HYDRAULIC CYLINDER DRIVES FOR EXISTING BASCULE BRIDGES

by James M. Phillips 111, P.E. Senior Engineer

Parsons Brinckerhoff Quade & Douglas, Inc.

INTRODUCTION

Purpose

For t h e p a s t f i v e years , t h e a u t h o r has been i n v o l v e d w i t h movable b r i d g e p r o j e c t s , most n o t a b l y , bascu le b r i d g e p r o j e c t s . D u r i n g t h i s t ime, t h e use o f f l u i d power, o r h y d r a u l i c s , as a source o f b r i d g e a c t u a t i o n has become more prominent i n t h e movable b r i d g e i n d u s t r y . The expanding use o f h y d r a u l i c s i n a f i e l d f o r m e r l y dominated by heavy machinery has l e a d t o t h e development and imp lementa t ion o f new des igns and concepts . T h i s paper p r e s e n t s a r e v i e w and e v a l u a t i o n o f t h r e e s p e c i f i c a p p l i c a t i o n s o f f l u i d power on e x i s t i n g bascu le b r i d g e s .

The pu rpose o f t h i s paper i s t o share w i t h t h e movable b r i d g e i n d u s t r y s e v e r a l h y d r a u l i c c y l i n d e r powered bascu le b r i d g e a p p l i c a t i o n s f a m i l i a r t o t h e a u t h o r . The r e v i e w o f each a p p l i c a t i o n i n c l u d e s a d i s c u s s i o n o f t h e concepts developed f o r t h a t p a r t i c u l a r b r i d g e and an e v a l u a t i o n o f t h e a p p l i c a t i o n from a r e t r o s p e c t i v e view. The focus o f t h i s paper i s on t h e l o c a t i o n , moun t ing and c o n n e c t i o n o f h y d r a u l i c c y l i n d e r s t o t h e movable span and bascu le p i e r r a t h e r t h a n t h e s p e c i f i c s o f h y d r a u l i c s . However, t o a i d i n unders tand ing t h e m a t e r i a l , some r e v i e w of f l u i d power f o r movable b r i d g e s i s presented.

Background

The a u t h o r ' s invo lvement w i t h h y d r a u l i c b a s c u l e b r i d g e s i n c l u d e s t h e d e s i g n o f new b r i d g e s , t h e des ign o f h y d r a u l i c systems as a replacement f o r e x i s t i n g machinery, t h e d e s i g n o f temporary h y d r a u l i c systems f o r r e h a b i l i t a t i o n p r o j e c t s , and t h e i n s p e c t i o n o f e x i s t i n g b r i d g e s . The s p e c i f i c b r i d g e s p resen ted i n t h i s paper a re :

M e r r i l l Barber B r i d g e Over t h e I n t r a c o a s t a i Waterway, Vero Beach, FL - A Temporary System f o r o p e r a t i o n d u r i n g r e h a b i l i t a t i o n

Hobe Sound Bridge, Hobe Sound, FL - A Temporary System f o r operat ion dur ing r e h a b i l i t a t i o n

Parker Bridge, West Palm Beach, FL - A Replacement System f o r an e x i s t i n g mechanical d r i v e

I n each o f these pro jec ts , the author was employed by t h e consu l t i ng engineer ing f i r m contracted t o design the hyd rau l i c c y l i n d e r attachment and i n s t a l l a t i o n d e t a i l s . The author i s g r a t e f u l f o r t h e consent granted by h i s former employer t o present t h i s ma te r i a l .

HYDRAULIC CYLINDERS AS A BASCULE BRIDGE ACTUATOR

Hydraul i c Power

Hydraulic power, use of pressurized fluid to perform work, is well suited for operation of large mechanisms such as movable bridges. Hydraulic systems provide the mechanical advantage required to move the large masses of swing, vertical lift, and bascule bridges with relatively efficiency. The fiexabil ity of hydraulic power transmission systems, as evidenced by elements such as pressure hoses and piping, makes them well suited to the various geometric constraints encountered in movable bridge design and construction. This flexibility is especially valuable when developing a concept for installation on an existing structure.

As noted earlier, this paper will not attempt to cover all of the various types of movable bridges, but will focus on bascule bridges and in particular trunnion bascules. Figure 1 is an example o f a common configuration for a hydraulic cylinder actuated bascule bridge. In addition, only systems with hydraulic cylinders as actuators will be discussed. Other hydraulic drive systems such as low speed high torque hydraulic motors are not covered.

Hydraulic cyl inders are l inear actuators, that is, the motion produced by a cylinder is along a straight line with respect to the cylinder. The force developed in the cylinder is the result of fluid pressure acting on the piston. Double acting cylinders, which are commonly used in bridge applications, have pressurized fluid on both sides of the piston, the rod side and cap side (see Figure 2). The force in the cylinder is the result of the difference between the pressure on each side of the piston multiplied by the effective area of the piston. Note that the area on the rod side of the piston is less than the area on the cap side by the cross sectional area of the rod. Pressure on both sides of the piston is necessary to provide adequate control of the cylinder during operation.

SPAN TRUNNION

HYDRAULIC CYifNDER

HYDRAULICALLY ACTUATED TRUNNION BASCULE

FIGURE i

CAP ,rHEAD

DOUBLE ACTING HYDRAULIC CYLINDER

FIGURE 2

Geometric Constraints

Hydraulic bascule bridges are usually very similar to trunnion bascules operated by conventional machinery. Instead of a rack attached t o the main girders, a cylinder or group of cylinders is attached t o each main girder and supported off the machinery platform or the front wall of the bascule pier. Other arrangements are possible but this configuration is most common. The cylinder is usualiy attached to the main girder because it is the primary load carrying member of the span. It also happens to be a longitudinal member which is advantageous when considering the forces from the cylinder. Loads from the cylinder remain within the plane of the member only for longitudinal members. This will be more obvious after reviewing the motion of a bascule bridge.

Motion o f a bascule bridge is primarily one of rotation. With the exception of rolling lift bridges, most bascule bridges pivot about an axis or trunnion. Rotation of the leaf mass is the result of torque applied about that axis overcoming friction, angular inertia, wind, and unbalanced loads. Rotation of a pivoting mass caused by a linear actuator, such as a hydraulic cyl inder, involves a combination of several movements.

Motion of the bascule span or leaf and the hydraulic cylinders is defined by three points; the leaf pivot or trunnion, the hydraulic cylinder pivot, and the connection of the cylinder to the leaf. As the bascule leaf rotates, the pivot point at the rod end of the cylinder follows the leaf on a circular path. At the same time, the cylinder must rotate about its support on the fixed portion of the pier in order to follow the rod end as it moves along its arc. As a consequence of this motion, the effective moment arm of the cylinder force about the trunnion varies continuously throughout the rotation of the bridge. A1 so varying is the angle at which the cylinder force intersects the mounting supports and attachments at either end of the cylinder. This variation is most severe at the connection to the leaf because greater rotation takes place between the rod and leaf than between the cylinder and pier. The difference in rotation occurs because the leaf rotates 60 to 80 degrees typically, while the cylinder rotates only the few degrees required to follow the path of the rod end. It should also be noted, that the force exerted by the cylinder is subject to complete reversal as the actuation may be to push the bridge open or to pull the bridge closed.

If the hydraulic system is to utilize an economical cylinder size, then the effective moment arm of the cy:inder force aboct the leaf trunnion should be made as large as i s practical. The limiting factor to this length will be t h e maximum length of cylinder which can fit between the machinery platform and the connection to the leaf. Maximum cylinder lengths may also be limited by the buckling strength of the rod specified, depending

on the mounting style. The length of the closed cylinder is the sum of the fixed dimensions of the cylinder including head, cap, and rod end plus the stroke and stop tube. Therefore, the length of cylinder will increase directly with an increase in stroke or stop tube. The greater the moment arm the greater the stroke and cylinder length. Similarly, the smaller the effective moment arm the larger the cylinder force required to operate the bridge.

Cylinder Mounting Styles

In order to accommodate leaf rotation, hydraulic cylinders must be mounted so that they can pivot about their supports and the connection the the leaf. This is commonly accomplished by using a trunnion mounted cylinder with a pinned rod end. This is one of several mounting styles commercially avalable. Some of the most common styles found in bascule bridges are shown in figure 3. Other mounting styles used on movable bridges include the cap clevis and the cardanic ring. Cardanic rings consist of two rings with trunnions mounted at right angles one within the other to provide freedom of rotation in all directions.

When considering the cylinder as a structural compression member, it is apparent that the head trunnion provides the least unbraced length of the three trunnion positions available. Minimizing the unbraced length minimizes the rod diameter required for a given axial load and will also limit the requirements for a stop tube. A stop tube is used to limit the stresses on the rod bushings and improve the life of the cylinder. Determination of stop tube requirements is beyond the scope of this paper, but it can be generally assumed that the length of stop tube will increase as the tendency for buckling or bending of the cylinder increases. The disadvantage of a long stop tubes is an increase in the overall length of the cylinder equal to the stop tube.

While the head trunnion i s preferable from a structurai standpoint, it does have one major drawback. The pivot point of the cylinder trunnion is located only a few inches from the rod end when the rod is fully retracted and the bridge is in the closed position. During span operation, as the rod retracts near the closed position, motion or improper alignment of the leaf and cylinder transverse to the plane of the cylinder trunnion, will result in development of stresses in the cylinder bushings and trunnion bearings. This geometry leaves little or no room for compensation of error in the alignment of the cylinder and leaf. For this reason, an intermediate trunnion mount is preferable. Intermediate trunnion mounts caii be located near the head of the cylinder to limit the unbraced length of the cylinder, but far enough from the rod end to allow for minor misalignment.

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CAP PIVOT

GAP CLEViS

HEAD TRUNNION

CAP TRUNNlON

C Y L I N D E R MOUNTING STYLES

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FIGURE 3

TEMPORARY HYDRAULIC SYSTEMS

The first two projects presented are examples of a temporary hydraulic system designed to operate an existing bascule bridge while its permanent drive system was under rehabilitation. When a bascule bridge is in need of repair or rehabilitation, one of the most critical issues is that of maintaining traffic during the work period. In some cases, the demands for maintenance of marine traffic dictate that the owner must keep the bridge operational. For projects requiring major work on the main drive system, use of a temporary operating system may be the most economical method of satisfying the maintenance of traffic requirements.

The following are examples of rehabilitation projects where the requirements for maintaining bridge operation lead to the design and installation of a temporary hydraulic system utilizing hydraulic cylinders. Once again, the emphasis of this paper will be on the mounting of the cylinders rather than the selection of the operating unit itself.

Development of temporary hydraulic systems in Florida was initiated by the Florida Department of Transportation (FDOT) for use during a large scale rehabilitation program involving over 40 bascule bridges. The prototype system was designed by the FDOT for use at the Oania Bridge over the Intracoastal Waterway in Dania, Florida. This system consisted of a power unit and two hydraulic cylinders for each bascule leaf. The concept was to construct several of these systems for modification and reuse on other similar bridges in the program. Attachment brackets for the cylinders where to be designed and installed on each bridge and left i n place for future use if required.

Most of the bridges in the rehabilitation program cross the Intracoastal Waterway and are therefore similar in span size and geometry. This provided the FDOT with the opportunity to utilize one hydraulic power unit design (i .e. pump, motor, reservoir, etc.) to operate most of the bridges under normal operating conditions. This simplification limited the mcdifications to the temporary system for each bridge to the length, location, and mounting of the cylinders, including installation of brackets and supporting structures. Recognizing that this system was temporary, the FDOT did not specify a system which could meet the requirements for permanent operating systems. This was necessary

to limit the system to one of economical and transportable size. This did produce some concerns over projected span operating speed because of the units limited size and power.

Cylinder attachments for the temporary systems were designed for I50 percent of the cylinder force at relief valve pressure, in accordance with the American Association of State Highway and Transportation Officials (AASHTO), Standard Specifications for Movable Highway Bridges, I978 edition. This force was selected as the design load rather than the operational forces computed in accordance with the M S H T O specifications. The relief valve setting was selected not only to protect the hydraulic system, but Lo provide adequate forces for leaf operation under AASHTO condition A, normal operation. Conditions B and C where disregarded because of the temporary nature of the system. Again, the disigners recognized that the temporary system would have some limitations in order to remain economical.

Each installation of a temporary hydraulic system provided the designer with several problems. First of all, where could the cylinder be attached to the movable span and fixed pier? The attachment points had to be selected to satisfy both structural and operational requirements such as existing member capaci ty and cylinder stroke. Next, the brackets and attachments had to be selected and designed to withstand the design load at any angle it could be applied at during operation. The design also had to include complete load reversal. In addition, these problems had to be solved within the restraints of the existing bridge geometry and the power available from the specified hydraulic power unit. In these examples, this meant that the unit and cylinders had to fit around the existing machinery, so that it could be rehabilitated without interruption.

BRIDGE NO. 1 - MERRILL P. BARBER BRIDGE

The Merrill P. Barber Bridge is a four lane double leaf Hopkin's Trunnion bascule bridge crossing the Intracoastal Waterway in Vero Beach, FL. In the process of developing a rehabilitation sequence for the structure and operating system, it became apparent that a temporary operating system would be required. The system to be used was a modification of the hydraulic cylinder system designed for use at the nearby Dania Boulevard bridge by the Florida Department of Transportation. However, while the hydraulic system itself was previously designed, the mounting details for the hydraulic cylinders had to be designed for the specific geometry of the Merrill P. Barber 6ridge.

The Merrill P. Barber Bridge is typical of Hopkin's Trunnion Bascules in Florida having two main girders and two trunnion girders on each leaf. The bridge is powered by an electric motor and gear system mounted on a Hopkin's Frame, Power is transmitted

t o t h e l e a f by two rack and pinion s e t s located between t h e trunnion g i r d e r s . This same design i s frequently seen throughout F lor ida , i nc lud ing the Bridge a t Dania f o r which t h e FDOT developed t h e i r f i r s t temporary hydraul ic system.

AS an eng inee r f o r the design consul tan t , t h e author developed a mounting system s imi l a r t o the one used a t Dania. There i s one geometric f e a t u r e , common t o most Hopkin's Frame Bascules, which severe ly l i rni ts the p rac t i ca l l oca t ions ava i l ab le f o r mounting t h e temporary hydraul i c cy l inde r s . This geometric r e s t r a i n t i s t h e r e i a t i vely small v e r t i c a l c learance between t h e machinery platform and the bottom of t h e main g i r d e r s or t runnion g i r d e r s (usua l ly about 5 o r 6 f e e t ) . A cy l inde r length which would maintain a resonable e f f e c t i v e momentarm about t h e l ea f t runnion, would not f i t between t h e machinery platform and t h e main g i r d e r o r trunnion g i r d e r . To f i t the cy l inder between t h e machinery platform and the l e a f , a connection had t o be designed up near the top of t h e main g i r d e r .

The connect ion developed f o r t h e Dania Bridge was mounted on a channel i n s t a l l e d between t h e top of t h e trunnion g i r d e r and main g i r d e r . This loca t ion provided v e r t i c a l c learance f o r t h e cy l inder length required and an adequate moment arm about the l e a f t runnion . The reason f o r not reusing t h i s same design was a matter of s t r u c t u r a l adequacy. A t Dania, t h e channel was loaded in i t s major ax is when the bridge was in t h e closed pos i t i on , but as the l e a f ro t a t ed the angle of t h e load ro t a t ed towards the channel 's weak axis and to r s ion in t h e channel developed as a r e s u l t of t h e o f f s e t of t h e rod eye with respect t o the channel. While t h i s configurat ion functioned well a t Dania, i t i s not p re fe rab le because of the channel 's r e l a t i v e weakness in b iaxia i bending and t o r s i o n .

For t h e Merri l l P . Barber Bridge a s i m i l a r hydraul ic system was used with modif icat ions t o t h e mounting bracket support t o e l iminate t h e inadequacies mentioned above. Instead of mounting t h e eye bracket on a channel, i t was mounted on an HP sec t ion . This member provided adequate sec t ion p rope r t i e s i n bending and to r s ion and a wide f lange sur face f o r bracket attachment. I n an e f f o r t t o e l iminate to r s ion in t h e member d i r e c t l y supporting t h e eye bracket , i t was decided t o mount t h e bracket t o a longi tudina l member which would then be loaded in i t s s t rong ax i s regard less of l e a f r o t a t i o n . For t h i s reason, t h e HP sec t ion was connected between the f l o o r beam and a wide f lange beam i n s t a l l e d between t h e trunnion g i r d e r and main g i r d e r . This conf igura t ion provided the r i g i d connection t o t h e main members of t h e span and t h e longitudinal o r i en ta t ion des i r ed .

The connection t o t h e new longi tudina l beam was a rod c i e v i s and eye as shown in Figure 4 . This attachmeat allows f o r the r e l a t i v e ro t a t ion of t h e l e a f and cy l inde r and y e t provider t h e s t rength required t o t ransmi t 150 percent of t h e cy l inde r force a t r e l i e f valve pressure. The r e l i e f valve s e t t i n g was computed based on cyl inder forces required f o r operat ion under AASWTO

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NEW W SECTION SPANNlhlG FROM MAIN TO TRUNNION GIRDERSI r N E W H P 5ECTIOM

TEMPORARY WYDRAULlC CYLINDER

FOR MERRILL P. B A R B E R BRIDGE

FIGURE 4

condit ion A , normal operat ion.

As was t h e case with t h e Dania Bridge, the cy l inder was spec i f i ed with a cap c l e v i s mount f o r attachment t o a c l e v i s base on t h e machinery platform below. The c l e v i s base was t o be anchored t o the platform with high s t r eng th anchor b o l t s . The cy l inde r rod end and cap c l e v i s were se lec ted with commercially ava i l ab le spherical plain bearings t o allow some r o t a t i o n t r ansve r se t o t h e plane of l e a f r o t a t i o n . These bearings not only reduce bending and buckling tendencies in t h e cy l inde r , but simp1 i f y i n s t a l l a t i o n by allowing some minor misalignment i n t h e range of one t o two degrees t o e i t h e r s i d e .

Evaluation of t h e Merr i l i P. Barber Bridge Mounting System

A t t h e time of t h i s wr i t i ng , t h e temporary system had been in place and operat ional f o r several months. While operat ion has been mostly normal, t he re has been one instance of note.

The programmable c o n t r o l l e r used t o operate t h e temporary system i s inter locked with t h e t r a f f i c ga tes and span locks t o prevent span operat ion while t h e g a t e s a r e open t o t r a f f i c o r t h e locks a re c losed . During one p a r t i c u l a r operat ion, a t r a f f i c ga t e was manually opened. This t r ipped a l i m i t switch and signaled t h e programmable c o n t r o l l e r t o s top operat ion. The programmable c o n t r o l l e r immediatly shut o f f power t o t h e power u n i t . This caused the hydraul ic system valves t o shut t i g h t while the bridge was opening a t fu l3 speed. With t h e f l u i d locked in t h e cy l inder , i t became an almost r i g i d member un t i l the in t e rna l pressure exceeded t h e r e l i e f valve s e t t i n g . The force generated in the cy l inder was s u f f i c i e n t t o pull one of t h e c l e v i s bases o f f the machinery platform.

Upon f u r t h e r examination i t was determined t h a t t h e force had not exceeded t h e design force (150% of r e l i e f valve pressure) o r anchor b o l t s t rength ( se l ec t ed t o exceed design load within manufacturers spec i f ied s t r eng th ) . The anchor used was an epoxy type and the f a i l u r e was in t h e bond t o the concrete . This combined with t h e f a c t t h a t none of t h e o ther th ree c l e v i s bases moved ind ica t e s a loca l ized problem in manufacturing o r i n s t a l l a t i o n of the anchors. The connection t o t h e l e a f and support brakes exhibi ted no s igns of d i s t r e s s .

The concerns over slower opera t ing speeds never developed as t h e temporary hydraulic system was found t o operate the span f a s t e r than the o r ig ina l mechanical d r i v e system under normal condit ions.

BRIDGE NO. 2 HOBE SOUND BRIDGE

Hobe Sound B r i d g e i s a new double l e a f bascu le b r i d g e over t h e I n t r a c o a s t a l Waterway i n Hobe Sound, F l o r i d a . Even though t h e b r i d g e i s c o n s i d e r e d new, t h e bascu le spans and machinery were from a t e m p o r a r y b r i d g e b u i l t n e a r l y t e n years e a r l i e r . The spans and d r i v e machinery were p a r t i a l l y d isassembled and p u t i n s t o r a g e f o r s e v e r a l y e a r s b e f o r e t h e Hobe Sound B r i d g e became a p r o j e c t . When t h e p r o j e c t f e l l beh ind schedule, t h e F l o r i d a Department o f T r a n s p o r t a t i o n , e l e c t e d t o i n s t a l l a temporary h y d r a u l i c o p e r a t i n g system i n o r d e r t o b r i n g t h e b r i d g e i n t o o p e r a t i o n b e f o r e t h e h o l i d a y season o f 1985.

The d e s i g n c o n f i g u r a t i o n o f t h e d r i v e system was f l o o r mach inery c o n s i s t i n g o f a p r i m a r y d i f f e r e n t i a l r e d u c e r and two secondary r e d u c e r s , one l o c a t e d a t each o f two d r i v e p i n i o n s . The two d r i v e p i n i o n s engage r a c k s on each o f t h e two main bascu le g i r d e r s . An i m p o r t a n t f e a t u r e t o n o t e i s t h e m in ima l c lea rance between t h e secondary reducers and t h e f r o n t w a l l o f t h e b a s c u l e p i e r . T h i s space i s n o t s u f f i c i e n t f o r c o m f o r t a b l e passage o f a n average s i z e person.

The a u t h o r ' s invo lvement i n t h i s p r o j e c t was as an e n g i n e e r f o r t h e c o n s u l t a n t h i r e d t o des ign t h e s u p p o r t s and connec t ions f o r t h e temporary o p e r a t i n g system. The F l o r i d a Department o f T r a n s p o r t a t i o n dec ided t o use t h e same h y d r a u l i c power u n i t s p e c i f i e d f o r t h e i r r e h a b i l i t a t i o n p r o j e c t s equ iped w i t h two 6 i n c h d i a m e t e r h y d r a u l i c c y l i n d e r s . The rema in ing parameters t o be de te rm ined by t h e c o n s u l t a n t where t h e l e n g t h and l o c a t i o n of c y l i n d e r s and t h e method o f c y l i n d e r c o n n e c t i o n and s u p p o r t .

U n l i k e t h e p r e v i o u s temporary i n s t a l l a t i o n s , t h e space between t h e main g i r d e r and t r u n n i o n g i r d e r was n o t a v a i l a b l e on t h e Hobe Sound B r i d g e because o f t h e presence o f t h e secondary reducers . T h i s f a c t , combined w i t h t h e d i sadvan tages o f t h a t l o c a t i o n d i scussed e a r l i e r , l e a d t o a new e v a l u a t i o n o f t h e l o c a t i o n o f t h e temporary h y d r a u l i c system. Another i m p o r t a n t d i f f e r e n c e between t h e Hobe Sound Br idge, w i t h f l o o r machinery, and t h e t y p i c a l Hopk in 's Frame Bascu le b r i d g e i s i n t h e v e r t i c a l c lea rance between t h e machinery p l a t f o r m and t h e bascu le l e a f . Whereas on t h e Hopk in 's Frame b r i d g e s t h e c y l i n d e r a t tachment t o t h e l e a f needed t o be made up n e a r t h e t o p o f t h e main g i r d e r t o accommodate c y l i n d e r l e n g t h requ i rements , on t h e Hobe Sound Br idge , t h e v e r t i c a l c lea rance was s u f f i c i e n t t o c o n s i d e r s e v e r a l o t h e r l o c a t i o n s .

As p r e v i o u s l y noted, t h e c y l i n d e r l o c a t i o n g e n e r a l l y cons ide red most i d e a l i s d i r e c t l y underneath t h e main g i r d e r s . On t h i s b r i d g e , t h e area beneath t h e main g i r d e r s was n o t a v a i l a b l e because o f i n t e r f e r e n c e w i t h t h e p i n i o n s and secondary reducers . Another span member w i t h good p o t e n t i a l f o r c y l i n d e r c o n n e c t i o n because o f i t s s t r e n g t h and l o n g i t u d i n a l o r i e n t a t i o n , i s t h e t r u n n i o n g i r d e r . W i th t h i s i n mind, a scheme was deve loped w i t h t h e one c y l i n d e r p l a c e d d i r e c t l y under each t r u n n i o n g i r d e r .

- i5-

However, t h e e f f e c t i v e moment arm of t h e cy l inders about t h e span trunnion a x i s had t o be s u b s t a n t i a l l y reduced in order t o maintain a cy l inder shor t enough t o f i t under the g i r d e r . This concept was workable within t h e geometric c o n s t r a i n t s of t h e bridge, b u t was found t o r equ i re excessive cy l inder fo rces because o f t h e small moment arm. A l a r g e r cy l inder would have been requi red with an 8 o r 10 inch bore. This increase was not acceptable because i t would a l t e r t h e power u n i t requirements.

In looking f o r an a l t e r n a t e attachment point which would provide t h e des i red s t rength and longi tudina l o r i e n t a t i o n , the f l o o r beam and s t r i n g e r s became poss ib le candidates . The f l o o r beam had t h e s t rength and could be at tached t o a t many poin ts along t h e machinery platform. The s t r i n g e r s were not s t rong enough t o withstand the cy l inde r forces b u t did have t h e ideal o r i e n t a t i o n . Af ter some ana lys i s t h e s t r i n g e r loca t ion was se l ec t ed . While s t r u c t u r a l modif icat ions were necessary t o s t rengthen t h e member and i t s connection t o the f l o o r beam, t h e s impl i c i ty of the mounting configurat ion made i t t h e winner. In order t o connect the eye bracket t o t h e f l o o r beam, t h e member would have required t h e addi t ion of l a r g e s t i f f e n e r s t o withstand t h e to r s iona l loads induced by t h e o f f s e t of t h e cy l inde r forces . Every attempt was made t o avoid connections which would induce t o r s i o n on ex i s t ing members not designed f o r such loads.

The qext s t e p was t o develop a bracket t o s t rengthen t h e s t r i n g e r and provide a sur face t o accept t h e eye bracket . The r e s u l t , shown in Figure 5, provided t h e increase in t h e s t r i n g e r s bending s t r eng th and shear s t rength a t the connection t o t h e f l o o r beam. Bearing s t i f f e n e r s were provided t o prevent web buckling. To reduce t h e e f f e c t s of bending in t h e s t r i n g e r , t h e eye bracket was located as c lose t o t h e f l o o r beam as was p r a c t i c a l .

While t h i s configurat ion l i m i t s out of plane loading of t h e s t r i n g e r , one more s t e p was taken t o e l iminate poss ib le to r s ion on t h e composite member. Once again, t h e cy l inde r s were spec i f ied with spherical p la in bearings in t h e rod end and cap pivot t o allow f o r few degrees of un res t r i c t ed r o t a t i o n t ransverse t o t h e plan of r o t a t i o n , thus e l iminat ing out of plane bending s t r e s s e s .

The cap pivot of the cy l inde r was designed t o connect t o a c l e v i s base located on t h e machinery platform between t h e primary d i f f e r e n t i a l reducer and t h e secondary reducer . This l oca t ion did not i n t e r f e r e with the operat ion of t h e e x i s t i n g mechanical system. However, t h e mechanical systems could not be operated unless t h e cy l inders were detached from t h e l e a f because they would r e s t r a i n the l e a f aga ins t movement.

EXISTING FLOOR BEA

E X l S T l N G STRINGER

EYE B R A C K E T NEW BUILT UP BRACKET

+CLEVIS MOUNT / HY DRAULlC CYLINDER

TEMPORARY H Y D R A U L I C S FOR HOBE SOUND B R I D G E

FIGURE 5

E v a l u a t i o n o f t h e Hobe Sound Mount ing System

Most i m p o r t a n t l y , t h e temporary h y d r a u l i c system f o r Hobe Sound was i n s t a l l e d and b rough t i n t o o p e r a t i o n i n t i m e f o r t h e h o l i d a y season. As f o r t h e mount ing and i n s t a l l a t i o n o f t h e c y l i n d e r s and b r a c k e t s , e v e r y t h i n g went accord ing t o p l a n . The s t r i n g e r b r a c k e t s and eye b r a c k e t s where mounted f i r s t . Then t h e c y l i n d e r s were a t t a c h e d t o t h e new b r a c k e t s and a l l owed t o hang f r e e . The c l e v i s bases on t h e machinery p l a t f o r m s were l o c a t e d under t h e hang ing c y l i n d e r s t o o b t a i n p r o p e r a1 ignment. The s p h e r i c a l b e a r i n g s i n t h e r o d ends s i m p l i f i e d t h i s a l i gnment .

S ince i n i t i a l i n s t a l l a t i o n , ad jus tment , and t e s t i n g , t h e temporary system has opera ted w e l l . There a re no s i g n s o f s t r u c t u r a l d i s t r e s s i n t h e mount ing b r a c k e t s o r s t r u c t u r a l members o f t h e spans. Once again, t h e concerns o v e r s lower o p e r a t i n g speeds d i d n o t deve lop because t h e temporary h y d r a u l i c system was found t o opera te t h e span f a s t e r t h a n t h e o r i g i n a l mechanical d r i v e system under normal c o n d i t i o n s .

REPLACEMENT HYDRAULIC SYSTEM

PARKER BRIDGE

The Parker Bridge i s a four l e a f bascule bridge in Palm Beach County, F lo r ida over the in t r acoas t a l Waterway. Four lanes of U.S. 1 a r e c a r r i e d by two p a r a l l e l double l e a f Hopkin's Trunnion bascule br idges staggered t o accommodate a skewed channel. As pa r t of t h e bridge r e h a b i l i t a t i o n program, the FDOT decided t o replace the d r i v e systems on a l l four leaves r a t h e r than proceed with e x t e n s i v e r e h a b i l i t a t i o n . Consultants were contracted t o prepare a l t e r n a t e cont rac t documents f o r two new d r i v e systems, one hydraul ic and one mechanical. This decsion was fueled by t h e extensive wear of the ex i s t ing gearing and Hopkin's Frame.

This p r o j e c t presented several problems in addi t ion t o those encountered during design of t h e temporary systems previously mentioned. Because t h i s was t o be a permanent system, i t had t o be designed f o r the f u l l loads of AASHTO condit ion C. in addi t ion , i n order t o reduce the in t e r rup t ion t o marine and vehicular t r a f f i c during cons t ruc t ion , the new system had t o be located so t h a t i t could be i n s t a l l e d while t h e e x i s t i n g system remained i n operat ion.

Af ter summarizing the loads on t h e span from condi t ions A and C, i t became apparent t h a t t h e r e were t h r e e span member types located above t h e machinery platform which could possibly withstand t h e loads , including t h e main g i r d e r s and trunnion g i r d e r s . Unfortunately, the bottom f langes of t h e main g i r d e r s and trunnion g i r d e r s were located only about s i x f e e t above the machinery platform. This was not near ly enough space f o r t h e cy l inder length required i f a p rac t i ca l moment arm about the span trunnion was t o be maintained.

The o the r member type with adequate s t rength and longi tudina l o r i e n t a t i o n was the rack g i r d e r , which supports t h e rack and t r a n s f e r s t h e dr iv ing loads from t h e machinery t o t h e movable span. The two rack g i r d e r s span between t h e counterweight g i r d e r and t h e l a s t f l o o r beam, one a t each pinion of t h e Hopkin's Frame. In add i t ion , t h e rack g i r d e r ' s bottom f lange i s located several f e e t above the bottom f lange of t h e main g i r d e r which provides g r e a t e r v e r t i c a l c learance f o r t h e cy1 inder . Unfortunately, t h e Hopkin's Frame i s loca ted between t h e rack and f ron t p i e r wall , bellow t h e rack g i r d e r s . A cy l inde r loca t ion

could not be found under the rack g i r d e r which d id not i n t e r f e r e with t h e opera t ion of t h e e x i s t i n g machinery. Therefore, t h i s loca t ion was dismissed.

Some cons idera t ion was then given t o loca t ing the cy l inders between t h e counterweight end of t h e main g i rde r s and t h e bascule p i e r below t h e machinery platform. This loca t ion was dismissed because t h e cy l inder s t roke was too long and the cy l inde r s would be in an inaccess ib l e a rea .

After f u r t h e r i nves t iga t ion , an acceptable loca t ion was discovered. By modifying the concrete niachinery platform a conf igura t ion was devised t o mount the cy l inde r s under t h e trunnion g i r d e r s . To make room f o r the cy l inde r under t h e g i r d e r , a ho le was t o be cored through the platform so t h a t t h e cap end of t h e cy l inder could be located below t h e platform ( see Figures 6 & 7 ) . This concept was not appl icable t o t h e main g i r d e r because t h e concrete p i e r was s o l i d under t h e g i r d e r . The cyl inders were se lec ted with intermediate trunnion mounts located t o f i t i n t o a bearing support anchored t o t h e t o p o f t h e machinery platform. This provided s u f f i c i e n t length between the pivot point of t h e cy l inder and t h e connectjon t o t h e l e a f t o allow f o r minor e r r o r s in allingment o r sidesway.

The support f o r t h e cy l inde r s was designed t o allow r o t a t i o n pa ra l l e l t o t h e l e a f ro t a t ion and d i s t r i b u t e t h e loads t o sound concrete around t h e cored hole. Rotational freedom was provided by a pa i r of pillow blocks with bronze bushings f o r t h e cy l inde r t runnions. To d i s t r i b u t e t h e loads t o sound concrete and anchor the cy l inde r s aga ins t u p l i f t , the pillow blocks were a t tached t o a s t e e l frame bolted through the platform. The s t e e l mounting surface f o r t h e bearings was a l s o designed t o provide a shimming surface f o r adjustment of the cy l inde r loca t ion in t h e f i e l d . A high s t r eng th polymer grout leve l ing pad was loca ted between the support frame and the e x i s t i n g concrete su r face t o s implify i n i t i a l a1 ignment and provide shock load d i s t r i b u t i o n .

An ana lys i s of t h e trunnion g i r d e r under load condit ion C revealed i t t o be of adequate s t rength in shear and bending. The only design modif icat ions required were t h e i n s t a l l a t i o n of bearing s t i f f e n e r s a t the point of loading. A pivot bracket designed t o accept t h e c l e v i s rod eye was bolted t o t h e lower f lange of the g i r d e r . This bracket included a spher ica l p la in bearing f o r t h e c l e v i s pin. Lubrication f i t t i n g s were spec i f i ed f o r these bearings and t h e pillow blocks below.

The cyl inders se l ec t ed were 8 inch bore high pressure cy l inde r s with 4 inch diameter rods and a 105 inch s t roke . Because of t h e intermediate t runnion mount only a 12 inch s top tube wds required desp i t e the l a r g e design loads. The rod c l e v i s f o r each cy l inde r was a special design r a t h e r than t h e commercially ava i l ab ly one t o accommodate t h e loads and t h e spher ica l bearing of the pivot bracket. The commercially ava i l ab le rod c l e v i s was not l a rge enough t o accept t h e width of the spher ica l bearing se l ec t ed in

-20.

FIGURE 6

YE BRACKET ATTACHED 0 TRUNNION GIRDER

REPLACEMENT HYDRAULICS FOR PARKER B R I D G E

FIGURE 9

accordance with AASHTO. A set screw was specified to prevent the clevis from backing off the rod end.

Eva1 uat ion of the Parker Bridge Rep1 acement Hydraulic System

This hydraulic replacement drive system has been selected as the alternate for construction and will be installed on the Parker Bridge in the winter of 1988.

From t h e example pro jec ts presented here i t i s apparent t h a t hydraul ic c y l i n d e r operating systems can be adapted t o f i t many types of e x i s t i n g trunnion bascule br idges. These systems can be used as temporary operating systems while t h e bridge i s under r e h a b i l i t a t i o n o r as replacement systems f o r ex i s t ing mechanical d r i v e systems.

The geometr ic and s t ruc tu ra l r e s t r a i n t s of an e x i s t i n g bridge may l i m i t t h e use and loca t ion of such hydraulic systems but in most cases wi l l not prevent t h e design and i n s t a l l a t i o n of an acceptable system. However, each bridge must be evaluated on an individual b a s i s t o determine t h e system which bes t meets t h e p a r t i c u l a r needs of t h a t p ro jec t .

During t h e course of these t h r e e p ro jec t s t h e r e were several i ssues w h i c h were common t o a l l designs. Resolution of these i s sues became t h e bas is f o r development of a r a t iona l design so lu t ion s p e c i f i c t o each p ro jec t . These i ssues and the parameters developed t o resolve them a re summarized below.

A. S e l e c t i o n of t h e cy l inder loca t ion

- provide s u f f i c i e n t clearance f o r cy l inder length - l o c a t e under main span member i f poss ib le - l imi t in ter ference with e x i s t i n g s t r u c t u r e s and machinery - provide maximum e f f e c t i v e moment arm about l e a f ax i s

B. Se l ec t ion of the cy l inder mounting s t y l e

- l i m i t r e s t r a i n t s on r o t a t i o n - l i m i t cy l inder buckling length - provide s u f f i c i e n t length between support poin ts f o r minor

misalignment - provide f o r l e a f r o t a t i o n

C . S t ruc tu ra l design of brackets and support s t r u c t u r e s

- s e l e c t design force based on s p e c i f i c use - apply forces as near t h e main members as poss ib l e

- a p p l y f o r c e s w i t h i n t h e s t r o n g a x i s o f t h e member - do not l o a d e x i s t i n g members i n t o r s i o n un less t h e y

have been designed f o r such l o a d i n g s - d e s i g n f o r l o a d d i r e c t i o n s which v a r y th roughou t o p e r a t i o n

F o l l o w i n g these gu ide1 ines , t h e temporary h y d r a u l i c systems f o r t h e f i r s t t w o p r o j e c t s p resen ted were s u c c e s s f u l l y des igned and i n s t a l l e d . E v a l u a t i o n o f t h e rep lacement system f o r t h e Parke r B r i d g e w i l l have t o w a i t u n t i l t h e system i s i n s t a l l e d and t i m e t e s t e d .