Apparatus for erecting a bridge arch

March 26, 1968

H. J. MEHEEN

3,374,497

APPARATUS FOR ERECTING A BRIDGE ARCH

Filed June 1, 1965

Sheets-Sheet 1

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A ttorneys

March 26, 1968 h. j. meheen 3,374,497

APPARATUS FOR ERECTING A BRIDGE ARCH

Filed June 1, 1965 4 Sheets-Sheet 2

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A ttorneys

March 26, 1968

H. J. MEHEEN

3,374,497

<img file="US3374497A_D0003.tif" />

Attorn eys

March 26, 1968

H. J. MEHEEN

3,374,497

APPARATUS FOR ERECTING A BRIDGE ARCH

Filed June 1, 1965

Sheets-Sheet 4

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INVENTOR.

³³ Homayoun J. Meheen

BY

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Attorneys

United States Patent Office

3,374,497

Patented Mar. 26, 1968

3,374,497 APPARATUS FOR ERECTING A BRIDGE ARCH Homayoun J. Meheen, 6464 N. 14th Ave., Denver, Colo. 80214 Filed June 1,1965, Ser. No. 460,170 11 Claims. (Cl. 14—26)

ABSTRACT OF THE DISCLOSURE

A method and apparatus for erecting an arch bridge, wherein each half of the arch is erected separately as a seif 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 tne tie rods and the tie back cable downwardly and in alignment with the tower insofar as possible to proon 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 cantiievered from an abutment. The invention is especially applicable for erecting the primary arch members of an arch bridge and for supporting 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.

Anotner 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 or a bridge arch, which permits erection of the span to proceed m an efficient 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 uic 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 wuiie it is being 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 incomnleted “^ldge i^irC1 °,^r.^simi!ar 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-to-use means for releasing one tie as another is ®^e.’P^erm'^{is ea}sy adjustments to the position of the ^rld°^{e a}5^{cb as 1S being cons}t^ructed, which is a lightweight, balanced structure capable of being quickly and it is nnr^reCte^ α¹¹⁴ T T-^{lckIy and easi!y dism}antled after it is not needed, and which permits a bridge to be erected 35 at a reasonable cost.

Further objects of the invention are to provide for a ?°;^e! ^and miproved head for a tie back tower for holdof’tie?^^{1 s}?^{pportlng tlss}&gt; ^which may hold a plurality , . . . -------— ™ °^£, ^e ,er m sequential arrangements or in erouns duce, a minimum of eccentricity of compressive loading ⁴θ which provides a simple and easy means for adding on tne tower when a maximum null .....moving and adjusting the length of individual tie mem’ the^toZeSsrif^to^{5 MbaIanced bend}mg reactions on one H i ¹ λ ^{the su}PP°^rtlng loads are shifted from one tie to another and which is a simple, easily-built lo” 45 cost arrangement of components.

hail ;^{Urtller objeci of the} invention is to provide in a tie in- aid^Wadiu-Hn ^improY^{ed ar}™gement for holdWith the Λ _l^oad-supportmg ties mounted therein. With the foregoing and other objects in view all of ⁵⁰ Sis ”X^ein^fUc^{y h}n^{nafter my} invenlfon^ommfnt ^c&lt;,^rtailC0‘^lst™^cil0ns&gt; combinations and arrange“^efi^nts °^f Γ ^{aDd eIements} as hereinafter describld emh H-^{ln tbe}.^apP^ended claims and illustrated in preferred

FlcSr ^the,^accompa^mg drawing in which:

1GURE 1 is a diagrammatic, transverse section of the upper portion of a canyon whereacross an aS“ bridge is being erected, the figure illustrating further a side elevational view of arch members of the bridge,’ a highhne and an improved arrangement of tie back towers 60 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 arch 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 70 3 at FIG. 2 but on an enlarged scale, and illustrating one manner of interconnecting prefabricated structural segments forming the arch.

3.374.497

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 an enlarged 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-constructed bridge arch A, illustrated at FIG. 1, exemplifies one application of the invention. It is adapted to span a canyon C and to be supported upon abutments 20 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 5 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 2 5 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 20 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 25 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 30 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 procedures 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 40 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 ⁰ 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 ₅θ 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 affixed 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 „₀ 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 rigidify 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.

„θ It follows that the critical factor of design of the tower 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 75 back cables 32 and also resist bending moments acting

3,374,497 ίο uppn the leg because of any eccentricity of the reaction points of these cables with respect to the axis of the Ibeam 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 40 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 30 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 ------------uuw width at the base of the head to a rounded end. A hole 35 ^a conventional fluid pressure-supply source, not shown ex45 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 . - _____________ ______uuo tie back cable to effectively align the pull of the tie back 40 rod extends through a central passageway in the cap 69 cable 32 with the pull of the ties 31, to produce a re- * ----*-----^J — ’ -......

action 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 75

55, of each rocker plate includes a pair of half-round rocker bearings, 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 40. 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 60 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 efficiently 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 _ --------- otijLzpij 0MU1VU, UUl auuwu, CA tending 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 — --- Λ------o— · —J MXV ^VUF 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 70 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 5j. 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 suacers 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 53.

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 60 is then replaced on rod 53 to bear against the head 70 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 hold60

3,374,497 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 apparatus above described, is now described. Normally, the upstream and downstream arches will be erected together, with ties 31 extending from the two legs of the tower. As the frame segments of each arch are assembled, the cross braces 24 and diagonal braces 25 are set in position to keep the system laterally rigid, and such will be inferred in the following description. Also, preferably, both sides of the bridge arches will be erected at the same time, but with one side being ahead of the other so that the one side may use inner and intermediate ties 31 as they are removed from the other.

The drawing exemplifies a 16-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 20 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 to the head of the tower to be affixed 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 sufficient 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 sufficient 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 outer ties 31 will be tightened sufficiently so that each half-span will be well above its final closing position and separated from the other. A gradual, controlled release of the tension in each outer tie will permit an easy, con8 trolled lowering of the spans to their final closure at the crown 23.

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 effect 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 80 which is offset from the center line 81 of the beam web to produce a bending moment illustrated as being a counter-clockwise moment vector 82. The bending moment, dependent upon the product of the resultant and the offset 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 80 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 of the 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 37α is illustrated at FIG. 14 where the saddle 44α of the plates 40α is formed as a series of distinct inclined steps 85, 86 and 87, with each step being sloped at a position normal to the slope which the respective inner, intermediate and outer ties, carried by that head 37α 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 42α, 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 43α 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 flat saddle portions 85, 86 and 87, a different form of rocker plate 54α 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 portion 87.

I have now described my invention in considerable detail; 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 only by the proper scope of the appended claims.

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