(text taken from Advantage Steel, Issue
No. 6, December 1996)
Much admired for its attractive design, the Humber
River Pedestrian Bridge is a sparkling new addition to Toronto's waterfront.
Casual observers of the bridge – awed by its soaring tubular
arches and intricate cross-bracing motif – are likely to overlook
the structural innovations that made it all possible.
Unique configuration
Located at the mouth of the Humber River, the 139-metre pedestrian-bicycle
bridge connects several trails along a scenic waterfront park. The project
is the first step in a multi-year plan by Metro Toronto Transportation to replace
a series of existing structures. With architectural expression and engineering
design joining forces to achieve its unique configuration, the new bridge was
built using 1200 mm diameter high strength steel pipe bent into twin arches
rising 21.3 m above grade. The spacing between the arches is wide at the base
but narrow at the top, imparting a futurist basket-handle appearance to the
superstructure. A total of 44 stainless steel hangers of 50 mm diameter were
strung from the arches to support the 22 floor beams forming the bridge deck,
creating what architect Peter Sisam calls a "room-like quality" for visitors
crossing the bridge.
The bridge was designed in accordance with the 1993 Ontario Highway Bridge
Design Code. Due to the low bearing capacity of the soil, a tied-arch concept
was adopted with the horizontal arch thrusts carried by high-strength steel
cables within the deck. The bridge abutments rest on five concrete-filled caissons
socketed into the bedrock 30 m below grade.
The tapered shape of the floor beams was dictated by both structural and architectural
reasons. According to structural engineer Ken Price, "The floor beams are simple
spans in the transverse direction, suspended at each end by a single hanger.
These beams are efficient in bending when tapered to the hanger sup-ports.
In addition, they required sufficient depth to accommodate the primary post-tensioning
contained in the central deck spine. The rounded ends and non-prismatic shape
were otherwise de-tailed for aesthetic reasons."
Delcan structural engineers designed the entire bridge with the assistance
of the finite element program SAP 90, while Rowan Williams Davies & Irwin
of Guelph, Ontario, carried out an independent assessment of the bridge aerodynamics.
Innovative construction
McCabe Steel fabricated the tubular structure using gas-fired ovens to heat
the steel pipe sections. Bending operations involved a system of saddles, supports
and weights devised to achieve the pro-per curvature. Tight tolerances on the
parabolic arch geometry (out-of-round and camber) required careful consideration
of the chemical and physical material changes due to heat bending. A pre-assembly
of the pipe sections end-to-end in the fabricator's shop provided the necessary
dimensional verification.
The bridge was completely erected on land and launched over the Humber River
by Dominion Bridge, Inc. Arriving at the site in separate pieces, the steel
arches were welded together on the river bank and equipped with cross-bracing,
hangers and cross-beams. "The alignment of the sections, which had to be accurate
within mm, would have been very difficult if not impossible if it had been
done in the air over water. By assembling the complete bridge on land, half
the individual arches could be pre-assembled and welded flat on the ground
with easy dimensional control," explains Brian Thompson of Dominion Bridge.
Temporary cables were added to tie the ends of the structure together, and
the complete 40'' assembly was erected in one piece.
The launching operation began with the leading end of the bridge placed on
a floating barge and pulled across the river by winches and cables while travelling
cranes supported the trailing end. Lifting gantries at both ends of the 300-tonne
arches then took over the erection sequence as anchor plates fitted to the
bases were fastened onto receiving thrust blocks. The concrete deck and stainless
steel railings were installed after the bridge was erected.
Corrosion protection
The different types of structural components and their exposure to the elements
called for special corrosion protection measures. For the arches and floor
beams, the standard three-coat protection system recommended by the Ministry
of Transportation of Ontario was used. Stainless steel was specified for the
hangers and railing system primarily for architectural and maintenance reasons.
To avoid a premature corrosion failure arising from galvanic action, the stainless
steel hangers were isolated from the mild steel hanger hardware by means of
a sacrificial zinc coating, waterproofing and fabreeka pads.
The natural environment presented numerous challenges to designers who also
had to contend with interest groups and government regulations. Planners considered
the environmental impact of the bridge, river navigation, protection from Lake
Ontario storms and erosion, public safety and problems related to the soft
organic soil along the river bank.
Historic site
The double-arched bridge forms a gateway between Toronto and Etobicoke along
an aboriginal trading route linking Lake Ontario to the north. Emphasizing
the cultural history of the site, native imagery was skillfully woven into
the design of the bridge. The steel cross-bracing between the arches exhibits
an elegant cutout pattern suggesting a repeating Thunderbird motif, an important
symbol for the Eastern Woodland First Peoples who inhabited the site for centuries.
Additional cultural elements are evoked by the bridge abutments with their
decorative panels depicting turtles, canoes, and other native icons.
Recipient of the 1995 Ontario Steel Design Award from the Ontario Region of
the CISC and recognized as a local landmark, the Humber River Pedestrian Bridge
proves once again that structural steel can be as beautiful as it is functional.
Architect: Montgomery
and Sisam Architects,Toronto
Structural Engineer: Delcan
Corporation, North York
Artwork: Environmental
Art works Studio, Toronto
Owner: The Municipality
of Metropolitan Toronto Transportation Department, Toronto
General Contractor: Sonterlan
Construction Corporation, Richmond Hill
Steel Fabricator: McCabe
Steel, Stoney Creek
Steel Erector: Dominion
Bridge, Inc., Oakville |
