THE MEMPHIS BRIDGE.

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adjustable or movable parts in the open air above the masonry in plainsight, there being no unexposed portions and no ironwork buried inchambers, the only iron out of sight being the long rods and the I beamplatforms, which being buried solid in Portland cement are protected fromoxidation, while there is a large excess of material in the rods.

3. The superstructure and moving load throw on each of the twopoints of support on Piers I, II and III a weight of about 2000 tons, thisweight, however, being thrown directly over the axis of the pier. Todistribute this weight properly on the masonry requires an area of about100 square feet and a sufficient height between the masonry and thechords of the superstructure to distribute it with some degree of uni-formity. On Piers I and III the bearings are fixed. They are shown onPlate 38. In each instance the weight is transferred first to a 14 inch pin,passing through the centre of the chord. The three posts, one verticaland two inclined, rest on this pin, being made with semicii’cular pin bear-ings at the ends, no pin plates passing around the pins, this arrangementbeing adopted partly for convenience in erection and partly because theweight always carried here is so great that nothing approaching to tensioncan ever exist. This 14 inch pin rests on a steel casting cast with ribsplaced directly under the bearings of the posts on the pin. The inclinedposts are two-web posts and the vertical posts four-web posts, but theyare so packed that the whole weight is transferred to six ribs in the cast-ings. The steel casting rests on two iron castings which are packed withlong bolts and locked together with a grooved steel plate between them.These two castings rest again on four castings which are locked togetherin the same manner at the centre. The whole was fastened together byturned bolts passing through drilled holes. The actual weight of eachpedestal casting between the pin and the masonry is about 45 tons. Thecentre of the 14 inch pin is 9 ft. 10 in. above the masonry, whichbrings the top of the stringer of the bridge 13 ft. above the masonry.

4. On Pier II the bearing, which carries the same weight as thoseon Piers I and III, had to be made an expansion bearing to allow of theexpansion of the central span, temperature alone here representing anexpansion of about eight inches. It is shown on Plate 39. The piers areslender and the expansion great; it was, therefore, deemed necessary toprovide a joint which would move with the least possible friction, andwhich should not be liable ever to become clogged or stopped by the col-lection of dirt. The joint must also be so arranged that the structurewould be held laterally and the motion limited longitudinally. It wastherefore determined to use rollers 15 in. in diameter, and to make these

segmental rollers of a pattern resembling the European practice, the rollersbeing placed 6 in. between centres.

Each expansion bearing as constructed consists, first, of a steel castingprecisely similar to those used at the fixed ends. This steel casting restson a bolster composed of a horizontal top plate, then of eleven 12 in.I beams ranning transversely, of a second horizontal plate, of sixteen12 in. I beams running longitudinally, and of a third and thicker horizon-tal plate, the lower surface of which is polished smooth. It had origi-nally been intended to make this bolster of two steel castings with aplaned steel plate between them, and this arrangement would have beenpreferred; but the delays in getting the castings and the uncertainty ofsecuring the finished product in time made it necessary (after three cast-ings had been made and rejected) to change the plan.

Under the bolsters came the rollers. The rollers are in two lengths,and are separated by two steel guide plates, one of which is built inbetween the polished bottom plates of the bolster and the other betweenthe iron castings below. These serve as the transverse guides. Thereare thirty rollers in each joint, fifteen on each side of the central guides.The rolling surfaces of these rollers are polished, and each roller has twoholes drilled completely through it, through which pass turned rods, sothat the rollers on one side of the centre must always work with those onthe other, while the distances between the rollers are kept constant bytwo side plates drilled to fit the rods, the rods being held in place by nutsoutside the side plates. The upper side plates are made with hookedends, which, striking against the ends of the lower side plates, limit thepossible motion of the rollers and prevent any possible overturning.

Under the rollers is the bearing plate (commonly called the railplate). This is of the pattern which the engineer has used universallyfor the last ten years. It is formed of T rails riveted on a plate below,the tops of the rails being about £ in. apart and the top surfaces planedand polished. This arrangement makes a stiff surface for the rollers toroll upon and provides adequate means for cleaning. The rail plates arein two parts, divided by the centre plate which guides the rollers.

The motion of the top bearing is further limited by a lock platewhich is fitted over the lower guide plate, the upper jaws of which wouldstrike the edges of the top bearing plate before a motion could occurwhich might cause the top plate to slide on the rollers.

Under the rail plates are the castings, which bear directly on themasonry and are like the lower sections of the fixed end castings.

The result of this arrangement is a very sensitive and very powerful

expansion bearing. For convenience of observation it was fitted withvernier scales, and the record, which has been kept of the motion so far,indicates that this bearing works practically without friction.

5. A somewhat different form of support was used at the end of theintermediate span over Pier IV. This is shown on Plate 40. Here, also,the weight is distributed on the masomy by iron castings, and the weightis transferred to the iron castings through a pin on a steel casting; butthe pin is placed below the level of the chord, the whole weight beingtransferred by a small upright support, which also holds the floor beam.While this works fairly well, it is not as satisfactory a detail as the otherbearings. It was adopted with a view to keeping the construction of thisparticular panel point as nearly uniform as possible in the four placeswhere it occurs, this supported upright taking the place of the suspendeduprights at the other three points.

6. The expansion between the cantilevers and the suspended spansis taken up by sliding joints in the top and bottom chords, the long sus-pender swinging. No special addition was made to this suspender inconsequence of this swinging motion, the extra strain which can possiblybe produced in this way being less than that which is usually producedin horizontal eye bars by their own weight. The sliding joint in thebottom chord is placed in the last panel of the cantilever. The slidingjoint in the top chord is placed in the first panel of the suspended span.As the end web members of cantilever and suspended spans are parallelcompression members, both of these sliding joints come where no strainexists.

The sliding joint in the bottom chord is shown on Plate 43. Itslides between polished steel surfaces with a play of only fa in. Thisjoint is placed near a floor beam and the lateral system of the cantileverarm ends at this floor beam, which forms the lateral strut. The lateralsystem of the suspended span ends on the other side of the sliding joint,there being an independent strut to hold the two chords in position; thisstrut was put in after the erection of the superstructure, the pin holesbeing reamed in position. There is no observable lost motion at thisjoint. The top lateral strains are transferred at the ends of the suspendedspans to the bottom chord by the portal bracing, and there is no lateralsystem in the top chord of the end panels of the suspended spans. Thesliding joint in the top chord is made, therefore, simply an oblong pinhole between two sets of stiffened tension members. The same oblongpin hole was used in the bottom chord joint, but principally for necessi-ties of erection.