US3394420A

US3394420A - Bridges

Info

Publication number: US3394420A
Application number: US483951A
Authority: US
Inventors: Popov Vladimir
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-08-31
Filing date: 1965-08-31
Publication date: 1968-07-30
Anticipated expiration: 1985-07-30

Description

July 30, 1968 v POPQV 3,394,420

BRIDGES Filed Aug. 3l, 1965 5 Sheets-Sheet l July 30, 1968 V, POPOV 3,394,420

BRIDGES Filed Aug. 3l, 1965 5 Sheets-Sheet 2 i ,4770 Ey July 30, 1968 Y v PQPOV 3,394,420

BRIDGES Filed Aug. 31, 1965 5 Sheets-Sheet 5 INVENTOR. 25 25 m40/MH? ,D0/20V United States Patent O 3,394,420 BRIDGES Vladimir Popov, 511 W. 184th St., New York, N.Y. 10033 Filed Aug. 31, 1965, Ser. No. 483,951 15 Claims. (Cl. 14-42) ABSTRACT F THE DISCLOSURE A prestres'sed lift bridge having supporting members spaced inwardly from each end to form cantilever ends of said bridge, each supporting member resting on piers and lift means, comprising cables to lift said support-s with the end of said cables extending to the outer ends of said bridge and counterweights-equalizers suspended from the ends of the cantilevered portions.

This invention relates to lift bridges.

Lift bridges comprising counterweighted vertically movable spans are well known to the art. It is an object of this invention to provide improved lift bridge constructions.

Other objects of this invention will be apparent from the following detailed description and claims.

In accordance with one aspect of this invention, there is provided a lift bridge of cantilevered construction. In this aspect of the invention there is also a unique cooperation, to be explained below, between the cantilevered construction and locking devices at the ends of the cantilever portions of the span.

Another aspect of the invention relates to the novel feature of applying the downward force of the counterweight to a portion of the span in such a manner as to prestress the span against deflection resulting from liveloads thereon.

Still another aspect of this invention relates to the arrangement of counterweight ropes so that high lift towers are unnecessary.

Another feature of this invention relates to the provision of a laterally adjustable counterweight. This is particularly suitable yfor counterin-g the effects of high wind loadings.

Still another aspect of the invention deals with an arrangement for prestressing the movable span of the bridge by applying a horizontal prestress to a d-epending column attached to that span.

Yet another aspect of the invention relates to an arrangement for equalizing the effects of temperature changes.

Another aspect of the invention relates to the prestress` ing of the span of the bridge by the application of an upward force between the ends of the span while' restraining the ends of the span.

According to another feature of the invention, provision is made for removal of the bearing pres-sure between the movable portion of the bridge and the footings which bear the live load.

Still other aspects ofthe invention will be apparent from the following detailed `description and claims, and the accompanying drawing.

In the drawings, which are generally schematic,

FIGURE 1 is a simplified side view (partly in section) of a lift bridge showing, at the right, the movable span of the bridge in its normal lowered position and at the left, in phantom, the movable span in its raised position.

FIGURE 2 is a simplified end view (partly in section) of a column attached to the movable span of FIG. 1, and illustrates in somewhat greater 'detail the support for the movable span; here too the raised span is shown in phantom.

FIGURE 3 is a simplified end view (partly in section) of the end of the cantilever portion of the span of FIG. l, illustrating the counterweight construction and the locking apparatus.

FIGURE 4 is a cross-sectional vie-W showing the plate girder construction of the span of FIG. 1.

FIGURES 5 and 6 are schematic views of the lower portion of a column, showing the fixed trac-k for movement of the column and a temperature-equalizing and prestressing means which applies pressure between the track and the lower part of the column, FIG. 5 being a side view and FIG. 6 being a plan View, partly in section.

FIGURE 7 is a simplified View, partly in section, of details of a bearing which supports the live load of the movable span.

FIGURE 8 is a side View, partly in section, of a modification of the bridge of FIG. l, in which piers are ernployed to support the driving mechanism.

FIGURE 9 is a side view, partly in section, of a modification in which cantilevers are not employed.

FIGURE l0 is similar to FIG. 9, but shows the span in raised position.

In the embodiment illustrated in FIGS. 1 to 7 of the drawing, the movable span 9 of the bridge includes a pair of spaced supporting members or vertical columns 11, fixed to a center portion 12 (extending bet-Ween the spaced col-umns and over a waterway 13) and to two cantilever portions 14, extending from each end of the center portion 12.

The approaches on both sides of the bridge comprise roadways 15, suitably supported, as on a series of piers 16, and terminating above end piers 18. These end piers are preferably so constructed as to receive counterweights 19 in a manner to be described below.

The lifting and counter-balancing system associated with each column 11 of the bridge (as illustrated schematically in FIGS. 1 and 2) includes a rope 21 having one end fixed to the base 22 of a column at an attachment point 23, and its other end attached to the counterweight 19. The rope passes upward and over a rotatable drive roll 24 (which may be -a lgrooved sheave) having a fixed axis of rotation situated atop a fixed support which may be at ground level, then down and under a second rotatable sheave 25 mounted at the base of the supporting column 11, then upward over a third rotatable sheave 26 also mounted on the movable span of the bridge, near the upper part of the column 11, and then more or less horizontally to, and over, an outer sheave 27 mounted at about the outer end 28 of the cantilever portion 14.

The drive lroll 24 is adapted to be driven, through suitable reduction gearing, by a motor 31 which may be mounted at ground level also. When the motor is driven to rotate the drive roll in the direction shown in the arrow at the right side of FIG. 1, the length of rope between the drive roll 24 and the attachment point 23 is shortened, thus raising the bridge, as shown in phantom at the left of FIG. 1. At the same time, the tension exerted by the counterweight on the rope keeps taut the length of rope between the drive roll and the counterweight, and the counterweight falls. The rope 21 is under high tension, since it carries the weight of the bridge :and counterweight, and it therefore presses against the surface of the `drive roll 24 with considerable force, which makes for adequate friction between the rope and the drive roll to enable the rope to be driven by the rotation of the drive roll. As can be seen from FIG. 1, when the movable portion 9 of the bridge rises the counterweight 19 falls an equal distance, so that the center of gravity of the system of counterweights 19 and movable portion 9 remains at a substantially constant height. The total Weight of the counterweights is advantageously about equal or slightly less than to the total weight of the movable span, generally about 97% of the weigth of the span. Thus, the mechanism for raising and lowering the movable portion of the bridge essentially need merely overcome the frictional forces.

To minimize tilting forces, there are advantageously two counterweights, one at each end of the movable portion 9 and four ropes, two for each counterweight, the ropes and sheaves being laterally spaced and arranged symmetrically at both sides of the bridge, as shown in FIG. 2. As is conventional in lift bridges, provision may be made for varying the weights of the counterweights to keep them in balance with the movable span of the bridge; for example, when the weight of the movable span is increased owing to a snow deposit on the bridge, the weights of the counterweights should be increased accordingly, under the control of the bridge operator. Controls of a standard type may be employed for synchronizing the drive motors to insure even lifting at the four ropes.

In one aspect of this invention provision is also made for varying the weight distribution of each counterweight to compensate for tilting forces, e.g., for vary high wind loads, so as to facilitate lifting of the bridge even in strong winds. To this end, each counterweight 19 includes an auxiliary weight 33 (FIG. 3) mounted for lateral movement on the main body of the counterweight (as on tracks 34), and there are suitable means for shifting the position of this auxiliary weight in the directions shown by the arrows in FIG. 3; the shifting means may, for example, comprise an electric motor 35 mounted on the main body of the counterweight 19, connected to the auxiliary counterweight in any suitable manner as by a gear and rack arrangement, and having its controls in the oce of the bridge operator. If desired, the movement of the auxiliary weight may be effected automatically by making the motor 35 responsive to suitable means for measuring the tilting forces on the movable span, c g. a strain gauge 36 (shown schematically) mounted on the movable span 9.

When the bridge is in its normally lowered position, the ends of the cantilever portions 14 rest on the tops of the end piers 18. Suitable locking devices 38 (FIG. 3) are provided to secure the cantilever ends 28 to the free ends of the stationary approach roadways 15. These locking devices may be of the usual type; thus there may be a horizontal lock bolt or pin 39 supported for horizontal movement along its axis at the end of the roadway and transverse thereto, the pin being releasably engaged in a corresponding recess 40 (of slightly larger diameter than the pin) at the end of the cantilever. Advantageously, there are four such locking devices, one at each side of each end pier.

Each end pier is yadvantageously constructed to receive the counterweight 19. The pier may be made hollow so that the counterweight is completely enclosed (except at the top of the end pier); this helps to avoid undesired displacement of the counterweight by strong winds or other agencies, and gives the bridge a neater appearance, the counterweights being thus hidden from View. Alternatively, each end pier may be constructed of two latterally spaced columns with the counterweight moving up and down between these columns, as on vertical tracks. It will be appreciated that a pit may be provided to receive the counterweight at the bottom of its travel.

The dead load of the movable span of the bridge is substantially supported by the counterweight ropes, which are supported on the drive rolls 24. The live load on the bridge is supported on suitable footings 43 (FIG. 2) which may be at about ground level, the columns 11 having transverse girders 44 which rest on the upper portions of these footings.

The construction of the movable span is advantageously such that it behaves as a unit in response to forces applied thereto. One preferred form is a plate girder type of span. With such a structure, the presence of the cantilever portion makes it possible to substantially decrease the weight of the girder for the center portion; that is, a center span of the same length would need a heavier girder if the cantilever portions were not present. Also, when the center portion 12 is under a heavy live load, there is a tendency for the center portion to be deflected downward at the middle, and for the outer ends 28 of the cantilever portions to rise accordingly. (The height of the span atop the columns 11 is substantially constant when the span is in its normal, unlifted, position.) Any such tendency for the ends 28 to rise is, however, resisted by (a) the counterweights 19 which hang from the cantilever portions, and (b) the locking devices 38. Accordingly, the tendency for deflection at the middle of the center portion of the span is greatly reduced. The cantilever portions, counterweights and locking devices thus help to x or rigidify the main portion of the span. In effect, the presence of the counter weights on the cantilever portions pre-stresses the span of the bridge.

The end piers 18 are advantageously so constructed 'e.g. of suitably reinforced concrete which can resist tension forces) and the locking devices 38 are so anchored to these end piers that the locking devices can resist any upward forces on the ends 28 of the cantilever poritons resulting from live loads on the center portion of the span. It will be appreciated that the pins of the locking devices ordinarily move into locking engagement with the cantilever ends 23 at a time when there is substantially no live load on the bridge; that is, the locking is effected when the previously raised, and empty, span has been lowered to its normal position, just before the resumption of the movement of traffic on the bridge.

The presence of the cantilever spans 14 also makes it possible to return the bridge to its normal position more rapidly and smoothly without damage after it has been lifted. Each cantilever span has inherent flexibility and it can deflect to absorb the energy of the impact of its end 28 on the end pier 18 or locking device 39 when the span is dropped quickly from the raised position; the end 28 can, because of this flexibility, also be readily accommodated to the location of the locking device. This feature also makes for smooth and rapid operation at the start of the lifting process.

In a typical plate girder type of construction (shown in FIG. 4) the girder 46 is made up of a series of steel plates 47 situated in a vertical plane and running lengthwise of the span. Steel angle members 48, also running lengthwise of the span, are fastened at the upper and lower edges of the plates 47 and a steel cover plate 49, also running lengthwise of the span, is fastened to the upper angle members. It will be understood that these elements are ordinarily not made in sutiicient length to provide continuous single elements running the whole length of the span, and that the plate girder is therefore composed of a series of such elements arranged end-to-end and fastened together. Fastening may be by welding, riveting, bolting, or other suitable technique. In the illustrated embodiment, the span has two parallel plate girders, one under each side of the roadway carried by the span. Resting on the plate girders are a series of steel floor beams 50, arranged transverse to the plates and spanning the distance between the two parallel plate girders. The floor beams may rest within corresponding cut-out portions of the plates and may be rigidly fastened to the girders as by welding, riveting, etc. Carried atop the floor beams is a decking 51, advantageously made of steel and having longitudinal stiiening ribs 52, which runs the length of the span and serves to support the roadway.

In the illustrated embodiment, each of the vertical columns 11 includes a pair of laterally spaced downwardly tapering feet 54 (FIG. 2), rigidly joined by an upper supporting web S5 and by the lower transverse girder 44. The plate girders 46 on each side of the movable portion of the bridge may be welded, riveted or otherwise attached to the sides of the columns 11.

The rigid columns 11 are advantageously maintained under a stress tending to urge their lower portions in the direction of the waterway 13. In one arrangement for `accomplishing this, there is a vertical rail or track 58 (FIGS. 5 and 6) mounted in xed position, and secured to the foundation wall 59 adjacent to the outer side of each foot 54 (there being four such tracks), and each foot carries a flanged wheel 60 mounted on an axle 61 which is supported for horizontal movement with respect to the foot; for example, the axle may be received in a horizontal slot 62 in the foot, the slot being of such dimensions that this horizontal movement is relatively small (say, a few inches). Each anged wheel 60 is urged forcefully against the track 58, as by a hydraulic system including a yoke 64 engaging the axle 61 and a piston 65 attached to the yoke and operating in a cylinder 66 fixedly mounted on the foot S4 and supplied with hydraulic tluid under a constant predetermined pressure. The hydraulic line 67 leading to the cylinder 66 may include a hydraulic pump 68, a valve 69, a surge tank 71 and a manometer 72; and there may be a suitable reservoir 73 for the hydraulic fluid.

The pressure applied to the wheel 60v through the hydraulic system pre-stresses the foot 54, and the whole column 11, in a direction tending to move the lower portion of the foot towards the waterway. Since the column is rigidly attached to the movable span this also prestresses the center portion of the span upwards. When a live load is then applied to the center portion of the span, the resulting downward deflection of that center portion causes the lower portion of each foot 54 to be pressed extremely firmly Vagainst the fixed tracks 58; the force on, and deflection of, the foot is such that the foot moves to a point Where the end 75 of the slot 62 bears very firmly against the axle, so that the pressure between the wheel 62 and the track 58 is no longer determined by the hydraulic pressure in the system but is the direct,4

much higher, pressure resulting from the deflection of the foot. This horizontal reaction thus rigidifies the center portion of the span and tends to limit its deflection.

When the bridge is closed to traflic, before lifting, the foot is no longer deflected under the force of the live load, and the pressure between the wheel 60 and the track 58 is at the predetermined level which is low enough to permit the wheel to ride evenly on the tracks. This is the case even when the temperature rises and falls, changing the length of the bridge, since the slot 62 is of sucient length to Iaccommodate the movement of the foot resulting from such changes in length. The pressure in the hydraulic system may, if desired, be kept at a predetermined constant value by any suitable means or may be regulated under the control of the operator of the bridge. It will be seen that the system just described acts as an equalizer to accommodate changes in temperature. The deflection of the foot resulting from changes in temperature is a small fraction (eg. 1&0) of the deilection which would result from the live loading of the center portion of the span were the lowered part of the foot unrestrained horizontally, and the horizontal force on the wheel resulting from the spressure in the hydraulic system is much less than that resulting from such live loading (e.g. -2 kips vs. 100 kips).

It is advantageous to maintain the wheels 60 under the force of the predetermined hydraulic pressure throughout the raising and lowering movements of the bridge.

Another aspect of this invention relates to the means for supporting the -bridge load on the footings 43. Advan- 1 tageously, there are provided means for removing the bearing pressure between the movable portion of the bridge and the footings and for prestressing the movable portion of the bridge. To this end, each footing 43 may carry a pair of laterally spaced pads 77 (FIG. 7) underneath the bearing zones 77a of each transverse girder 44, which pads may be hydraulically supported; thus each pad 77 may be supported on a piston 78 resting on the hydraulic fluid 79 in a cylinder 80. The cylinder may be part of a hydraulic system including hydraulic lines 81 and 82, normally open valves 83, 84 (for use in special situations) another valve 85, a surge chamber 86, a manometer 87 and a hydraulic pump 88 and a reservoir 89 for hydraulic fluid. Advantageously each cylinder also contains a relatively small bubble of a gas, preferably a. gas substantially insoluble in the hydraulic liquid (e.g. helium). This serves to increase the elasticity and resilience of the liquid. (Similar bubbles may be present in the cylinders 66, previously described.) There are Aalso provided shims for transmitting the live load on the bridge to the footings; suitably there is a main shim 89a and a thinner, readily movable adjustable shim 89b, which may be movable manually or by any suitable motor-operated device.

After the movable portion of the bridge has been lowered to its normal position and the lock bolts 39 have been placed in their locking positions (but before there is any traflic on the bridge), the pressure in the cylinder 79 is increased to a predetermined value to apply a previously calculated upward force to the empty span, which upward force is resisted, at the ends 28 of the cantilever portions, by the locking devices. The valve is then closed and is advantageously kept closed thereafter until just before the span is to be lifted again (although, at the discretion of the bridge operator, suitable adjustments lin the pressures in the cylinders 79 may be made). Accordingly, the span is bowed upward, and prestressed against the tendency for downward deflection resulting from the subsequent liveload. The movable shim 89h is then placed in position to substantially fill the space between the bottom of the transverse girder 44 and the main shim 89a. Then trafic is permitted on the bridge; the resulting load causes the span to settle so that the bottom of the girder 44 is in firm bearing contact with the top of the adjustable shim 891;. The pads 77 naturally move downward in response to this load, since the pressure in the bubble-containing hydraulic system is insuflicient to resist the heavy live load.

Then, just before the bridge is to be lifted, when the live load has been cleared from the span of the bridge, the parts return substantially to the positions they had prior to live loading, the movable shims 89b are removed, and, thereafter, the hydraulic fluid is permitted to leave the cylinders 79, so that the bridge merely floats in balance with the counterweight. This permits the span to accommodate itself to the effects of uneven solar heating, which may cause one side of the bridge to tend to rise relative to the other side. Such a tendency, which is resisted during live loading by the bearing pressure between the transverse girder 44 and the footing 43, may cause very high vertical pressures at the lock bolts 39 making it extremely di'icult or impossible to slide these bolts out of locking position. By eliminating the bearing pressure before releasing the locks, the force on the lock bolts can be greatly reduced permitting their easy operation.

The hydraulically controlled pads 77 also serve as stress equalizers to compensate for possibly dangerous unequal stresses resulting from uneven heating, uneven settling of footings, uneven positioning of the locking devices, etc.

The effects of unequal solar heating of the movable span are also counteracted by moving the laterally movable auxiliary counterweight 33, previously described, to compensate for the lateral movement of the center of gravity of the span, in a manner similar to the movement of said auxiliary counterweight in response to wind loading.

It will also -be appreciated that the interconnection of the two cylinders through open line 81 assists in laterally equalizing the bearing pressures in these cylinders.

The cantilevered arrangement described above is particularly suitable for long-span lift bridges, such as those having a center portion (between the columns 11) on the order of about 300 feet, or more, in length. It permits the construction of a bridge which uses less steel and which is lighter, and more economical, than a conventional lift bridge. It helps avoid the use of very heavy trusses and makes possible lift bridges whose view ratio (ratio of total span to width of span at center) is well above 25:1, e.g. 48:1. The precise dimensions of the parts of the bridge will, of course, depend among other things on the weight of traffic for which it is designed, the anticipated wind loads, etc. The lengths of the cantilever portions will generally be less than those usually employed in continuous fixed bridges (in which the length of each of the outer cantilever portions is often on the order of about 70% of the length of the center portion); for example, in one design of a lift bridge according to this invention and having a 400 foot span for its center portion, the length of each cantilever portion is about 90 feet. Generally, as is usual for highway or railroad bridges, the Width of the span will -be appreciably less than its length (e.g. a fraction of its length). It will be appreciated that the economy of the bridge of this invention is particularly marked when it is used for wide roadways; here its capacity to support heavy loads with the use of less steel is especially advantageous.

Another aspect of this invention relates to lift bridges whose movable spans do not have any substantial cantilever portion. In this case, illustrated in FIGS. 9 and 10 (more suitable for shorter span bridges, e.g. spans of 100-200 feet in length), the movable span 91 has two depending vertical columns 92 (only one being shown) similar to those described in relation to the cantilevered span. At each end of the span there is a lifting and counterbalancing mechanism which includes a drive roll 93 mounted similarly to drive roll 24 and a rope having one end attached to the base of the column and its other end attached to a counterweight. On rotation of drive roll 93 (as by an electric motor 9S, through gearing, in a manner similar to that described for drive roll 24), the span is raised (FIG. 10) or lowered (FIG. 9), depending on the direction of rotation of the roll. It will be understood that, as is conventional, each end of the movable span above the column is aligned with a suitable approach roadway, when the span is in its normal lowered position, and that each end is locked in that position lby suitable locking devices which may be of the same type as those described in connection with the cantilevered span. For the shorter span bridges, the non-cantilevered construction, with the columns 92 rigidly connected to the plate girder span 91, is generally adequate to fix and rigidity the span. It will be understood that the various auxiliary features previously described, such as the use of the auxiliary counterweight, the stressing of the lower portions of the columns (as by means of the hydraulically pressed wheels) and the removable live load supports and the prestressing of the span (e.g. by the hydraulically supported pads), may all be employed with the noncantilevered construction.

The ropes used in the bridges of the invention may be of conventional type. For example, metal wire cables may be used, and each rope may be made up of a number of parallel separate individual cables. Advantageously the paths of the ropes from the attachment points 23 up to the drive rolls 24, then down to the sheaves 25 and up to the sheaves 26 are substantially vertical (although in some cases these paths are shown in the drawings at an angle to make the mmore distinguishable from the sides of the columns).

In FIG. 8 there is illustrated an arrangement in which the drive rolls 24 are supported on piers 97. It will be appreciated that the choice of whether or not piers will be used will depend on the terrain, but the basic construction will otherwise be substantially the same in either case. The piers can be relatively simple and light, particularly since their upper portions do not have to resist any great horizontal load. Also, since they do not extend substantially above the normal height of the movable span they are not, unlike the high towers of conventional lift bridges, subject to very high wind loads.

It is also within the broader scope of this invention to use the counterweight shown in FIGS. 9 and l0 with a cantilevered movable span. Here the cantilevered portions of the span add some rigidity to the center portion of the span, particularly for spans of medium length (e.g, center spans about 200 to 300 feet in length). The various features previously described, such as the use of the auxiliary counterweight, the prestressing at the lower portions of the columns, and especially, the controlled prestressing of the span by upward pressure at the supports (e.g. by the hydraulically controlled pads) and the removable span supports, may all be employed with this type of span, without mounting the counterweights at the ends of the cantilever portions.

Various other modifications and varied applications of the novel features of the apparatus in the detailed description above will occur to those skilled in the art, and consequently this invention should be construed broadly in accordance with its full spirit and scope.

I claim:

1, A lift bridge over a waterway comprising a movable unitary span having a longitudinal roadway, said movable span having a pair of spaced depending supporting members positioned at opposite sides of the waterway, said supports being spaced inwardly from opposite ends of the span defining opposite cantilever portions extending beyond said supports, said cantilever portions having ends aligned with respective ends of approach roadways when the span is in a normally lowered position, a counterweight for each of said supports, located below the ends of cantilever portions, a pair of rope means connecting a bottom portion of each of said supports to its respective counterweight, a pair of sheaves mounted for rotation on a fixed axis adjacent each of said supports above said bottom portion rope connection, first rope guide means mounted on each of said support bottom portions below each of said sheaves, second rope guide means comprising a pair of sheaves mounted on the end of each of the cantilevers, each of said ropes passing from said support connection upwardly over said sheave, downwardly under said first guide means, upwardly over said second guide -means and downwardly to the respective counterweight, and power means for rotating each of said first mentioned sheaves to drive said ropes for raising said span by lowering the counterweight to provide increased clearance over said waterway and to lower the span by raising the counterweights, said counterweights serving to prestress and equalize said lift bridge.

2. The lift bridge, as defined in claim 1, including releasable means for locking the free ends of said cantilevers against upward movement thereof caused by downward deflection of the center portion of the span under live load conditions.

3. The lift bridge, as defined in claim 1, in which said span, supports and cantilevers are of a unitary plate members rigid construction, and said supports serving to prestress said span.

4. The lift bridge, as defined in claim 3, having a pier support for the end of each cantilever portion, auxiliary force-applying means acting on said support for urging the central portion of the span upwardly, locking means between the end of each cantilever portion and its pier support serving as a fulcrum against live load defiection.

5. The lift bridge, as defined in claim 1, and fixed bearing means located adjacent the outer bottom portion of each of said spaced supports, and force-applying means acting between each of said support bottom portions and said fixed bearing means for urging said bottom portions of said supports toward each other to defiect the center of said movable span upwardly for prestressing against a downward force of liveload thereon.

6. A lift bridge, as defined in claim 1, said counterweight extending substantially the width of the span, and

means carried by each of said counterweights for varying the lateral distribution of the mass of the respective counterweights to counteract wind forces and solar heating effects.

7. The lift bridge, as defined in claim 6, in which said counterweight mass lateral distribution means includes an auxiliary weight mounted on the counterweight for lateral movement with respect thereto, and a motor for effecting said lateral movement.

8. The lift bridge, as defined in claim 6, in which the connection between each pair of ropes and counterweight are adjacent to the counterweight center between the lateral sides of the counterweight.

9. The lift bridge, as defined in claim 5, in which each of said fixed bearing means is a vertically extending track and said force-applying means comprises a wheel riding -on said track having a supporting axis of rotation mounted for movement with respect to said support bottom portion, and hydraulic means mounting said wheel on said support bottom portion for said axis in a slot hole.

10. The lift bridge, as defined in claim 9, in which resiliency is imparted to said force-applying means by gas bubbles in the hydraulic fluid of said hydraulic means.

11. A lift bridge, as defined in claim 1, including releasable means for locking the free ends of said cantilevers against upward movement thereof, a fixed bearing means for each of said supporting members, and hydraulic means for exerting pressure in an upwardly direction from each of said bearing means against its respective supporting member when the span is in said normally lowered position to effect additional prestress conditions in the span counteracting defiective forces by partial load thereon.

12. A lift bridge, as defined in claim 11, said hydraulic means being relatively resilient under said load, and adjustable shim means between each of the supporting members and said fixed bearing means for limiting said load defiection to a predetermined level.

13. The lift bridge, as defined in claim 12, in which said hydraulic means includes gas bubbles in the hydraulic fluid for imparting said resiliency thereto.

14. A lift bridge over a waterway comprising a pair of opposite fixed approaches and a vertically movable unitary span therebetween, said approaches and span each having a roadway section which are in alignment at a predetermined level when the movable span is in a normally lowered position, a support member depending from substantially each opposite end of the movable span, a counterweight for each `of said supports mounted beneath said roadway level, a pair of rope means connecting a bottom portion of each of said supports at opposite sides thereof to its respective counterweight for raising the span to -provide increased clearance over said waterway and lowering the span to said normal position, power means acting on said rope means for effecting said raising and lowering, a fixed bearing means located adjacent the outer side of each of said support bottom portions, and hydraulic force applying means acting between each of said support bottom portions and the respective fixed bearing means for urging said support bottom portions toward each other to defiect the center of said movable span upwardly for prestressing against a downward force of a live load thereon, each Voff said hydraulic force applying means including a fixed track, a wheel riding on the track and having an axis horizontally movable in relation to its respective support bottom portion by said hydraulic force applying means.

15. The lift bridge, as defined in claim 14, in which said hydraulic force applying means includes gas bubbles in the hydraulic fluid for imparting predetermined resiliency thereto.

References Cited UNITED STATES PATENTS 1,285,696 11/1918 Harrington 14-42 1,367,115 2/l921 Blondel 14-28 2,373,072 4/ 1945 Wichert 14-24 X 2,652,783 9/ 1953 Skinner. 2,889,565 6/1959 Harty 14-42 2,040,445 5/ 1936 SakamotO 14-42 JACOB L. NACKENOFF, Primary Examiner.