The length record in conventionally fabricated and shipped prestressed concrete bridge girders continues inching towards 200 feet. By Concrete Products' records for plant-cast members hauled over the road, Calgary-based Con-Force Structures Ltd. has stepped to the front with delivery of 57.5-meter (188.6-ft.)-long girders to the Oldman River replacement bridge along Highway 864 just north of Taber, Alberta. The 2,800-mm (9.17-ft.)-deep girders top the 175-ft.-long bulb tees CSR Hydro Conduit produced in 1994-95 for a Wabash River crossing near its Lafayette, Ind., plant (Concrete Products, January 1995).

The Oldman River crossing is the first in Alberta to use the NU Girder section, which can optimize weight and length with slender cross section and prestressing strand concentration at the base. Con-Force's Bridge Department pursued the NU profile as part of its new-product development role. The girder specification was validated when the Oldman River precast concrete solution beat out a bid for a steel alternate. (More recently, and again at the expense of steel, the producer has landed a contract to fabricate 2,800-mm-deep NU members for the Deerfoot Trail Bridge just south of Calgary.)

This new girder has been dubbed “NU” to reflect its initial development by University of Nebraska Professor Dr. Maher Tadros and his Department of Construction Systems Technology colleagues. In Alberta, the design will result in phasing out a bulb tee section the Province began using in the early 1980s.

Size and season matter

While the Oldman River job proved an ideal target to debut the streamlined girder, a dead-of-winter construction schedule had the potential to chill most anyone connected to it. Girder fabrication was timed so that Calgary carrier Burnell Contractors Ltd. could capitalize on a late December to early February window in which frozen ground enables heavier than normal loads on the TransCanada Highway and other Province routes.

Delivery scheduling minimized extra handling of finished product, as nesting of up to 10 girders was possible in Con-Force's loading area. A typical delivery sequence had girders loaded in advance of a nighttime escort from the plant to just outside the city limits of Calgary. At first daylight, Burnell drivers would begin an eight-hour trek to the job site.

Girder erection was hampered on only two occasions by wind. During the 28-piece schedule, Burnell used three tractors and three specialty gooseneck jeeps, towing rear hydraulic steering trailers. The largest girders traveled on rigs bearing 96 wheels. Bridge erection ran from January 15 to February 6. Deck and approach work are scheduled for early spring, with Alberta Infrastructure officials planning a mid-summer 2001 opening.

Zero tolerance

Con-Force's new NU Girder form required a web thickness of 175mm +0 mm versus typical PCI tolerances of +3/8 in. (9.5mm), -1/4 in. (6mm). To accomplish this Bonnybrook agreed to build within a +0 in. -3mm tolerance. Team members analyzed the form design using Finite Element Analysis to determine the optimal diaphragm and horizontal stiffener spacing. They calculated that the deflection in the form, resulting from hydrostatic pressure, would require manufacturing the web width to 172mm +3mm. The analysis showed that the deflection would be 1 mm per side, allowing for 1 mm production tolerance. Con-Force crews poured a test section to evaluate form behavior; the cross section of the web was the required 175mm +0mm.

The Finite Element Analysis is integrated with 3-D modeling software, allowing engineers to analyze the form, or a section of it, as it will be used. A variable hydrostatic pressure, equivalent to that of fresh concrete, is applied to the skin surface of the form. The form is restrained at the top and bottom to represent the top and bottom tie and the deflection in the form can be calculated. The results from the analysis of a section of the girder show that the maximum deflection of the form is approximately 1mm. Shown here is the deflection at 300X, making the actual deflections easier to view. Another Finite Elements Analysis was performed on the top tie, bottom tie and soffit with stress and deflection being the primary design criteria.

No standing steel

The girder and post tensioning scheme behind the Oldman River replacement bridge is consistent with technology producers have exploited over the past decade to incorporate prestressed concrete into longer span practice. At Con-Force Structures, displacing steel in such applications is a tradition that dates to at least 1959 when, through what would now be considered value engineering, the company secured a bridge contract in eastern Alberta.

In “Red Deer River Bridge,” a paper presented at the Precast/Prestressed Concrete Institute's October 1961 convention and featured two months later in the PCI Journal, Con-Force's George Adam and Structural Engineering Services' D. de Wolff note, “A complete study of the most economical structure indicated that the (Red Deer) bridge should be designed with five equal 148-ft. spans. As these were considerably longer than any previous spans using precast girders, the Bridge Branch of the Alberta Provincial Department of Highways proceeded with the design of a continuous built-up steel girder section. Con-Force commissioned Structural Engineering Services to prepare a simple span alternate design using prestressed concrete. When tenders were opened, the prestressed alternate was considerably lower than the lowest steel bid.” Con-Force handled girder fabrication on site.