Stephen J. Seguirant, Concrete Technology Corporation
The Methow River Bridge, currently under construction in Okanogan County, Washington, is a replacement for a seven-span, cast-in-place concrete Tbeam bridge that had become functionally obsolete. The new bridge consists of two 180.5-ft (55.0-m) long spans using precast, pretensioned, 176.5-ft (53.8-m) long Washington State Department of Transportation (WSDOT) W83G girders at 6.1-ft (1.85-m) centers.(1) The girders are 82.7 in. (2.10 m) deep and weigh 1.11 kips/ft (16.2 kN/m). The cast-in-place deck is 7.9 in. (200 mm) thick. Each span consists of seven girders for a roadway width of 43.3 ft (13.20 m). The bridge is being constructed in two stages. Stage 1, with three girders, is complete and open to traffic. The existing bridge has been demolished and Stage 2 construction is proceeding. The new bridge was subjected to many environmental restrictions, including a 10-month closure on construction in the river for fish protection.
The original design used a three-span bridge with WSDOT W74G, 73.5-in. (1.87-m) deep girders. However, this concept placed one of the piers in the middle of the main river channel, which is heavily used for recreation in the summer months. The conversion of the original concept to a two-span structure removed the pier from this channel, reduced the amount of work required in the river, and placed the pier in a location that is dry for much of the year.
Handling, Shipping, and Erection of Girders
To date, the Methow River Bridge girders are the largest to be constructed and transported in Washington State. Permanent pretensioning consists of sixty-eight, 0.6-in. (15.2-mm) diameter strands tensioned to 75 percent of the guaranteed ultimate tensile strength. The length and weight of the girders required special planning for handling and shipping.
Without the aid of temporary top strands, stability during stripping would require that the lifting devices be placed 12.3 ft (3.75 m) from the ends of the girders with a concrete strength at transfer of 8700 psi (60.0 MPa). With six temporary top strands, tensioned to the same stress level as the permanent strands, the lifting devices could be moved to 9.5 ft (2.90 m) from the ends, and the required concrete strength at transfer could be reduced to 8220 psi (56.7 MPa). The temporary top strands are placed 2 in. (50 mm) from the top of the girder, and are bonded for only 10 ft (3 m) at both ends of the girder. These strands are cut through pre-formed blockouts after the girders have been erected and stabilized.
In general, temporary top strands are not required for stability during handling, but in many cases, they are required for stability during shipping. Though temporary, these strands can affect the longterm camber, since much of the camber growth due to creep occurs early in the life of the member. The design of the Methow girders considered this effect. The thickness of the cast-in-place haunch at the girder ends was reduced by an estimated amount of 1.25 in. (30 mm) due to the reduction in camber.
For shipping, the contractor was given several options for support configurations and overhangs to fit the three different truck configurations used to haul the girders. Calculations indicated that a rotational stiffness of approximately 50,000 in.-kips/radian (5.65 MN-m/radian) was needed per truck to assure an adequate factor of safety against rollover. Based on previous measurements and the number of axles required to carry the weight of 197,000 lb (876 kN), the trucks were deemed adequate for the haul. All girders were safely delivered from Tacoma to the site; however, the journey along the 250-mile (400-km) long route took from 9 to 13 hours. The girders were erected in as little as 20 minutes each with a crane positioned at each end.
Concrete
The specified 28-day concrete compressive strength for the girders was 10,000 psi (69 MPa). The concrete mix proportions included 752 lb/cu yd (446 kg/cu m) of Type III cement and 50 lb/cu yd (35.6 kg/cu m) of silica fume, placed with a water-cementitious materials ratio of 0.27. State-of-the-art electrically heated forms allowed for optimum curing conditions. The actual concrete strengths at transfer ranged from 8300 to 12,500 psi (57 to 86 MPa) at one to three days. The actual 28-day strengths ranged from 10,600 to 15,200 psi (73 to 105 MPa). Reference
- Seguirant, S. J., “New Deep WSDOT Standard Sections Extend Spans of Prestressed Concrete Girders”, PCI Journal, V. 43, No. 4, July-August 1998, pp. 92-119.
Further Information
For further information, contact the author at [email protected] or 253-383-3545