APPENDIX ."E— Continued.

35

nitude I do not think it would be wise to attempt to do so. It must be remem-bered that the season which can be depended on for working in the river atMemphis is only five months long.

CANTILEVER BRIDGE.

The main span of the Cantilever Bridge would occupy the portion of theriver spanned by the two easterly spans of the Three Span Bridge, the lengthof this span being 1300 feet. This is one half greater than the span of anyrailroad bridge now in existence, but 300 feet less than the East River Suspen-sion Bridge and 400 feet less than each of the two main spans of the ForthBridge now building in Scotland.

I have estimated on making the cantilevers 150 feet deep at the ends andbuilding them with curved upper and lower chords, the masonry to finish tenfeet above high water. This arrangement is not strictly in accordance with therequirements of the charter hitherto granted ; it gives the required height (65feet) for a distance of about 400 feet at the center, but this height is reduced ateither side. It would, however, accommodate the navigation interests of theriver perfectly, as there is abundant room in the center for the upper works oflarge boats and abundant width for tows which are never high. There can belittle doubt that if this design should be decided on there would be no difficultyin getting the authority to build it.

To secure lateral stability I have proposed to put the cantilever trusses 75feet apart at the base and to build them in inclined planes, these planes to beput 15 feet apart at the highest point. To avoid concentrating too much weighton one point I should put the support of the anchorage span 75 feet from thesupport of the main span. The result of this arrangement would be that thecantilever structure would be supported on each side of the main span by agroup of four piers, these piers to occupy the corners of a 75 feet square. Thisarrangement is similar to that adopted for the Forth Bridge in Scotland.

In the structure I have estimated on, the center of the easterly group ofpiers is placed at station 104 -f- 12.5, and the center of the westerly group of piersat station 117 -f- 87.5, the length of the span, measured from centers of piers nextto the channel being 1300 feet, the distance between centers of towers 1375 feetand the distance between centers of piers including towers 1450 feet. Thedesign provides for shore or anchorage arms 375 feet long, which places theeast anchorage 240 feet back of the edge of the bluff and places the westanchorage at station 122, or about 150 feet from the low water shore line. Itwould probably be economical to shorten the east shore arm and to lengthenthe west shore arm. With the arrangement as it is now designed, a 225 footspan will be required to reach from the west anchorage to the end. of the ironviaduct.

The substructure of this Cantilever Bridge will consist of (1) the eastanchorage pier, (2) the group of piers under the east tower, (3) the group of

piers under the west tower, (4) the west anchorage piers and (5) a small pierunder the west end of the approach span.

The east anchorage pier will be of the simplest kind, being merely a pieceof masonry built in the bluff and of sufficient size to give the weight requiredfor the anchorage. I have estimated on 1200 yards of masonry, but of lessexpensive character than that of the piers in the river.

The group of piers under the east tower would consist of four piers, each16 feet in diameter under the coping, resting on caissons 36 feet square andfounded at the same depth (— 55) as Pier I of the Three Span Bridge.

The group of piers under the west tower would be similar to those underthe east tower, except that the caissons would be 40 feet square and the foun-dations sunk to — 79.

The west anchorage pier would be founded on a caisson 35 feet by 75 feetat a depth of — 55, and though of considerable dimensions would offer no specialdifficulties in construction.

A small pier would be required back of the shore line to support the heavybent at the end of the trestle, which would sustain the shore end of the 225 feetspan.

The same shore protection would be required for the east tower as forPier I of the Three Span Bridge, the same protection around the west tower asaround Pier III and the same amount of protection on the west shore as withthe other design, the total cost of protection being perhaps two thirds of thatrequired in the Three Span Bridge.

A general plan and strain sheet have been made for this structure, and anapproximate estimate of weights. This design, however, has not been workedout with the same degree of detail as the design for the Three Span Bridge, andthe estimates, though probably ample, are not as accurate.

The estimated cost of this bridge complete would be as follows:

East Anchorage Pier.....

$20 000

East Tower:

750 c. y. Masonry at $20.

$15 000

16 000 c. ft. Cribwork “ 40 cts. ..

6 400

26 000 “ “ Caisson “ 80 cts.

20 800

Sinking.

7 500

$49 700

Group of 4 Piers.

198 800

West Tower:

910 c. y. Masonry at $20.

18 200

32 000 c. ft. Cribwork “ 40 cts.

12 800

32 000 “ “ Caisson “ 80 cts.

25 600

Sinking.

10 000

$66 600

Group of 4 Piers.

266 400

West Anchorage Pier:

3 300 c. y. Masonry at $20.

66 000

27 000 c. ft. Cribwork “ 40 cts.

10 800

52 000 “ “ Caisson “ 80 cts...

41 600

Sinking.

12 000

130 400

West Approach Pier...

5 000

30 000

70 000

Outfit for Foundation Work. .

Protection Work.

Tot AT, STrRfiTRTTrTTTTiir.. . .

$720 600

Cantilever Superstructure.

One 225 ft. Span.

.14 000 000 lbs.

400 000 “

Floor and Painting...

14 400 000 lbs. at 6 cts.

$864 000

Total Supebstbuctube

879 000

Total Bbidge Pbopeb. .

$1 599 600

West Annroach as for Three Soan Bridge..

Total Bbidge and Approaches.

$1 794 600

Add 10 per cent for Contingencies

Engineering, etc.

‘ O'-. --

$1 974 060

50 000

$2 024 060

This is the entire estimated cost of the bridge ready for the rails from thebluff at Memphis to the foot of the grade on the west side of the river. Com-paring it with the estimate of the Three Span Bridge, there is but a slightdifference in the cost. The main cost of this bridge is the great cantileversuperstructure, the accurate weight of which can only be determined afterworking out a careful design in detail. While I fully believe that the estimatenow made is ample, I think the chances of saving the 10 per cent contingencyallowance are much less in this last plan than in the former, and that it isprobable that the Cantilever bridge would cost $200 000 more than the ThreeSpan Bridge.

In the matter of time the cantilever structure could probably be builta little quicker than the Three Span Bridge. The foundations for the easttower and anchorage could easily be put in this year and the east half of thestructure erected during the Spring of 1888. The foundations for the westtower and anchorage could be put in in the low water season of 1888, the westhalf of the superstructure erected immediately thereafter and the bridgeopened for traffic in the early Summer of 1889.

CONCLUSION.

Comparing the two structures when once completed, I think the ThreeSpan Bridge would be the better one for the railroads. It would be a per-fectly simple structure, the expense for maintaining which would be a mini-mum. It would involve no complicated details, and as it consists simply ofstraight trusses resting on masonry piers, would be subject to a minimumdegree of disturbance should any slight settlement occur in the foundations.In brief, it would fulfill the universal requirement that the simplest structureis the best.