"The Blackfriars Street Bridge in London, Ontario, Canada is a wrought iron, through, bowstring truss or tied arch bridge, placed across the North Thames River in 1875 and still carrying frequent vehicular and pedestrian traffic. At 216 feet (65.8 meters) it is the longest working span of that kind in North America.
Engineering
According to bridge historian Nathan Holth,
Bowstring bridges are one of the rarest types of truss bridges, and most date in the 1870s. They fell out of favor due to the limited weight they could support. Any bowstring truss bridge that survives today is a miracle. Truss bridges are always intricate structures, but bowstring trusses are even more so. There [are] lattice, v-lacing, and members all over. This large amount of complexity is balanced by the simple, graceful appearance of the arched top chord. The result is a bridge with incomparable beauty and appeal. Among the rarest and oldest bridges in Canada is this breathtaking iron bowstring truss. Keystone Columns form the top chord. A sidewalk on the south side appears to be original. The bridge has undergone extensive repairs and modifications. Most notably, the top chord has had plates of steel welded to the top of the column. Numerous rods and bars have been welded onto many of the vertical and diagonal members as well. A couple of added bars of steel run lengthwise through the middle of the truss. These modifications have affected the historic integrity of the bridge, but have no doubt helped keep it standing over 130 years. The original lattice guardrails remain on the sidewalk, albeit with a metal pole welded above them.
The deck surface (see illus. gallery below) is presently of renewable planking: a double file of approximately one-thousand, five hundred eight-foot two-by-fours each, on edge, upon a framework of nine longitudinally laid stringers of one-foot iron I-beams, topped with bolted-on wooden cladding, whose ends rest on the two abutments. Attached beneath these are fifteen transverse floor-beams, from which vertical lattice pillars, under tension, translate the live thrusts of traffic to the bowed upper chord, which transfers this back as tension along the bottom-chord "string" of the bow. This bottom chord consists of two sets of four 10 cm x 3 cm wrought-iron eyebars, running along, outside, both sides of the deck. Although originally two-lane, due to the weight and frequency of modern traffic, the Blackfriars is at present two-way but single-lane. Because of damage to the wooden deck surface and to the iron structure, there is recurrent consideration of closing it to four-wheeled vehicles.
Historical, natural and cultural aspects
The Blackfriars Street Bridge (BSB) was manufactured by the Wrought Iron Bridge Company (WIBC) of Canton, Ohio, although erected by local London contractor Isaac Crouse. There is evidence for its being prototypical for a revised design by WIBC, incorporating a double-panel web. The BSB is the successor to a series of fixed, wooden structures at the site since 1831, which were damaged mainly by spring "freshets" of the river. It has been designated a historic structure under Part IV of the Ontario Heritage Act. The BSB is the northernmost and oldest of a company of eight bridges of different ages, constructions and uses, surrounding the confluence of the North Thames and Thames rivers, which fixes the historic center of London. The Bridge is sited at the east end of a short Blackfriars Street, which turns sharply south and up a slight grade, as the downtown Ridout Street, upon crossing it. To the BSB's west is the lower ground of the previous town of Petersville, protected by an extensive dike embankment, due to a history of flooding; to its east a terrace rises above the North Thames River to central London. The river there is bordered on both sides by extensive bicycle and walking paths, and the Bridge is well framed by a variety of second-growth trees.
Much of the "beauty and appeal" of the BSB is its appearing to float, from many profile views provided on both banks of the gently winding river, up and downstream. That is owing not only to its strung-bow shape but also to its light placement, at its very tips, upon modest granite abutments—a feature rare among more modern tied-arch bridges. These abutments bear the passive vertical load of the open structure itself. However, the varying ‘live’ thrust forces of traffic downward on its deck are translated by the bowed chord above into horizontal tensions along the longitudinal iron eyebars of the 'string' or bottom chord running parallel to the deck.
The Blackfriars Street Bridge has figured in various artistic works, visual and literary, including a series of stained glass windows by Ted Goodden. Nathan Holth praises the City of London and the Province of Ontario for maintaining its old iron bridges. Regarding the nearby, larger pony-truss Kensington Bridge he praises the City for "keeping the Thames River looking wild and natural," even near its center, with the look of "a rural bridge in an urban environment, which further enhances the beauty of the bridge", a remark that would apply as well to the Blackfriars. With recent improvement of the rivers, wild nature seems to confirm the point."
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