By Associated Engineering
Three Crossings On HendayEngineering
Whitemud Creek Arch
The Anthony Henday Drive’s southwest quadrant crosses Whitemud Creek, located in a ravine 300 metres wide and 18 metres deep. The site was once used for underground and strip coal mining, so the foundation conditions were very poor.
The crossing had to protect the wildlife and fish habitat and a pedestrian and bicycle path had to be provided.
Completed in 2005, the structure is a unique semi-circular arch spanning 20 metres, supported on 7 metre high walls and closely spaced drilled rock-socketed piles. The pedestrian-bike path is raised within the arch, 7 metres above the creek.
The design provided substantial life cycle cost savings over more traditional bridge structures and had other important advantages:
• The arch structure, being a closed structure supported on bedrock, is robust and is not prone to sudden failure. Effects of fill settlement will be gradual and the structure will show signs of distress long before the threat of collapse.
• The height of the arch structure allows for a raised walkway so that pedestrian traffic is separated from wildlife.
• Operationally, the soil-covered arch structure involves very low maintenance when compared to a bridge structure.
• Provision for future road widening was made.
• The structure is unique and aesthetically pleasing.
The design was developed from a basic arch form with a desire to minimize its overall length and provide openness and light. The bridge has sloping ends and a central oculus that opens upwards into the highway median. The soffit of the arch was finished with a bright white-pigmented sealer to maximize daylight within the arch. The pedestrian/bicycle path has a curved plan complementing the arch theme. The entire structure is constructed from reinforced concrete, poured in place.
The ends of the arch consist of wall segments perpendicular to each other in order to achieve stability against the overturning effects of earth pressure.
Blackmud Creek Crossing
The crossing over Blackmud Creek on Anthony Henday Drive’s southwest portion required three bridge structures, one for each of the two-lane divided highways, and a third for a ramp from the nearby Calgary Trail Interchange.
The crossing, completed in September 2005, comprises three individual cast-in-place concrete, deck stiffened, open spandrel arch bridges. The thrust from each arch is used to partially support 16-m high embankments on each side.
The geotechnical conditions permitted this type of arch structure and it was more economical than using traditional steel or precast girder bridge forms since they would have required much longer bridge lengths.
The roadway is elevated 17 metres above the creek bed and at a 45-degree skew. The creek was realigned to minimize the bridge spans.
Each 76-m long bridge structure has a 15-degree skew and consists of a 40-m middle span, with 18 metres cantilevers. The cellular, post-tensioned concrete box girder deck is designed to carry an additional slab for future widening. The deck slab is cantilevered up to 4 metres from the main girders.
Each structure consists of two arch ribs, with a 40 m span x 7.5 m rise, 7.0 m wide x 0.4 m thick. The intrados of the arch is essentially parabolic, composed of eight straight-line segments. The side spans are cantilevered approximately 18 m and the ends of the cantilevers are tied back to the top of the pile caps with struts. Because of the skew, the struts are at different slopes.
The deck section consists of a 1.5-m deep cellular slab (five cells in each bridge) with a cantilevered top slab.
Using the same geometry and construction for each structure allowed the arch formwork to be reused. In order to reduce the unequal load on the arch, the east and west sides of the deck were specified to be cast at the same time.
Trellis Beam Structures at Calgary Trail Interchange
The Calgary Trail interchange on Edmonton’s Anthony Henday Drive is Alberta’s first three-level interchange. At this point the southwest and southeast quadrants meet.
Flyovers were needed to provide high-speed directional changes for vehicles, and the geometry required four gradeseparated structures at varying and high angles of skew. On two of the structures, for example, the skew varies from 50 to 80 degrees from south to north.
The challenges were met by a structural system dubbed the “Trellis Beam” concept. It gave substantial savings over more conventional bridge designs.
The Trellis Beam concept is an arrangement wherein the main girders spanning the roadway are perpendicular to the underpassing roadway, thus resulting in the shortest possible span and shallowest superstructure depth.
The deck slab is cast to the required roadway width of the flyover and spans between the beams. The roadway above may be easily widened in the future by casting additional deck slabs with stay-in-place formwork or precast slabs, min- imizing traffic disruption. There are no piers adjacent to the roadway, which improves traffic safety.
Each of the four trellis beam structures is supported on integral abutments. All the trellis beams have the same narrow trapezoidal cross section, varying in width from 600 mm at the bottom to 1,000 mm at the underside of the top flange, and with a 1,300-mm flange width. The tops of the beams are sloped to shed rain and snow, while an overhang provides a drip.
By maximizing the trellis beam spacing from a normal 6 metres to 8 metres and using a narrow cross section, the design allows for more light and openness under the structure. For the same reasons, spans of 44 m were used instead of a minimum required span of about 22 metres.
Each beam is post-tensioned. Box abutments consisting of trapezoidal beams with a cross-section similar to that of the trellis beams support the bridge deck at both ends.
Owner: Alberta Infrastructure & Transportation
Design engineer, Whitemud/Blackmud/Trellis Beams: Associated Engineering, Edmonton (Steve Croxford, P. Eng., Bala Balakrishnan, P. Eng.)
Road design & project management: UMA Engineering
Geotechnical & environmental: AMEC Earth & Environmental
Contractors: Kiewit (Whitemud Creek); PCL Maxam (Blackmud Creek); Graham Industrial Services (Trellis Beam structures)