24

THE MEMPHIS BRIDGE.

for a final adjustment after the half span had been built out half itslength. Before beginning the erection of the half span, the distanceacross the opening was again measured and the wedges placed at nearlymid-position, the measurement having checked previous measurementsand computations within a small part of an inch. After the wedges hadi been placed and the erection of the half span commenced, the adjustmentwas not changed until the span had been released at the adjustmentpoints at the west end.

When an attempt was made tff move these wedges, the doublewedge was found defective. One of the wedges would move first; theimmediate result was a great increase of pressure on the other wedgeapplied at its small end; this fixed it in place, and as the screw wasturned to continue the withdrawal, the movement of the first wedge con-tinued until some means were used to prevent its movement.

For the lower chord adjustment at the east end of -the span ahydraulic jack (Plate 44) was provided for each truss. Each jack hada plunger 14 in. in diameter, with a stroke of 3f in. One force-pumpwas provided for operating these jacks; it was of special design and verysimple in construction. The ordinary leather cup-packing proved in-adequate and was replaced by soft lead. The adjustment in the lowerchord was simply the thrusting out of the hydraulic jacks to the properdistance to give the end of the half span, when built out, the same eleva-tion as the end of the west half span already built. The stroke of thejacks was not enough for this, but this was provided for by placing cres-cent-shaped fillers in the elongated pin holes between one of the pins andthe webs of the chord. When the half span was built half its length,the jacks were used to increase the distance between the adjustment pinsi in. With this adjustment the limit of the capacity of the pump was^reached ; and as the erection proceeded, this half span, like the west one,became unadjustable. When the half spans met at the center they wereso nearly the same elevation that little difficulty was met in driving thecenter pins of the upper chords which closed the span and completed theconnection between the upper chords and the web systems.

To supply the place of the end adjustments ordinarily provided forswinging the span, the Chief Engineer decided to depend mainly on ex-pansion by temperature to extend the upper chord, and' on a toggle jointTo shorten the-lower chord, so' that the adjustment pins could be with-drawn. For one full truss panel of the lower chord there were substi-tuted two lengths of eye bars connected to adjoining sections of bottomchord by temporary pins and to each other by short coupling bars. A

short vertical rod 2 in. square, having a nut at the lower end and an eyeat he upper end, passed between each pair of coupling bars and througha washer which took bearing on the lower edge of the coupling bars.The toggle joint was completed by lifting on the 2 in. square rods. Theeye bars and couplings were placed to permit the insertion of the rivetedlower chord sections after swinging the span, the top flange angles beingremoved and riveted back again after placing. The arrangement of thebars is shown on Plate 45. The eye bars were called the toggle bars,the joint at center was called the toggle joint, and the rods by which thetoggle joint was pulled up were called the toggle rods,i: ■ As the ends of the half spans were too high, the opening in thelower chord between the ends of the riveted sections (one section at thecenter being left out), when the joints in the upper chord were justbrought into bearing, was nearly 4 in. less than the length of the closingsection. It was necessary to raise the toggle joint 3 ft. before the togglebars became taut. By loading the adjoining cantilever arms the lowerchords were compressed so that the toggle bars became taut when thetoggle joint was lifted 1£ ft.

The erection of the. east half span ready for the closing sections wascompleted April 8th; the traveler and all superfluous material were re-moved ; a derrick was erected on the end of each arm to handle the clos-ing sections. The season was well advanced, and for several days thetemperature had been too high to close the upper chord sections, if allpreparation had been made; but cooler weather came opportunely, andat 9 a.m. of April 10th they were inserted with an opening of only in.more than required.

During the next three days the inclined rods (Plate 45) for liftingthe toggles were placed; in addition, two pairs of heavy triple blocksrove with manilla line 2 in. in diameter were placed in each truss, con-necting the upper and lower chords at the center of the span, each pairof blocks having a capacity of 25 tons. The derricks were taken down,and all was in readiness to swing the span; but the weather became coldwhen heat was wanted.

During the swinging of the span changes in strain causing changesin length must occur. The total shortening thus caused in the upperchords amounted to about 4f in. The distance between the fixed pointson Piers I and <III was 1411 ft. 5 J in. ! The'change in length for 1°Fahr. was 0.0095 ft. To produce a change in length of 4f in. re-quired a change of 41°. The top chord at the center of the intermediatespan became closed at 56°. A temperature of 97° was necessaiy to re-

lease the adjustment pins in the upper chord. This natural temperaturecould not be had, and various means were tried to swing the span at alower one, all tending to increase the length of the upper chord.

During the several days of cool weather which followed the connec-tion of the top chords at the centre of the span, the cantilever anus be-tween Piers I and II were loaded with timber, rails, locomotives, etc., toabout 6000 pounds per lineal foot. The toggle rods were screwed up dailyand the adjustment pins watched closely, but there was no sign of loosen-ing. The resident engineer, Mi*. Alfred Noble, adopted the expedient ofheating the upper chord of the intermediate span with steam. All inchpipe was placed in each chord for the entire length of the span. Canvaswas placed under the pipe on the lacing to retain the heated air in thechord; on the 19th of April steam from the locomotives was turned onthe pipes, the toggle rods were screwed up, and as heavy a strain as theywould stand was put on the lines, and blocks placed to assist in raisingthe toggle joint. As the temperature approached its maximum for theday it was found that Piers I and III were being pushed apart, themovement of Pier I being f in. and of Pier III, 1 £ in.; the possibility ofthis motion had been considered, both in its effects on the masonry of thepiers and in swinging the span. Masonry, like all other construction, iselastic; the piers could move much more than these amounts withoutinjury. The motion of the piers, however, worked against the shorten-ing of the top chord by compression, and this attempt to swing the spanfailed. It almost succeeded, however, as was shown by the fact thatsome of the upper chord web bearings could be moved on the pins.

During the next two days four rods, each If in. round, were placedon each truss, to connect the toggle joints with the center of the upperchord and aid in lifting the toggles; steam pipes were laid in the upperchords of the central span extending half way from Pier II to Pier III.Clamps (shown on Plate 44) were placed at the adjustment points ineach of the upper chords at the west end of the intermediate span; eachclamp consisted of four rods, 2| in. diameter, with upset ends and nuts,bearing against pieces of rail and through oak blocks bearing against theedges of the tie plates of the chord sections.

On the morning of April 22d the operation of lifting the togglejoints began ; there were 24 toggle rods, and the work of turning up thenuts required several hours of steady work; the joint was finally raised3.75 feet above the line of the lower chord. At 4 p.m. the steam pipeswere connected to the locomotives and a heavy strain put on the clamps;in an hour the adjustment pins at the west end of the upper chords be-