Canada’s Confederation Bridge
Completed in 1997, the Confederation Bridge connects the Provinces of Prince Edward Island (PEI) and New Brunswick (NB) on the east coast of Canada. Design requirements issued by Public Works Canada required design criteria specifically developed for this unique structure. The challenges of harsh environmental conditions, short construction time, and the Public Works Canada requirements were met through innovative design features, large-scale precasting of exceptionally large concrete superstructure and substructure elements, and the most advanced construction techniques.
The total crossing is divided into three major sections: the 4,330 foot New Brunswick approach, the 36,050 foot main bridge, and the 1,870 foot Prince Edward Island approach. The main bridge spans are 820 feet and have a depth varying from 15 to 48 feet. Both approach structures have typical spans of 305 feet and a depth varying from 10 to 17 feet. Water depths in the Strait vary along the alignment, with a maximum depth of about 115 feet. Shallower water depths at the approaches limit access by deep draft vessels and heavy floating equipment.
The bridge superstructure is a single cell, precast, prestressed trapezoidal box girder; the substructure is precast or cast-in-place concrete. The roadway width is a constant 36 foot barrier-to-barrier; total bridge width is 40 feet. The profile grade of the main bridge is 134 feet above the project datum, increasing to 194 feet at the midpoint of the navigation span.
Main Bridge Components
The structural scheme of the main bridge consists of a series of 21 two-column portal frames connected by 22 drop-in spans. These frames and drip-in expansion spans form 43 main bridge spans, each 820 feet long. The spans are fabricated and constructed of four basic components:
- Pier base,
- Pier shaft,
- Double-cantilever deck, and
- Drop-in span.
Precast, prestressed concrete was selected for the construction of these components. Precasting satisfied the following major constraints: total project construction schedule of three years with completion by a fixed date; environmental limitations imposed by ice and weather on the time available for marine work in the strait; and durability requirements dictated by design for a 100-year service life.
Special Design Requirements
Design requirements issued by Public Works Canada had a direct impact on the structural scheme chosen for the bridge. The most important of these requirements are:
- The facility shall be designed to provide a 100-year service life.
- A 564 foot wide navigation channel with 160 foot vertical clearance and a minimum 43 feet water depth shall be provided.
- The roadway width shall be able to accommodate three traffic lanes.
- Failure or collapse of any one span shall not lead to progressive failure or collapse of other spans.
- Environmental loads such as ice, wind, wave and current, earthquake, and temperature shall be taken into account.
- The structure shall be able to withstand a certain magnitude of ship collision.
- Consideration shall be given to the fundamentals of esthetics.
One Hundred-Year Service Life
To achieve a 100-year service life, specific project criteria were developed for design, material selection, workmanship, and quality control. Special load combinations and load and resistance factors for ultimate and serviceability limit states were derived for the bridge design. For this a full calibration process using probabilistic reliability techniques was performed.
The design target safety index, which is a measure of the probability of failure of a structural member, is 4.0 for multi-load-path components and 4.25 for single-load-path components. For serviceability limit states, crack control and concrete cover for corrosion protection of reinforcing steel and prestressing tendons were evaluated for the different structural elements.
With a design life of 100 years, the use of high performance concrete and careful attention to production and construction practices were imperative. More than 520,000 cubic yards of concrete was used for the structure. The proposed high-performance concretes were extensively tested for durability, especially through freeze-thaw cycles, sulfate resistivity and chloride diffusivity testing, checking of alkali content and alkali/aggregate reactivity, evaluation of curing regimes for the huge components, etc. Precasting was chosen for improved quality, as well as reduced construction time.
To monitor the progress of this bridge built to last 100 years, visit Confederation Bridge.
High-Performance Concrete is the Centerpiece in Reconstruction of Historic Wacker Drive
Concrete Technology, November 2001, describes the renovation of historic Wacker Drive in Chicago, Illinois. An L-shaped stretch of bi-level roadway that runs through the heart of downtown, Wacker Drive underwent a $200 million reconstruction of most of its 1.5-mile length. Determined to build maximum longevity into this heavily traveled thoroughfare, the city set an ambitious goal: a 75- to 100-year design life.
The material of choice? High-performance concrete. The concrete for the columns and the 13-inch thick, post-tensioned elevated deck contains 525 lb/yd3 of portland cement, as well as 10 percent by mass of cement Class F fly ash, five percent silica fume, and 15 percent ground granulated blast furnace slag. The mix was developed by Wiss, Janney, Elstner Associates Inc. (WJE) and the University of Illinois-Chicago.
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