Coraopolis Bridge

The first bridge to cross the Allegheny at Sixth Street was a six-span, flat-roofed, covered wooden bridge constructed in 1819. This bridge had a total length of 1,037 feet (316 m), consisting of four 185-foot (56 m) spans, a 170-foot (52 m) span, and a 137-foot (42 m) span. The design and construction of this bridge have traditionally been attributed to a Mr. Lothrop; it was presumably a Burr arch truss, like the Ninth (Hand) Street Bridge, another of Lothrop's products. Physical evidence for this assumption survived until the 1890s, when an observer reported that the skewbacks from the wooden arches remained visible in the north abutment.

In 1860, a wire-suspension bridge, designed by John A. Roebling, replaced the decaying timber structure. This bridge was four spans long, with two 344-foot (105 m) main spans, and approach spans measuring 177 and 171 feet (54 and 52 m) each. The bridge was dominated by cast iron towers, each formed by four cast iron columns.

Although the Roebling bridge, with its iron superstructure, was generally believed non-flammable, it fell victim to fire on June 19, 1881. The bridge was not destroyed, but its floor system of white pine and white oak was severely damaged. John Harper, president of the Allegheny Bridge Company, which owned and operated the structure, stated that he believed the fire was the result of "sparks and perhaps flame from the stacks" of passing steamboats igniting bird nests located under the superstructure of the bridge. After heavy electric streetcars were introduced to the bridge in 1890, concern over its functional capacity grew, and in 1891 plans were begun to replace the suspension bridge with one that could better handle the steadily increasing traffic, as well as permit trolleys to cross without reducing their speed.

Coraopolis Bridge, from below deck, showing deck girders, eyebar tension members, lattice girders of truss superstructure

The third Sixth Street Bridge was designed by the nationally significant engineer Theodore Cooper (1839–1919). This bridge has been identified as the last surviving structure entirely designed by Cooper, whose involvement extended even to such details as the bridge's handrail, lamps, and fascia.

The third Sixth Street Bridge, like most roadway river bridges in the 19th century, was privately built and operated as a toll crossing. The total cost of construction was $560,000. This cost was underwritten by the Sixth Street Bridge Company and Fidelity Title and Trust Company, successors to the Allegheny Bridge Company, which had received its charter in 1810. When the bridge was opened in 1893, the toll was set at two cents for each man, while women crossed at no charge. Beginning in the late 1890s, Allegheny County and the City of Pittsburgh undertook a systematic program of acquiring the privately owned bridges within their jurisdiction and eliminating their tolls. The Sixth Street Bridge was purchased by Allegheny County in 1911 along with nine other bridges at the combined price of $2,851,000; all ten of these bridges were subsequently declared free.

The trusses for the third Sixth Street Bridge were fabricated by the Union Bridge Company. This company had been formed in 1884 by the merging of the Central Bridge Company of Buffalo, New York, and Kellogg and Maurice of Athens, Pennsylvania. The Buffalo plant was closed around 1890, so presumably the trusses were produced in the shops which remained at Athens.

Coraopolis Bridge with newer pony truss at left, from SE (Coraopolis side)

The superstructure was erected by the Baird Brothers, John and William, who first advertised in the Pittsburgh and Allegheny City Directory in 1886 as contractors located at Home and Valley Streets. In the 1891 and 1892 editions of the Directory they were listed as bridge builders; their listings no longer appeared by 1900, and by 1911 William Baird had left the construction business and had begun managing a hotel.

The Baird Brothers had been involved with numerous bridge projects and companies in the late nineteenth century. William Baird had worked on seven bridges crossing the Missouri River, the Merchants Bridge across the Mississippi River at St. Louis, two bridges crossing the Ohio River at Wheeling, West Virginia, the bridge carrying the Baltimore & Ohio Railroad across the Susquehanna River, and bridges at Steubenville, Ohio, and Poughkeepsie, New York. John Baird had also been involved in the construction of the Eads Bridge in St. Louis and the Cairo bridge in Memphis, and had been employed by the McCann Construction Company, the Keystone Bridge Company and American Bridge Company.

This bridge was also outgrown.

While the Pittsburgh City Council and the Municipal Affairs Committee of the Pittsburgh Chamber of Commerce argued for the retention and upgrading of the existing bridges, the issue was ultimately decided by the Department of War, acting under the authority of Section 18 of the Rivers and Harbors Act of March 3, 1890 (30 Stat., 1121–1153). This act empowered the Secretary of War to require the removal or alteration of any bridge which " ... is an unreasonable obstruction to the free navigation . . . on account of insufficient height, width of span, or otherwise". So, unfortunately, the War Department decreed that the numerous bridges over the Allegheny with their differing spacing of piers, main spans, and generally low clearances, were an impediment to navigation, and decreed that all bridges would have to be replaced. Although the decree was discussed in the early 1900s, and first issued in 1917, serious work did not begin till 1924. As part of the Three Sisters (Pittsburgh) project, the bridges at Sixth, Seventh, and Ninth Streets were to be demolished and replaced. The other two bridges were destroyed and scrapped, but the Cooper Bowstring trusses were moved to Coraopolis and reused.

The Cooper spans remained in good shape, and concerns for economizing in public works projects led to recycling of the 1892 bridge. Commissioner Armstrong took credit for proposing to reuse the Cooper bridge instead of building an entirely new structure in nearby Coraopolis, saving Allegheny County $350,000. The Foundation Company bid on a contract to move the Sixth Street Bridge from its site in downtown Pittsburgh to Coraopolis, twelve miles away. Winning the contract at a bid of $316,200, the company also assumed any risk that the spans would sink or be damaged during the project. The company allowed pedestrians to cross the structure while roadway removal proceeded in the fall of 1926 in order to reduce the inconvenience for residents. Workers took off half of the roadway at a time, leaving pedestrian areas accessible until final segments were taken away

Coraopolis Bridge with newer pony trusses in background, filigree work and decorative mesh of arched openings in foreground. Built up compression girder members at side, and tie rod bracing also visible at top of arch

The bowstring trusses, in addition to weighing 1,600 tons each, presented the difficult problem of being slightly too tall to fit under two bridges along the journey. Because of the stresses inherent in a bowstring truss, the Foundation Company could not merely disassemble them; instead, the firm had to transport the entire 450-foot (140 m) spans, which were 44 feet (13 m) wide and 80 feet (24 m) high.

Each truss consisted of sixteen eye-bar panels pinned together. Removing one part would break the structure's rigidity and make moving it very difficult. Instead of shifting the spans off piers for lowering or pivoting them from their present support, the Foundation Company lowered the structure in position, taking off the masonry and using substitute supports for resting the structure without getting in the way of the process. The contractor attached a frame to each of the piers and abutments, used straps to bind the trusses to each frame, and lowered them using the straps.

With twenty-six 7-inch (18 cm) holes punched in the strap, the company used a matching chain to counter the eight straps. Using pins to move the strap by hole sets, the company brought the spans downward 15 inches (38 cm) at a time with jacks. The pins attached to the plungers of eight 500-ton jacks. The jacks remained in place while the pins moved 15 inches (38 cm). The water-cylinder jacks were also 15 inches (38 cm) high, capable of exerting 3,200 pounds per square inch (22 MPa) after pumping. By bleeding water out of the cylinders of the jacks all at once, workers used the four jacks on each side of the bridge to lower the spans on alternate sides to the full depth of 16 feet (4.9 m).

Workers made a pontoon out of two pairs of barges, spaced to create a platform 400 feet (120 m) long and 52 feet (16 m) wide, which carried the bridge with 20 feet (6.1 m) of overhang.^[5] Stringers supported the bridge in forty-two places, with a 40-ton screw jack at each stringer for easier loading and unloading. When workers reached the Manchester Bridge, they had to adjust the bridge to fit the clearance, which was 14 feet (4.3 m) and 12 feet (3.7 m) less than the bridge's height. After supporting the bridge under the floor beams, they disassembled the top chord and stabilized each panel point on the trusses. Steering the bridge under the Manchester Bridge and the railroad bridge at Brunot's Island, the tugboat captains went through the Ohio River lock and up the back channel toward Coraopolis, where piers and abutments had been constructed.

Reversing the process and using the same jacks and steel frames to raise the structure 32 feet (9.8 m), the company erected the Cooper bridge 30 days after the project began.

The Farris Engineering Company, a Pittsburgh company located in the Empire Building, performed a great deal of work on the bridge at its new location. The original approved contract for their work had a price total of $271,811.50. They built portions of the substructure that the Foundation Company had not done. Farris also erected the two pony truss approach spans, repaired and painted the main superstructure, and paved the bridge deck and the two approaches.

The approach spans, and the end bearings for the main spans, were fabricated by the American Bridge Company at its shops located in Ambridge, Pennsylvania.