THE MEMPHIS BRIDGE.

17

in., 56 ft. 51 in. became a common divisor. The lengths were, therefore,slightly changed, and a panel 56 ft. 51 in. long was made the unit for theentire bridge, this panel being divided into two panels in the floor sys-tem. Each cantilever arm was divided into three panels, each suspendedspan into eight panels, and the central span into 11 panels.

As the bluff rose rapidly back of Pier I a long anchorage span wasnot required, and it was thought best to limit it to such length that therewould be no reversal of strains; it was made four panels, or 225 ft. 10in., long.

The anchorage span consists of four panels; the channel span of 14panels, three in each cantilever and eight in the suspended span; the cen-tral span of 11 panels; and the west span of 11 panels, of which threeare in the cantilever and eight in the suspended span. The total numberof panels is, therefore, 40; and the total length of the continuous super-structure is as follows;

Anchorage arm.

10 in.

Channel span.

5 “

Central span.

0* “

West span.

0i“

Total.

. 2 258 ft.

4 in.

The arrangement of these spans is given on Plate 4.

As each truss panel is divided into two floor panels, there are 80panels in the floor system. Moreover, the floor system is extended east-ward on a viaduct of three floor panels beyond the anchorage pier, and adeck span of 12 floor panels reaches from the shore side of Pier IV toPier V which is located back of the shore line. The entire floor systemof the bridge, therefore, consists of 95 panels and is 2681 ft. 9| in. long.

The lengths of the spans being fixed, the next thing to determinewas the width. The principal limit in determining this was the length ofthe central span. In the matter of transverse stiffness the position of thisspan corresponds with the separate spans of a common bridge, whereasthe longer span by its cantilever construction was held rigidly at theends. The central span being 620 ft. long, it did not seem wise to makethe width between trusses less than 30 ft. This corresponded withwidths which have been adopted with good results in shorter spans.The channel spans of the Cairo bridge are 518.5 ft. long and the trussesplaced 25 ft. between centres, the ratio between length and width beingalmost exactly the same as between 621 ft. and 30 ft. A width of 30 ft.was adopted.

The next feature to determine was the depth of the trusses, and

in determining this other considerations than economy of metal workgoverned.

The difficulties of erection made it important to keep the dimensionswithin limits to which ordinary falsework could be adapted. The mag-nitude of the structure and its cantilever design made it important to usesuch proportions that vibrations would be reduced to a minimum. Itwas not thought best to make the depth at any part of the superstructuremuch more than two and one half times the breadth of base. Thebreadth of base could not be increased without increasing the length ofthe piers, which would have added to the already excessive cost of thefoundations. By fixing the depth of both the central and the suspendedspans at one eighth of the span and making all three of the cantileversalike, the depth of the structure over each of the piers and for the wholelength of the central span became 77 ft. 7|f in., and the depth of thesuspended spans, 56 ft. 5J in. These dimensions were adopted.

It was at one time proposed to make the central span in eight panelsof greater length than those used elsewhere, but as the plans matured thisseemed unwise, and a uniform panel length was adopted throughout.

The form of trusses adopted was the double triangular or doubleWarren girder. It was intended to erect the channel span without false-work, building out from either end, and this form of truss was thoughtto have advantages in the manner in which it sustained the upper chordfrom which the work would be done.

The floor system is suspended from the intersection points of the twosystems of web members, and the upper chord is supported from thesame points by struts.

As the depth of the cantilevers over the piers corresponds to thedepth of the central span, while their outer ends correspond to the depthsof the suspended spans, the chords are not parallel. The same may besaid of the anchorage arm. In order to keep the floor panels uniform, itwas necessary to keep the points of intersection of the web members overthe panel centres. This made the panel lengths of the upper chord irregu-lar, but as these chords were formed of eye bars the irregularity was notmaterial.

The cantilever construction made it possible to bring the weight ofthe spans on each side of each pier together in a single point and transferthis weight directly to the centre of the pier, instead of taking it nearerthe edge of the pier, as is always done with separate spans. This al-lowed the use of somewhat lighter piers than would otherwise have beenneeded, and the increased cost of the superstructure design over three

equal separate spans was in a measure balanced by the decreased cost ofthe substructure.

The only span which is continuous from pier to pier is the centralspan, and provision had to be made at one end of this span for expansionand contraction. All other expansion was taken up by sliding joints atthe ends of the cantilevers. As there are two sliding joints in the chan-nel span and only one in the west span, there was a double chance to takeup expansion in the chords and floor system of the channel span. Forthis reason the west end of the central span was fixed and the east endplaced on expansion rollers.

Principal Details.— The general dimensions and form of trusshaving been adopted, it next became necessary to fix the character of theprincipal details. The American form of construction with pin connec-tions was adopted from the first. The chords of the central span, beingsubject to reversals of strains from the action of the cantilevers, werenecessarily both made stiff, and the same stiff bottom chords were neces-sarily extended out as the bottom chords of the cantilevers. The topchords of the cantilevers being always in tension were made of eye bars.As the east suspended span (in the channel span) was to be erected with-out falsework, the two halves being built out as continuations of the can-tilevers, it was necessary that the bottom chord of this truss should alsobe made stiff for the greater part of its length, ivhile the top chord, beingin compression in the finished structure, was of course stiff. It was de-termined to sacrifice some material and to make the bottom chord of thesuspended span stiff throughout, and to make both suspended spans (orintermediate spans, as they were called on the drawings) with stiff chordsthroughout. The chords of the anchorage arm correspond to those of thecantilevers. The bottom chord is therefore stiff throughout the entirelength of the superstructure, and the only portions of the top chordwhich are made of. eye bars are in the anchorage and cantilever arms andin the end half panels of the central span, this special arrangement beingadopted from the difficulties of packing a stiff chord into an eye barchord at the extreme ends of the span.

The stiff chord provision undoubtedly adds to the lateral stiff ness ofthe bridge and, to a certain extent, to its vertical stiffness. The slidingjoints at the ends of the cantilevers are carefully fitted, and in spite ofits cantilever construction the bridge is very free from vibration.

The lateral bracing is formed of diagonal rods attached to pin platesriveted to the chords and so placed that the axial lines intersect at thecentre of each chord. The lateral bracing is adjustable, and is the only