GB2109040A

GB2109040A - Cable stayed bridge

Info

Publication number: GB2109040A
Application number: GB08231033A
Authority: GB
Inventors: Dr Ulrich Finsterwalder
Original assignee: Dyckerhoff and Widmann AG
Current assignee: Walter Bau AG
Priority date: 1981-11-03
Filing date: 1982-10-29
Publication date: 1983-05-25
Also published as: IT8268278A0; GB2109040B; IT1156222B; DK469982A; DE3207340A1; CA1174403A; DE3207340C2

Abstract

In a cable stayed bridge a continuous roadway slab 16) includes an end span (II) supported on and extending between an abutment (9) and a tower (3) and a middle span (I) extending halfway between the tower (2) and another tower (3). Both the towers and the roadway slab are formed of reinforced concrete. The roadway slab cantilevers outwardly on both sides from the tower. The tower is constructed so that it can absorb approximately two-thirds of the bending moment from one sided live loads by redistributing the resultants of the vertical loads within its cross section. Inclined stays (7,7') extend between the tower and the roadway slab includes first stays (7') extending between the upper end of the tower and the ends of the spans remote from the tower and second stays (7) also extending between the tower and the roadway slabs, however, on the tower their ends are located below the upper end of the tower and on the roadway slab they are located between the tower and the end of the span to which the first stays are connected. The first stays are constructed stronger relative to the second stays. The roadway slab 16) is formed as a plural hollow cell member of relatively low height constructed as a Vierendeel truss. <IMAGE>

Description

SPECIFICATION Cable stayed bridge The present invention is directed to a stayed cable bridge with at least one tower fixed to a foundation body and a roadway slab supported from the tower with both the tower and the slab constructed of reinforced concrete. The roadway slab cantilevers out from both sides of the tower in the long direction of the bridge and it is connected to the tower so that it is bending-resistant, however, it is also relatively flexible and it is supported by inclined cables extending in parallel vertical planes.

The basic concept in the classical cable stayed bridge involves suspending the roadway slab and its stiffening girder, usually in a form of a single unit, with stays extending in an inclined downward direction from the tower. The horizontal components of cable force are introduced into the stiffening girder as a compressive force while the vertical components of cable force, to a large extent, compensate for the dead weight of the combined roadway slab or deck and its stiffening girder.

Stretching of the cables is balanced through pretensioning.

The stiffening girder experiences bending moments in the long direction as the result of one-sided live loads, including wind loads and snow loads, in particular when both towers are bent in the same direction through loading of the end span and half of the middle span. To appreciate the action of such one-sided loads they will be divided into two load groups. Accordingly, the first load group is uniformly and symmetrically distributed over both the middle spans and the end spans, while the second load group is not symmetrically distributed. The first load group loads the towers centrically, while the second load group bends the towers to one side.In the classical cable stayed bridge, the first load group is carried by a tower while the second load group only loads the stiffening girder and for this loading the stiffening girder must be dimensioned as a freely supported girder having a length equal to the end span and half of the length of the middle span. In addition, the height of the girder amounts to at least 1/100 of the length of the middle span in consideration of the limitation of bending. Accordingly, the height of the girder is comaparatively great, and as a result, the girder is very rigid and, is it is constructed of reinforced concrete, very heavy.

As compared to a cable stayed bridge constructed of steel, such a bridge where the towers and the roadway carrier are formed of reinforced concrete has the advantage that the compression members are low cost though of considerably greater weight.

In particular, the successful reduction of the weight of the roadway slab and support can save on costs.

In this context an incomplete cable stayed cable bridge project is known in which a relatively flat, completely reinfoced concrete roadway member is used, note the magazine "Der Bauingenieur", 1967, No. 11, pages 405-406. In this cable stayed bridge extending over several spans, only the middle spans are described. The towers are fixed to foundation piers and must completely absorb the bending moment resulting from one-sided live loads. The inclined stays located along both sides of the bridge are composed of steel rods located inside a steel casing. The opening spaces within the casing around the steel rods are filled with cement mortar.

To improve the resistance to vibration at the anchorage of the stressed tension members for large loads in a concrete structural part, for instance the anchorage of an inclined stay in a cable stayed bridge, it is known to arrange the tension members together within a jacket which extends into the concrete structural part and is composed, at least in the input area in the concrete structural part, of a metal jacket, where, in addition to the bond existing between the metal jacket and the tension members, a bond exists between the metal jacket and the concrete structural part by means of the introduction of cement grout or the like into the jacket after the members have been tensioned (German Patentshrift 2114863).

Therefore, the primary object of the present invention is to provide the design of a cable stayed bridge as described above which is both esthetically and economically advantageous and where the roadway slab is supported on one end on an abutment so that it is prevented from deforming or warping as a consequence of one-sided live loads.

In accordance with the present invention, a cable stayed bridge includes an end span where the roadway slab is supported on an abutment so that it is free to more horizontally and of a half middle span or an additional end span where the bridge tower is constructed so that it can absorb within its core cross-section approximately two-thirds of the bending moments from one-sided live loads by redistributing the resultants from the vertical loads. In such a bridge, the inclined cables connecting the ends of the roadway carrier to the top of the tower are strengthened rleative to the other inclined stays or cables to take overthe remaining portion of the live loads.

The inclined cables or stays are made up of a plurality of steel rods having an average yield point and positioned within a steel pipe or casing and bonded with the steel pipe after tensioning by introducing a hardening material, such as a cement grout, in the space within the steel pipe not filled by the steel rods.

Advantageously, the roadway slab is formed as a plural hollow cell member of relatively low height in accordance with the Vierendeel system where the tension members are preferably prestressed.

The underlying concept of the invention is that, in a bending-resistant design of a reinforced concrete tower fixed on a foundation pier, the one-sided live loads can be supported to a great extent by the cantilevered support of the roadway slab and only a small part of such loads supported by the abutment and/or towers need to be absorbed by the inclined stays secured to the ends of the roadway slab and to the top of the towers.Therefore, in accordance with the present invention, the support is not more or less completely effected by the reinforced roadway member as in classic cable stayed bridges, rather the predominant part, approximately two-thirds of the moment is carried by the tower, and only a small pert, approximately one-third is taken up by the reinforced inclined stay3 connected to the top of the tower and to the midpoint of the middle span.

Aside from the advantage of better vibration resistance in the anchorage which is a problem in suspended stays because of the alternating loads acting on the bridge, the special design of the inclined stays, formed of steel rods and a tubular steel casing with a bond provided by cement grout filled into the casing about the rods, affords the advantage of benefiting the redistribution of the moment caused by one-sided live loads from the roadway slab into the tower.

A roadway slab member designed according to the Veirendeel system is particularly suitable for compensating the movement at the end of the cantilever arm for the tower resulting from the bending of the tower and the stretching of the cables by means of elastic bending. The height of the roadway slab member is fixed along its edges by the stay anchorages which may not exceed a minimum.

The upper side of the slab member is provided with an outwardly and downwardly inclined slope while its lower surface is preferably arranged in a level plane to afford a resisting moment which is as great as possible relative to the bending resistance.

The interior of the roadway slab member is divided into a number of hollow cells by vertical ribs which are very low between the slabs forming the upper and lower sides of the hollow roadway slab.

The vertical members of the Vierendeel system act as a frame and can be designed wide enough to receive the thrust of the frame without resulting in a member of great weight. By prestressing the tension members of the system a reduction in avoidable secondary stresses can be obtained.

The main object of the Vierendeel system is the elimination of diagonal members conventional in such roadway structures and the weight of such diagonal members load the total construction to a similar degree as the traffic. Accordingly, the entire concrete mass of the roadway slab member is available for transmitting the longitudinal force and thus results in large cost economy. Other advantages are gained from the reduction in height of the roadway slab member by reducing the applied wind forces as well as the possibility of locating the roadway surface at a lower level and thereby effecting a cost saving in the approach to the bridge.

Moreover, designing the roadway slab member in accordance with the Vierendeel system enables a simplification and acceleration of the production of cantilever sections so that a very favourable incremental construction system is provided which further reduces costs.

The following is a description of a specific embodiment of the invention reference being made to the accompanying drawings in which: Fig. 1 is a schematic side view of a cable stayed bridge embodying the present invention; Fig. 2 is a cross-sectional view through an inclined stay on an enlarged scale as compared to Fig. 1; Fig. 3 is an enlarged detail view of the anchorage of an inclined stay in the roadway slab member; Fig. 4 is a cross-sectionai view through the roadway slab member; and Fig. 5 is a partial enlarged cross-seofion through the roadway slab member shown in Ffg. 4.

The basic construction of a cable stayed bridge is shown schematically in side view in Fig. 1. Towers 2 and 3 extend upwardly from the water surface 1 and are fixed, below the water surface, to foundation piers 4, 5. A roadway slab member 6 is suspended on inclined stays or cables 7, 7' arranged in a pair of vertically extending lateral planes 8, note Fig. 4. At one end the inclined stays 7, 7' are anchored to the towers 2,3 and, at the other end, to the roadway slab member 6.

The cable stayed bridge, shown in Fig. 1, has a middle span I considerably longer than the two end spans 11. The roadway slab member 6 extending from each of the opposite ends of the bridge are connected at the bridge midpoint 8 by a shearing force joint. The outer ends 9 of each section of the roadway slab member is supported on a land abutment 10 so that the roadway slab member is freely displaceable in its elongate direction. Each inclined stay is made up of a number of steel rods 11, preferably with average yield strength, arranged within a steel pipe or tubular casing 12. Preferably, steel rods 11 are provided with hot-rolled ribs forming a screw thread so that correspondingly designed anchoring bodies or connecting bodies can be screwed onto the rods. The open spaces within the steel pipe 12 around the steel rods 11 is filled with a cement grout 13.The other inclined struts 7' connected to the top of the towers 2,3 and to the ends of the end and middle spans most remote from the towers, are reinforced relative to the stays 7.

In Fig. 3, an anchorage for an inclined stay 7 in the roadway slab member 6 is illustrated. The upper surface of the roadway slab member 6 is provided with an upwardly projecting enlargement 14 and its lower surface is provided with a similar downwardly projecting enlargement. The steel pipe 12 enclosing the steel rods 11 extends into the upper enlargement 14 a sufficient distance for transmitting the live loads carried by the steel pipe, by means of the bond, to the concrete of the roadway slab member 6. An anchorage 16 for the steel rods 11 is located in the lower enlargement 15 on the underside of the roadway slab member.

In Fig. 4 a transverse cross-section is illustrated through the roadway slab member 6. The roadway slab member is designed according to the Vierindeel system as a closed multi-vertical member frame in which the roadway slab 17 forms the upperhonzon- tal member and the base slab 18 forms the lower horizontal member with the vertical webs or ribs 19 forming the vertical members of the frame. This arrangement is also known as a Vierendeel truss.

The upper surface of the roadway slab member is inclined outwardly from the center line Sto the elongated edges or sides of the member for the drainage of surface water.

Fig. 5 is an enlarged section of half of the roadway slab member illustrated in Fig. 4 showing how the base slab 18 of the member, forming the tension member of the Vierendeel system, is prestressed by means of the tendon strands 20, 21. The tendon strand 20 is located below the tendon strand 21. In accordance with the size of the bending moments, the anchorages for tendon strands 20, 21 are offset relative to one another, so that the strand 20 has an anchorage 22 located along the outside edge of the roadway slab member in the vertical plane of the stays along one side of the bridge while the anchorage 23 is provided in the region of the lower end of the inner vertical member 19' closest to the outside of the roadway slab member.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

Claims

1. A cable stayed bridge comprising at least one tower formed of reinforced concrete, a foundation pier supporting said tower, an elongated reinforced concrete roadway slab member extending in the long direction of the bridge and cantilevering outwardly in the elongate direction thereof from both sides of said tower, means for supporting said roadway slab from and connected to said tower (that is so that said roadway slab is bending-resistant and comparatively bendingly flexible) said means including inclined stays arranged in spaced paraliel relation in vertical planes, wherein a pair of abutments are spaced apart in the elongate direction of the bridge and each being located on an opposite side of and spaced from said tower, said roadway slab members comprises a first span extending between one of said abutments and said tower and a second span extending from said tower the other said abutment, said first span is horizontally movably supported at the said one abutment, said stays comprising first stays and second stays with said stays secured at one end to one of the ends of said first and second spans spaced remotely from said tower and at the other end to the uppermost part of said tower, said second stays secured at one end to one of said first and second spans between said tower and the end of said span spaced remotely from said tower and at the other end to said tower below the point of secu rement of said first stays, said first stays are reinforced relative to said second stays, and said tower is constructed to absorb approximately two-thirds of the bending moment from one-sided live loads acting on said first and second spans by redistributing the resultants of the vertical load into the core cross-section of said tower, and said reinforced first stays absorb the remaining portion of the one-sided live loads.

2. A cable stayed bridge, as set forth in claim 1, wherein each of said first and second stays includes a plurality of elongated steel rods with an average yield strength, a steel tubular member laterally enclosing said steel rods, and a cement grout filling the open spaces within said tubular member and around said steel rods after prestressing rods for effecting a bond between said steel rods and said steel tubular member.

3. A cable stayed bridge, as set forth in claim 1, wherein said roadway slab member is formed of an upper slab, a lower slab, and a plurality of vertically extending support ribs extending between said upper and lower slabs with said support ribs disposed in laterally spaced relation and extending in the long direction of the bridge and dividing the interior of said roadway slab member into a plurality of hollow cells, and said vertical support ribs having a relatively low height.

4. A cable stayed bridge, as set forth in claim 2, wherein said roadway slab member is formed of an upper slab, a lower slab, and a plurality of vertically extending support ribs extending between said upper and lower slabs with said support ribs disposed in laterally spaced relation and extending in the long direction of the bridge and dividing the interior of said roadway slab member into a plurality of hollow cells, and said vertical support ribs having a relatively low height.

5. A cable stayed bridge, as set forth in claim 3 or claim 4, wherein tension members located within said lower slab and extending transversely of the long direction of the bridge, and said tension members are prestressed.

6. A cable stayed bridge, as set forth in claim 5, wherein said tension members comprise first tension members extending across approximately the full width of said lower slab and said first tension members being anchored close to and inwardly from the longitudinal extending outside edge of said lower slab, and second tension members having a length less than the length of said first tension members with said second tension members being anchored in said lower slab in the lower region of said vertical support rib spaced laterally inwardly from the outer longitudinal edge of the lower slab.

7. A cable stayed bridge as set forth in any of claims 3 to 6, wherein the upper surface of said upper slab slopes outwardly and downwardly from the center line to the longitudinal outer edge of said roadway slab member, and the lower surface of said lower slab is a flat horizontal surface.

8. A cable stayed bridge, as set forth in any of claims 3 to 7, wherein said roadway slab member has a number of anchorage locations spaced apart in the long direction of the bridge along the longitudinally extending sides of said roadway slab member, each of said anchorage locations has an upwardly extending projection extending upwardly from the upper surface of said roadway slab member and a downwardly extending projection on the lower surface of said roadway slab member, each of said stays having the steel tubular member thereof extending into and anchored within said upwardly extending projection and the steel rods thereof extending from the end of said tubular steel member into the downwardly extending projection and said steel rods being anchored within said downwardly extending projection.

9. A cable stayed bridge as set forth in any of claims 3 to 8, wherein said roadway slab member comprises a Vierendeel truss.

10. A cable stayed bridge substantially as described with reference to and as illustrated in the accompanying drawings.