Title:
Bridge and hanger system
United States Patent 2380183


Abstract:
This invention relates to bridges having a web system, all component parts of which are under tension and prestressed;: and particularly to a novel hanger and web system arrangement which in spite of the absence of members under compression provides great dynamic rigidity against resonant vibration...



Inventors:
Maney, George A.
Application Number:
US38203941A
Publication Date:
07/10/1945
Filing Date:
03/06/1941
Assignee:
Maney, George A.
Primary Class:
Other Classes:
14/24
International Classes:
E01D11/02
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Description:

This invention relates to bridges having a web system, all component parts of which are under tension and prestressed;: and particularly to a novel hanger and web system arrangement which in spite of the absence of members under compression provides great dynamic rigidity against resonant vibration in a vertical plane and renders suspension bridges virtually immune to complex vibrations due to .Von Karman Vortices.

Bridges may be broadly classed as arched bridges, in which the principal members are in compression; suspension bridges, in which the principal members are in tension; and, girder or truss bridges, in which half the components of the principal members are in compression and half in tension. The present invention relates primarily to suspension bridges, and as pointed out, the principal members of such bridges are in tension. The introduction of iron link chains about the end of the 18th century, and later of wire ropes of still greater tenacity, permitted the construction of road bridges of this type with spans at that time impossible with any other system of construction.

The suspension bridge dispenses with the conipression members required in girders and with a good deal of the stiffening required in arches.

On the other hand, suspension bridges have one great inherent disadvantage, and that is, their flexibility. They can, of course, be and have been -stiffened by relatively massive horizontal stiffening trussed, but in such cases they lose much of their advantage and economy. Suspension bridges are used today usually only where there is a very large span and where the dead load of the bridge is relatively large compared to the live load. A number of large and supposedly well-engineered suspension bridges have failed and broken down by resonant vibrations set up in the bridge due to a live load passing thereover.

Resonant vibrations in a, suspension bridge cable are of two different general types; first, a pure undulation in a vertical plane; and second, a combination twist or oscillation and an undulation. The greatest amplitude of vibration occurs when an impact is received at the quarter point of the span which causes an undulation or vibration having a node at the center point. A resonant vibration will also occur, however, when an impact is received at the center point, and in such event, there will be two nodes. While the amplitude of vibration is relatively large under an impact at the center point, the resonant vibration is damped out much faster than where the impact occurs at the quarter point. Failure of the suspension bridge at Brighton, England, in 1836; the Niagara Falls bridge in 1889; and the Tacoma bridge in 1940, all started at the quarter point.

Complex resonant vibration, or in other words, vibration which includes not only undulations in a vertical plane, but also a transverse twist of the bridge between the two sides, are generally caused by a wind phenomenon known as Von Karman Vortices. These occur only when the wind is blowing within an angle of plus or minus 2° above and below the floor system of the bridge.

For the purposes of this application, it is sufficient to say that Von Karman Vortices cause alternate high and low pressure areas on opposite sides of the bridge, which causes an up and down lift dn the bridge, and is the cause of flutter of airplane parts and particularly of airplane wings.

Efforts have been made to stiffen suspension bridges by putting relatively large horizontal stiffening trusses across the span. While these stiffening trusses are not as large as would be necessary if they were employed by themselves, they nevertheless greatly increase the cost of the suspension bridge, and failure of suspension bridges with such stiffening trusses have occurred in spite of their stiffening effect. Efforts have 1;0 also been made to overcome the effect of the Von Karman Vortices by employing wind shields which prevent the wind from blowing across the floor system within the relatively small angle which causes complexed vibrations to be set up.

'"' It is an object of the present invention to provide a novel web or hanger system which retains the advantages of economy inherent in a suspension bridge, but which reduces undulations in a vertical plane and complex resonant vibrations to a negligible amount.

Another object of the present invention is to provide an economical stiffening system for suspension bridges and other suspended members which gives such bridges substantially the characteristics of a truss or girder bridge.

A further object of the present invention is to provide a novel bridge construction which is economical to construct, which is rugged and reliable in use, and which is substantially free of resonant vibrations set up by live loads or by wind conditions.

A still further object of the present invention is to provide a novel web or hanger system all of whose component parts are under tension and 5 .prestressed.

Another and further object of the invention is to provide a prestressed all-tension double diagonal web system.

Another and still further object of the invention is to provide a novel means for supporting the floor system of a suspension bridge.

A further object of the present invention is to provide means for eliminating the danger of aerodynamic vibration and also to provide means in which all lifts due to ordinary angles of wind attack are zero or negative on the floor system of the bridge.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and manner of construction, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which: Figure 1 is a diagrammatic view of a portion of a suspension bridge embodying the novel web system of the present invention; Figure 2 is a greatly enlarged view of a portion of the web system shown in Figure 1; Figure 3 illustrates one manner in which the vertical and diagonal tension members may be secured to the floor system of the bridge; Figure 4 illustrates a vibration record of a suspension bridge of conventional design having only vertical tension members and in which an impact load was dropped at the quarter point; Figure 5 illustrates a vibration record of a suspension bridge of the same dimensions and characteristics as the bridge employed for the vibration record of Figure 4, but in which the novel web system of the present invention 'is incorporated and in which an identical impact load was dropped at the same quarter point; Figure 6 illustrates the deflection of the floor system of a suspension bridge of the type shown in Figure 1 when a large dead load is permitted to rest at the quarter point; Figure 7 illustrates the position of the floor system of a conventional type of suspension bridge when a dead load of the same mass as employed for the curve of Figure 6 is permitted to rest at the quarter point; Figure 8 is a diagrammatic illustration of an aerodynamic shield for the floor system of a suspension bridge, the view being taken transversely through the floor system; Figure 9 is a diagrammatic view of a web system employed to suspend a floor system from an arch; Figure 10 illustrates transverse cross bracing of the hanger system of Figure 1; and, Figure 11 illustrates a modified form of web system in which the diagonal tension members have multiple points of intersection with each other.

In Figures 1, 2 and 3 of the drawings the suspension bridge diagrammatically illustrated embodies a novel web or hanger system constructed in accordance with the teachings of the present invention. As shown in Figure 1, the suspension bridge includes a pair of towers II (only 1 of which is shown in the drawings) cables 12 and a floor system 13. It will of course be understood that the cable 12 is suitably anchored at the opposite ends of the bridge. The floor system 13 of the bridge includes horizontal side girders 14, to which the vertical and diagonal tension members are secured and upon which the actual floor system is mounted. It should be clearly understood in connection with these horizontal side girders that they are not the equivalent of the heavy horizontal stiffening trusses which have -5 sometimes been employed in connection with suspension bridges, but on the other hand, are relatively small inexpensive members.

As will best be understood from an examination of Figures 2 and 3, the floor system 13 is suspended from the cables 12 by a web system 15 which includes diagonal intersecting tension members 16 and 17 and vertical tension members 18 in each panel.

It has been found that where a continuous system of double diagonals or X's are employed entirely across the span of a suspension bridge, the bridge no longer possesses the characteristics of a suspension bridge, but on the contrary, possesses the characteristics of a truss or girder bridge insofar as stiffness and manner of deflection under loads is concerned. It has further been found that isolated diagonal bracing members or ties will not obtain this effect, and indeed, if there is a break anywhere in the continuous series of X's the effect of a truss is lost. It will thus be understood that the intersecting diagonal tension members or X's must be a continuous series throughout that portion of the bridge which is to be given the effect of a truss.

::0 One of the novel features of the present invention is the particular manner in which the diagonal tension members 16 and 17 and the vertical tension members 18 are secured to the side girders 14 and also the manner in which these :.i members are prestressed. It will be remembered that when a load is secured to and suspended from the ends of two flexible hangers the stress in the two flexible hangers is always statically determinate and may readily be figured when the 4( angles of the flexible members to the vertical is known. When a load is secured to the ends of three flexible members coming together at a point the stress in the hangers is statically indeterminate. (By that, of course, it is meant that you .5 dannot forecast in advance the amount of stress which each hanger will take without elaborate - deformation calculations.) The present invention contemplates the prestressing of the diagonal tension members 16 and 17 and the vertical ten.n sion member 18 in a statically determinate manner simply by erecting the floor system thereon.

In other words, the relative percentage of the load which each of the three tension members will assume is statically determined when the floor 5s system of the bridge is connected thereto in a manner now to be described.

The diagonal tension members 16 and 17 and the vertical tension members 18 are secured to the horizontal side girders 14 in the manner shown in the left-hand part of Figure 3. More specifically, the vertical tension member 18 is secured to the plate 19 as at 20. This plate is provided with pins, pulleys or their equivalent, diagrammatically illustrated at 21, over which the diagonal tension members 16 and 17 are free to slide. The lower ends of the diagonal tension members 16 and 17 are secured to the side girder 14 as at 22 and 23. Initially, the plate 19 is not connected to the girder 14 or any other portion of the floor systems; nor does it bear thereagainst in a stress transmitting manner.

Since all of the loading of the floor system at this point is carried by the two vertically downwardly extending portions 24 and 25 of the tension members 16 and 17, and since the diagonal web members 16 and 17 extend freely around the pulley or pin points 21, it will be understood that the loadings in the diagonal portions of the tension members 16 and 17 are exactly equal and opposite to the downward pull exerted by the vertical portions 24 and 25. Since the summation of the downward forces in the portions 24 and 25 must equal the vertical components of the forces in the tension members 16, 17 and 18, it will at once be understood that the amount of prestressing in the'tension member 18 is equal to the total loading at this junction placed upon it by the floor system minus the vertical component of loading in the diagonal tension members 16 and 17. With the diagonal tension mem- 1 bers disposed at the angles shown in Figure 3, the loading in the vertical tension member is less than the loading in either of the diagonal members. The unit stressing in the vertical tension member 18 can of course be controlled by controlling the diameter of the vertical member 18 with respect to the diagonal members 16 and I7.

From the above description, it will at, once be apparent that the amount of prestressing in the tension members 16, 17 and 18 is statically determinate and therefore the relative distribution, of loads between the three hanger members may be controlled.

After the prestressing of the tension members 16, 17 and 18 has been effected by the erection of the floor system on the ends of the tension members 16 and 17, the plate or member 19 is preferably welded or otherwise suitable secured to the side girder 14. This has been found desirable under certain circumstances in order to prevent relative movement between the member 19 and the floor system caused by live loads passing over the bridge. It will of course be understood that when the member 19 has been secured to the side girders 14 any further loading such as the live load will not be statically determinate in the members 16. 17 and 18. In other words, under these circumstances, the amount of prestressing in the tension members 16, I17 and 18 is statically determinate but the subsequent loading in these members caused by the live load on the bridge is statically indeterminate. In certain instances it will be found that it is not necessary to secure the member 19 to the side girder 14. and in such event the amount of loading in the tension members 16 and 17 and 18 will be statically determinate at all times-not only in prestressing these members but also in the loading therein caused by the live load.

At this time, it is well to point out that while some additional material is of course required to employ supporting members disposed at a diagonal, the increased cost is relatively slight and very much less than the saving gained by not having to employ horizontal stiffening trusses.

In addition to the principal diagonal tension members 1-6 and 17 and vertical tension members 18, a secondary web arrangement may be incorporated in the construction and indeed is advisable in large constructions, which includes vertical tension members 26 which extend down to the floor system from the point of intersection of the diagonal tension members 16 and 17, additional diagonal tension members 27 and 28 which extend from substantially the midpoint of the vertical tension member 18 to diagonal tension members 16 and 17 respectively; and vertical tension members 29 and 30 which extend down from the point of connection of the diagonal tension members 27 and 28 to the side girders 14. The vertical tension member 30 is shown connected to the side girder 14 as at 31 in the right-hand portion of Figure 3. It will be understood that the vertical tension members 26 and 29 may be similarly secured to the side girder 14.

The side girders 14 are each made of a plurality of sections 33 which are pinned together o as at 34, which permits some flexure thereof in a vertical plane, but which ties the floor system together in a horizontal plane.

The secondary web system includes the tension members 27, 28, 29 and 30, and they are .5 preferably employed only in the portions of the web system where the distance is relatively large between the floor system 13 and the cable 12, or in other words, in the portions of the web system adjacent the towers II. Near the center 2o of the span the web system is preferably composed only of the vertical tension members 18 and the diagonal tension members 16 and 17 and the short tension member 26. At the point in the span where the secondary web system inj5 cluding the tension members 27, 28, 29 and 30 is discontinued, a diagonal tension member 35 is preferably connected from the cable to the diagonal tension member 16. In addition to the diagonal tension member 35 a vertical tension memio ber 36 is connected between the floor system 13 and the point of connection of the tension member 35 to the tension member 16.

In order to prevent out of phase oscillation of the cable on one side of the bridge with respect :5U to the cable at the opposite side of the bridge, a transverse bracing, such as that shown in Figure 10, is provided at points of maximum oscillation. More specifically, a tension member or tie 48 is connected between the cable 12 on op1 posite sides of the bridge, and diagonal tension members 49 and 50 are connected between the cable 12 and vertical hangers 18. This transverse bracing of the bridge by tension members 48, 49 and 50 is preferably located at the two quarter ' points since maximum vibration occurs at these points and an impact load is set up at one of the two quarter points. The transverse bracing by members 48, 49 and 50 is also preferably em5, ployed at the one-sixth point and the center point since maximum oscillation occurs at these points when the cables 12 are vibrating with two nodes., The ties 48 are preferably tensioned so as to cause a slight inward bowing of the main cables 12 when viewed from the top. The ties 49 and S50 are also preferably prestressed in tension and indeed may, if desired, be tightened to a point where the intermediate points of attachment on the hangers 18 are pulled in slightly toward the center of the bridge when viewed from the top.

From an inspection of Figure 10, it will be observed that if the cable on the left side starts to move up and the cable on the right side starts to move down an increased tension occurs in the diagonal member 49. When the cable 12 on the ~5 left side and the cable 12 on the right side moves up tension in the diagonal member 49 is decreased and tension in the diagonal member 50 is increased.

Figures 4, 5, 6 and 7 illustrate the difference between a conventional bridge and a suspension bridge having a double diagonal prestressed web system of the present invention.

Figure 4 shows a vibration record of a model bridge which simulated in every detail both dimension and loading, of the suspension bridge at Tacoma, Washington, which recently failed, the vibration having been set up by an impact load applied at the quarter point. Figure 5 shows the vibration of a bridge having identical dimensions and loading as the bridge employed to make the vibration record of Figure 4, but in which the web system of the present invention was employed, the vibration having been set up by an impact load of the same magnitude as that employed on Figure 4 and applied at the same point in the bridge. From a comparison of Figures 4 and 5, it will at once be observed that not only is the magnitude of vibration many times less, and indeed almost negligible, in a bridge employing the novel web system of the present invention, but it will also be observed that the vibrations are damped out much more rapidly.

A well known characteristic of suspension bridges which is not present in girder bridges or trusses is the fact that when a heavy static load is placed on the bridge at the quarter point the floor system of the bridge at the opposite end of the span is lifted up. This typical characteristic of the conventional type of suspension bridge is illustrated diagrammatically in Figure 7, the dotted line 37 illustrates the normal floor line of the bridge under its own distributed dead load.

The full line 38 illustrates on an amplified scale the position of the floor system under a concentrated static load placed at the left quarter point.

It will be noted from an inspection of the position of the floor line 38 that the floor line below the load is depressed while the floor line opposite the other quarter point is lifted above its normal position as at 39.

Figure 6 of the drawings is a diagrammatic sketch similar to Figure 7 but shows the position of the floor line under a concentrated static load at the quarter point in a suspension bridge having a double diagonal prestressed web system.

Figure 6 is drawn to the same scale as Figure 7 and has the same concentrated static load applied to a quarter point thereof and represents comparison tests made on the two bridges previously referred to in connection with Figures 4 and 5.

Note that in the suspension bridge having the double diagonal prestressed web system the concentrated static load at the quarter point does not cause the floor system of the bridge at the opposite quarter point to be lifted up as is the situation in the conventional type of suspension bridge.. It will furthermore be noted that the extent of depression of the bridge at the quarter point is nowhere near as great under the same load as in the conventional type of suspension bridge. It will still further be noted that the deflection curve of the bridge of the present invention is a characteristic curve of a girder bridge or truss and not that of a suspension bridge in spite of the fact that all the component parts of this bridge are under tension.

Although the double diagonal prestressed web system of the present invention substantially eliminates all objectionable resonant vibration by either the live load on the bridge or by Von Karman Vortices, additional means may be provided to further reduce any danger from the latter cause. More specifically, an aerodynamic shield such as that diagrammatically illustrated in Figure 8 may be provided under the floor system of the bridge and around the sides thereof. In Figure 8, the aerodynamic shield 40 extends from the top of the side girders 14 around under the floor system 30 of the bridge. The shield 40 is relatively light and may be secured to the side girders 14 and floor system 13 in any suitable manner (not shown). The outer surface of the shield 40 closely simulates the upper surface of an airplane wing, it being noted, however, that in this case the surface is inverted. A shield having a cross-sectional surface configuration substantially as shown in Figure 8 will provide a zero or negative lift (downward pull) at all times irrespective of what angle to the horizontal the wind is blowing at.

A modification of the present invention is illustarted in Figure 9 of the drawings. More specifically, Figure 9 illustrates how a double diagonal prestressed web system may be employed to suspend a floor system 41 frpm an arch 42. If the arch 42 approximates a parabolic curve the entire arch will be under compression while the tie in the floor system 41 is in tension and the web system is stressed only by the tensile prestressing due to carrying the floor. As shown in Figure 9, the web system includes diagonal tension members 43 and 44 and vertical tension members 45. Additional secondary vertical tension members 46 are also preferably employed. Since the two outer corners of the arch 42 tend to spread away from each other the floor system .41 is preferably in the form of a tie under tension between these two points. Under this arrangement it is not necessary to have obliquely disposed abutments at the two ends of the bridge, but it is simply necessary to have vertical supports 47 therefor.

A further modification of the present invention is shown in Figure 11 of the drawings. In Figure 11 of the drawings, which shows a portion of a suspension bridge having diagonal tension members in addition to the vertical hangers and which diagonal tension members have multiple points of intersection with respect to each other. More specifically, the hanger system includes vertical tension members 51 and diagonal tension members 52 and 53. This type of construction functions in a manner similar to that described in connection with the construction of Figure 1, and it will be found desirable in certain large types of bridge construction. The type of construction shown in Figure 11 will be found particularly useful, however, in stiffening suspension bridges which have already been 'no erected. In such cases it will be assumed that the tension members or hangers 51 are the existing hangers of the bridge. The diagonal tension members 51 and 52 will then be connected in the hanger system in the manner shown in Fig.r ure 11. When the diagonal tension members 51 and 52 are erected in the bridge they will preferably be tightened or stressed to take at least a small part of the load carried by the vertical hangers 51. This tensioning or tightening of o0 the diagonals 51 and 52 may be done in any suitable manner, such as by a turn buckle (not shown), or the like.

While I have shown particular embodiments of my invention, it will, of course, be understood 1t5 that I do not wish to be limited thereto, since many modifications may be made, and I therefore, contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

I claim as my invention: 1. In a suspension system having a vertical hanger and a pair of oblique hangers, all three of which converge to a common location point, means for securing a load thereto comprising a member having means thereon over which said oblique members are free to hang, said vertical hanger being secured to said member and said oblique hangers being secured to said load, whereby the relative distribution of loads between said three hangers is statically determinate.

2. In a suspension system having a vertical hanger and a pair of oblique hangers, all three of which converge to a common location point, means for securing a load thereto comprising a member having means thereon over which said oblique hangers are free to hang, said vertical hanger being secured to said member and said oblique hangers being secured to said load, whereby the relative distribution of loads between said three hangers is statically determinate, said member being secured to said load after said load has been hung on said oblique hangers.

3. In a suspension bridge span, a pair of bridge cables, a floor system, a hanger system for suspending said floor system from said cables, and at least one transverse bracing system including a flexible member extending transversely across said bridge and secured at opposite ends to said cables, and a pair of intersecting flexible members extending obliquely downwardly across said bridge from the points of connection of said first transverse member and secured at opposite sides of said bridge to said hanger system.

4. In a suspension bridge span, a pair of bridge cables having known points of maximum resonant vibration, a floor system, a hanger system for suspending said floor system from said cables, and a plurality of transverse bracing systems, each bracing system including a flexible member extending transversely across said bridge and secured at opposite ends to said cables, and a pair of intersecting flexible members extending obliquely downwardly across said brace from the points of connection of said first transverse member and secured at opposite sides of said bridge to said hanger system, said transverse bracing systems being located at certain said known points of maximum resonant vibration of said cables.

5. In a suspension bridge, a pair of bridge cables, a base structure, a hanger system for suspending said structure from said cables, and at least one transverse bracing system including a prestressed tie extending transversely across said bridge and pulling said cables slightly toward each other, and a pair of intersecting ties extending obliquely downwardly across said bridge from the points of connection of said o0 first tie and secured at opposite sides of said bridge to said hanger system, said oblique ties being prestressed and pulling intermediate points of said hanger system on opposite sides of said bridge slightly toward each other. 6. A bridge having a suspension system for the floor thereof comprising vertical and oblique members, such members forming a continuous web with each vertical member intersecting a pair of oblique members at or near the floor, said suspension system including means for distributing the dead load of the bridge floor in a statically determinate manner, said means causing the oblique members of a pair to be prestressed an amount equal to the dead load of the floor at the point of intersection.

GEORGE A. MANEY.