WINDBREAK BARRIER FOR A SUSPENSION BRIDGE STRUCTURE, COMPRISING FLUTTER DAMPING MEANS

Abstract: Windbreak barrier for a suspension bridge comprising a substantially rigid flat structure, the top surface of which forms the roadway for the vehicles crossing the bridge, and a suspension system formed of catenary cables and of a plurality of vertical stays or hangers (P) to which is anchored said structure. The windbreak barrier comprises, on one hand, aerodynamic elements consisting of a pair of horizontal wing surfaces (8, 9) having a symmetrical profile and forming part of a framework, and on the other hand, a vertical grille wall (10) having a transparency of the order of 70 % and being mounted on said framework. Said wing surfaces have a wing chord of 100 to 150 cm and a thickness, in correspondence of the highest point, of 20 to 30 cm.

Description: "WINDBREAK BARRIER FOR A SUSPENSION BRIDGE STRUCTURE, COMPRISING FLUTTER DAMPING MEANS" ===oOo=== The present invention concerns suspension bridges comprising an essentially flat main structure, or framework, the top surface of which forms the roadway for the transport means crossing the bridge, and a suspension system formed of catenary cables anchored to end piers of the bridge and of a plurality of vertical stays or hangers to suspend the bridge framework to the catenary cables. As known, these suspension bridges have vibration frequencies of their own; normally, with no wind, the basic flexural vibration frequency differs from the basic torsional vibration frequency, both being general¬ ly very low. Nevertheless, the action of side winds varies said typical vibration frequencies, particularly because - especially in bridges with large transversal dimensions and/or a wide span, for instance motorway bridges - the flat suspended structure behaves, when actually exposed to side winds, similarly to a wing surface, hence with a "lifting" effect which greatly varies from one moment to the next. As wind increases its strength, the two aforespecified vibration frequencies tend to approach, up to the point of coinciding: in these circumstances, the structure is thus subjected to so-called "flutter" conditions, i.e. to flexural-torsional stresses which may even result dangerous for the stability of the whole bridge. These flutter conditions, and the problems connected therewith, are described in more detail in EP-A-0.233.528, filed by the same Applicant, to which reference is made for a better understanding of the present invention. The main object of EP-A-0.233.528 is a wing structure, which is rigidly fixed to the lateral edges of the bridge and is meant to increase its flutter speed beyond the top speed of the wind expected in the bridge area. The same EP-A-0.233.528 also discloses a windbreak barrier, whose main function is to reduce the crosswind pressure on the vehicles travel ¬ ling along the bridge; such a barrier essentially consists of a grating, having a typically convex surface apt to cooperate with the wing struc¬ ture, so as to control the action of the wind. The object of the present invention is to propose a windbreak bar¬ rier having a remarkable aerodynamic damping action on the bridge struc ¬ ture, such as to limit the use of additional wing structures, or even - at least in certain cases - give it up completely. This result is essen ¬ tially achieved due to the fact that said windbreak barrier comprises a grille wall, mounted onto a carrying framework formed of a plurality of uprights and of at least two ledgers, at least one of said ledgers con¬ sisting of an aerodynamic element formed as a wing surface with a sub¬ stantially horizontal axis. According to the invention, said wing surface has a wing chord of 100 to 150 cm and a thickness, in correspondence of its highest point, of 20 to 30 cm, while said grille wall has a transparency of about 70%. Further characteristics and advantages of the windbreak barrier according to the present invention will anyhow be more evident from the following description of a preferred embodiment thereof, illustrated by way of example on the accompanying drawing, in which: Fig. 1 is a diagrammatic, partial, cross-section view of the late¬ ral edge of a bridge carrying a windbreak barrier according to the invention; Fig. 2 is a diagrammatic, enlarged scale, cross-section view of one of the wing surfaces forming part of the windbreak barrier of fig. 1; and Fig. 3 is a partial side view of the barrier, in the direction of arrow A in fig. 1. The bridge structure to which the present invention refers substan¬ tially corresponds to that described in EP-A-0.233.528, whereby a detail¬ ed description thereof need not be given herein. Fig. 1 thus illustrates only the lateral edge 1 of the bridge, with the roadway 2 and the conven¬ tional guardrails 3, fixed on uprights A the height of which may vary between 1 and 1.5 m. The hangers P to suspend the bridge framework are connected to an anchor pLate P1 fixed to the edge 1. As also shown in fig. 1, to the edge 1 there are fixed, at regular intervals, arms 5 outwardly projecting from the bridge and supporting, at their free end, uprights 6; the arms 5 also carry surface gratings 7 forming a service lane. According to the present invention, on the uprights 6 there are fixed ledgers 8 and 9 forming therewith a framework to support a grille wall 10 acting as windbreak barrier. As clearly shown on the drawing, said ledgers 8 and 9 are shaped as a wing surface. The height h of the uprights 6 is such that their top is at a level more or less corresponding to the highest of the vehicles supposed to cross the bridge. For instance,, the height h of the uprights 6 may be of about A to 5 m from the level of the service lane surface 7, which latter may find itself at a height h of about 1 m below the level of the roadway 2. The wing surface 9 is fixed at the top of the uprights 6, while the wing surface 8 is fixed at about half-way along said uprights. These wing surfaces preferably have a symmetrical section in respect of both their vertical and horizontal axes; the wing chord I of such wing surfaces is of 1 to 1.5 m, and their thickness in correspondence of the highest point is of 20 to 30 cm. The windbreak grille wall 10 is preferably mounted directly on the framework formed by the uprights 6 and ledgers 8, 9, and is thus positio¬ ned perfectly vertical, as shown on the illustrated embodiment. It is however possible to mount said wall 10 with additional support elements - for instance in the form of ribs (not shown) - apt to give to its surface an arcuate or anyhow profiled configuration, allowing a better control over the wind stream. The grille wall 10, shown in fig. 3, has a transparency (ratio between mesh-holes surface and overall surface) of the order of 70%. Practical tests carried out in a wind tunnel, on a dynamic model of the bridge according to the described embodiment, have allowed to ascer ¬ tain an optimal behaviour both as far as protecting the traffic from wind gusts and, above all, as far as damping the flutter phenomena. In practice, the windbreak barrier according to the invention is apt to produce a damping effect allowing to settle the wind stream just in a position in which - owing to the bridge configuration and to the traffic - turbulence conditions may more easily arise. The tests carried out in a wind tunnel have shown, in particular, that the flutter speed is of the order of 90 m/sec (while the arrangement according to the cited EP-A-0.233.528 allowed to obtain flutter speeds not exceeding 70 m/sec). It is interesting to note, moreover, the improved efficiency of the structure according to the present invention: the wing profiled ledgers 8, 9, have in fact a considerably reduced surface - by about a quarter - compared to the wing surface of EP-A-0.233.528; in spite of this, as said, they allow to increase the flutter speed. As well as an optimal behaviour from the functional point of view, the windbreak barrier according to the invention provides an additional advantage from the constructive point of view - in respect of a system as that described in EP-A-0.233.528 - in that its construction costs are far lower and its assembly is far easier: said barrier can in fact be cons¬ tructed into separate, prefabricated sections. Another considerable advantage lies in the fact that the windbreak grille wall can be cons¬ tructed flat and can be supported directly by the uprights 6 and ledgers 8, 9, thus, without having to necessarily provide for special arc-shaped supporting ribs. It is anyhow understood that the invention is not limited to the heretofore described and diagrammatically illustrated embodiment, and that there may be other embodiments, differing from the same, all within easy reach of an expert of the art, but all obviously falling within the scope of the above inventive idea, as defined in the following claims. - A

Claims: C LA I MS 1) Windbreak barrier for a suspension bridge structure comprising: a suspension system formed of catenary cables and of vertical stays or hangers (P); a substantially rigid flat framework anchored to said sus¬ pension system; and aerodynamic elements, rigidly fixed to the bridge structure, to control the action of the wind on said structure; said windbreak barrier being characterized in that it comprises a grille wall (10) mounted onto a framework formed of a plurality of uprights (6) and of at least two ledgers (8, 9), at least one of said Ledgers consisting of an aerodynamic element formed as a wing surface with a substantially horizontal axis. 2) Windbreak barrier as in claim 1), wherein said wing surfaces have a wing chord of 100 to 150 cm and a thickness, in correspondence of the highest point, of 20 to 30 cm.' 3) Windbreak barrier as in claim 1), wherein said grille wall (10) has a transparency of the order of 70%. A ) Windbreak barrier as in claim 1), wherein said ledgers (8, 9) are both formed as wing surfaces, with a symmetrical profile, and are positioned in a parallel arrangement. 5) Windbreak barrier as in claim A), wherein the longitudinal plane of symmetry of said wing surfaces is positioned horizontal. 6) Windbreak barrier as in claim 1), wherein the top of the barrier is substantially on the same level as the highest of the vehicles cross¬ ing the bridge. 7) Windbreak barrier as in claim 6), wherein the height of the bar¬ rier is of A to 5 m. 8) Windbreak barrier as in claim A), wherein said two wing surfaces are positioned, one (9) at the top of the grille wall (10) and the other one ( 8) at about half-way along said wall. 9) Windbreak barrier as in claim 1), wherein to the uprights (6) supporting the grille wall (10) there are associated ribs projecting outwardly of the bridge and having an arcuate profile onto which is applied a correspondingly profiled grille wall.

Inventors: DIANA GIORGIO, FALCO MARZIO, DIANA, GIORGIO, FALCO, MARZIO
Applicants: DIANA GIORGIO, FALCO MARZIO, STRETTO DI MESSINA SPA, DIANA, GIORGIO, FALCO, MARZIO, STRETTO DI MESSINA S.P.A
Application year: 1993
Main classification: E01D11
IPC codes: E01D1, E01D11, E01D19