Record-Breaking Precast Nu Girders Installed In Alberta

These new twin bridges are part of the Deerfoot Trail extension connecting to Highway 2. Both structures cross the Bow River 8 km south of Calgary. The bridges were designed and tendered with steel plate girder and precast concrete NU girder superstructure alternatives. The low bid for the NU girder system provided about a 10% cost saving ($9.6 million versus $10.5 million).

The span arrangements for both bridges are 53.0 - 65.0 - 65.0 - 53.0 m. The original design used segmental precast concrete NU girder segments, 48 m long for the end spans and 54 m for the center spans. A 12 m long post-tensioned hammer head (cantilever) section was designed to be integral with the piers.

An alternate proposal, submitted by the contractor, eliminated the pier cantilevers. Precast NU girders were proposed to span from support to support. The continuity for LL and partial DL is achieved by post-tensioning after the end anchorages and pier diaphragms were poured in place. After extensive reviews, the proposal was accepted and considered equivalent to the original design. Although there was a small financial benefit, the primary reason for Alberta Transportation's accepting the alternate proposal was to enhance the precast concrete technology and provide flexibility in bridge design alternatives by developing long span precast girder design solutions.

The 64 m (210 ft) long main span girders are believed to be the longest precast concrete plant-cast girders ever manufactured in North America, if not in the world. Sixteen of these 2800 NU girder main span sections, each weighing over 130 tonnes, were shipped over 10 km from the precast plant and installed at the jobsite. Sixteen 52 m (171 ft) long end span girders completed the bridge superstructure.

NU Girder Sections

NU girder sections have a wide bottom flange to increase the compression flange capacity in the negative moment region and to provide better stability during handling, shipping and erection. Up to 58 pretensioning strands can be placed in the bottom flange. Fourteen straight pretensioning strands can be placed in the relatively thin top flange.

Post-tensioned NU girders are produced using the same side forms as pretensioned NU girders (160 mm web) but with a 175 mm thick web to accommodate the post-tensioning ducts. For the Bow River Bridge, full height 2800 mm deep side forms were installed in the plant to cast the girders.

Design
The bridges are designed for CSA S6-00, CL 800 loading. No tension was allowed in the concrete at the top flange level or at the top of deck level for a post-tensioned superstructure at Serviceability Limit State. Where practical, tension was also limited to zero at the bottom flange level.

No end blocks were provided in the precast NU girders. Post-tensioning anchorages were installed in 1.0 m thick cast-in-place concrete end diaphragms at both abutments. The elimination of integral precast end blocks greatly simplifies precast formwork, and is a major factor in the economy of the system.

Galvanized steel angles, used at the piers and intermediate diaphragms, were significantly more economical than concrete diaphragms.

Materials
The girders were cast using normal weight concrete (2400 kg/m3) that contained 10 % silica fume by weight of cement. The concrete strength was 35 MPa at release and 60 to 65 MPa at 28 days. Minimum air entrainment was 5 %. The prestressing strands are seven-wire, low relaxation with a nominal diameter of 15.0 mm. The strands have a minimum ultimate strength of 1860 MPa and the allowable initial stress before losses was 0.7 fpu. CSA G30.14 Grade 480W Welded Wire Fabric (WWF) and CSA Standard G30.18M Grade 400 reinforcing steel were used. Tack welding of reinforcing steel was not allowed.

Fabrication
Con-Force Structures' Calgary plant has the capacity to strip and load girders to a maximum weight of 136 tonnes (including rigging). This lifting capacity generally governs the maximum allowable length of girders. Handling is accomplished with various combinations of overhead cranes.

A key component was to simplify the girder manufacturing to allow girders to be stripped out and a new girder cast each day. Maintaining this daily casting cycle provided several challenges: The girder's inherent ability to accommodate large prestressing forces required a very high early concrete release strength. This was achieved with an optimum concrete mix design and curing the concrete in a heated form. The concrete mix attained the required release strength within 12 hours of placement of the concrete. The actual concrete placement for large girders takes up to 3 hours. Release cylinders were taken from the last batch of concrete.

Daily casting also leaves limited time for form set-up, stressing and installing the reinforcing steel, post-tensioning ducts and other embedded items. This work was accomplished with the following strategies: Deformed Welded Wire Fabric (WWF) reinforcement (up to 13 mm diameter) was delivered to the plant as pre-bent sheets ready for placement into the open form and was easily handled by the workers. The use of 15mm prestressing strands required less pulling and stressing labour per unit of force than traditional 12.7 mm strands. Standard cross bracing hardware was embedded at standardized locations. Fixed form soffits did not require daily adjusting to accommodate camber control. Eliminating end blocks allowed quick girder length changes.

Shipping and Erection
The lateral stability of the sections due to handling and transportation was fully analyzed. The NU girders for this project were 52 m and 64 m long. The girders were temporarily trussed with steel sway bracing attached to the top flange for trucking to the site. This bracing had to be narrower than the final girder spacing to facilitate installation. The wide bottom NU girder flanges provided substantial lateral restraint during both shipping and erection. Wind loads did not present any problems during the shipping and installation. The girders were set down on temporary bearings and cross braced immediately upon the erection of adjacent girders. The permanent steel cross bracing, designed for structural integrity, facilitated girder stabilization during the erection.

Conclusions
Valuable experience was gained on the *Taber Bridge. Although this bridge had fairly long girders, confidence was gained that even longer girder sections could be produced. Therefore, a value engineering proposal was submitted for the Bow River Bridge that proposed to eliminate all pier cantilevers and erect 65m long girders without any splices across the Bow River. The knowledge acquired and proactive strategies developed were sufficient to enable Alberta Transport and the precast concrete industry to achieve this goal.

Some of the steps implemented for this project include:

Partial length, full depth and fully reinforced/prestressed mockup girders were produced to help find ways to overcome cracking in high stress areas.
Hardware (such as bearing and gusset plates) was securely bolted to the girder formwork.
Various types of sleeves through the webs of the girders were tested for lifting and installing the girders without cracking the girders. This resulted in the development of a 'spool' type steel assembly to be selected that provided satisfactory and crack-free performance.
Bearing shoe plates at girder ends were cast in to satisfy design requirements and to eliminate any cracking in the bottom flanges.
The local WWF supplier was able to supply close stirrup spacing of 75 mm c/c. A combination of WWF and normal reinforcing steel stirrups at 50 mm c/c at the ends of the girders eliminated almost all web cracking at this location.

NU girder bridges are being quickly accepted in Alberta. Several more projects are being designed using NU girders for the superstructures. Alberta Department of Transportation, Bridge Engineering, as well as major cities in Alberta have been very receptive to working with the precast industry in developing new products and solutions.

Owner: Alberta Transportation
Prime Consultant: Associated Engineering Edmonton Ltd.
Redesign Engineer: Campbell Woodall & Associates
General Contractor: Penn-Co Construction
Precast Contractor: Con-Force Structures, Calgary, AB

 
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