Precast concrete speeds infrastructure upgrade

By Kurt Weber, P.Eng.

Linking Highway 8 and the Trans-Canada Highway, the West Calgary Ring Road North Project is one of the final pieces in the completion of the Calgary Ring Road. The entire ring road will provide a corridor of over 100 kilometres giving convenient access to all parts of the city while also helping to support Alberta's economy through the efficient movement of people and products around the city.

Planning for the Calgary Ring Road began in the 1970s, and the northwest, northeast, and southeast sections of the road (known as Stoney Trail/Tsuut'ina Trail) have been in use for some time. The West Calgary Ring Road North Project is part of the final section of the Stoney Trail/Tsuut'ina Trail freeway construction and incorporates 20 bridges. EllisDon is responsible for the construction of the North Project, with Lafarge Precast in Edmonton providing the precast concrete girders. AECOM provided the bridge design. The North Project is being delivered under a DesignBuild contract, meaning the design and construction happen concurrently. This contract structure allows the contractor to be innovative, and shortens the construction duration of a project, thus reducing overall costs.

Of the 20 bridges in the North project, 17 utilized precast prestressed concrete NU girders. NU girder sections have a wide bottom flange to increase the compression flange capacity, and to provide better stability during handling, shipping and installation. With up to 58 pretensioning 16 millimetre diameter (5/8-in.) strands in the bottom flange and up to 14 in the thinner top flange, the NU girder has a large span-to-depth ratio which can help to simplify bridge design. All of the NU girder structures were constructed using three girder sizes – NU2000, NU2400, and NU2800 – for a total of 134 girders. Other NU girder sizes like NU1200 and NU1600 are also available in Alberta.

SUMMARY OF PRECAST CONCRETE GIRDERS USED

• 70 at 31 to 45.5 metres (102 to 149 feet) long NU2400 girders
• 37 at 44.6 to 54.5 metres (147 to 178 feet) long NU2800 girders
• 27 at 52 to 60 metres (170 to 197 feet) long NU2000 girders

The various bridges are either single- or double-span, with a number of prestressing or connectivity conditions. Specifically, nine of the bridges incorporate prestressed NU girders with integral abutments, four of the structures incorporate single-span post-tensioned girders with conventional abutments, two structures are two-span single stage post-tensioned with conventional abutments on a 300-metre radius alignment, and two structures are two-span post-tensioned superstructures with semi-integral abutments. The long-standing tradition of using NU girders on Stoney Trail/Tsuut'ina Trail, the aggressive nature of the schedule, and the cost-competitiveness of precast concrete were major contributing factors in the material choice.

Precast concrete offered a shorter lead time and higher productivity to help advance construction to the point where the majority of the girders on the project were erected within 18 months of project award. The versatility offered by the range of NU girder spans, and the positioning of the bridge structures in existing fill or in cut slopes, eliminated the use of mechanically stabilized earth (MSE) retaining walls for the bridge abutments in 14 of the bridge structures. As MSE walls are not typically constructed during winter, eliminating the need for MSE walls on some of the bridges represented a significant saving to the overall schedule of the project.

The large NU girders were moved to the construction sites and positioned using trucks and cranes. Installation of the girders over areas of live traffic occurred at night to minimize disruptions to the travelling public. Availability of the oversized-overweight corridor within Alberta was critical in successfully moving these girders from the manufacturing site to the project site. The project incorporates the heaviest NU girders ever made and transported in Alberta. Each girder weighs 136,658 kilograms each (301,279 pounds), stretch to just under 60 metres (197 feet) in length, and measure 2.8 metres (9 feet) tall. The transport trucks had to take the provincial oversized-overweight corridor in their journey south from the Lafarge precast plant in Edmonton.

Kurt Weber, P. Eng. is Senior Bridge Engineer, Transportation, Western Canada at AECOM. Information also provided by Shane De Lorey, P.Eng, M.Eng, Project Director, Alberta Transportation, Government of Alberta.

PHOTOS Lafarge Precast Edmonton and Alberta Transportation, Government of Alberta