New innovative precast bridge girders are being introduced in Alberta. Con-Force Structures' Calgary plant has completed production of 28 giant 2800-mm deep NU girders for the Oldman River Bridge, at Taber, Alberta. This project represents another made-in-Alberta success story. All of the labour and most of the materials are sourced right in Alberta, including the girder forms built at Bonnybrook Steel in Calgary.
The precast industry is introducing new girder sections for Alberta designers. Girder sections vary in height from 1200 to 2800 mm in 400-mm increments. The side forms are spaced to provide a 150-mm web for pretensioned girder applications (Type I) and for a 175-mm web when post-tensioning ducts are cast in the webs (Type II). The flanges are slender - top 65-mm deep by 1250-mm wide and bottom 135-mm deep by 1000-mm wide (Type II). Spans of 70-m or more (with girder spacing at 3500-mm) can be realized using spliced NU girder construction.
The key to this new construction is simplicity. End blocks are not required in the girders. The form is opened and closed mechanically for easy access to install the prestressed and non-prestressed reinforcement.
Shear and flange reinforcing is custom-formed welded wire mesh. Wire sizes and the spacing of full depth shear mesh can be varied in different regions of a beam. The mesh is supplied in panels and assembled in the form. A small amount of additional reinforcement is added at critical locations.
Diaphragms are exposed galvanized steel angles fastened to plates cast in the girders. This allows girders to be precast daily - even with length and depth changes.
Oldman River Bridge Project
Campbell Woodall & Associates designed the 301-m long bridge with 5 spans (3 main spans of 62-m and 2 end spans of 57.5-m). The composite deck roadway is supported by 4 lines of 2800 NU girders spaced at 2500-mm c/c. Environmental constraints forced the longer end spans of 57.5-m.
The bridge was bid using steel and precast girders. The precast alternative came in 10% less than steel. The cost of the bridge is $5.5 million.
The precast design by Campbell Woodall Associates was done in consultation with Con-Force. The design took advantage of how the girders were to be fabricated and determined how the deck was to be cast to reduce the amount of post-tensioning.
The design was calculated for 3 stages of prestressing:
1.Pretensioning during girder casting for all five spans,
2.First stage longitudinal post-tensioning with the centre portion of the composite bridge deck in place, and
3.Stage two longitudinal post-tensioning for the completed bridge.
Using this approach and low friction ducts, the savings for the post-tensioning and girder fabrication were in the order of 20% over the preliminary design. "Co-operation and consultation between the designers and the precast industry during the design phase of the project made this project possible", said designer David Thompson, partner, at Campbell Woodall.
The end span girders at 57.5-m long, weighing 110 tonnes, are the largest plant precast girders ever manufactured in Canada. Girders for the other 3 spans are erected in segments on temporary supports. Two sets of 20-m long hammerhead girders are erected at piers 2 and 3. Drop-in girders are 42-m long in the centre span and 51.5-m long in the two adjacent spans.
Straight pretensioning was used in the bottom flanges of the long girders and in the top flanges of the hammerhead girders for shipping and to support dead loads. Some unbonded prestressing strands were placed in the top flanges of the longest girders to satisfy transportation and construction loads. These strands will be released before longitudinal post-tensioning. Post-tensioning consists of three 12-15 mm tendon ducts through all girders. The post-tensioning anchorages will be placed in cast-in-place end diaphragms at both abutments.
2800 NU Girders for the Oldman River Bridge
8 pieces - 20,000-mm long hammerhead girders
8 pieces - 57,500-mm long clear-span end span girders
8 pieces - 51,500-mm long drop-in mid span girders
4 pieces - 42,000-mm long drop-in centre span girders
Bridge owners, bridge design engineers, university faculty and students were invited to an open house at Con-Force Structures' Calgary plant on January 11, 2001. The tour was timed to allow visitors to see the form ready for casting just before buttoning up (to view the reinforcing) and see the start of the concrete placing. Finished girders in the yard and a girder being loaded for shipment on a multi-axle truck were on display. Interest was high. About 70 people showed up at the plant for the demonstration.
Alberta has always been a leader in precast prestressed concrete in Western Canada. Con-Force was the precast contractor for the Ross Creek Bridge in Medicine Hat, Alberta, built in 1954. This 60-ft (18.3 m) span bridge is the first segmental precast concrete bridge in Canada and the first prestressed concrete bridge in Alberta. The bridge was completed in March 1954 and is still in service. The owner is the City of Medicine Hat and the designer was Structural Engineering Services of Calgary (now Lamb McManus Associates).
Owner: Alberta Infrastructure - M.D. Taber
Designer: Campbell Woodall & Associates, Calgary, AB
Project Manager: Torchinsky Engineering Ltd., Sherwood Park, AB
General Contractor: Getkate Construction, Lethbridge, AB
Precast Contractor: Con-Force Structures, Calgary, AB