Apparatus for erecting a bridge arch

Abstract

Claims

March 26, 1968 H. J. MEHEEN 3,374,497 -APPARATUS FOR ERECTING A BRIDGE ARCH Filed June 1, 1965 4 Sheets-Sheet l INVENTOR. Homayoun J. Meheen zl/ zw g/it A Horneys March 26, 1968 H. J. MEHEEN APPARATUS FOR ERECTING A BRIDGE ARCH 4 Sheets-Sheet 2 Filed June 1, 1965 mm bl mm mm l mm INVENTOR. Z Z/ e 7 a m 0 H A Horneys March 26, 1968 H. J. MEHEEN 3,374,497 APPARATUS FOR ERECTING A BRIDGE ARCH Filed June 1, 1965 4 Sheets-Sheet 5 INVENTOR. Homayoun J. Meheen a wi /Z401 Attorneys March 26, 1968 H. J. MEHEEN APPARATUS FOR ERECTING A BRIDGE ARCH 4 Sheets$heet 4 Filed June 1, 1965 INVENTOR. H o mayoun J. Meheen Wz/Q Attorneys United States Patent 3 374,497 APPARATUS FOR EliECTING A BRiDGE Aiifiii Homayoun J. Meheen, 6464 N. 14th Ave, Denver, Colo. 86214 Filed June 1, 1965, Ser. No. 469,170 11 Claims. (Cl. 14-26) AESTRACT OF THE DECLOSURE A method and apparatus for erecting an arch bridge, wherein each half of the arch is erected separately as a self supporting, cantilevered structure by holding it with rods extending from the top of an erection tower upstanding from the bridge abutment. Each tower is laterally stabilized and held by a tie back cable extending to an anchor behind its abutment. Completion of the erection is effected by bringing the arch halves together at the crown of the arch. Each half of the arch is formed by erecting and joining sequentially an array of segments commencing with a first segment at the abutment at the base of the tower. A tie rod extends from the top of the tower to hold the completed arch portion of segments in place and to permit the next segment in the outward sequence to be erected and secured to the completed segments to cantilever outwardly therefrom. Another tie rod is then extended to this outward segment and pulled to lift the completed arch portion to permit the first mentioned tie rod to be released. The operation is repeated by the construction of another outward segment and replacement of ties until the arch half is completed. As a structural feature, the tower is provided with a head having a jack supporting saddle, to hold the several tie rods in predetermined positions and to permit easy shifting of a pulling jack from one tie rod to another. The saddle is arranged to direct the resultant pull forces of the tie rods and the tie back cable downwardly and in alignment with the tower insofar as possible to produce a minimum of eccentricity of compressive loading on the tower when a maximum pull is occurring, with the completed arch half. This invention relates to methods and apparatus for erecting bridges and similar structures, and more particularly to apparatus for supporting a partially-completed bridge span while it is cantilevered from an abutment. The invention is especially applicable for erecting the primary arch members of an arch bridge and for su porting the opposing sides of an arch before it is completed and the span closed. Accordingly, the invention will be referred to as a method and apparatus for erecting a bridge arch and will be described in connection therewith. However, such is not to be construed as a limitation, for the invention may be used to erect other similar structures. A primary object of the invention is to provide a novel and improved towers and tie members for holding the sides of an incompleted bridge arch While they are outreaching from their abutments and for permitting the components forming the bridge arch to be assembled and set in place by a simple, logical sequence of operational steps. Another object of the invention is to provide a novel and improved arrangement of towers and tie members for holding the opposing reaches of an incompleted span of a bridge arch, which permits erection of the span to proceed in an efiicient manner and which, at the same time, completely eliminates the need for temporary under supports, such as underpinning or cribbing. Accordingly, the invention is especially adapted to facilitate the construction of an arch bridge over a deep, inaccessible canyon. Another object of the invention is to provide a novel and improved construction of towers and tie members for holding an outstanding side of an incompleted bridge arch and an improved method for using these members which permits a bridge arch to be efficiently erected in situ with prefabricated segments of the bridge arch, to extend each side of the structure towards the center, and with the segments being of a size which are easily carried and positioned for erection by a highline trolley. Another object of the invention is to provide a novel and improved apparatus for holding an outreaching side of an incompleted bridge arch during its erection which incorporates a tower at the abutment from whence the incompleted side of the bridge arch outstands, and an improved and simplified arrangement of supporting ties extending outwardly and downwardly from the head of this tower to connect with and support the bridge arch While it is bein constructed. Other objects of the invention are to provide a novel and improved tower and head structure thereon for holding support ties which extend downwardly and outwardly from the head of the tower to support an incompleted bridge arch or similar structure, which permits the ties to be changed in a simple sequential manner as the construction of the bridge progresses, which provides a simple, easy-tomse means for releasing one tie as another is set, which permits easy adjustments to the position of the bridge arch as it is being constructed, which is a lightweight, balanced structure capable of being quickly and easily erected and as quickly and easily dismantled after it is not needed, and which permits a bridge to be erected at a reasonable cost. Further objects of the invention are to provide for a novel and improved head for a tie back tower for holding structural supporting ties, which may hold a plurality of ties, either in sequential arrangements or in groups, which provides a simple and easy means for adding, removing and adjusting the length of individual tie members, which minimizes unbalanced bending reactions on the tower itself as .the supporting loads are shifted from one tie to another and which is a simple, easily-built, lowcost arrangement of components. A further object of the invention is to provide in a tie back tower, a novel and improved arrangement for holdmg and adjusting load-supporting ties mounted therein. With the foregoing and other objects in view, all of which more fully hereinafter appear, my invention comprises certain constructions, combinations and arrangements of parts and elements as hereinafter described, defined in the appended claims and illustrated in preferred embodiment in the accompanying drawing in which: FIGURE 1 is a diagrammatic, transverse section of the upper portion of a canyon whereacross an arch bridge is being erected, the figure illustrating further, a side elevational view of arch members of the bridge, a high line and an improved arrangement of tie back towers and ties for holding the bridge arch portions to permit the erection of the bridge arches to proceed in situ. FIGURE 2 is an isometric view of a portion of the showing at FIG. 1, as taken from the indicated arrow 2 at FIG. 1, but on an enlarged scale to better depict a partially-constructed side of an upstream and downstream bridge arc-h as being supported by the tower. FIGURE 3 is a somewhat diagrammatic side view of a fragment of an arch, as taken from the indicated arrow 3 at FIG. 2 but on an enlarged scale, and illustrating one manner of interconnecting prefabricated structural segments forming the arch. FIGURE 4 is a side elevational view of a tower and a partially-completed side of a bridge arch, similar to the showing at FIG. 2, but on an enlarged scale, with the additional arch segments being connected to the side and with another support tie being added, the broken lines indicating the positioning of additional arch segments and another tie which are to be added to complete the side of the arch and the raising of the arch side to depict certain operational steps in the erection of the bridge. FIGURE 5 is a side elevational view of the head of the tower, similar to that portion of the showing at FIG. 4, but on anenlarged scale. FIGURE 6 is an end elevational view of the head of the tower as taken from the indicated arrow 6 at FIG. 5, but on a slightly enlarged scale. FIGURE 7 is a sectional, elevational view of the head as taken substantially from the indicated line 7- 7 at FIG. 6. FIGURE 8 is an outline of the tower head with the innermost tie cable being installed thereon and showing in heavy lines a representative vector diagram indicating the forces and'bending moments acting upon the tower when this inner tie cable is holding the incompleted side of the bridge arch. FIGURE 9 is an outline and diagram similar to FIG. 8, but with the outermost tie cable being installed thereon and with heavy lines showing a representative vector diagram indicating the forces and bending moments acting upon the structure when this outer cable is holding the side of the bridge arch. FIGURE 10 is a longitudinal view of an upper portion of a tie within the head and a sectional view of a portion of the head and of the holding and adjusting apparatus associated with the tie and head, as taken from the indicated line 10-10 at FIG. 7, but on an enlarged scale. FIGURE 11 is an isometric view of the upper portion of the tower head, per se. FIGURE 12 is an isometric view of a tie holding block, per se, which is adapted to receive a tie and to bear against the tower head. FIGURE 13 is an isometric View of a jack saddle, per se, for holding a tie pulling jack. FIGURE 14 is a side elevational view, similar to FIG. 5, but, showing an alternate construction of the tower head member and of support blocks holding the ties. Referring more particularly to the drawing, the partially-constructedbridge arch A, illustrated at FIG. 1, at opposite sides of the canyon. The bridge arch is formed as a series of segments 21 interconnected together to constitute right and left half-arch sides 22R and 22L which commence from their respective abutments and come together at the arch crown 23. The right half-arch 22R is illustrated as being completed by the interconnection of eight selected arch segments 21, while the left half-arch 22L is illustrated as being partially completed with only four segments 21 being in position, but with a fifth'segment being transported to its erection position by the trolley of a highline H. The highline need not be described, for it is a conventional unit installed before other construction begins and extends across the canyon above all other constructions. A completed bridge will require at least two arches A which are substantially identical in form and are disposed side by side in spaced-apart parallel, vertical planes traversing the canyon. One may be referred to as an upstream arch and downstream arch, as illustrated at FIG. 2. The two arches are held apart by cross braces 24 and diagonal braces 25, all of which are suitably secured in their proper place as erection proceeds, preferably by erection of both arches simultaneously. Each arch frame is formed by individual prefabricated segments 21, and each segment is light enough to be lifted and carried to position by the highline trolley. The segments forming one side of the arch, as illustrated at FIGS. 2 and 3, are interconnected in tandem sequence, end to end, commencing with the first segment adjacent to the abutment 20. Each segment is connected to the next by plates 26 which lap the abutted ends of the segments and the plates are secured to the segments by bolts or field rivets 27, as in the manner suggested at FIG. 3. Structurally, an arch frame theoretically resists a uniformly applied load across its span in compression and without bending moments acting upon the arch itself. However, as a practical matter, each arch span must resist substantial bending moments because of unbalanced loading during the construction of the span and also when the bridge is being used. Accordingly, each arch segment is designed, not only to resist compression, but also bending moments. Likewise, the connections, at the plates 26, must resist such bending moments. This resistance to bending by the segments and the connections thereof permits each side of the bridge to be erected in situ by supporting a partially-completed span with holding ties. Moreover, a substantial portion of each side of an arch may be cantilevered beyond any outermost supporting tie. Each side of a bridge arch is thus erected and is held by ties until it extends outwardly and over the canyon to the crown 23 to there be connected and closed with the other side of the arch. When both the upstream and downstream arches are erected and closed and all cross braces 24 and diagonal braces 25 are in place, the final bridge construction is completed by providing a road-supporting framework upon the arch frames, finishing the road abutment at each side of the bridge and laying the final road itself over the bridge. Such features are not shown nor described since they do not form part of the invention. The present invention resides in apparatus and pro cedures to simplify the erection of the upstream and downstream bridge arches. This apparatus includes a tower 30 at each side of the canyon which upstands from the respective bridge abutments 20. Cable ties 31 extend from the top of each tower to connect with and support a side of each bridge arch over the canyon, and tie back cables 32 extend in the opposite direction, away from the canyon, to hold the tower upright against the pull of the cable ties 31. Each tower 30 includes a pair of upstanding legs 33 which are in spaced parallelism and define the plane of the tower which is ordinarily normal to the plane of bridge arches. The legs 33 are suitably spaced apart to permit the base of each to rest upon its abutment 20 in line with the vertical plane of an upstream or downstream arch. Cross braces 34 and diagonal ties 35 hold the legs together as a laterally rigid framework. A ladder means, not shown, is provided for climbing the tower, and a working platform 36 may be located between the tops of the legs to provide access to cable-holding head members 37 afiixed to the top of each leg. Each tower leg is shown as an I-beam having its web 38 in the plane of the bridge arch and normal to the plane of the tower. The flanges 39 are thus parallel to the plane of the tower. The web of each I-beam not only lies in the plane of its bridge arch, but its centerline axis lies in the central plane of the tower. With this arrangement, a maximum stability of the legs 33 is obtained for any given tower height, for the braces 34 and ties 35 rigidity each I-beam leg against buckling in the direction transverse to the plane of the web, while the greater strength of the I-beam leg, in the plane of the web, resists buckling of the tower in the unsupported direction normal to the plane of the tower. It follows that the critical factor of design of the will concern bending or buckling in this direction, normal to the plane of the tower. Each leg must be heavy enough to prevent its buckling under the compressive loads imposed upon the tower by the cable ties 31 and the tie back cables 32 and also resist bending moments acting tower upon the leg because of any eccentricity of the reaction points of these cables with respect to the axis of the I- beam web. It is important to keep eccentricity of the loading to a minimum to minimize the size and Weight of the tower leg, and the novel construction of the head members 37 accomplish this desired result. Each head 37 is formed as a pair of comparatively heavy, flat plates mounted at the top of a tower leg 33. The plates are set in spaced parallelism with and at each side of the web of the I-beam forming the leg. The top of the leg 33 is positioned between the lower portion of the plates so that the plates overlap this leg and are welded or otherwise secured to the edges of the beam flanges 39. The upper portion of these spaced-apart plates 40, above the leg 33, are also reinforced and held apart by suitable transverse clip members 41 positioned along the edges of the plates Where they Will not interfere with ties or cables. The ties 31 and the tie back cables 32 are attached to this head 37, and they extend between the plates 40, as Will be described. It is contemplated that if the flanges 39 of the beam forming the legs 33 space the plates too far apart, as where a wide-flange I-beam is used, the flanges may be cut and narrowed where they hold the plates. The general side configuration of each head 37 includes an upright post 42 at the side adjacent to the bridge arch for connection with ties 31 and a spur 43 outstanding from the opposite side of the tower leg for connection with the tie back cable 32. The outward edge of the post 42 is straight and upright, while its inner edge, at the portion in the crotch between the post and spur, is curved and sloped to form a saddle 44 to properly receive tie connectors, as hereinafter described. The outstanding spur 43 is tapered from a maximum width at the base of the head to a rounded end. A hole 45 at the end of the spur extends through each plate to hold a pin 46 which, in turn, holds a clevis link 47 connecting with an eye 48 in the tie back cable 32. The configuration of this spur is correlated with the slope of the tie back cable to effectively align the pull of the tie back cable 32 with the pull of the ties 31, to produce a reaction on the tower legs which is substantially at the web axis of the beams forming the legs. It is to be noted that the tie back cable 32 must be at a predetermined slope to produce such a selected reaction at the tower head. Such involves extending the tie-back cable a proper distance from the tower depending upon the ground levels. At the selected position, it is anchored in any suitable manner in a concrete block 49, as illustrated at FIG. 1. Each head 37 and its ties 31 are formed and arranged to permit the construction of a specified bridge arch to proceed in a predetermined sequence of operations with selected ties holding selected segments of the bridge arch, as will be further described. Each tie 31 is formed as a cable of selected length which includes a means to connect with the bridge segment. Such means may be an eye 50 at the lower end of the cable, a clevis 51 on the selected bridge arch segment 21, and a removable pin 52, as illustrated at FIG. 3. The upper end of each cable 31 is adapted to extend upwardly from its segment connection to the top of the tower and it terminates as an extended, threaded holding rod 53. This rod 53 is adapted to extend through and beyond the plates forming the tower head at the same inclination which the tie assumes when it is connected to its proper segment of the bridge arch and extended to the top of the tower. It is contemplated that the rod will be secured to the saddle of the head at a selected position where the slope of the saddle will be substantially normal to the inclination of the tie rod. The rod is held on the saddle by a rocker plate 54 and slidably extends through a central orifice 55 in this plate. The rocker plate is a substantially square member, having a transverse width sufficient to overlap the plates forming the head 37. The underside of each rocker plate includes a pair of half-round rocker hearings, in common transverse alignment at each side of the central orifice 55, so that each bearing will rest upon the saddle 44 with a portion thereof overhanging the plate 44). This permits the rocker plate 54 to be self aligning to avoid any bending of the rod 53 due to its possible misalignment on the saddle. To further insure the proper positioning of the rocker plate 54 on the saddle, opposing pairs of finger plates 57 are secured to the outer sides of each head plate 40, as by bolts 58, to embrace the overhanging portions of the rocker bearings. A holding nut 59 is turned on the threaded rod 53 to bear against the outer face of the rocker plate 54 to hold the tie in tension and a lift nut 66 may be turned upon the rod to secure the holding nut in position and to effect pulling of the rod, as hereinafter described. Each tie is adapted to be pulled to lift and adjust the position of the bridge arch it is supporting, and the holding nut 59 may be turned and set as such adjustment proceeds. Such adjustment is by a jack 61 set over the extended end of the tie means, as will be hereinafter described. The pulling and releasing of a rod 53 of a tie 31 to raise or lower the arch span it supports is conveniently and efiiciently accomplished by a double-acting hydraulic jack 61 of an improved type characterized by having an axiallycentered passageway 62 extending through it, which is adapted to receive the rod 53. This jack, illustrated in section at FIG. 10, is formed within an outer cylindrical body 63 upstanding from a base end 64. A concentric inner cylindrical sleeve 65 extends from this base and through the body of the jack to define the passageway 62 therethrough. A tore-shaped piston 66 is adapted to move in the annular space between the cylinder and sleeve to extend and retract responsive to hydraulic pressure from a conventional fiuid pressure-supply source, not shown, extending into lines 67 and 67', respectively, at opposite ends of the cylinder. A tubular rod 68, connecting with the piston 66, slidably embraces the sleeve 65. The outward end of this rod extends through a central passageway in the cap 69 at the outward end of the cylinder opposite to the base 64. The rod 68 extends beyond this sleeve to terminate as a sliding head 7%) having a central passageway 62 therein through which a rod 53 may extend. The jack 61 is conveniently mounted upon a cradle carried by the rod 53. This cradle 71 includes a transverse base plate 72 having a central hole 73 through it to receive the rod 53 and hold the jack base 64. A pair of opposing spacers 74 outstand from one face of this plate 72 at opposite sides of the hole 73. These spacers are adapted to abut against the rocker plate 54 to provide an access space between the rocker plate and the base plate for reaching and adjusting the holding nut 59. A pair of arms 75 outstand from one edge of the other face of the base plate 72, which are adapted to conveniently hold the jack 61 in its properly aligned position upon the rod In using this apparatus, the lift nut is first removed from the rod 53. Next, the cradle and the jack are placed on the threaded rod 53. The lift nut 61) is then replaced on rod 53 to bear against the head of the jack. As pressure is applied against this nut by the jack, it causes the tie rod to commence moving to lift the side of the bridge arch to which it is connected. The holding nut 59' is then loosened and is turned to move it towards or away from the rocker plate to loosen or tighten the tie 31 as desired. Tightening of the tie rod to raise the bridge arch may be effected by continuing to extend the jack piston while the holding nut is being turned to keep it against the rocker plate until the desired position is attained. Loosening or lowering of the bridge arch follows a different procedure, for once the cradle, jack and lock nut are properly set upon the holding rod, the jack is first extended to a desired amount and the lock nut 60 placed against it. The jack is then extended to loosen the hold- Z ing nut 59 and then slowly retracted, as the holding nut is turned upon the rod to continue to loosen it. The erection of one side of a bridge arch to the point where the segments 21 extend to the crown 23, using the the one side may use inner and intermediate ties 31 as they are removed from the other. The drawing exemplifies a l6-segment type bridge arch, and the erection of one side of the upstream and downstream arches will proceed as follows: The first portion of one side of the upstream and downstream bridge arches, the first and second segments 21, may be extended from the abutment 2i and held in position by any conventional rigging since their weight is not great. The third segment is provided with a clevis 51 near its outer end, and once it is placed in position by the highline trolley and secured to the second segment, an inner tie 31 is connected to the clevis. This tie extends upwardly t the head of the tower to be afiixed to the saddle 44 by a rocker plate located at the base of the saddle 44, as illustrated. A jack 61 is then placed upon the rod to apply tension to the rod until the bridge section is at a selected position. The rod is then secured by the holding nut 59 and the jack is removed. Thence, fourth and fifth segments 21, carried by the highline trolley, extend the upstream and downstream arch portions. As they are secured in position, they cantilever from the tie-supported third segment of each arch. Each fifth segment is provided with a clevis 51 near its outer end and a second intermediate tie 31 is attached to this clevis to extend to its tower head, as illustrated at FIG. 4. The rod 53 of each second tie is positioned in its saddle above the inner tie rod at a spacing sufficient to permit clearance for placing a jack 61 over it. The jack, once positioned, pulls the second tie with snfiicient force as to lift and hold the entire side of the partially erected bridge arch and to loosen the first, inner tie. After the upstream and downstream arch portions are thus held, the first, inner ties are loosened and removed. They may then be used to erect the arch portions at the opposite side of the canyon. Subsequent erection will include the setting of a sixth and seventh segment to extend each arch. These segments will cantilever from the tie-supported fifth segment. Each seventh segment is provided with a clevis 51 near its outer end and a third, outer tie 31 is attached to this clevis to extend to its tower head, as illustrated in broken lines at FIG. 4. The rod 53 of each third tie is positioned in its saddle above the intermediate tie rod at a spacing sufficient to permit clearance for placing a jack 61 over it. The jack, once positioned, pulls the third tie with sufiicient force as to lift and hold the entire side of the partially erected bridge arch from its abutment. Thus held, the intermediate ties are then loosened and removed and may be used to erect the arch portions at the opposite side of the canyon. Subsequent erection steps will include adding the eighth segment of each arch to complete the half-span of the bridge arches to the crown 23. When both half-spans of the bridge arches are completed, they may be closed to complete the arches. Preliminary to closing, however, the trolled lowering of the spans to their final closure at the crown 23. v Ideally, the spur 43 of the head reaches outwardly and upwardly to align the tie back cable 32 with a tie 31 so that the reaction point of the forces acting on the cable and tie will be at the centerline of the beam web 38 of the tower leg 33 to avoid a bending moment on the tower leg. However, the use of several ties and the placing of one tie 31 above the other in the saddle of the head 37 renders this balancing of forces impossible for all tie positions encountered. To minimize the efifect of this disparity, the spur 43 is formed to direct the pull of the tie back cable to an intermediate manner, as between the inner tie and the outer tie so that, for example, a counterclockwise bending moment will act on the tower leg when the inner tie is holding the arch span and a clockwise moment will act on the tower leg when the outer tie is holding the span. Such is diagrammatically represented at FIGS. 8 and 9. When the innermost tie 31 is used, the force vectors 31' and 32' of the tie and of the tie back cable 32 produce a resultant 8%) which is offset from the center line 81 of the beam web to produce a'bending moment illustrated as being a counterclockwise moment vector 82. The bending moment, dependent upon the product of the resultant and the olfset distance 83 can be established by the loading on the tie 31. When using the outermost tie 31, the forces on the tie are considerably increased because of a heavier load. The vectors of the tie 31" and of the tie back cable 32" produce a resultant at the opposite side of the tower leg center line 81. The resulting bending moment is illustrated as a counter-clockwise moment vector 82". The bending moment, dependent upon the product of the resultant 80" and the offset distance 83" can also be established. These moments 82 and 82 can be correlated with the reactions 84 and 84" ofthe tower leg 33 so that the same critical stresses for the design of the tower leg will occur with either loading. However, such critical stresses will be a minimum since they are oppositely directed and a minimum the possible size of tower leg 33 may be used. An alternate construction of the tower head 37a is illustrated at FIG. 14 where the saddle 44a of the plates 40a is formed as a series of distinct inclined steps 85 86 and 87, with each step being sloped at a position norrnal to the slope which the respective inner, intermediate and outer ties, carried by that head 37a will assume when holding a bridge arch portion. The stepped saddle portion 85 for the inner tie 31 is adjacent to the base of the post portion of the post 42a, while the stepped portion 87 for the outer tie is at the top of the post portion to provide an overall configuration similar to that heretofore described. The spur 43a may also be slightly modified to provide for a flat top portion which facilitates placing a platform upon it. Reinforcing angles 88 may also be secured to the head plates to rigidify them or to support a structural platform mounted on the head. In using the fiat saddle portions 85, 86 and 87, a different form of rocker plate 54a may be provided as where the underside of the plate is conveniently arched across its entire section, as in the manner illustrated. Also clip angles 89 may be positioned at the bottom of each sloping saddle portion to hold these rocker plates in position, but such is 'not necessary where the sloping portions are offset as at 90, as located between intermediate saddle portion 86 and outer portion87. I have now described my invention in considerable de- 7 tail; however, it is obvious that others skilled in the art can build and devise alternate and equivalent constructions which are nevertheless within the spirit and scope of'the invention. Hence, I desire that my protection be limited, not by the constructions illustrated and described, but y by the p p r Sc pe of the appended claims. What is claimed is: 1. Apparatus for facilitating the erection of a bridge arch, of a type which is constructed by a sequence of prefabricated arch segments adapted to be interconnected together in situ, commencing with segments adapted to be set at the base of the arch and other segments cantilevered therefrom, and including, in combination therewith, a tower adapted to upstand from the bridge abutment, tie rods adapted to connect to the top of the tower and to extend outwardly and downwardly from the tower to connect with bridge segments, a tie-back cable extending outwardly and downwardly from the tower in the opposite direction to oppose the pull of the ties as they support the bridge segments, and jack means at the top of the tower adapted to selectively pull the tie rods, whereby to pull one rod to loosen another rod and to permit the bridge segments to be raised and lowered. 2. In the apparatus set forth in claim 1, wherein said tower includes a vertical leg member having a tie-retaining head at its top, the head including abutments adapted to hold a plurality of ties, one alongside the other, with each tie being held against an abutment, with a portion thereof extending from the abutment, and being adapted to be connected to the jack means when the jack means is set upon the abutment. 3. In the apparatus set forth in claim 1, wherein said tower includes a tower head at the top formed as a pair of plates held apart in spaced parallelism to permit the ties to extend therethrough, said head including a post portion adapted to receive a plurality of ties, one above the other, in a sequential arrangement and an opposing spur portion adapted to hold the end of the tie-back cable, the post portion having abutment surfaces alongside each tie position, a plurality of rocker plates with each having a hole therethrough and being adapted to rest upon an abutment surface, with the extended end of a tie being extended through the hOle in the plate and secured thereto, and said jack means being adapted to be set upon the rocker plate and connected to the extended end of a tie when pulling it. 4. In the apparatus set forth in claim 3, wherein each extended tie end is .a threaded rod and is secured to the rocker plate by a nut turned thereon. 5. In the apparatus defined in claim 1, including a tieholding head at the top of the tower having a substantially-upright post portion at the side adjacent to the bridge span and a spur portion at the opposite side, said post having a seating edge thereon adapted to hold selected ties at selected, spaced-apart positions, and said spur being adapted to hold the tie-back cable at a fixed position thereon, and wherein said spur extends outwardly from the axis of the tower a distance suflicient to hold the tieback cable at a position which extends the line of action of this cable between the lines of action of the spacedapart ties at their intersection with the axis of the tower, whereby to minimize unbalanced moments between the tie-back cable and any one of the ties which act on the tower. 6. In the apparatus defined in claim 1, including a tieholding head at the top of the tower formed as a flat, planar member lying substantially in the vertical plane of the bridge arch, with the portion adjacent to the arch forming an upright post adapted to hold the ties at selected, spaced-apart positions, one above the other, and with the portion of the head opposite to the arch forming an outstanding spur adapted to hold the tie-back cable. 7. In the apparatus defined in claim 1, including a tieholding head at the top of the tower formed as a pair of flat plates oriented in spaced parallelism and substantially in the vertical plane of the bridge arch, and with each tie lying between the head plates, the head portion adjacent to the bridge arch forming an upright post portion adapted to hold selected ties in spaced-apart positions, one above the other, between the plates, a transverse rocker plate carried upon each tie adapted to bear against edges of the post portion of the plates opposite the bridge arch, with said edge being saddle-formed to provide a sloped portion at each selected tie-holding position, which is substantially normal to the direction of the tie. 8. In the apparatus defined in claim 7, wherein the portion of each tie which extends between the plates is formed as a threaded rod and a holding nut turned thereon, and being adapted to abut against the rocker plate to hold the plate in position and to permit adjustments to the length of the tie. 9. In the apparatus defined in claim 1, wherein each tie includes a threaded portion at the top of the tower and holding nuts turned thereon, and wherein said jack means comprises a jack having a hollow body adapted to be threaded over the threaded tie portion and secured thereto by a nut turned onto the tie. 10. In the apparatus defined in claim 1, including a tie-holding head at the top of the tower formed as a pair of flat plates oriented in spaced parallelism with the vertical plane of the arch, each tie being adapted to lie between the plates, and with said end portion being threaded, a rocker plate having a central hole therethrough adapted to lie against the edges of the head plates, with the tie end extending through the hole therein and a holding nut turned on the tie and bearing against the rocker plate, whereby to secure the tie to the rocker plate and hold the rocker plate against the head plates. 11. In the apparatus defined in claim 10, including a cradle having legs and a central portion to form a U- shaped body with a central passageway through the central portion adapted to be threaded upon the end of the tie, with the legs thereof abutting against the rocker plate at each side of the holding nut, and said jack means includes a hollow jack adapted to be threaded upon the end of the tie to abut against the cradle, and a second nut adapted to be turned onto the end of the rod to bear against the jack, whereby extension of the jack against the said second nut permits the first said holding nut to be lifted from the rocker plate and to be adjusted by turning it on the tie to take-in or let-out the tie as it supports the partial bridge span. References Cited UNITED STATES PATENTS 159,315 2/1875 Fulton 14-25 886,666 5/1908 Thomas 14-26 2,415,240 2/ 1947 Fouhy 52-745 3,236,020 2/ 1966 Totfolon 52-745 FOREIGN PATENTS 3,069 12/ 1864 Great Britain. 507,893 9/ 1920 France. CHARLES E. OCONNELL, Primary Examiner. NILE C. BYERS, IR., Examiner.

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