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A model for the mechanical behaviour of the railway track in the lateral plane
No full textThis paper deals with a mechanical model for the analysis of the railway track behaviour built by exploiting the periodicity of the track-structure. The starting point of this study are the inner forces transferring modes. They have been determined by the unit principal vectors analysis of the base cell transfer matrix. The main outcome on bending moments is that they are transferred through the track without deforming the sleepers and fasteners and are composed of two parts: the first one, referred as primary bending moment, is generated by the couple of axial forces acting on each nodal section while the second is due to the curvature changes of the rails. Shear forces are engendered by two independent mechanisms that are associated respectively with the unit changes of the total and primary bending moments. The constitutive properties of the equivalent medium are derived by an averaging and limiting process of the transferring modes strain energies, avoiding any a priori assumption on the kinematics of the substitute medium. Finally, the equilibrium equations are achieved by the virtual work principle.
The proposed model is able to reproduce accurately the track behaviour in transferring its inner forces. However, solutions that are equilibrated but not kinematically admissible are obtained from it when transversal loads are applied. In additions, only boundary conditions compatibles with the track transferring modes can be satisfied. This inconsistency is eliminated by superposition of some corrective deformed shapes. These are derived by the eigenvectors of the transfer matrix pertaining to self-equilibrated systems of bending moments decaying along the track.
The application field of the proposed track model is also discussed, and the results of a validation study carried out by F.E. analysis are finally presented
A Micro-Polar Model for Buckling Analysis of Vierendeel Periodic Beams
No full textA method is presented for evaluating the axial critical loads and deformed shapes of a Vierendeel periodic girder. They are obtained by solving the large-deflections equilibrium problem of a micro-polar equivalent model. The elastic properties of this model have been derived from those of an ideal girder whose cells may deform only according to the inner forces transferring modes of the unit cell. In particular, the strain energy density of the equivalent medium is obtained by evaluating the limit of the girder elastic energy for the cell to girder size ratio tending to zero. To solve the Engesser/Haringx discord, the large-deflections equilibrium equations are deduced by the virtual work principle, without any a priori assumption on the shear force. For this aim, also the external work of the substitute medium is evaluated by the same procedure as the strain energy density. Actually, it is first written for the ideal girder under the assumption of negligibility of the axial effects at the cell scale and then specialized for the continuous medium by examining the limit value for cell to girder size ratio going to zero. Closed form solutions for the critical deformed shapes are given, together with an accurate formula for the girder buckling load
Dynamic shear behaviour of truss towers for wind turbines
No full textThe global interest in renewable energy sources has increased the attention to the manufacturing of wind turbine towers, since they are largely diffused in seismic areas too. Different types of towers have been produced in recent years. Among them, the truss structures assure a reduced mass and the modular characteristics necessary for easy transportation. Reduced costs of production, installation and maintenance are typical of these structures. Nonlinear dynamics is an efficient framework to analyze structures subjected to variable actions, i.e. to assess the seismic safety of wind turbine towers in case of earthquake actions. This study outlines a procedure to evaluate the post-elastic behavior of truss towers for wind turbines. Rigid-plastic behaviour is taken into account to develop approximate solutions for the problem of a tower modeled as a vertical cantilever beam and subjected to harmonic base motion. A comparison with the results of a finite element model is proposed
Dynamic shear behaviour of truss towers for wind turbines
Full text linkThe global interest in renewable energy sources has increased the attention to the manufacturing of wind turbine towers, since they are largely diffused in seismic areas too. Different types of towers have been produced in recent years. Among them, the truss structures assure a reduced mass and the modular characteristics necessary for easy transportation. Reduced costs of production, installation and maintenance are typical of these structures. Nonlinear dynamics is an efficient framework to analyze structures subjected to variable actions, i.e. to assess the seismic safety of wind turbine towers in case of earthquake actions. This study outlines a procedure to evaluate the post-elastic behavior of truss towers for wind turbines. Rigid-plastic behaviour is taken into account to develop approximate solutions for the problem of a tower modeled as a vertical cantilever beam and subjected to harmonic base motion. A comparison with the results of a finite element model is proposed
Operative treatment of heterotopic hip ossification in patients with coma after brain injury
No full textIn 13 joints of 12 patients who sustained traumatic brain injury, heterotopic ossifications of the hip were surgically excised at an average of 15 months after the patients' recovery from coma. All of the patients were referred by neurologists of the postcoma unit who deemed surgery necessary to speed up the rehabilitation program of the patients. Before surgery, three patients were able to ambulate, whereas nine were not ambulatory. Eight hips were ankylosed, whereas five had a severe painful limitation of joint motion. All of the patients were checked at regular intervals after the operation, and the final followup averaged 38 months. In an attempt to prevent postoperative recurrence of ossification, 100 mg of indomethacin was administered daily after surgery for 6 weeks. At followup, 10 patients could ambulate, and two were able to sit in a wheelchair. Patients with poor neuromuscular control tended to lose part of their postoperative range of motion, and heterotopic periarticular ossification recurred in two of them (three hips). No correlation was found between recurrence and the time that elapsed from head injury to the operation, but old ossifications continued to show osteogenic activity at the histologic level
Resection of elbow ossification and continuous passive motion in postcomatose patients
No full textHeterotopic periarticular ossifications were surgically excised in 16 elbows of 14 traumatic brain injury patients an average of 18.9 months (range, 4-67 months) after the end of coma. In 11 elbows the ulnohumeral joint was ankylosed in a position that ranged from 0 degrees to 100 degrees of flexion (group 1); in 5 elbows the arc of flexion ranged from 10 degrees to 25 degrees (group 2). Full pronation and supination were present in 15 of the elbows; in 1 the radiocapitellar joint was fixed at 30 degrees of pronation by a partial ossification of the interosseous membrane. The arc of flexion attained after surgery averaged 115 degrees (range, 90 degrees to 145 degrees) in the group 1 elbows and 128 degrees (range, 115 degrees to 140 degrees) in the group 2 elbows. In an attempt to prevent postoperative loss of motion and recurrence of ossification, continuous passive motion was applied to the affected elbow for 6 weeks before starting a fully active rehabilitation program. All the patients were examined at regular intervals after the surgery. The follow-up period ranged from 12 to 60 months (average, 30.7 months). During the follow-up period, all the elbows showed improvement in range of motion and the arc of flexion averaged 95 degrees (range, 30 degrees to 135 degrees) in the group 1 elbows and 116 degrees (range, 80 degrees to 145 degrees) in the group 2 elbows. Patients with poor neuromuscular control lost part of their postoperative range of motion and partial recurrence was observed in 3 elbows. We believe that our improved results, compared with those obtained by previous investigators, may have been due to the prolonged application of continuous passive motion after surgery
Excision for the treatment of periarticular ossification of the knee in patients who have a traumatic brain injury
No full textPatients who are comatose after a traumatic brain injury often have heterotopic periarticular ossification that can be treated with excision to improve the range of motion of the joint
A direct technique for the homogenization of periodic beam-like structures by transfer matrix eigen-analysis
No full textTo homogenize lattice beam-like structures, a direct approach based on the matrix eigen- and principal vectors of the state transfer matrix is proposed and discussed. The Timoshenko couple-stress beam is the equivalent continuum medium adopted in the homogenization process. The girders unit cell transmits two kinds of bending moments: the first is generated by the couple of the axial forces acting on the section nodes, the other one is due to the moments directly applied at the node sections by the adjacent cells. This latter moment is modelled as the resultant of couple-stress. The main advantage of the method consists in to operate directly on the sub-partitions of the unit cell stiffness matrix. Closed form solutions for the transmission principal vectors of the Pratt and X-braced girders are also attained and employed to calculate the stiffnesses of the related equivalent beams. Unit cells having more complex geometries are analysed numerically. As a result, the principal vector problem is always reduced to the inversion of a well-conditioned (3×3) matrix employing the direct approach. Hence, no ill-conditioning problems, affecting all the known transfer methods, are present in the proposed method. Finally, comparing the predictions of the homogenized models with the finite element (f.e.) results of a series of girder, a validation of the homogenization method is performed
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