AASHTO Design Criteria Applied to Hydraulic Bascule Bridges

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Article 2.5.3, "Power Requirements and Machinery (Hydraulic)uesign" established specific load conditions and combinations forwhicu tne hydraulic system shall be proportioned and power provided.Condition A is tne combination of loads for normal time ofopening, the 60-75 second time frame. For bascule bridges the loadcombinations include the effects of frictional resistance, inertia,unbalanced load conditions and a wind load of 2 1/2 pounds persquare foot acting normal to the floor with the bridge in any position,for open decked grid floor bridges the floor area may be reducedby 15%.Condition B is the combination of loads for whicn an extendedtime of operation is allowed, specifically 1 1/2 times the nornaltime. This combination requires an additional 2 1/2 pounds persquare foot for ice loading be applied. In addition to those loadsspecified under condition A above. Obviously, this load conditionneed only be explored where ice is likely to be encountered.Condition C, again allowing for an extended time of operation,specifically twice the normal time, requires the following combinationof loads: Frictional resistance, inertia, unbalanced loadconditions, and a wind load of 10 pounds per square foot on anyvertical projection of the open bridge and an ice load of 2 1/2pounds per square foot. Again, the ice loading need only be appliedin areas where ice is anticipated. When calculating theactual time of operation, at no time should the wind loads be lessthan 2 1/2 pounds acting normal to the bridge as compared to the 10pounds per square foot acting on the vertical projection of theopen bridge.A brief discussion and consideration of electric motors andpumps is important at this point. In most hydraulic applications asomewhat higher speed motor is generally provided than that usedfor the machinery design systems. It has been our practice toselect an 1800 rpm motor. The hydraulic pump that we intend to usewill require horsepower control. With this pump the monitoring ofhorsepower in terms of flow (gallons per minute) and pressure willbe provided. With this type of pump we will obtain automaticstroking or ramping of the pump to insure that the combination offlow and pressure generated by the system will provide a uniformhorsepower output. Thus the motor is generally brought up to speedprior to energizing or stroking the pumps. With this motor pumpcombination the system has the ability to operate under a varietyof load combinations and the effect would be to regulate flow thusextending the time of operation. This feature is a perfect matchfor the AASHTO loading combinations in which, as the loads are increased,the times for operation are extended. . The followingseries of steps are suggested in sizing cylinders and selectingpower:
5) Would first require the calculation of the forces generatedby each of the three load conditions, A, B, and C. Generallyeach lift consists of two main girders and it is thereforeimplied tiiat a miniurn of 1 cylinder be required undereacll main girder. It has been our practice on the somewhatlarger bridges, that two cylinders be located under eachmain girder, or a total of 4 cylinders per leaf. This providesthe designer and the owner with the ability, duringrepair and servicing of cylinders, to remove one or even twocylinders and still have the capability of bridge operationunder normal loading conditions. We have generally foundthat load condition C will dictate the number of cylindersor size of cylinders that must be provided and by reviewingtne operating pressures developed for load condition A whentwo cylinders have been removed from the system will findthat operating pressures are within safe limits.b) Determine the total operating force generated by conditionA based on normal time of operation. This should be donefor both the opening procedure and for the closing procedureby calculating the surface area of the piston for allcylinders and dlviding that into the opening force generatedone can determine the operating pressure required, andsiu~ildrly calculating the surface area on the rod side oftne cyllnder and dividing that into the force required toclose the leaf will generate the operating pressure that results.Generally operating pressures for normal time of operationshould be around 500-600 psi.7) Determine operating force, and tile pressures for condition Bwith an extended time of 1 1 /2 tne normal time in a similarfashion to 6 above, a pressure of approximately 1000 psiwould be appropriate.8) Determine operating force, and the pressures for condition Cwitn an extended time of twice the normal time in a simlarfashion to b above, a pressure in the range of 1500 to 2000psi.9) The next condition of consideration is the ability of thecylinders to hold the bridge in the fully open position forthe 20 pound wind force. Under this condition the maximumpressure of between 2500 and 3000 psi would be appropriate.10) Having calculated the various combinations of loads andcylinder pressures and thus determined the appropriate numberand size of cylinders, and having positioned them, ournext step would be to determine stroke length and verifythe time of operation for the various load conditions.Since loads and moment arms vary as the bridge
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AASHTO Design Criteria Applied to Hydraulic Bascule Bridges

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