Historic American Engineering Record
Smithfield Street Bridge, Pittsburgh (PA-2)
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Removal of Old and Erection of New Bridge
"The new north abutment wall was located 40 feet back of the old one. In preparing the foundation for the same it was necessary to remove the anchorage of the old cables, and to construct temporarily two anchor chains attached to the second pair of old towers. Previous to this wrought-iron anchors had been imbedded into the foundation of new pier No. 1, which had been built up to obtain the requisite weight for the temporary anchorage.
"These anchor chains were composed of steel eye-bars, which were on hand from the intended suspension bridge. Each chain was made adjustable in length by means of a transverse screw rod, and four sets of eye-bars, forming a funicular machine. The chain could thereby be shortened with comparatively little power. To the cable the chain bars were attached by means of two wrought-iron plates. Between these plates were cast-iron friction clutches holding the cable, and pressed and held together with bolts passing through the plates. These were attached to the cable as near to the towers as possible. To prevent slipping of the clutches on the cable, the wire wrapping was removed, and spikes driven through the cable wires behind the clutches.
"The transfer of the anchorage was done without mishap while travel as usual was going over the old bridge on both tracks. The pull per anchor chain was at times 160 tons.
"Under the first north span of the old bridge, false works had been built, which, after the transfer of the anchorage, supported the old roadway, and at the same time served for the erection of the iron girders for the new bridge. No other part of the old bridge was removed till after the erection of the new channel spans.
"In the false works for the latter an opening 100 feet wide near the Pittsburgh and was left for navigation, and temporarily bridged over with wooden Howe trusses. The false works were further so arranged as to clear one track on the old bridge, on which the team-travel moved in squads in alternate directions.
"To prevent accidents from anything falling from above on pedestrians or teams below, the false works were covered with a platform of planks, which were afterwards used for the new floor. The upper staging was built up on the outside of, and to half the height of, the trusses to be erected; at that height a traveling derrick, 30 feet high, moved on a track of iron rails. All material for the channel spans was lifted (by a hoisting engine near the south Pittsburgh end) to the platform, on which a temporary track wa laid, and all material transferred on push-cars.
"With another hoisting engine, conveniently located on the up-stream end of pier No. 3, the material for both spans could be handled and put in place without moving the engines.
"The Pittsburgh span was erected first. After the pier posts were put into position, the bottom chords and connecting web-members were put in place. The top chord sections, weighing from 7 to 9 tons, were picked up and placed on the verticals, one after the other, from each end in each truss. For closing the top chord, the two middle chord-sections were raised at one end till they met, and then sprung into line by pulling down these ends towards the bottom chord with block and tackle acting as a funicular machine.
"The false works of the Pittsburgh spans had settled more than was anticipated. Before it was possible to close the top chords the different panel points had to be jacked up 2 to 6 inches. No such trouble was experienced with the other span.
"During the erection of the channel spans no little anxiety existed at the possibility of an accident from some heavy weight dropping to the platform and breaking through to the constantly crowded old bridge below. Fortunately, the work was completed without such accident, but there were two casualties, which both resulted luckily. One man fell from a height of 80 feet into the river, but was picked up and next morning was at work. Another man fell from a height of 50 feet into shallow water; he was able to report for work after two days.
"The iron floor construction was suspended to the trusses after these were swung.
"The detail of connections in the Pauli trusses being simple, the erection of the steel and iron work went off smoothly, and with no more expense than in parallel chord trusses. It commenced in the middle of September, 1882, and was completed December 31st, 1882.
"To the new iron floor construction the old bridge floor was temporarily suspended with iron plate-girders at the south end of the bridge. These down-stream towers were removed first, together with the cable they supported. The old bridge floor where it was not suspended from the new bridge was held up on wooden trestles.
"Three plate-girders in each span, supporting the down-stream track and sidewalk, equal to half the width of the new bridge, were put into position, and the paving for one car-track finished for the entire length of the bridge, without interrupting travel on the old bridge below.
"Temporary wooden trestle approaches, with plank floors for one track, were built at both abutments, because the filling in would have interfered with travel on the old bridge. All this work was much retarded by a stormy and severe winter. Travel was turned over the new bridge on the down-stream track on March 19th, 1883. (33)
"During a high water, February 22, 1883, a heavy mass of ice came down the river on a swift current, and tore away a part of the false works supporting the old bridge in a place where it was not suspended from the new one. The old bridge was then in danger of falling into the river; but by promptly suspending the old floor to the new one, first with ropes and chains, and then with iron rods, the old bridge, after one and a half day's interruption, was again safe. This was the only interruption of travel throughout the whole work.
"After travel was turned on the new bridge, the gaps and openings in the abutments and piers were walled in, as stated before. The remaining old towers, cables and bridge floor were removed, and the up-stream half of the plate-girder approaches completed.
"This was done by placing in position the remaining three plate-girders in each span, and the iron columns (supporting the girders near the abutments). At the same time the erection of the hand-railing and of the ornamental cast-iron tower progressed. The adjustment nuts of the diagonal ties in the channel spans were covered with ornamental castings, which prevent tampering with the sleeve-nuts.
"The filling in and regrading of the approaches at both ends, and the building of the toll-houses and bridge office, were completed simultaneously with the superstructure.
The Flooring of the Roadway and the Sidewalk
"This consists of preserved wood, namely, gum-wood and white pine, preserved by the zinc-tannin process. On both the roadway and sidewalks the bottom planking distributes the weight on the iron and girders, so that the top sheeting or paving forms merely the wearing surface.
"To the top of iron floor girders are bolted wooden bolsters, to which is spiked the bottom cross-planking, 3 inches thick for the roadway, and 2 inches thick for the sidewalks.
"No provision is made to carry off the water sideways. The grade of the bridge is sufficient to carry off all surface water lengthwise. Besides, the durability of the preserved gum-wood is increased by keeping the floor moist (by sprinkling during the dry season).
"The space between the track rails and in the middle of the bridge is paved with preserved gum-wood blocks, 3 inches thick and 3 inches high, laid with 1/4 inch strips between.
"Every paving block is fastened down to the bottom planking with diagonal spikes. The paving blocks for the tracks rest on a 1 inch longitudinal sheeting of preserved white pine, which serves to distribute lengthwise any uneven pressure to the cross-planking beneath. The joints between paving blocks were filled with a hot mixture of tar, pitch, rosin, lard, lime and sand in such proportions as to run freely from the ladle.
"The space between the tracks and sidewalks is covered with a lengthwise top planking 3 inches thick.
"The sidewalks are 9 inches higher than the roadway. The wearing surface consists of white pine, 1 inch thick, on the bottom planking of gum-wood, 2 inches thick. In the curb are openings 30 inches long, and on the average 3 feet apart for cleaning the roadway of mud and snow. Under the sidewalk, on the down-stream side, extends a box with a movable cover, the entire length of the bridge. This contains the water and gas pipes and telegraph cables. Every 150 feet are covered opening for hose attachment, provided for sprinkling the floor and for use during a fire.
"A small fire occurred in April, 1883. It originated on the old bridge, and scorched the floor of the new bridge near the southern end. It showed the necessity of guarding against fire on the new bridge. All wooden flooring is to be protected by a paint of quicklime and glue water, and all crevices and joints in the wooden floor to be filled with it.
"For the preservation of the lumber by the zinc-tannin process, the specifications stipulated that steaming in the curing cylinder should continue at 18 pounds pressure for four and a half hours; the vacuum should not be more than two pounds per square inch. Gum-wood should absorb 25 percent, and white pine 12-1/2 percent of the antiseptic solution under a pressure of 30 pounds per square inch. The solution is to be 5 parts (in bulk) of chloride of zinc to 95 parts of water. The lumber was to be left in it till each cubic foot of gum-wood had absorbed one and a half gallons, and each cubic foot of white pine 1.05 gallons of the antiseptic. After this a solution of tannin was forced into the cylinder, and the lumber kept immersed in it for 3 hours, under 80 pounds pressure.
"Borings from the end of a stick which were analyzed, contained 0.370 percent metallic-zinc in weight, equivalent to 0.789 percent of zinc chloride. This was rather a high showing, as 0.5 percent of zinc chloride was all that was expected.
"Borings taken from the middle of a stick 26 feet long, 12 inches by 6 inches, were found, on analysis, to contain 0.125 percent of zinc chloride, or only one-quarter of the amount intended to be injected; it is doubtful whether in long sticks the desired percentage can be attained, without very materially increasing the strength of the solution, which again would probably increase the percentage at the ends to such an extent as to render the lumber brittle after a while.
"The borings were from freshly treated lumber. It is probable that the percentage is gradually increased to a limited extent in the heart of a long stick, owing to interchange of the solution by capillary attraction along the grain of the wood. The real antiseptic substance is the zinc chloride, while the tannin serves only to increase the adhesion of the precipitate to the wood fibers.
"Care was required in the inspection of the lumber before treatment. Sap, loose knots, cracks, windshakes, are of course as much a defect in treated as in untreated lumber. Any unsound, weak or soft wood will not be improved by the treatment, which aims merely to make the lumber more durable, by preventing rotting. It may be added that lumber, treated by the zinc-tannin process, will not lose anything in its value as a combustible, as the experience with the fire at Monongahela Bridge proved.
"The track rails on the bridge are 12 inches wide, and composed of flat bars 7 x 3/4 inches, 2 feet long under the joints, and 3/4 inch round spikes, with conical heads, countersunk to the full thickness of the rail (3/4 inch), so that the spikes may hold down the rail, no matter how thin it may have worn.
The Ornamental Towers
The ornamental towers are built of cast-iron, the roofs being of wrought-iron; they support merely their own weight; they incase the steel posts, which, to the eye, would seem very slender supports, and would appear out of proportion in comparison with the heavy piers and high trusses. The end posts can rock inside of the towers, which are not in any way connected with them. Where the trusses pass through the towers, room is left for expansion from temperature changes.
"The architecture of the towers is so planned, and the composing parts so arranged, that the portals may be widened out to suit the entrance to a wider bridge, should it be required.
"Besides painting the metal with raw linseed oil at the mills, and iron oxide paint at the bridge shops, two coats of white lead paint were applied to the erected steel and iron work. The white lead paint was used without any dryer, and mixed with boiled linseed oil only. All joints and crevices where water might collect, were puttied all around and raw linseed oil poured in, as much as they would hold.
"As the erection took place mostly in inclement weather, the shop paint came off in many places by dragging the pieces through slouch and mud, which, especially in Pittsburgh, rusts iron rapidly.
"Rusty places were coated with a thin lime paste, which, after drying, was scrubbed off with wire brushes and freshly painted.
"All iron work under the flooring has been painted brown, all iron and steel work above the flooring is blue. The towers have a stone color.
Loads and Unit Strains
Beginning from the north end, there are:
1. One 40 foot span, six equal plate-girders, proportioned for a live load of 10,800 pounds per lineal foot of bridge.
2. One 81 foot span, six equal plate-girders, proportioned for a live load of 9,000 pounds per linear foot of bridge.
3. One 87 foot span, six equal plate-girders, proportioned for a live load of 9,000 pounds per lineal foot of bridge.
4 & 5. Two channel spans, 360 feet each, two equal Pauli trusses of steel and floor construction of iron, proportioned for a live load of 4,500 pounds per linear foot of bridge and in addition a concentrated load of 40 tons on a 20 foot wheel base for each track; of these loads the sidewalks were assumed to carry 100 pounds per square foot.
6. One span, 88 feet 3 inches, six equal plate-girders.
7. One span, 84 feet 9 inches, six equal plate-girders.
8 & 9. Two spans, 60 feet each, six equal plate-girders in each. All of these plate-girder spans proportioned for 9,000 pounds live load per linear foot of bridge.
The wind truss and lateral bracing under the floor is proportioned for a wind force of 400 pounds per lineal foot of bridge.
The above live loads, in addition to the load of the superstructure in the different spans, produce no greater strains per square inch of useful metal areas than:
8,000 pounds in compression flanges of all plate-girders, floor beams, stringers, etc.
9,000 pounds in tension flanges of all plate-girders, floor beams, stringers, etc.
8,000 pounds tension in suspenders and hangers of channel spans.
4,000 pounds shear in iron web-plates.
12,500 pounds bearing strain on iron in rivet and pin-holes.
9,800 pounds to 13,200 pounds in compression members.
15,000 pounds in steel eye-bars.
10,000 pounds shear on steel rivets and steel pins.
20,000 fibre strain on steel rivets and pins from bending-moment.
18,000 pounds bearing strain on steel in rivet and pin-holes.
Table of Quantities
The following quantities of material were consumed in the construction of the Monongahela Bridge:
Lumber, feet B.M. (board measure) . . . 594,000
Foundations Piles, lineal feet . . . 10,800
Concrete, cubic yards . . . 1,280
Iron, tons . . . 32
Stone masonry, cubic yards . . . 10,500
Iron, tons . . . 1,070
Steel, tons . . . 740
Cast-iron of towers, pedestals, etc., tons . . . 196
Preserved lumber for floor, feet B.M. . . . 358,000
Steel rails, tons . . . 134
Filling, cubic yards . . . 10,000
Sidewalk pavements, square yards . . . 1,400
Street pavements, square yards . . . 2,200
The total cost of construction amounts to about $460,000.
In 1890-91 the bridge was widened by utilizing the provisions built into the original bridge. (34) Lindenthal was both designer and contractor for this change. A third truss was added to each span on the up-stream side of the bridge which increased the width of the structure by twenty feet, eight inches and provided a second roadway. When this was done the street railway tracks ran on each side of the center line, but twenty-one years later the up-stream trusses were moved four and a half feet to the eastward and the additional width made it possible to put both the electric car tracks on that half of the bridge and devote the opposite roadway to other vehicular traffic. Sidewalks eleven feet wide projected beyond the truss work. The floor system, beneath the car tracks, was also modified in 1911, but the other half remained essentially the same as in 1883.
Between 1911 and 1915 the elaborate wrought-iron bridge portals were removed and such simpler gateways of cast-steel, designed by Stanley Roush, were substituted. Here we can see the old ideas of the monumental bridge portal in process of disappearing, but even these later portals are highly ornamental. (35)
In 1895 the City of Pittsburgh determined to secure title to the bridge and throw it open to the public, an action which was in accordance with the trend of the times. After the appointment of viewers and the taking of testimony on both sides, the Commissioner's Report was filed in Court, and no exception being taken, the City assumed complete ownership of the Corporation through purchase of the outstanding stock. (36) The purchase price of the bridge was $1,000,000. (37)
After 1911-15 there were few changes in the bridge, but by the early 1930's it began to be in need of repair. In order to lighten the load on the structure, it was proposed to install an aluminum deck on the vehicular roadway and this was carried out in 1934. According to The Engineer of London -- "This is, as far as we have been able to ascertain, the largest bridge undertaking in aluminum that has yet been carried out. It has afforded engineers an opportunity to gain experience in the use of aluminum on a large scale. Regarded as an experiment in bridge building we suggest that its importance cannot be overrated." (38)
In the early 1960's the bridge was once again exhibiting signs of wear and stress. The Pittsburgh Post Gazette of April 10, 1964, announced that "Approximately $700,000 will be spent next year to rehabilitate the Smithfield Street BridgeÉWeight limits have been placed on the bridge."
The Pittsburgh Press for May 7, 1967 stated that the bridge would close in June to permit the installation of a new deck -- "the contract for the repairs had been awarded to the Mosites Construction Company in July, 1966, and since that time substructure work has been completed as well as the fabrication of new aluminum deck panels. A polyester non-skid coating is to be applied to the panels". The Post Gazette of November 16, 1967 announced that the bridge "closed since last June will reopen today at three o'clock. The total cost of repairing the structure was $712,615".
In 1970 the Pittsburgh History and Landmarks Foundation placed one of its historic landmark plaques on the structure, and on May 28, 1974 the bridge was named an official city landmark by the City Planning Commission under the city's landmarks ordinance. (39)
Perhaps the most famous double lenticular truss span in the world is the Saltash Railway Bridge spanning the River Tamar in Cornwall which was designed by the great engineer Isambard Kingdom Brunel and built in 1857-59 just before his death. (40) Perhaps the Smithfield Street Bridge deserves no lesser fame. Impressive as is the Brunel Bridge, the former is at least the more graceful and beautiful.
According to David Plowden -- "The Smithfield Street Bridge was the first and largest bridge in the New World to employ the Pauli system of lenticular trusses, it remains the only example of this type in America". (41)
The Monongahela Bridge at Smithfield Street is now within a few years of attaining its centenary, that magical state which should ensure its veneration by all who care about our technological monuments. Rumors of demolition still trouble the local air, but our great bridge, now the oldest on our three rivers at Pittsburgh as well as the oldest steel through-truss span in America, if it cannot continue to bear increasing burdens, should be, like the great bridges at Wheeling and Cincinnati, eased into an honorable quasi-retirement. Lindenthal's splendid spans have served long and well, they are now an almost indissoluble part of the city-scape, and it is profoundly to be hoped that this tough, but graceful structure will, as it begins its second century, enter upon a new period of usefulness.
1. J. N. Boucher, A Century and a Half of Pittsburgh. (New York, 1908), 11, p. 387. See also C. W. Dahlinger, Pittsburgh, Sketches of its Early Social Life. (New York, 1916), pp. 29-30.
2. The route of Jones' Ferry appears on the McGowan map of Pittsburgh of 1852, together with other ferry routes plying the local rivers.
3. Herbert duPuy, "A Brief History of the Monongahela Bridge, Pittsburgh, Pa.", Pennsylvania Magazine of History and Biography, XXX: 2 (1906), pp. 187-188.
4. A 1795 map of the City of Pittsburgh shows the sand bar but on the Molineux map of the city of 1830, it has disappeared.
5. Krasmus Wilson Standard History of Pittsburgh. (Chicago, 1898), pp. 112-113.
6. Act of March 19, 1810.
7. Act of February 17, 1816.
8. Pittsburgh Gazette, October 27, 1818.
9. Pittsburgh Gazette, November 24, 1818.
10. Llewellyn Edwards, A Record of the History and Evolution of Early American Bridges. (Orono, Maine, 1959), P. 198.
11. Richard S. Allen, Covered Bridges of the Middle Atlantic States. (Brattleboro, Vermont, 1959), p. 75. There is also a description of the bridge in "A View of Pittsburgh" in The Franklin Magazine Almanac for 1820. pp. 51-52, and in Rebecca Raton, Geography of Pennsylvania. (Philadelphia, 1837), P. 235.
12. D. B. Steinman and S. R. Watson, Bridges and Their Builders. (New York, 1941), pp. 121-122.
13. Dictionary of American Biography, X, Pt. 2, pp. 2-3. See also Robert Fletcher and J. P. Snow, "A History of the Development of Wooden Bridges", Proceedings of the American Society of Civil Engineers, LVII (1932).
14. duPuy, p. 194.
15. Ibid, p. 198.
16. There is a contemporary oil painting by the Pittsburgh Artist, William C. Wall, "The Great Fire of 1845) on display at the Old Post Office Museum, Pittsburgh, (on loan from John H. Follansbee, Sr.) which shows the Monongahela Bridge in flames. Another canvas, attributed to the same painter, and in the possession of the Museum, shows the ruins of the city and the bridge just after the fire.
17. The Builders of the Bridge. The Story of John Roebling and His Son. (New York, 1945), pp. 89-100.
18. D.A.B., 16, pp. 86-87; Steinman; Hamilton Schuyler, The Roeblings - A Century of Engineers, Bridge Builders, and Industrialists, (Princeton, New Jersey, 1931; "John A. Roebling", Engineering News, 10 (May 26, 1883), p. 246.
19. John A. Roebling, "The Wire Suspension Aqueduct Over the Allegheny River at Pittsburgh", Journal of the Franklin Institute. Third Series, X (1845), pp. 306-09. The bridge is illustrated on p. 307.
20. "The Wire Suspension Bridge Over the Monongahela River at Pittsburgh", The American Railroad Journal, 19, (April 4, 1846), p. 216, also (June 13, 1846), p. 376. The version quoted here is a quote from a reprint in a periodical published at Pittsburgh and edited by Neville B. Craig, The Olden Time, 1:6 (June, 1846), pp. 286-288 (reprinted 1876 under a Cincinnati imprint). See also Gustav Lindenthal, "The Monongahela Bridge - Rebuilding of the Monongahela Bridge at Pittsburgh", Engineering News, (July 7, 1883), pp. 314-315; 11 (March 14, 1884). pp. 239-241-371; "The Suspension Bridge", American Railroad Journal 10 (February 21, 1846), p. 126; Col. S. M. Wickersham, "The Monongahela Suspension Bridge", The Scientific American Supplement 15 (1883). p. 6201; "The Monongahela Suspension Bridge at Pittsburgh, Pa.," Engineering News, 10 (May 26, 1883), pp. 243-244; The Iron Age, 31:25 (June 21, 1883), pp. 3, 5, 1. Roebling's American Railroad Journal article is also illustrated with a lithographed plate of his own drawings for the structure. Steiman in his biography also reproduced two Roebling drawings for the Bridge -- opposite p. 134.
21. duPuy, p. 201.
22. Joseph White and M. W. von Bernewitz, The Bridges of Pittsburgh. (Pittsburgh, 1928), p. 32.
23. See description of a scull-race in Harper's Weekly, XIX: 545 (June 8, 1867), pp. 363-364, with a wood engraving after a sketch by C. B. Reinhart, showing the Bridge.
24. Steinman, p. 100
25. dePuy, p. 202
26. Gustav Lindenthal, "Rebuilding of the Monongahela Bridge at Pittsburgh, Pa.", Transactions of the American Society of Civil Engineers, vol. XII, no. CCLXIII (September, 1883), p. 355.
27. Biographical Review, (Pittsburgh and Vicinity), (Boston, 1897) XXLV, pp. 475-477: Memoirs of Allegheny County, Pennsylvania, (Madison, Wisconsin, 1904) I, pp. 37-38; Pittsburgh Gazette Times, February 22, 1907.
28. Lindenthal, p. 355.
29. duPuy, p. 203
30. D.A.B., 21, Supplement 1, pp. 498-499: Transactions of American Society of Civil Engineers. CV (1904); Who's Who in America (1934-35); Who's Who in American Engineering (1931); Civil Engineering (September, 1935); Engineering News Record. (August 8, 1935); Electrical Engineering, (September, 1935); New York Times, (August 1, 1935).
31. Lindenthal, pp. 355 ff-
32. A lenticular, "bow-string" or "fish-belly" through truss named for the famous German engineer Friedrich August von Pauli (1802-1883). For Pauli see Zeitschrift fr Baukunde, VII: 6 (1884), pp.379-396; Allegemeine Deutsch Biographi, (Leipsig, 1887), 25, pp. 251-258; and Zeitschrift des Vereins Deutscher Ingenieuren (1865) -- "den Artikel von Gerber uber die Berechnung des Brckentrager nach Paulischen Systems".
33. The opening of the new bridge was noticed in the following: The Scientific American, XLIX: 12 (September 22, 1883), pp. 175-180. There is a large full-page wood-engraving of the north portal, with an insert showing a general view of the bridge; Monongahela-Brcke, zu Pittsburgh in der Verlangerung on Smithfield Street", Zeitschrift fr Baukunde, 11:4 (1884), pp. 258-260; "The Rebuilding of the Monongahela Bridge at Pittsburgh, Pa.", Engineering News, 11 (May 24, 1884), pp. 251-253, 265-266.
34. The Engineering News, 67:15 (April 11, 1912), pp. 676-680.
35. "Decorating a City Bridge With Structural Steel Portals:, The Engineering News, 74:23 (December 3, 1915), p. 1086.
36. duPuy, p. 204.
37. The Pittsburgh Bulletin, XXXII: 24 (18 April, 1896), p. 4.
38. "Aluminum Floor for an Old Bridge", The Engineer (London) CLVII: 1098 (27 July, 1934), p. 91.
39. Pittsburgh Press, May 29, 1974.
40. Eric De Mare, The Bridges of Britain (London, 1954), pp. 180-181. See also David Plowden, Bridges (New York, 1974), p. 66.
41. Bridges, p. 167.
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Historic American Engineeering Record (HAER) Text: James D. Van Trump, 1974