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The Alexandra Bridge was originally the named the Interprovincial Bridge and officially renamed "Royal Alexandra Bridge" only a year later. The bridge is a magnificent heritage bridge that spans the Ottawa River right next to the center of Canadian government, Parliament Hill. Parliament Hill and Alexandra Bridge each offer spectacular views of each other. It is also next to where the Rideau Canal, a World Heritage site, connects to the Ottawa River. In a setting with such rich heritage and iconic structures, this bridge is an essential element of the area and it contributes greatly to the rich heritage and culture found here. The bridge has been honored by the Canadian Society for Civil Engineering as a National Historic Civil Engineering Site.
Early View of Bridge From Parliament
Source: William James Topley/Library and Archives Canada/PA-008917
Aside from the iconic geometric beauty formed by its trusses, the bridge is historically and technologically significant as an extremely early surviving example of a large-scale cantilever truss bridge that represented a significant engineering achievement when completed. There are only a very small number of surviving cantilever truss bridges in North America that are older than this bridge. It was especially noteworthy in the context of Canada because it was designed and erected by Canadians without assistance of firms outside the country.
Alexandra Bridge In Its Early Years. View Showing Deck Layout.
Source: William James Topley/Library and Archives Canada/PA-009430
A bridge with more than just a truss as a cantilever, the deck of this bridge is extremely unusual because it includes a design where the floorbeams are cantilevered far out beyond the truss lines to provide lanes originally for both vehicular and electric rail, while the traditional roadway space between the trusses was reserved for steam rail. This feature is particularly unusual because it is an original design feature of the bridge. This sort of deck expansion is sometimes added to a bridge in later years, usually with deck trusses. With most original designs for through truss bridges, pedestrian sidewalks are usually the only things cantilevered out beyond the truss lines.
Cross Section of Bridge at Pier
Source: Engineering Record, Vol 44, 1901. Digitized By Internet Archive.
As the handsome plaques mounted on the end posts indicate, this bridge was the work of the railways, specifically the Ottawa and Gatineau Railway and the Pontiac Pacific Junction Railway. These railways later became part of Canadian Pacific, but Canadian Pacific had nothing to do with the construction of this bridge, contrary to some online sources. As originally built, the bridge had substantial approach systems carrying the rail line up to the bridge. These have since been removed, and the bridge only carries vehicular and non-motorized traffic today.
Alexandra Bridge from Major's Hill Park. Railway approach system visible.
Source: William James Topley/Library and Archives Canada/PA-008909
Today, the southern cantilevered deck section carries non-motorized traffic only. The northern cantilevered deck section carries westbound traffic, while the center section between the trusses carries eastbound traffic.
The bridge consists of a main cantilever structure, along with several approach spans, the most noteworthy of which are a Pennsylvania through truss span and a Pratt through truss span located west of the cantilever spans.
Drawing Showing Entire Bridge
Source: Waddell, J.A.L., Bridge Engineering Volume 1, 1916. Digitized By Internet Archive.
The design of the cantilever portion of the bridge includes not only a top chord that slopes upward to form a point or tower above the cantilever piers, but its bottom chord also slopes downward to form a point at the same location. This is a design detail that only some through truss cantilever bridges display, such as the Quebec Bridge. In the case of the Alexandra Bridge, the purpose of this was to reduce the height of the piers. By reducing the needed masonry, costs were reduced. It also had the effect of allowing the bridge to be built with a shorter tower above the deck.
Construction of Bridge
Source: Topley Studio / Library and Archives Canada / PA-013866
The erection of the bridge was accomplished using a "traveler crane" which was a tower structure that ran along tracks and had cranes hanging from it which were used to put the steel pieces into place. The use of travelers was common for erection of bridges like this during this period in history, although the exact design of the custom-built travelers varied from bridge to bridge. The design of the traveler used for this bridge is shown below.
Erection Traveler Crane (Left) and Diagram Showing Erection of Bridge (Right)
Source: Engineering Record, Vol 44, 1901. Digitized By Internet Archive.
The cantilever spans of this bridge follow a subdivided Pratt (Baltimore) truss configuration. The suspended span at the center of the bridge is a Pratt truss.
Construction of the bridge began in February 1898 and the bridge was carrying trains in December of 1900. One of the more noteworthy and unusual aspects of this bridge was the challenges which were overcome during construction. Work on the bridge continued even during terrible winters with ice filling the river including the unusual frazil ice crystals. Another very unusual challenge was a massive amount of sawdust, logs, and other debris from mills which had literally filled in a significant part of the river. This debris, which apparently compressed, along with logs and other obstacles, made installing some of the piers a significant challenge as workers had to excavate through this material to reach the bedrock below. All this debris sitting in the river caused gasses to accumulate in some areas, and sawdust explosions occurred during construction, although no injuries occurred.
Alexandra Bridge In Its Early Years
Source: William James Topley/Library and Archives Canada/PA-008971
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