assembly and erection of pamunkey river bridge bascules

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Trunnion Shafts and HubsThe size and weight of the span-mounted machinery components presented their ownfabrication challenges. The trunnion shafts were forged at a facility utilizing a 5,000-tonpress—the largest in the country. Careful consideration had to be given to the sequencefor fitting the trunnion shafts to the hubs and the hubs to the bascule girders, both ofwhich were specified as ANSI class FN3 interference fits. (Several papers have beenwritten on this topic and varied sequences used in the past.) The size of the pieces,available shop handling equipment, heating and cooling capabilities, and the internalstresses in the hubs caused by the heating and cooling process were all factors to beconsidered.The decision was made to fit thehubs to the bascule girders first andthen the shafts to the hubs. Thissequence reduced the possibilityof cracking the hubs due to the hightensile stresses experienced if thehub-and-shaft assembly were cooledexternally as a single unit. Thesestresses would be caused by thefaster cooling of the hubs and theslower cooling of the shafts due totheir higher thermal mass.After the hubs were fit to the basculegirders in the horizontal position,these assemblies were heated to270 degrees F and the shafts cooledto minus 90 degrees in order to fitthem to the hubs. (Figure 6) Thegirder-and-shaft units were thentransported outside, rotated into thevertical position, and lifted ontotemporary supports on the bargesfor the initial stages of assembly.Maneuvering the four bascule girdersections with the 12-ton, 17-footlongtrunnion shafts installedperpendicular to the webs requiredspecial lifting considerations.Figure 6 – Fitting the Cooled Trunnion Shaft into the Heated Bascule-Girder Hub6
4. ONSITE ASSEMBLY AND ERECTIONBascule Span Assembly and Lock BarsOverall bascule geometry and dimensions were controlled by standard survey methodsduring the initial stage of assembly in Florida. An as-built survey of the leaves wasprovided to the contractor for use in establishing the trunnion centerlines on site, fromwhich the remaining machinery layout was derived. Once the barges containing thepreassembled sections arrived at the jobsite, the remaining assembly was completed.The lock bars and sockets were installed and bolted in their final configuration on thefirst leaf, while the receiving sockets were temporarily attached to the opposing leaf withundersized bolts until achievement of final alignment. Splice plates were attached on oneside of the structural connections and as much of the electrical hardware and lubricationsystempiping installed as possible.Racks and PinionsThe machinery plans prohibited the finalalignment, drilling, reaming, and attachmentof the racks to the underside of the basculegirders until the spans were set in place andaligned. The racks were originally to bedelivered to the steel fabrication yard duringassembly in Florida and transported to thejobsite bolted in position on the underside ofthe bascule girders with temporary bolts insub-sized holes.Figure 7 – Temporary Positioning of Rack and PinionHowever, fabrication took considerably longer than anticipated because of factorsassociated with the size of the racks and pinions. The machine shop had to manufacturecustom tooling and use single-index cutting methods for the gear teeth, for instance, andthe machining work was limited to the largest equipment in the shop. As a result, theracks and pinions were trucked directly to the jobsite just prior to setting each bascule inplace. Temporary supports were designed and fabricated to hang the 16-ton racks 35 feetabove the bascule pit floor so that they could be set in place prior to the counterweightsections. In order not to interfere with the setting of the bascules, the racks were set oneinch below and about 10 inches forward of their final position. (Figure 7)The pinion-gear assemblies also had to be set in place, offset forward of their correctposition, before the racks and bascule spans were set. The pinions could not be installedin their final configuration, as they would engage the rack and be back-driven when thebascule was opened during initial testing. The pinions could easily be damaged duringthis back-driving, and their placement would also hinder the rack alignment process.Therefore, coupled anchor bolts were used so the pinions could be slid into positionafter the racks were aligned and then the upper portion of the anchor bolts installed.7
Page 4 and 5: The engineer’s estimate for this
Page 8 and 9: Trunnion BearingsEach trunnion bear

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