US3132363A - Suspension bridges - Google Patents

Description

May 12, 1964 G. ROBERTS SUSPENSION BRIDGES 3 Sheets-Sheet 1 Filed May 3, 1961 My 12, 1964 G. ROBERTS 3,132,363

SUSPENSION BRIDGES Filed May 5, 1961 5 sheets-Sheet 2 0.2 0.4 0/57J4/VC`F FROM 70K/5? EXPRESSED /45 A FAC/O/V 0F THE SPA/V.

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and lower chords of the trusses.

United States Patent 3,132,363 SUSPENSON BRIDGES Gilbert Roberts, London, England Filed May 3, 1961, Ser. No. 107,520 Claims priority, application `Great Britain May 16, 1960 3 Claims. (Cl. 14-18) It is now Igenerally accepted that the most important factors .in the design of a suspension :bridge from the viewpoint of aerodynamic stability are:

1) That the suspended deck should be so designed that it has a minimum tendency to oscillate or flutter.

(2) That the system as a whole should have maximum vertical stiffness.

(3) That the suspended deck should have maximum torsional stiffness.

ln most suspension bridges the suspended deck consists of two vertical stiffening girders braced together by top and bottom lateral bracing in the plane of the upper Stiilening girders of lattice or truss construction have hither-to been considered to be more resistant aerodynamic oscillation than those of soli-d web construction, which on some bridges have been found, when used in combination with Ia continuous deck in the plane of their bottom flanges, to be unstable. The function of the top and bottom lateral bracing is to give the system torsional rigidity las well as to resist the lateral static wind forces or drag.

It has been found desirable to provide, in a suspended deck of this construction, longitudinal spaces or slots between the roadways, and between the roadways and the iootways, if any, to prornote aerodynamic stability.

The deck is suspended from the cables by vertical wire rope hangers which permit relative movement longitudinally between the deck and the cables and therefore contribute nothing to the stiffness of the structure as a Whole. To prevent or reduce this relative movement the cables are sometimes attached to the stiffening trusses at the center of the bridge, by diagonal stays in the plane of the hangers or by rigid clamps. These devices complicate the ldesign by introducing a very large horizontal rforce between the cables and the trusses at the point of attachment.

The present invention provides a suspension bridge in vwhich the suspended deck is constituted by a hollow closed box extending throughout the span between the supporting towers of the bridge, the box presenting a continuous unbroken surface to the wind and having convex sides.

The box constituting the deck is closed at top and bottom, at the ends and at the sides in contradistinction to the tmssed ginder-construction hitherto adopted which is open Iat top or bottom even when the side girders are of solid web construction. The box is accordingly torsionally stiff land will normal-ly incorporate internal transverse stiifening trames and longitudinally extending stringers. sides of the box are convex. However, they need not necessarily be curved and for ease in fabrication they may be constituted by upper and lower flat plates inclined outwardly trom the top and bottom surfaces of the box and meeting along sharp edges. Thus the sides of the box may consist of upper flat plates, extending downwardly and outwardly from the upper surface, and meeting longer flat plates extending upwardly ffrom the bottom surface of the box (which is accordingly of less Width than the upper surface) at sharp edges In the interests of aerodynamic stability the Y y ice situated slightly above the mid-depth of the box. The box is thus to a large measure streamlined, so that the wind drag may be reduced by as mulch as two-thirds as compared with a trussed girder construction, with corresponding reduction in the lateral forces which must be taken by the towers, while the aerodynamic stability is nevertheless as good as that of a trussed girder construct-ion.

The box will be fabricated from a number of sections extending over the complete span of the bridge, e.g. 3,000-ft., and in a typical case may be 60 ft. wide and l10 it. deep. Similar hollow closed boxes with convex sides are normally provided in la bridge according to the invention in the side spans outside the supporting towers at the extremities of the main span. The ends of the boxes are preferably supported on the towers by articulated linkages.

One embodiment of suspension bridge according to the invention will now be described in more detail, by way of example, with reference .to the accompanying drawings in which:

FIG. 1 is a side elevation of the bridge,

FIG. 2 is la cross section on a larger scale,

FIG. 3 is a diagram illustrating the disposition of the hangers along the main span of the bridge and FIG. 4 is an enlarged perspective View ot part of the bridge near the center of the span.

The deck of l e bridge consists of a main span 10 and side spans 11 supported on the towers 12 by articulated linkages 13. 'I'he deck is suspended from the cables 14 in both the main and the side spans by hangers 15. As shown most clearly in FIG. 4, each hanger 15 comprises -two wire ropes looped over the cable 14 Aand connected at their lower ends to the deck, said wire ropes intersecting Where they loop over the cable 14 and being spaced apart at their deck level connections. Each of the spans 10, 11 is constituted, as shown, by a closed lbox having streamlined sides constituted by upper land lofwei outwardly inclined plates 16, |17 meeting at sharp edges 18 situated slightly above the mid-depth of the box. The boxes include internal transverse stiiening frames 19, formed at intervals with holes 20 and provided with vertical stiieners 21, and longitudinal stringers 22.

The bridge has carriageways 23, the surfacing material of which is applied directly to the upper surface of the deck, railings 24 and external tootways 25 yand cycle tracks l27, fitted to the deck at the level of the edges 18 and having side railings 26.

Normally the hangers of a suspension bridge extend vertically between the cables and the deck structure. While triangulation of the hangers is desir-able, in order to increase the stiffness of the structure and therefore its natural 4frequency of vibration, this is not possible when the deck is supported on a trussed lgirder construction, because the bottom yattachments of the hangers must be made at the intersections of the truss system; which are too far apart near the center of the span.

With the Ibox deck according .to the invention, however, the attachments for the hangers 15- can be made at any point in its length, -and according to a feature ofthe invention each hanger 15 is divided, as shown, into two parts inclined to each other Iby not more than about 35 in the plane of the cables 14, the two parts being attached at the same point to the cable. Thus each point of attachment on the cable 14 is triangulated to two points on the suspended deck by the divided hanger l15 and relative llongitudinal movement between the cable and the deck is thereby restrained though not entirely prevented.

The amount of restraint and the consequent stress induced in the hangers can be varied by appropriate selection of the included angle a (IFIG. 3) between the two parts of each hanger, and will normally be such that under no conditions of live loading can the dead load tension in either par-t of the hanger be reduced to zero, and also such that the maximum stress that can occur in either part of the hanger is limited to a suitable, safe and economical value. The variation of the inclination of the hangers along the center span as chosen to suit the particu lar example illustrated is shown in FIGURE 3. As there shown `for about one third A of the span measured inwardly from each tower 12 the hangers 15 (which are at uniform spacing) intersect at their points of attachment to the deck, their inclination to the vertical thus increasing with increase in their distance from the tower. lFor the middle third of the span (one half of which is indicated at B in FIG. 3) the hangers do not meet at their point of attachment to the deck, and over this central portion of the span the inclination of the hangers diminishes towards the center.

One of the hazards to which a suspension bridge is subjected is torsional oscillation arising from the action of aerodynamic forces. Such torsional oscillations can only be absorbed by the provision of damping in the system and in conventional suspension bridges, the only damping available is that provided by the hysteresis of the structural steelwork and slight slip which may occur in the riveted joints. The damping available Iirom these causes is very small and is not susceptible of measurement.

As the result of the torsional oscillation arising from wind action on the -bridge there is relative longitudinal movement between the cables and the deck. The amplij tude of this movement is a maximum at the center of the span of the bridge and diminishes progressively towards the towers. Hangers of wire rope which extend vertically from the cables to the deck impose negligible restraint upon such oscillations since they do not resist the above mentioned relative longitudinal movement. Damping of torsional oscillation of the bridge can, however, be effected to a determined and calculable extent by the use .at each side of the bridge of triangulated hangers, i.e. hangers each of which is formed of two parts which intersect at their upper points of attachment to the cable and which are spaced apart at their lower points of attachment to the deck. With triangulated hangers, relative longitudinal movement of the cables and the deck results in a decrease in tension in one part of each hanger and -an increase in tension in the other part. This change in stress in the `hangers involves absorption of energy by hysteresis, because the wire rope hangers are not truly elastic, and the hangers accordingly act as dampers by absorbing some of the energy of the aerodynamic oscillations of the bridge.

The difference in stress between the -two parts of a hanger, and therefore the electiveness of the triangulated hanger in damping oscillation, increases with the included angle between the parts. However, safety factors impose a maximum, normally about 35, upon the included angle, because if the included angle exceeds this value there is a substantial risk of the tension in one part of the hanger becoming zero, notwithstanding the dead load imposed by the deck, so that this part of the hanger becomes slack. The critical included angle Ivaries with the amplitude of the relative longitudinal movement in relation to the deck of the portion of the cable to which the triangulated hanger s attached and is therefore a minimum at the center of the span. It is impossible safely to nse triangulated hangers when the deck is of truss construction because in .this case the lower ends of the hangers must be attached to the deck at the points of intersection of the members of the truss to enable 4the deck to withstand the stresses imposed upon it. These points are normally equally d spaced and while they may enable triangulated hangers connected to high points of Ithe cables near the towers to extend at included angles of less than 35, they necessarily require the hangers near the center of the span and connected to the low portions of the cables t0 have included angles greatly in excess of 35.

When, however, as in accordance with the invention the deck is of box construction the lower ends of the hangers may be connected to the deck at any desired points in its length and these points of attachment need not be uniformly spaced. It is accordingly possible to use triangulated hangers for suspending the deck Ifrom the cables.

As will be seen from FiG. 3 of the drawings, triangulated hangers with a small included angle of the order of 15 are used at the center of the span, where the amplitude of the relative longitudinal movement is a maximum. Nearer the towers the included angle is increased, i.e. to some 30, but beyond this and as the towers are approached the included angle is progressively reduced. This is because it is inconvenient to cross the hangers and therefore in order to accommodate in these zones suicient hangers to support the deck safely, it is necessary for the points of attachment of each triangulated hanger to the deck to be close together.

The -invention thus provides, firstly a suspension bridge in which the suspended deck consists essentially of a hollow box member streamlined on its edges to reduce wind drag, and secondly the provision in such a suspension bridge of triangulated hangers in which the inclination of the cons-tituent parts of the hangers to the vertical at their points of attachment to the cable is adjusted to a predetermined value, varying along the span and such that their tension due to dead load is not entirely counteracted by stresses induced by live loads.

What I claim as my invention and desire to secure by Letters Patent is:

l. A suspension bridge comprising spaced supporting towers, a deck constituted by a hollow closed box which extends between and rests upon the towers, said towers having legs supporting the ends of said deck, a pair of suspension cables situated above the deck and extending between the towers, one at each side of the deck, and triangulated hangers extending between each cable and the deck, disposed side by side throughout the span of the deck between the towers and constituting the sole connections between the cables and the deck, each triangulated hanger consisting of two parts extending between the cable and the deck and attached to the deck at their lower ends, said parts looping over the cable and intersecting where they rneet the cable and being spaced apart at their points of attachment to the deck, the included angle between the parts of all said hangers being restricted to a maximum of about 30, the parts of adjoining hangers having common points of attachment to the deck in zones adjacent said towers, the included angles between the parts of the hangers in said zones increasing with increase in the distance of the hangers from the towers, the points of attachment of adjacent hangers to the deck in the central portion of the span of the deck between the towers being spaced from one another, and the included angle between the par-ts of the hangers in said central portion of the span `diminishing towards the center of the span.

2. A suspension bridge comprising spaced supporting towers, a deck extending between and resting upon the towers, the towers having legs supporting the ends of said deck and said deck being a hollow closed box having sides constituted by upper fiat plates extending downwardly and outwardly from the upper surface of the box and lower fiat plates extending upwardly and outwardly from the lower surface of the box, said upper and lower dat plates meeting at edges located adjacent the mid-depth of the box, a pair of suspension cables situated above the deck and extending between the towers, one at each side of the deck, and triangulated hangers extending between each cable and the deck, disposed side by side throughout the span of `the deck between the towers and constituting the sole connections between the cables and the deck, each triangulated hanger consisting of two parts extending between the cable and the deck and attached tothe deck at their lower ends, said parts looping over the cable and intersecting where they meet Ithe cable and being spaced apart at their points of attachment to t-he deck, the parts of said hangers being attached -to the deck at points spaced apart by difieren-t distances at different zones of the deck and the included angle between the parts of all said hangers being restricted to a maximum of 'about 35.

3. A suspension bridge as claimed in claim 2, wherein the sides of the box carry outwardly extending platforms situated at the level of said edges.

References Cited in 6 the le of this patentA UNITED STATES PATENTS Sullivan May 8,

Maney July 10,

Beretta June 23,

Stoltenburg Nov. 212,

FOREIGN PATENTS France Dec. 2,

OTHER REFERENCES Civil Engineering, March 1952, page 25.

Claims (1)

1. 2. A SUSPENSION BRIDGE COMPRISING SPACED SUPPORTING TOWERS, A DECK EXTENDING BETWEEN AND RESTING UPON THE TOWERS, THE TOWERS HAVING LEGS SUPPORTING THE ENDS OF SAID DECK AND SAID DECK BEING A HOLLOW CLOSED BOX HAVING SIDES CONSTITUTED BY UPPER FLAT PLATES EXTENDING DOWNWARDLY AND OUTWARDLY FROM THE UPPER SURFACE OF THE BOX AND LOWER FLAT PLATES EXTENDING UPWARDLY AND OUTWARDLY FROM THE LOWER SURFACE OF THE BOX, SAID UPPER AND LOWER FLAT PLATES MEETING AT EDGES LOCATED ADJACENT THE MID-DEPTH OF THE BOX, A PAIR OF SUSPENSION CABLES SITUATED ABOVE THE DECK AND EXTENDING BETWEEN THE TOWERS, ONE AT EACH SIDE OF THE DECK, AND TRIANGULATED HANGERS EXTENDING BETWEEN EACH CABLE AND THE DECK, DISPOSED SIDE BY SIDE THROUGHOUT THE SPAN OF THE DECK BETWEEN THE TOWERS AND CONSTITUTING THE SOLE CONNECTIONS BETWEEN THE CABLES AND THE DECK, EACH TRIANGULATED HANGER CONSISTING OF TWO PARTS EXTENDING BETWEEN THE CABLE AND THE DECK AND ATTACHED TO THE DECK AT THEIR LOWER ENDS, SAID PARTS LOOPING OVER THE CABLE AND INTERSECTING WHERE THEY MEET THE CABLE AND BEING SPACED APART AT THEIR POINTS OF ATTACHMENT TO THE DECK, THE PARTS OF SAID HANGERS BEING ATTACHED TO THE DECK AT POINTS SPACED APART BY DIFFERENT DISTANCES AT DIFFERENT ZONES OF THE DECK AND THE INCLUDED ANGLE BETWEEN THE PARTS OF ALL SAID HANGERS BEING RESTRICTED TO A MAXIMUM OF ABOUT 35''

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