AASHTO Design Criteria Applied to Hydraulic Bascule Bridges

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A) The effect of acceleration or retardation generally consideredto be between 5 and 15 seconds. The obvious force hereis overcoming inertia.B) The effect of frictional resistance which generally consistsof the frictional resistance in the main trunnion bearing,the efticiency of the hydraulic system which should accountfor pressure losses in lines, fittings, and components.C) Any unoalanced condition of the bridge. It is generallyaccepted that some unbalance be required for basculebridges. This unbalance is normally to make the bridge noseheavy, thus requiring somewhat less force for closing and toprovide a positive downward force at the live load shoe.For hydraulic cylinder this may also help even-out operatingpessure when co~riing down since the rod end of the pistonyields a smaller area upon which to develop the total operatiunalforce.D) Wind loads for which three load conditions are generallyconsidered. A 2 1/2 pound wind used for the selection ofpower and design of hydraulic system to operate the bridgeIn tne normal time frame, 60-75 seconds. A 10 pound windused for selection of power and design of hydraulic systemto operate the bridge in twice the normal time, 120-150seconds, and a 20 pound wind force used to establish maximumhydraulic pressure for holding the bridge in any position.Division I1 of this specification is entitled, "Design" and isdivided into two groups, structural and machinery. Of interest inthe structural section is Article 2.1.11 entitled, "Impact". Hydrauliccylinders are supported by some part of the structure andwill be affected by the operating forces. This section requirestnat stresses in structural parts caused by the machinery/hydraulicor by forces applied by moving or stopping the span, shall be increased100% as an allowance for impact.Article 2.1.12, "Fatigue", refers the designer to the basic designcriteria and allowable fatigue stresses as given by the currentedition of AASHTO Standard Specifications for Highway Bridges.Article 2.1.14 "Hydraulic Cylinder Connection". The stresseson the structural connecrion to the cyliners shall be based on acylinder pressure of 150 percent of the setting of the pressure-reliefvalve controlling the maximum pressure available at thecylinder.Continuing on into the Machinery Design section - the designersfirst series of steps is to determine tne location and size of hydrauliccylinders and the following steps would thus be necessary:
1) Establish normal time for opening - 60-75 seconds.2) Establish the Maximum angle of opening generally 74 to 77degrees.3) Positioning of hyraulic cylinders. Obviously the firstchoice would be to locate hydraulic cylinders under maingirders. This would not be the only location available andin the case of bridge renovation where a hydraulic systemwere to replace a machinery system, other locations mayprove to be more suitable. The connection of the rod eye attne end of the piston rod via pillow blocks (bearings) tothe main girder or structure should be located well belowtne trunnion and as far forward of the trunnion as is possible.The bottom surface of the bottom flange of the maingirder provides the designer with both these features. Thebasic object is to create a situation where the moment arm,being the distance between the piston rod and the center ofrotation (trunnion), is maintained at a maximum throughoutthe opening and closing process. This, in effect, will producethe minimal amount of operating force needed during theentire opening process. Cylinders can be supported on themachinery platform at the end with a clevis type device orat intermediate points between the bottom and the top of thecylinder. In effect, one consideration in determining theultiitiate length of the cylinder would be the stability ofthe cyli~lder and piston rod when fully extended. Theshorter the distance between the rod eye and the point ofsupport of the cylinder which would result in an overallshorter cylinder. In essence, the cylinder is made longerin order to leave the piston deeper within the cylinder whenthe piston rod is fully extended. The term for this is"stop-tube distance", a distance added to the length ofstroke required to fully actuate the bridge. It has beenour finding that a center mounted trunnion cylinder providesthe best features for design. Supporting the cylinder tooclose to the top also results in the need for a very accuratealignment between the vertical axis of the main girderand the horizontal positioning of the cylinders.4) At this point one should calculate or estimate the variousindividual load considerations as mentioned earlier, such asovercoming inertia, resistance in the trunnion bearing, efficiencyloss in the hydraulic system, unbalanced momentsdue to unbalanced load conditions, and the loads created bythe 2 1/2 pound wind, the 10 pound wind and the 20 poundwind.
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AASHTO Design Criteria Applied to Hydraulic Bascule Bridges

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