Balancing of Movable Bridges by the Strain Gauge Technique

At4ERICM CONSULTING ENGINEERS COUNCIL'SHEAVY MOVABLE STRUCTURESMOVABLE BRIDGES AFFILIATE3RD BIENNIAL SYMPOSIUMNOVEMBER 12TH - 15TH, 1990ST. PETER'SBURG HILTON b TOWERSST. PETERSBURG, FLORIDASESSIONWORKSHOP NOTESSesslon (1-3)"Balancing of Movable Bridges by theStrain Gauge Technique", Ed Roland,Specialty Measurements,Inc, NJDisclaimerIt 1s :he policy of :he Rffi;:a::an r3 ?r~v;ie a ;?an f ~ ::fonaj~::. r:,r?r;!iange. :; 3C3S SOTo r ~ are ~ a recormend or endorse any 3f rne :nfor~,a;:on x:srcnar.?eo as 1: :?;aces :: :?s:;?princ;p;es, processes, or 3 3 UC:S presEitea a .the pmpos:.+3 aar./sr r3n:a:r.eJ. i?rerr.. A.: ;araare tiieauihor's and XC: the ;ff ljaiiox'j. Aok,icai;on of infomatio;, :nter:aangeiresponsibilitv of the :iser t3 va.:Sate ana verify its :ntegri:y pr~3r 23 :se.

Table of ContentsINTRODUCTIONTEST METHODDATA REDUCTIONCASE STUDY NO. 1CASE STUDY NO. 2CASE STUDY NO. 3CASE STUDY NO. 4

IntroductionSeveral moveable bridges were built in the United States during themid and early 20th century. To prolong the life and reduce costlyequipment replacement and refurbishing, the bridge should bemaintained in a good operating balance state.The two major bridge types considered here are vertical liftbridges and bascule span bridges. Vertical lift, much like anelevator, are raised by cables at each of the four corners of thespan. The cables are suspended from pulleys supported by towers.The weight of the span are balanced by a counterweight attached tothe cables on the opposite side of the pulleys.There are two major types of bascule bridges. These are trunnionand roller types. A trunnion type consists of a deck supportingleaf and a counterweight that pivots about a fixed trunnionbearing. The roller type bascule bridge consists of a leaf andcounterweight that pivot about the axis of a large roller. Rollerbridges rotate and move horizontally during its operation.Both bridge types can develop excessive imbalances caused by deckrehabilitation, addition of utilities, dirt accumulation, paint,and numerous other reasons. These imbalances can increaseoperating power requirements and machine wear, and in extreme casescause equipment failure and create potential hazardous conditions.Several methods exist to balance a bascule bridge, but most providean incomplete picture of the full balance state of the bridge.This paper will present a method of determining the balance stateof a bascule bridge using strain gages on the final pinion shafts.The gages are used to measure operating torque during the raisingand lowering of the span. For vertical lift bridges the data canbe used to determine the present balance state and trunnionfriction. For bascule bridges the resulting data can be used to

determine the horizontal and vertical eccentricity of the imbalanceload, trunnion or roller friction, plus information on theperformance on the mechanical drive components.

Test MethodThis test method for balancing the movable bridge described hererequires the installation of strain gages on the final pinion driveshaft. Four (4) strain gages are installed on each final pinionshaft in a full wheatstone bridge configuration. The orientationof the gages used in this test measures shaft torque and cancelsout the effects of direct shear, moment and axial loading (SeeFigure 1). A full bridge is also temperature compensating. A fourconductor individual shielded pair cable with a common wireconnects the shaft gages to the strain gage signal conditioningsystem. The cable shielding is grounded at the signal conditioningsystem only.On bascule bridges an inclinometer is mounted to the span to detectthe angle of the bridge during its operation. The amplified straingage output and inclinometer voltages are recorded on an analogtape recorder during the test.Before testing of the bridge, the strain gage signals must becalibrated and then zeroed at a no load state of the pinion shaft.The gages are calibrated by the use of a shunt calibrationresistor. The resistor is temporarily connected in parallel acrossone of the gages to simulate a strain in the shaft (typically 50microstrain). The resistor can be connected at the gages whichwill account for the effects of lead wire resistance. The shuntresistor can also be connected at the signal conditioner and the

effects of lead wire resistance manually calculated. Thecalibration strain signals are recorded for each strain gage bridgeand is used as a reference when interpreting the actual shaftstrains.The gages are zeroed using the signal conditioning system when thepinion shaft is in a zero load state. To set a zero load statethe span locking pins are engaged while the braking system ismanually released. The operating machinery is manually rotated totake up the backlash in one direction, then the other. This willlocate the mid-position so that the pinion can float between thedrive rack or gear teeth. If the positioning is correct and thepinion is not loaded at this point the output of the gages will notrespond to the passage of a live load.Typically three tests are run with data being recorded during boththe raising and lowering of the span. Especially in the case ofbascule spans, it is advantageous to operate the bridge to its fulllimits of opening to collect as many data points as possible. Thebridge should also be operated at a constant speed to reduceinertia effects. Testing under severe weather or with ice and snowaccumulating should be avoided. A typical single leaf test traceis presented in Figure 2.

Data ReductionTo analyze the balance state of the bridge, shaft strain must beconverted into operating load for vertical lift bridges or leafmoments for bascule spans. For both bridge types, this begins withdetermining pinion shaft torque during operation.The equations to convert strain to pinion shaft torque are asfollows:EauationNo. 1: €cat = %/ [(%+R,) XG-F.)INo. 2: T - aG2A: c - a/ 22B: r - 2 EG2c: c - E/E2 x (1 + ~ ) 12D: T - 2€E/[2 x (1 + p)] = rE/(l + p)NO. 3: J - RD~/ 323A: c - D/ 23B: J/c - RD~/ 16NO. 4: r T, c/J4A: Ts - rJ/c-

€4= Calibration strain for R'% = Resistance of Strain Gage (OHMS)% = Resistance of Calibration Shunt (OHMS)G.F. = Gage Factor for Strain Gages (provided by Manufacturer)T = Torsional Shear Stress (PSI)Q = Shear Strain (inch per inch)G = Shear Modulus for Shaft MaterialE = Measured Strain (inch per inch)E = Young's Modulus for Shaft Material = 30 x lo6 PSIP = Poisson's Ratio = .3J = Polar Moment of InertiaD = Pinion Shafk Diameter (inches)c = Distance from Neutral Axis to Strain GageTsMG.R.rL= Pinion Shaft Torque (inch pounds)= Bascule Imbalance Moment (inch pounds)= Gear Ratio Between Pinion Shaft and Bascule Span Rotationor Lift Bridge Bull Gear Rotation= Radius of the Sheave Pulley (Lift Bridges)= Tension load on a cable group (Lift Bridges)Solving Equation 2D: r = eE/(l + P)Where :E = 30 * lo6p = 0.3Produces :r = (23.1) (PC)Inserting that value into equation 4A: T, = s J/cWhere :J/c = n~~/16 (equation 3B)

For bascule spans, leaf moment is determined by;For vertical lift, lift load is determined by load;In both cases, strain can be converted into the desired load by asingle conversion factor.The strain gage output during operation is a complex wave form.Much of the low frequency signal (.5 Hz to 10 Hz) is generated bythe individual teeth of the drive gears. The magnitude of thestrain due to the gears can be greater than the strain due to thespan imbalance. To extract the strain due to imbalance a smoothcurve can be drawn through the total strain trace. The signal canalso be played back through a low frequency filter when the signalis converted into a digital form. A computer can be utilized tofilter the signal by several various criteria, most often by pointaveraging.The operating friction of the bridge beyond the pinion shaft isdetermined by halving the difference of the raising and loweringload histories. To determine the imbalance loads without friction,.the raising and lowering load histories are averaged.For a vertical lift bridge the data reduction would be complete atthis point. For a bascule bridge, the vertical and horizontalcomponent of the imbalance must be determined. To do thisresulting load histories from all pinion shafts in a leaf are addedtogether. The cosine curve that best fits the data points must bedetermined. This is most often accomplished using a computerprogram.

The results of the program provide:1. The theoretical maximum moment (the angle of the lift thatthis moment would develop at may be beyond the actual rangeof angles the bridge operated within).2. The angle of lift that the bridge experiences the moment.3. The vertical and horizontal component of the theoreticalmaximum moment when the bridge is in the closed position.4. The operating friction.5. The degree of fit of the cosine curve.Refer to Figure 3 for items 1 - 5.Adiustinq the Balance StateAdjusting the balance state of a vertical lift bridge usuallyentail the addition or removal of weight from counterweight. Thisalone can produce the most desired balance states.The balancing of a bascule bridge requires considering the verticaland horizontal components of the bridge imbalance and of theavailable balancing weight locations.Operating load conditions can be predicted for any change in thebascule span balance state. This is performed by:1. Calculating the total of the weights to be removed and added.2. Calculating the center of gravity of the new in weight andits vertical and horizontal distanced referenced from thecenter of rotation of the bascule span.

3. Calculate the vertical and horizontal moments caused by thenew weights.4. Add these moments to the corresponding existing balance statemoments.5. Calculate the new maximum theoretical moment and its angle.6. Construct a cosine curve in phase with the lift angles witha magnitude equal to the maximum moment and a phase shiftequal to its angle.This will produce a new balance curve. Friction can be added toproduce the new opening and closing load curves. If the weight tobalance the bridge is significant the friction can be adjusted bythe factor of:New Dead Load-------------Old Dead LoadThese operating curves can them be examined to see if they fallwithin acceptable limits.

-120 KIP FEETA = HORrZONTAL MOMENTB = VERTICAL MOMENTWL= MAXIMUM MOMENTA= WL CDS ANGLEB= WL SIN ANGLEJfigure 3, EXAMPLE BASCULE BALANCE STATE

The Balance StateNo set standard of bridge balance has been widely established withthe condition of balance usually being determined by the Engineer.The factors considered in determining the best balance state areoften:SafetyWhen balancing the span, it is desireable that with the motor offand the brakes released the span should not raise from its loweredposition. On most bridges, this usually entails making the bridgeslightly span heavy. On many bascule bridges the counterweight ispart of the deck structure. A live load passing over thecounterweight will tend to lift the span. Increasing the leafweight or reducing the counterweight may be necessary to counterthis effect.Machine Wear and LoadMuch of the financial gains to be made by balancing a bridge arein increased equipment life. This is accomplished by reducedoperating loads. Span loads can be made to lessen with opening oreven made negative at the fully opened position if desired.Structural WearWhen lowered, the spans should be well seated on the live loadbearing to reduce pumping of the bearings. This minimizes anunwanted transmission of loads through the structure and thetransmission of undue load to the drive machinery. The spanlifting forces should be applied evenly to the structure to avoidtorquing of the span.

@Computerized Data Acquisition and Ana1jsis SystemThe test method described in this paper involves the recording ofan analog signal for playback during analysis. A computerized dataacquisition and analysis system has been developed to provide onsiteresults. The strain gage and inclinometer signals (whereapplicable) are entered directly to the system and converted to adigital form. At the end of each test, the system can provide thepresent balance state, predict the load curves for proposed weightchanges, and recommend weight changes based on available weightlocations.The system has the advantage of producing instant results. Thismeans results are available soon after changes in the weights havebeen made. These can be used to verify the effects of the weightchanges and if needed recommend further fine tuning of the weightplacement.The system is more compact and lighter than the current system andwith some training, a non-engineer can run the test.

Case Study No. 1The Route 52 Ocean City. New Jersey Bascule SpanUpon analysis, the span was found to have an extreme verticalimbalance moment. This would require placement of weight above thetrunnion to a degree not possible with the available weightlocations. Subsequent investigation found that the original woodand asphalt concrete deck was removed. A lighter steel grid deckis presently in place (See Figure 4). To correct this imbalance,major reconstruction of the span would be required. To minimizethe effect until corrections are made, operating the span at fullopening position can be avoided unless waterway traffic requiresit.Case Study No. 2Route 52 Somerset. New Jersey Bascule SDanIncorrect wiring of one of the wheatstone bridges produced a gageconfiguration sensitive to direct shear in one plane. The resultwas an output cosine wave in phase with the pinion shaft rotation.The mistake was identified, corrected by re-wiring of thewheatstone bridge and the test was performed.

FIGURE 4OCEAN CITYlia -tm -83 -63 -10 -ia -0 I I I I I

Case Study No. 3EBThe Eastern Boulevard Bridge carries five lanes of interstate Route278 traffic in both the east and westbound directions across theBronx River in Bronx, New York City. THe bridge has a 118 foot 84inch span between trunnion centerlines and a vertical clearance of244 feet at mean high water. The bridge is operated as four(4) independent trunnion bascule leafs with a north and south unitone each side of the river. Each leaf is comprised of two (2)separate leafs that have subsequently been bolted together to actas a unit. Therefore, each unit has two (2) sets of machineryincluding four (4) pinions and four (4) rack gears (See Figure).Mechanical deficiencies were clearly evident from the results incertain machinery. One leaf had a defective differential. Ingeneral the load sharing of the separate shafts was widelyscattered. When the torques for the four (4) separate shafts ineach leaf were total, the resulting leaf moments were similar tothe four (4) leafs. The leafs were generally span heavy by atleast 200,000 foot pounds. Representative data from the tests areprovided in Figure 6 and 7. The load verses angle traces beforeand after balancing are provided in Figure 8 and 9. Note thedecrease of the operating moments due to the addition of 10,000pounds to the counterweight.

......................I I 8 0 8t 8 I t w i - *t I I iKI??t I I I m I o mI I I IOI*YII I ,....I I I II I l l ,I , , I 0II t 1 > 1I f t a c h a t mI 1 0 1 i YiI f ,-I II I ,!-II I I D , I-.--I, I 1 0 ; u./ I 0t , ! I ,I 8 , I...., I / , /I i I & , *I E 8 ,>I k;I ! ,I , 1 1 1 0............I , , , a, I , ,I I II i 1 101I I I Y I m mI ' K b ?myI ! I l lI I m l h I i I I a I Y i m', II IUI ....l l ,I I I .I, O , Y II I t I I s I ,. nI40,I t - I II I C I I....II IEIZI Yi, O l S l '?I I I ' I X i m_I , ,II I I II**--I I II In, , P I Tt L t 1 2 1 c?, l l L 1 f If PI&: l ;om, I II J I ............1 0 4* / & I t u i h ~' K O I W I ' ??I iI ZIkI ! % I 2 5I W ! I Y I1st , ,.---I ILJEI I * : aI c I U I N I O l qI > ,I W : Y I % I a I a O, , a 1 - 1 f;r;fk; I....oII Izlal ?II , , !PI 5, I , ,, 1 81 I I I I-.--I ~ ~ U I Z I,/ I ,, , , I TI II I I I F :I I I cut, 4 4 ' Y " ~I , ..---.......I I IKEP,,0 , I ,& I , m i i n =I I IOIYIYII i I I U iI / , I I .. -., , I ,I i 1 . 1 0 ,I ,-lo,I , , ,>! '?oI I I Z I ~ ,I 80, I LCI I I#.! II It-, I-...I 1 1 0 8 1C Y l L l hI 15ia1 ?1 1 0 1I , , =, I , , ,I I / I....Ii I EL, -I I, I, I=; 7i l l l mf s t , , , I =I ......................4@II l l L W I i O C* a i m t o oI /1011i Z Q, a" It 0%...................

:------:-----------------------------------------------------------------------------------------------zLERF NO- 4BRLRNCE MOMENT (K-LBS-FT)-------:-------_----------------------------------------------------------------------------------------: LFRF : MOMENT TOTRLS I e 2 MOMENT TOTRLS 3 L 4 : MOMENT TOTRLS I. 2. 3 b 4!RNGLE ;-------------------------------'--------------------------------:-------------------------------,: : UP DOWN RUG. Cffi REG: UP DOWN RUG. COS REG: UP DOWN RUG. COS REG:R =llR.998 =-33.17WL =123.52Frict = 80.92Rngle =-15.58error =. 02 WLerror = 2.17R =106.898 = -0.65WL =106.89Frict = 34.20Rngle = -0.35error =.04 WLerror = 4.49R =225.08B =-33.81WL =229.40Frict =115.01Rngle = -8.51error =. 03 WLerror = 6.13Figure 7

Case Study No. 4poute 1&9 Vertical Lift B ridse Over the Passaic RiverThe Passaic River Route 1&9 vertical lift bridge carries two (2)lanes of traffic in both the east and westbound directions acrossthe Passiac River in Newark, New Jersey. Each end of the span isindependently operated by separate machinery controlled by thebridge operator. There is a pinion-bull gear-sheave assembly overwhich wire ropes run attached at one end to the span and the otherend to the counterweight.A 2000 pound generator was left on the bridge during the testprogram. This condition was not noted until the completion of thetest program. The generator load produce 0.07% of dead load errorin the northwest cable group. This effect was accounted for duringthe final balance calculations.The frictional forces were found to be substantially high on theeast tower, several times higher than the balance goal.The desired 1% span heavy condition was achieved when consideringeach end as a unit. Further attempts to improve the individualcorners would be questionable considering that the frictionalforces are substantially greater than balance goal.