6
THE
BLAIR CROSSING BRIDGE.
V.
SUPERSTRUCTURE.
The superstructure consists of three through spans and two short deckspans in the approaches. The through spans are 330 feet long betweencenters of end pins, and the deck spans were 110 feet long, but the westapproach span was subsequently replaced by a 176 feet span. The entiresuperstructure was manufactured by the pound from designs prepared bythe engineer of the bridge, complete shop drawings being furnished tothe contractor.
On November 6th, 1882, a circular was addressed to prominent bridgemanufacturers inviting them to make proposals for the construction of thesuperstructure. In response to this invitation, five proposals were received.These proposals were opened in New York on the 2d day of December,1882, and the work was awarded to the Keystone Bridge Company, whichwas the lowest bidder.
The trusses of the 330 feet through spans are divided into fifteenpanels of 22 feet each; they are of the double system Whipple type, withinclined end posts, are 44 feet deep, and spaced 20 feet between centers.The top chords, end posts, bolsters, rollers, bearing plates, pins, and theeye-bars in the nine central panels of the bottom chords are of steel; allother parts are of wrought iron excepting the heavy wall plates resting onthe masonry, the washers, and the ornamental work, which are of cast iron.The details of these spans are given on Plates 12-16. The deck spansare shown on Plates 17, 18.
The trusses were not proportioned to carry any particular class oflocomotives or cars now in use, but for general conditions which arebelieved to include the heaviest class of rolling-stock which is ever likelyto be used, and to make a provision for the increased effect of a movingload on the central members of the web. The dead load in the long spanswas taken at 30,000 pounds per panel. The actual weight of each span,including 450 pounds per foot of timber floor, and rails, was 929,035pounds, equivalent to 30,968 pounds per panel if averaged over the wholebridge. Inasmuch as a portion of this weight is carried directly by themasonry, and as the reduction in weight from the use of steel occursprincipally in the central panels, the assumed weight per panel is more
nearly correct than the average weight. The moving load provided forin the trusses was 3,000 pounds to the foot, except that, in proportioningthe web, the excess of maximum strain in any member over and abovethe strain in that member under a uniform load of equal intensity wastaken on a basis of 5,000 pounds per foot, instead of 3,000. The floorsystem is proportioned for a load of 5,000 pounds per foot throughout.The top lateral system is designed to resist a wind strain of 300 poundsper lineal foot and the bottom lateral system 500 pounds per lineal foot,the whole being taken as a moving load. The strain sheets are given onPlate 19.
The weights of iron and steel in the 330 feet spans are as follows: —
Three Spans.
Average per Span.
Steel.
Pounds.
Pounds.
884,548
Pounds.
294,849
“ “ Floor.
1,012,343
434,805
1,447,148
57,322
482,383
19,107
Total .
....
2,389,018
79 6 ,339
The weights of iron and steel in the deck spans are as follows : —
no' Span.
176 ' Span.
Pounds.
Pounds.
Steel.
4,979
6,753
Wrought Iron.
T 10,503
238,55°
8,955
4,325
Total.
124,437
249,628
The specifications under which the superstructure was manufacturedare given in Appendix G.
The steel blooms were manufactured by the Cambria Iron Company,at Pittsburg , Penn. , and the steel was rolled by Carnegie Brothers andCompany, Limited, at the Union Iron Mills in Pittsburg . A full recordof the tests of sample bars was kept, and fourteen full-sized bars were
broken at Watertown. The behavior of these full-sized bars was excellent,the only bars which failed to meet the requirements of the specificationsbeing bars which would have been condemned without the test. Theonly point in which the specifications were changed was, that the requiredreduction in the samples of tension steel was changed from 45 per cent,to 42 per cent. A summary of the records of tests of full-sized bars withthe analyses is given in Appendix H.
The superstructure was erected on staging, consisting of bents reach-ing to the level of the lower chord, with a large traveler and without theuse of upper false work. The false work under. the easterly long spanrested on the ground, that under the other two long spans on piles. Thewest approach deck span was raised in June, 1883. The erection of thefalse work for the easterly through span was begun August 5th, the raisingof iron began August 16th, the trusses were connected through by August2 2d, and the floor was all put in by the 24th. Pile-driving for the falsework for the central span was begun on August 13th, the raising of iron wasbegun September 5th, and the trusses were coupled up on September 9th.The false work for the westerly through span was begun September 17th,the erection of iron began on October 16th, and the trusses were coupledup on the 19th, the floor being put in by the 2 2d. The raising of the lastspan was delayed about a week by the accidental destruction of thetraveler. The east approach span was raised in September. The entireerection was handled by Mr. William Baird, for the Keystone BridgeCompany.
The floor system is shown on Plate 11. It consists of 9 by 9 oakties, generally 12 feet long, spaced about 15 inches between centers. Theseties rest on iron stringers 9 feet between centers. The track is laid witha 6o-pound steel rail. Inside the rails are placed two lines of 5 by 4 by-|-inch angle irons, bolted to every tie with i-inch bolts; these angle ironsacting as guard rails in case of derailment, and also holding the tiestogether. A timber ribbon 10 by 10 inches, faced 12 feet over all, is boltedto every fourth tie, and a narrow footway and wire rope hand-rail completethe floor.
The bridge was formally tested in the presence of a committee ofengineers, and opened for traffic October 27th, 1883. The official reportof the engineers who observed this test is given in Appendix I. A newwest approach span was erected in August, 1885, and the old span wastaken down and used at the crossing of White River.