The Blackfriars Street Bridge in London, Ontario

London, Ontario

London is a city in Southwestern Ontario, Canada, situated along the Quebec City – Windsor Corridor. The city has a population of 352,395, and the metropolitan area has a population of 457,720, according to the 2006 Canadian census; the metro population in 2009 was estimated at 489,274. The city...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 is a wrought iron

Wrought iron

thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...

, through, bowstring truss or tied arch bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

, placed across the North Thames River

Thames River (Ontario)

The Thames River is located in southwestern Ontario, Canada.The Thames flows west through southwestern Ontario, through the cities of Woodstock, London and Chatham to Lighthouse Cove on Lake St. Clair...

 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

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

.

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

Lattice girder

A lattice girder is a girder where the flanges are connected by a lattice web. This type of design has been supplanted in modern construction with welded or bolted plate girders, which use more material but have lower fabrication and maintenance costs...

, 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

Wrought Iron Bridge Company

The Wrought Iron Bridge Company was a bridge fabrication and construction company based in Canton, Ohio. It specialized, as the name would suggest, in the fabrication of iron truss bridges and was a prolific bridge builder in the late 19th century...

 (WIBC) of Canton, Ohio

Canton, Ohio

Canton is the county seat of Stark County in northeastern Ohio, approximately south of Akron and south of Cleveland.The City of Caton is the largest incorporated area within the Canton-Massillon Metropolitan Statistical Area...

, 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

Ontario Heritage Act

The Ontario Heritage Act, first enacted on March 5, 1975, allows municipalities and the provincial government to designate individual properties and districts in the Province of Ontario, Canada, as being of cultural heritage value or interest....

. 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

Stained glass

The term stained glass can refer to coloured glass as a material or to works produced from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant buildings...

 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. (See, below, resident Great blue heron

Great Blue Heron

The Great Blue Heron is a large wading bird in the heron family Ardeidae, common near the shores of open water and in wetlands over most of North and Central America as well as the West Indies and the Galápagos Islands. It is a rare vagrant to Europe, with records from Spain, the Azores and England...

in winter.)

Gallery