BRIDGES AND
TUNNELS OF
ALLEGHENY COUNTY,
PENNSYLVANIA
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HAER
Smithfield Street Bridge, Pittsburgh, PA
01 Cover Page
02 Foreword
03 Ferries
05 Monongahela
Bridge and
Fire
12 Masonry
17 Removal
of Old and
Erection of
New Bridge
18 Flooring
19 Ornamental
Towers and
Painting
22 Alterations
23 Footnotes
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Smithfield Street Bridge, Pittsburgh, PA Historic American Engineering Record PA-2 page 15
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Quality of Steel Used "Every heat of steel was tested and its quality determined before any more work was done to it. "For the compression members and pins, the steel was required to stand the following test on specimen bars 3/8 inch diameter: Elastic limit: 50 to 55,000 pounds per square inch. Elongation in 8 inches: Minimum 12 per cent. Reduction of area at fracture: Minimum 20 per cent. Cold bending: 180 degrees around its own diameter without crack. Cold punching of holes in flat 3 x 1/4 inch bars; 3/16 inch from the edge without crack or distention of metal. "All specimens and shapes were required to be finished at nearly the same heat, as it was observed that rods finished at a lower heat would give higher tension results than samples of same steel finished at a higher heat. "The Andrew Kloman firm in Pittsburgh had contracted to procure the steel and to furnish the steel shapes. "The intention was to use Bessemer steel for the compression members; a large lot of Bessemer steel was tested, but few samples were found to stand the required tests. The difficulty seemed to consist in controlling the uniformity of the steel within close limits for quality and strength. After a while the attempt was given up and open hearth steel was substituted. No trouble was then experienced in getting a uniform grade of steel of prescribed quality. "The top chord sections consist of four leaves, which were originally designed to be each a 20 inch steel plate with 4 x 4 inch singles for flanges. In ordering the steel it was discovered that enough plates of that width could not be procured in the required time. Therefore, the chord sections were changed to 10 inches and 12 inches steel plates, with 4 x 4 inch angles, composed as shown in the drawings. "Notwithstanding the great care used, the finished plates and angles were by no means a uniform product. According as they in rolling were finished at a higher or lower heat, they would have different degrees of hardness. Steel plates and angles finished at a lower heat had a smooth surface, and the noise of punching them resembled pistol-shots, while plates finished at a higher heat had a rougher surface, and there was hardly more resistance to punching than in wrought-iron. "The specifications for riveted steel work provided that the punched rivet-holes, 3/4 inch diameter, should in the assembled parts be enlarged to 1 inch diameter by reaming. The time for the delivery of the steel work growing short, the question was considered whether the reaming of the holes could be avoided, to hasten the completion of the work at the shops. Messers. Kellogg and Maurice, in Athens, Pa., had the contract for this part of the work. "To that end and the following experiments were made: "Ten specimens were cut from the same steel plate 1/4 inch thick; one specimen was tested to ascertain the tensile strength of the steel in the specimen. The nine other specimens, all alike in form, were prepared as shown in sketch, for the purpose of ascertaining the effects of punching holes, of punching and reaming, and of drilling. The tests were expected to show the amount of reaming required, and whether any annealing effects from the hot rivet on the injured steel around the punched hole could be observed.
"The conclusion from these tests was that the injured steel (of the quality used in this instance) around the punched hole was in part restored by annealing in contact with the hot rivets, the size of which was large in proportion with thickness of steel plates and angles as used in the chords. "The reaming of the punched holes to a greater extent than to make the rivet holes smooth and straight was therefore dispensed with, and a reduction in the price for the finished work agreed upon. "The same quality of steel as for the compression members was used for them; they were forged from solid steel billets, and turned to size. No appreciable difference in the hardness of the metal in the pins was observed. "For tension members and rivets, the steel was required to stand the following tests on specimen bars 5/8 inch diameter: Elastic limit: 45 to 40,000 pounds per square inch -- Yield. Ultimate strength: 70 to 80,000 pounds per square inch. Elongation in 3 inches: Minimum 18 per cent. Reduction of area at fracture: Minimum 30 per cent Cold bending: to a loop 360 degrees around its own diameter, without crack. Cold punching in 3 x 3/4 inch bars of 1 inch rivet holes: 1/4 inch from the edge without crack or distension of metal. Open hearth steel of the above and uniform quality was obtained without trouble. "The eye-bars were made by the Kloman process, i.e., the bars were rolled from billets between reversible and adjustable rolls, in such manner as to leave the ends thicker than the bar. The ends were then spread and forged to the proper shape of the eye, under a steam hammer. The heaviest steel bars for this bridge were 28 feet 6-1/2 inches long, centre to centre of eyes, and 1-13/16 inches thick. All steel billets and all steel bars required very close inspection for flaws, the detection of which was sometimes difficult. "It has been stated that for the detection of flaws in steel or iron, a magnetic needle had been used with success, though the manner of its use the writer has not heard stated. A device for the certain discovery of flaws in steel bars is certainly needed. Where the solid metal sections are proportioned very economically to the work they have to do, flaws are a source of great danger, especially in attenuated steel structures; flaws in wrought-iron are more likely to happen in the direction of the fibre, but in steel they can as well happen cross-wise to the direction of the tension strain as any other way. "Three steel bars 9 feet long between centres of eyes, and 4 inches x 1-1/16 inches in section were tested to ascertain the effect, if any, of annealing the finished bars. The results were as follows:
Pin-hole in one eye of bar C was bored 1/4 inch out of centre line of bar, and accounts for its lower ultimate and elastic limit. A specimen from the same heat of steel, of which the above bars were made, showed on a 3/4 inch round -- Elastic limit . . . 46,389 pounds per square inch Ultimate limit . . . 78,898 pounds per square inch Elongation in 8 inches . . . 18.0% pounds per square inch Reduction . . . 30.2% pounds per square inch The net section of the heads through the pin-holes for all eye-bars being at least 50 per cent more than the bars, and the good effects from annealing being doubtful in the above tests, it was thought not necessary to anneal the steel bars. For steel rivets the above quality of tension steel proved very suitable. The rivets were tough and tenacious. It was, however, observed that the manufactured rivet-heads would easily break off with few blows, the fracture in each instance showing a fine granulated appearance. "Rivet heads, however, made by hand or riveting machine were very tough, and could not be broken off; they had to be cut off. "The cause for the brittle rivet-heads was supposed to be the upsetting by blows, in forming the head at a high heat in dies, producing sharp corners under the rivet-head and around the rivet-stem.
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Introduction Last modified on
30-Sep-99
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