, and Design
9999Co-Authors: Saiidi, Mehdi S., Maragakis, Emmanuel M., Feng Shiping, Abdel-ghaffar, Saber M., O'connor DanAbstract:Report No. CCEER-93-6This report presents a summary of the important findings of a study aimed at several aspects of the behavior of Hinge restrainers used as a seismic retrofit measure. Details of the study are described in five other reports [15 to 19]. The study included field investigations, extensive analytical studies, and an evaluation of the restrainer design method. The objectives of the study were: To review the actual performance of bridge Hinge restrainers during the 1989 Loma Prieta earthquake To simulate the earthquake effect on the analytical models of several selected bridges and study their responses To carry out a parametric study of these bridges to determine the effect of stronger earthquakes and the effect of changes in the restrainer gaps To review the restrainer design procedure and recommend any needed refinements A data base of the bridges with Hinge restrainers which had been damaged by the 1989 ~Loma Prieta earthquake was formed. Twenty-three bridges were in the data base. The damage reports prepared by Caltrans maintenance Division were reviewed. Three bridges, namely, the Central Viaduct, the Route 580/24/980 Separation, and the Route 92/101 Separation were investigated in the field. Measurements were made of crack widths and patterns, and the condition of the restrainers was examined. An analysis of locally damaged components was subsequently made. The field investigations pointed out the need to consider the performance of the restrainer system and not merely the restrainers. In addition to the restrainers, the system includes (a) the connection between the restrainers and the superstructure including any diaphragms, and (b) the superstructure adjacent to the Hinge. The data base of the damaged bridges was also used to select four bridges for detailed nonlinear response history analyses using computer program NEABS-86. The four structures ranged from three to eleven spans. They had different number of Hinges and different substructure characteristics. The earthquake intensity also varied considerably from one bridge to another. Two groups of earthquake analyses were carried out: in one analysis the input acceleration records collected at sites near the bridges were used, and in the other a series of parametric studies with larger peak ground acceleration (PGA) was conducted. The field investigations and the analyses showed that the Loma Prieta earthquake activated the Hinge restrainers in the majority of the bridges investigated in this study. Except for a few instances, the restrainers and their supporting systems performed well. It was also noted that bridges with a small ratio of number of Hinges to the number of spans and in which the substructure is relatively stiff, are less likely to be susceptible to support loss. The evaluation of the current Caltrans restrainer design method consisted of two parts: (1) a study of the effect of refinement in the current methods, and (2) a large number of nonlinear analyses of the Caltrans example bridge for different earthquakes, Hinge gaps, and the number of restrainers. Based on these studies, a new method for the computation of the relative Hinge movement was proposed, and demonstrated for one of the four bridges which had been the subject of detailed nonlinear analyses. It was found that the current Caltrans method for restrainer design leads to a conservative and safe design in terms of the number of restrainers. However, the degree of conservatism for different Hinges is not uniform. It was also determined that a more refined method to compute relative Hinge displacements can lead to fewer restrainers even in Hinges with a nominal seat width of 6 in. The refined method would explicitly incorporate the nonlinearity of soil at the footings and abutments, plastic hinging of the columns, and the nonlinearity of the Hinges (Summary by authors)
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Experimental Evaluation of Seismic Performance of SMA Bridge Restrainers
9999Co-Authors: Johnson Rita, Saiidi, Mehdi S., Maragakis, Emmanuel M., Desroches ReginaldAbstract:Report No. CCEER-04-2Tests were conducted at the University of Nevada Reno Large Structures Laboratory, in cooperation with Georgia Institute of Technology, to determine the effect of shape memory alloy (SMA) cable and rod restrainers on the seismic performance of in-span Hinges of a representative multiple-frame concrete box girder bridge subjected to earthquake excitation. Another objective of this study was to compare the performance of SMA to steel restrainers as restraining devices to reduce Hinge displacement. Data collected from SMA restrainer experiments was compared to information gathered in a previous University of Nevada, Reno study on the performance of steel restrainers. The SMA restrainers showed promise as restraining devices to limit Hinge displacement in bridges and the ability to dissipate energy. Under equivalent loading, the steel restrainers produced relative Hinge displacement approximately three to four times that of the SMA restrainers. The hysteretic damping that was seen in the larger ground accelerations showed the materials ability to dissipate energy with small residual strain
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Behavior, Design, and Retrofit of Reinforced Concrete One-way Bridge Column Hinges
9999Co-Authors: Jiang Yang, Saiidi, Mehdi S.Abstract:Report No. CCEER-93-1This report describes an experimental and analytical investigation of one-way reinforced concrete Hinges, frequently used at the base of highway bridge columns, when subjected to axial compression, shear, and uniaxial moment transfer in the strong direction. Attempts were made to develop recommendations for more reliable Hinged column design, to conduct a preliminary study of the response of Hinged columns with inadequate reinforcement development length, and to develop and test a repair method for damaged columns. Many variables, including column aspect ratio, monotonic or cyclic loading, Hinge steel arrangement, and Hinge thickness relative to Hinge width, were examined to study their effects on Hinge flexural and shear strength, energy dissipation capacity, shear slip, and Hinge throat concrete confinement. A linear finite element analysis was performed to study stress distribution in the Hinge throat area. Analytical studies also included inelastic analyses of bridges with Hinged columns. Focus was placed on the influence of deck torsional stiffness and abutment spring stiffness on inflection point height. The inflection point represents the point where the lateral load should be applied for the purpose of calculating lateral Hinge strength. A new approach was developed for estimating the lateral load strength of Hinged columns. This approach can be used for design purpose as well as for calculating the shear capacity for existing Hinged pier columns (Abstract by authors)
Shumin Feng - One of the best experts on this subject based on the ideXlab platform.
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parameters in bridge restrainer design for seismic retrofit
Journal of Structural Engineering-asce, 1996Co-Authors: Mehdi S Saiidi, Emmanuel A Maragakis, Shumin FengAbstract:The primary objective of this study was to determine the effects of changing (1) the cross-sectional area of restrainers; and (2) the restrainer gap on the nonlinear seismic response of a representative bridge with several Hinges. The computer program NEABS-86 was used in the nonlinear analyses. The focus of the study was the relative displacements at the Hinges, restrainer stresses, and abutment forces. The input earthquake, the number of restrainers at each Hinge, the restrainer gap, and the Hinge gap were varied. It was found that restrainer forces can become critical in cold weather, when the restrainer gap is minimum. It is recommended that the design should be based on cases with and without restrainer gaps to encompass all the critical forces, stresses, and displacements.
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EVALUATION OF THE CURRENT CALTRANS SEISMIC RESTRAINER DESIGN METHOD
1992Co-Authors: Mehdi S Saiidi, Emmanuel A Maragakis, Shumin FengAbstract:The focus of this study was to understand the implications of the current Calif. Department of Transportation (Caltrans) Hinge restrainer design procedure. Two aspects were studied. One was the effects of changing the cross sectional area of restrainers and the restrainer gap on the nonlinear response of a bridge with several Hinges. The second was the sensitivity of the number of required restrainers to changes in some of the simplifying assumptions which are made in the current Caltrans restrainer design method. The focus of the nonlinear analyses was the relative displacements at the joints, restrainer forces, and restrainer stresses. The level of ductility demand in the piers was also examined to identify the extent of nonlinearity. Different earthquake records were used for insuring that conclusions reflect the effects of a variety of ground excitations. In addition to the input earthquake, the number of restrainers at each Hinge, the restrainer gap, and the Hinge gap were also varied. Only longitudinal bridge response was considered to be consistent with the design method. In order to study the effects of design assumptions on the required number of cables, several manual calculations of the design example in the Caltrans restrainer design guidelines were carried out. The deviations from the method included a different treatment of mass and stiffness as different Hinges closed as a result of the earthquakes. Another variable was the simultaneous change in the restrainer and the Hinge gaps.
Sanford I Bernstein - One of the best experts on this subject based on the ideXlab platform.
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alternative s2 Hinge regions of the myosin rod differentially affect muscle function myofibril dimensions and myosin tail length
Journal of Molecular Biology, 2007Co-Authors: Jennifer A Suggs, Anthony Cammarato, William A Kronert, Massoud Nikkhoy, Corey M Dambacher, Aram Megighian, Sanford I BernsteinAbstract:Abstract Muscle myosin heavy chain (MHC) rod domains intertwine to form alpha-helical coiled-coil dimers; these subsequently multimerize into thick filaments via electrostatic interactions. The subfragment 2/light meromyosin “Hinge” region of the MHC rod, located in the C-terminal third of heavy meromyosin, may form a less stable coiled-coil than flanking regions. Partial “melting” of this region has been proposed to result in a helix to random-coil transition. A portion of the Drosophila melanogaster MHC Hinge is encoded by mutually exclusive alternative exons 15a and 15b, the use of which correlates with fast (Hinge A) or slow (Hinge B) muscle physiological properties. To test the functional significance of alternative Hinge regions, we constructed transgenic fly lines in which fast muscle isovariant Hinge A was switched for slow muscle Hinge B in the MHC isoforms of indirect flight and jump muscles. Substitution of the slow muscle Hinge B impaired flight ability, increased sarcomere lengths by approximately 13% and resulted in minor disruption to indirect flight muscle sarcomeric structure compared with a transgenic control. With age, residual flight ability decreased rapidly and myofibrils developed peripheral defects. Computational analysis indicates that Hinge B has a greater coiled-coil propensity and thus reduced flexibility compared to Hinge A. Intriguingly, the MHC rod with Hinge B was ∼5 nm longer than myosin with Hinge A, consistent with the more rigid coiled-coil conformation predicted for Hinge B. Our study demonstrates that Hinge B cannot functionally substitute for Hinge A in fast muscle types, likely as a result of differences in the molecular structure of the rod, subtle changes in myofibril structure and decreased ability to maintain sarcomere structure in indirect flight muscle myofibrils. Thus, alternative Hinges are important in dictating the distinct functional properties of myosin isoforms and the muscles in which they are expressed.
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alternative s2 Hinge regions of the myosin rod differentially affect muscle function myofibril dimensions and myosin tail length
Journal of Molecular Biology, 2007Co-Authors: Jennifer A Suggs, Anthony Cammarato, William A Kronert, Massoud Nikkhoy, Corey M Dambacher, Aram Megighian, Sanford I BernsteinAbstract:Muscle myosin heavy chain (MHC) rod domains intertwine to form alpha-helical coiled-coil dimers; these subsequently multimerize into thick filaments via electrostatic interactions. The subfragment 2/light meromyosin "Hinge" region of the MHC rod, located in the C-terminal third of heavy meromyosin, may form a less stable coiled-coil than flanking regions. Partial "melting" of this region has been proposed to result in a helix to random-coil transition. A portion of the Drosophila melanogaster MHC Hinge is encoded by mutually exclusive alternative exons 15a and 15b, the use of which correlates with fast (Hinge A) or slow (Hinge B) muscle physiological properties. To test the functional significance of alternative Hinge regions, we constructed transgenic fly lines in which fast muscle isovariant Hinge A was switched for slow muscle Hinge B in the MHC isoforms of indirect flight and jump muscles. Substitution of the slow muscle Hinge B impaired flight ability, increased sarcomere lengths by approximately 13% and resulted in minor disruption to indirect flight muscle sarcomeric structure compared with a transgenic control. With age, residual flight ability decreased rapidly and myofibrils developed peripheral defects. Computational analysis indicates that Hinge B has a greater coiled-coil propensity and thus reduced flexibility compared to Hinge A. Intriguingly, the MHC rod with Hinge B was approximately 5 nm longer than myosin with Hinge A, consistent with the more rigid coiled-coil conformation predicted for Hinge B. Our study demonstrates that Hinge B cannot functionally substitute for Hinge A in fast muscle types, likely as a result of differences in the molecular structure of the rod, subtle changes in myofibril structure and decreased ability to maintain sarcomere structure in indirect flight muscle myofibrils. Thus, alternative Hinges are important in dictating the distinct functional properties of myosin isoforms and the muscles in which they are expressed.
Mehdi S Saiidi - One of the best experts on this subject based on the ideXlab platform.
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parameters in bridge restrainer design for seismic retrofit
Journal of Structural Engineering-asce, 1996Co-Authors: Mehdi S Saiidi, Emmanuel A Maragakis, Shumin FengAbstract:The primary objective of this study was to determine the effects of changing (1) the cross-sectional area of restrainers; and (2) the restrainer gap on the nonlinear seismic response of a representative bridge with several Hinges. The computer program NEABS-86 was used in the nonlinear analyses. The focus of the study was the relative displacements at the Hinges, restrainer stresses, and abutment forces. The input earthquake, the number of restrainers at each Hinge, the restrainer gap, and the Hinge gap were varied. It was found that restrainer forces can become critical in cold weather, when the restrainer gap is minimum. It is recommended that the design should be based on cases with and without restrainer gaps to encompass all the critical forces, stresses, and displacements.
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EVALUATION OF THE CURRENT CALTRANS SEISMIC RESTRAINER DESIGN METHOD
1992Co-Authors: Mehdi S Saiidi, Emmanuel A Maragakis, Shumin FengAbstract:The focus of this study was to understand the implications of the current Calif. Department of Transportation (Caltrans) Hinge restrainer design procedure. Two aspects were studied. One was the effects of changing the cross sectional area of restrainers and the restrainer gap on the nonlinear response of a bridge with several Hinges. The second was the sensitivity of the number of required restrainers to changes in some of the simplifying assumptions which are made in the current Caltrans restrainer design method. The focus of the nonlinear analyses was the relative displacements at the joints, restrainer forces, and restrainer stresses. The level of ductility demand in the piers was also examined to identify the extent of nonlinearity. Different earthquake records were used for insuring that conclusions reflect the effects of a variety of ground excitations. In addition to the input earthquake, the number of restrainers at each Hinge, the restrainer gap, and the Hinge gap were also varied. Only longitudinal bridge response was considered to be consistent with the design method. In order to study the effects of design assumptions on the required number of cables, several manual calculations of the design example in the Caltrans restrainer design guidelines were carried out. The deviations from the method included a different treatment of mass and stiffness as different Hinges closed as a result of the earthquakes. Another variable was the simultaneous change in the restrainer and the Hinge gaps.
Nicolae Lobontiu - One of the best experts on this subject based on the ideXlab platform.
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compliance based matrix method for modeling the quasi static response of planar serial flexure Hinge mechanisms
Precision Engineering-journal of The International Societies for Precision Engineering and Nanotechnology, 2014Co-Authors: Nicolae LobontiuAbstract:Abstract A matrix method is proposed to model the direct and inverse quasi-static response of constrained/over-constrained planar serial mechanisms with flexure Hinges under bending, axial, and shear planar (three-dimensional) loading and small-deformations. The method uses a basic three-point compliance matrix corresponding to one rigid link and one adjacent flexure Hinge that are subjected to one point load. This matrix connects the displacements at a point on the rigid link with the load that is applied at another point on it, and the deformations of the flexure Hinge at its distal point. The quasi-static model of planar serial flexure-based mechanisms with multiple links under single/multiple point loading results from linearly superimposing all relevant Hinge-link-load triads defined by their three-point matrices. A displacement-amplification planar device with right circularly corner-fileted flexure Hinges is studied using several refinement stages of the matrix method to generate a model whose predictions are confirmed by finite element simulation.
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planar compliances of thin circular axis notch flexure Hinges with midpoint radial symmetry
Mechanics Based Design of Structures and Machines, 2013Co-Authors: Nicolae Lobontiu, Matt Cullin, Jeffrey HoffmanAbstract:The new class of flexure Hinges with circular longitudinal axis and midpoint radial symmetry is introduced. Using rotation and mirroring, the symmetric flexure Hinge is obtained from one half flexure. The six planar-bending compliances of the full Hinge are determined analytically for small deformations by combining only three compliances of the half flexure. To illustrate the general flexure Hinge category, the novel circular-axis, right circularly corner-filleted design is introduced. Experimental and finite element results correlate well with the analytical model predictions. The new flexure Hinge design is compared to the circular-axis, constant-thickness flexure and the straight-axis, right circularly corner-filleted Hinge.
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design of circular cross section corner filleted flexure Hinges for three dimensional compliant mechanisms
Journal of Mechanical Design, 2002Co-Authors: Nicolae Lobontiu, Jeffrey S N PaineAbstract:The paper introduces the circular cross-section corner-filleted flexure Hinges as connectors in three-dimensional compliant mechanism applications. Compliance factors are derived analytically for bending, axial loading and torsion. A circular cross-section corner-filleted flexure Hinge belongs to a domain delimited by the cylinder (no fillet) and the right circular cross-section flexure Hinge (maximum fillet radius). The analytical model predictions are confirmed by finite element simulation and experimental measurements. The circular cross-section corner-filleted flexure Hinges are characterized in terms of their compliance, precision of rotation and stress levels.
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design of symmetric conic section flexure Hinges based on closed form compliance equations
Mechanism and Machine Theory, 2002Co-Authors: Nicolae Lobontiu, Jeffrey S N Paine, Ephrahim Garcia, Michael GoldfarbAbstract:The paper develops closed-form compliance equations for conic-section (circular, elliptic, parabolic and hyperbolic) flexure Hinges. Finite element simulation results confirm the theoretical formulation data. The main objectives are to predict the deformation/displacement field of a flexure Hinge under loading and to assess the precision of rotation for a specific conic flexure Hinge. A non-dimensional analysis is carried out to discuss both problems. Conclusions are formulated regarding the performance of circular, elliptic, parabolic, and hyperbolic flexure Hinges.
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Corner-Filleted Flexure Hinges
Journal of Mechanical Design, 2000Co-Authors: Nicolae Lobontiu, Jeffrey S N Paine, Ephrahim Garcia, Michael GoldfarbAbstract:The paper presents an analytical approach to corner-filleted flexure Hinges. Closed-form solutions are derived for the in-plane compliance factors. It is demonstrated that the corner-filleted flexure Hinge spans a domain delimited by the simple beam and the right circular flexure Hinge. A comparison that is made with the right circular flexure Hinges indicates that the corner-filleted flexures are more bending-compliant and induce lower stresses but are less precise in rotation. The finite element simulation and experimental results confirmed the model predictions.
