Engineering Marvel: The Amazing Story Of The Hunter Street Bridge

"The Hunter Street Bridge: A Photo Essay"

—Words by Peterborough historian/author Elwood Jones
—Photos by Scott Arnold
—Historical photos by Trent Valley Archives

The Hunter Street Bridge, officially known as Ashburnham Bridge, was an engineering marvel. Frank Barber, Ontario’s premier bridge builder of his generation, had the vision, imagination and skill to propel the project. Barber’s leadership and decisiveness was called upon at several crucial points. Because of him, the bridge was built of concrete. It was he who hired Claude Bragdon to do the design which included several ideas that had been researched by Barber. When City Council worried about the expensiveness of the project, it was Barber who kept them on course.

The necessity for the bridge arose as a direct consequence of the Quaker Oats fire of December 1916. Quaker Oats did not have to rebuild in Peterborough, and so it was necessary to address the problems of its site. Peterborough needed to be able to fight fires in eight-storey buildings. There had to be an end to the traffic jams caused when trains were unloading, for the trains blocked traffic that was using the bridge. Of course, the Hunter Street bridge installed in 1876 preceded the building of Quaker Oats and the invention of motor cars.       

Peterborough, home to the Lift Lock and the Inverlea Bridge, had two impressive public works projects built of concrete. But it was not certain that the new bridge would be built of concrete. Steel truss bridges were being built everywhere. After 1916, the longest steel arch bridge span was on the New York City Hell Gate Bridge over the East River. Its longest span was 1,017 feet, or 310 metres. This bridge was the inspiration for the Sydney Harbour bridge in Australia, which opened in 1932.

The president of Quaker Oats, John Stuart, made a persuasive case for building a steel cantilever bridge over the Otonabee River. Such a bridge had just been completed at Quebec City, over the much wider St Lawrence River. The Quebec bridge, while under construction, had collapsed and killed 82 workers. This was a reminder of the dangers of building bridges. However, the project was continued four years later and was completed in 1917. The Queensborough bridge built over the East River, 1901-1908, was an impressive engineering work, but when compared to other great bridges, it is rather ugly.

Even when considering that the Hunter Street Bridge would go from Sheridan Street to Burnham Street, it was easy to argue that Peterborough did not require such a long bridge. Barber also persuaded local politicians that a steel bridge would require frequent maintenance and painting. It would cost more in the long term. Moreover, Barber had researched the costs of building bridges in Ontario. In the few years since 1915, the price of building steel bridges had risen faster than for concrete bridges. Barber recommended that the city built a high level reinforced concrete arch bridge over the Otonabee.

Barber believed that since the first reinforced concrete bridge had been built in 1889, technology had improved. Engineers were able to calculate more closely the impact of traffic on bridges, and to design bridges with more elasticity. Concrete bridges could look very heavy, but engineers were more aware that the weight of the bridge could be lightened by having open spandrels. Claude Bragdon’s design of the river arch avoided the heavy look of concrete bridges by having open-spandrels that reflected the arches of the bridge. Bragdon applied his training in classical styles from Rome and Greece to produce graceful arches in the basic structure and in the spandrels.

Several articles, many written by Frank Barber, appeared in the Canadian Engineer from 1918 to the mid-1920s that discussed aspects of the concrete arch bridges. Barber also thought there would be less stress on the central arch if one applied principles of elasticity to the toes or skewback and to the crown. In another article in the Canadian Engineer, Barber discussed how to design concrete curves in arches to ensure even load-bearing.

After calculating the weight of the floor system and the depth of the arch ring, the engineer could then change the depth of the arch to reflect the dead load stresses, which he said in large arches was the major stress. In September 1918, he was appointed consultant for the proposed Hunter Street Bridge. Barber had the opportunity to apply his thinking to the central arch of the Hunter Street bridge.

Ratepayers approved a $245,000 bridge, but the city engineer, R. H. Parsons, estimated the cost would be $300,000. Tenders were called for Peterborough’s high-level concrete bridge in late 1918, and ten companies looked at the plans. Three companies submitted tenders just before Christmas. Canadian Engineering and Contracting was low bid at just under $340,000. John Toole bid over $350,000, while R. Sheehy tendered at over $380,000. As the city elections were held always the first week in January, the outgoing council passed the decision to the 1919 city council.

After some cuts initiated by the politicians, Frank Barber’s bridge still had eleven arches. It would use 14,000 cubic feet of concrete, and 170 tons of reinforcing steel; the original plans had called for 17,000 cubic feet of concrete and 250 tons of steel. The central arch over the river stretched 234 feet from springer to springer, which at the time was the longest span on an open-spandrel concrete arch bridge built in Canada. Worldwide, thirteen such bridges had longer spans.

The most impressive Canadian concrete bridge of the period was the bridge in Saskatoon, now known as the University Bridge but long known as the 25th Street Bridge. The new name is appropriate because that bridge was designed by engineering professors at the adjacent University of Saskatchewan. The Saskatoon bridge was seven spans of similar length, but the longest span was 150 feet. Peterborough’s longest concrete bridge was the 1910 Inverlea bridge with three spans each about 100 feet; the total bridge length was 385 feet.

Technically, the Hunter Street river arch was remarkable for its time and reflected lessons Barber had learned over a career that was already 20 years long. The first reinforced concrete bridge in Ontario was built in 1897, only 12 years before Inverlea and 22 years before the Hunter Street Bridge. Between 1909 and 1918, Frank Barber had built twenty concrete bridges with open-spandrel arches, nearly all in Peel, York and Ontario counties, and one in Haliburton. Barber’s earlier bridges had clear spans ranging between 40 and 91 feet.

The Hunter Street bridge was of reinforced concrete, mainly steel rods and lead alloy. In 1921 Canadian Engineer, April 7, 1921, Barber claimed: “The construction throughout, with the exception of the central or river arch, is of reinforced concrete; with the exception of the joints at the skewbacks and crown, there is no steel in the river arch. This arch is so designed that no possible loading can cause any part of the main ring to be in tension at any point.”

The bridge would always be in compression. In order to reduce the weight on the central arch, the roadway was filled with cinders rather than loam. As well, Barber said he could reduce the shrinkage in the concrete by shooting dense sand-cement mortar in the joints when the temperature was at 45 degrees Fahrenheit, deemed Peterborough’s average annual temperature.

Carl W. Condit, an American historian who wrote an excellent book called American Building, describes different ideas about reinforcement in concrete bridges. Clearly, some of the early efforts in the 1890s were almost steel beams covered with concrete. Engineers came to realize it was possible to use less. Almost all the force in arch bridges is compression; the weight of the structure is more significant than the traffic moving on the bridge. In the 1890s, the standard for reinforcement on bridges consisted of I-beams bent in the shape of the proposed arch, laid parallel near the undersurface of the arch. In 1897, two patents added I-beams in the deck slabs and reinforcement in the spandrels. The amount of steel got less over time.

Over the years, engineers have claimed that this was an unreinforced concrete bridge. The late Jim Hooper, and long-time city engineer, told me that when the concrete silos of the old Peterborough Cereal Company were taken down in 1974 for the building of the Red Oak Inn, the contractors were forbidden to use dynamite because of the proximity of the unreinforced Hunter Street bridge.

In 1993, the local engineering society prepared a plaque that is now mounted on the railing saying this is an unreinforced concrete bridge. The Professional Engineers of Ontario describe the distinction as follows: “The principal stresses in this arch are compressive stresses due to the dead load and are never neutralized or reversed by live loads or temperature stresses. Consequently, the entire rib is always in compression and there is no reinforcing whatsoever in the ring of the river arch.”

When I questioned how a bridge that used 250 tons of reinforcing steel could be considered unreinforced, I was told that engineers described bridges by its dominant characteristic, in this case, the river arch. Engineers have also assured me that all the iron in the bridge was not for reinforcing. My own preference is to describe the bridge as a reinforced bridge with a well-designed central arch that is less-reinforced.

What is clear is that Frank Barber looked at all the details related to building a concrete arch bridge. His expertise ensured that the bridge was built at the optimal weight for the purposes, and because of his planning the city saved money. That did not stop people from complaining of the rising expenses of the bridge. However, in retrospect, Peterborough was left with an outstanding concrete arch bridge, designed by Frank Barber working with an architect to ensure good looks. Frank Barber deserves the credit, both for the conception and fine-tuning, and for his ability to be persuasive whenever the local politicians tried to pull the plug on a project they believed was too grand for Peterborough.

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