Bridges – Suspension
Patent
1995-05-26
1997-04-01
Ferensic, Denise L.
Bridges
Suspension
14 73, E01D 1100
Patent
active
056154364
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
It is known that, when planning suspension bridges one must forcedly take into account the action of the wind, which not only involves a substantially static cross-wind pressure, but also vibrations. Two distinct planning theories are followed at present: framework is formed of a trestle structure which is particularly rigid and of considerable height compared to the width of the bridge; this type of structure opposes an essentially static resistance to the aerodynamic wind forces; framework is formed of a more slender and thus more flexible structure, which is however adapted to oppose an essentially dynamic resistance to the action of the wind.
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 generally 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 force, 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 be dangerous for the stability of the whole bridge framework.
A suspension bridge of this second type--and the problems connected therewith, particularly those concerning the flutter phenomena--is described 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 framework and is meant to increase the flutter speed of the bridge beyond the top speed of the wind expected in the bridge area. Nevertheless, said wing structure is conceived so as to be substantially independent from the framework; in fact, the EP-A-0.233.528 by no means describes said framework.
Studies carried out by the Applicant on these types of suspension bridges, including tests in a wind tunnel, have however proved how also the structure of the framework is of considerable importance for the behaviour of the bridge in strong wind conditions.
OBJECT OF THE INVENTION
The object of the present invention is to therefore propose a suspension bridge framework having an improved dynamic behaviour under the action of the wind.
SUMMARY OF THE INVENTION
This object is reached due to the fact that said framework consists of at least two parallel longitudinal box-like bodies--corresponding to two runways of the bridge--mutually spaced by an extent equal to their transversal dimension, said bodies being interconnected at regular intervals by stiff supporting transversal ledgers and the cross-section of said bodies having a profile adapted to favour the proper flow of the wind stream.
BRIEF DESCRIPTION OF THE DRAWINGS
Further characteristics and advantages of the bridge framework according to the present invention will be more evident from the following detailed description of a preferred embodiment thereof, given by way of example and illustrated on the accompanying drawings, in which:
FIG. 1 is a diagrammatic cross-section view of the suspension bridge according to the invention;
FIG. 2 is a diagrammatic plan view o
REFERENCES:
patent: 3132363 (1961-05-01), Roberts
patent: 4741063 (1988-05-01), Diana
Ferensic Denise L.
O'Connor Pamela A.
Stretto Di Messina S.p.A.
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