Karen A. Bexten and Shane Hennessey, Tadros Associates, LLC and Bill LeBlanc, Con-Force Structures Limited
Currently under construction near Calgary, Canada, the 774-ft (236-m) long twin structures of the Bow River Bridge use high performance concrete for their precast, prestressed concrete girders. Each structure consists of two 174-ft (53-m) and two 213-ft (65-m) long spans. The precast concrete alternative provided a cost savings of about 10 percent (CAN $9.6 million versus CAN$10.5 million) compared to the steel plate girder option.
This bridge marks the first time a one-piece 211-ft (64.25-m) long girder weighing 268,000 lb (122 Mg) has spanned the entire distance between permanent pier supports without recourse to segmental I-girders, intermediate splice joints, and temporary falsework towers. Another source of economy is the relatively wide girder spacing of 11.65 ft (3.55 m). This spacing resulted in fewer girder lines despite the relatively long spans and the uncommonly heavy design live load mandated in Alberta. The heavy equipment hauling demands of the oil refinery industry result in a maximum live load moment that is 1.5 to 2 times that produced by AASHTO HS20 loading.
An NU 2800 girder with a depth of 9.2 ft (2.8 m) and a standard web width of 6.9 in. (175 mm) was used for the 213-ft (65-m) long span. The thin web is one of the primary reasons for the minimized girder weight and the increased span efficiency. Previously, the Alberta bulb-tee girder was limited to 138-ft (42-m) lengths. The largest NU 2800 bridge girder to be used prior to this project was part of the Oldman River Bridge in Alberta. It had a length of 188.6 ft (57.5 m) and weighed 240,000 lb (109 Mg).
The Bow River Bridge girders were pretensioned to resist forces caused by lifting, shipping, and erection. After erection, the girders were post-tensioned with four tendons of twelve 0.6-in. (15.2-mm) diameter strands each in 3-in. (76-mm) diameter ducts. One tendon was stressed prior to placing the deck making the girders continuous for deck weight. The remaining tendons were post-tensioned after the deck was cast. This established continuity of the composite section over the intermediate supports for superimposed dead loads and live loads.
Advantages of I-Girder Post-Tensioning
The NU I-girder, developed at the University of Nebraska, is most suitable for installations where girders from adjacent spans are post-tensioned for continuity. This eliminates the need for staggered deck placement to control deck cracking. Post-tensioning of the entire bridge length significantly increases girder capacity and produces precompression in the deck in the negative moment regions. The latter effect eliminates transverse cracking in the deck. The balanced combination of pretensioning and post-tensioning allowed a relatively low concrete compressive strength of 5100 psi (35 MPa) to be specified for release of the prestressing strands, while requiring a compressive strength of 9500 psi (65 MPa) at 28 days.
Girder Handling
The girders were carefully evaluated for stability during lifting off the precasting bed, shipping, and erection until composite action with the deck was achieved. Special lifting devices were designed and used in conjunction with a top-flange bracing truss to mitigate the potential of the top flange buckling at time of lifting off the bed. The bracing truss and special trailer truck saddles were used during shipping. The stability analysis summarized in Chapter 8 of the PCI Bridge Design Manual proved valuable for this analysis. Delivery of the girders proceeded without significant problems.
Erection crews reported that the NU girder exhibited better stability during placing compared to other girder cross sections because of the wider bottom flange. Erection was temporarily halted when wind gusts reached 19 mph (30 km/hour). Immediately after each girder was placed and before the cranes were released, each girder was braced diagonally to the previously installed girders; stability of the first girder in each span was provided with diagonal braces to the tops of the pier supports.
Keys to Economy
A key to the competitiveness of the recent Alberta girder bridges was that the precaster serves as both the erection and post-tensioning subcontractor. The success of this project may prompt other contractors to consider undertaking the operations of erection and post-tensioning. The use of end-to-end post-tensioning without intermediate anchorages was important in the overall economy of the system. The NU girders were most economical because the simple post-tensioning scheme allowed their use without custom changes to the relatively expensive girder formwork.
Editor’s Note
See the following table for a comparison of the length of the Bow River Bridge girders with other applications:
