100-Year Bridges
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Lake Alvord Bridge, San Francisco,
CA
Constructed in 1889
 The first reinforced concrete bridge in America no longer carries the horse-and-buggy traffic for which it was built. But the 113-year-old prototype still arches strongly today over a pedestrian entrance to San Francisco's Golden Gate park, welcoming visitors to the Children's Quarters.
Known as the Lake Alvord Bridge, it was built in 1889 by Ernest L. Ransome, the great 19th century innovator in reinforced concrete design, mixing equipment, and construction systems. The bridge was constructed as a single arch 64-feet wide with a 20-foot span. Ransome is believed to have used his patented cold-twisted square steel bar for reinforcement, placed longitudinally in the arch and curved in the same arc. The face of the bridge was scored and hammered to resemble sandstone.
E.L. Ransome left San Francisco a few year's later, frustrated and bitter at the building community's indifference to concrete construction. Ironically, the city's few reinforced concrete structures, including the Lake Alvord Bridge, survived the 1906 earthquake and fire in remarkable shape, vindicating Ransome's faith in the method.
The Lake Alvord Bridge was designated a civil engineering landmark by the American Society of Civil Engineers in the 1970's.
Confederation Bridge, Canada Constructed in 1997
 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.
Bridge Description
The total crossing is divided into three major sections: the 1320 m (4330 ft) New Brunswick approach, the 10,990 m (36,050 ft) main bridge, and the 570 m (1870 ft) Prince Edward Island approach. The main bridge spans are 250 m (820 ft) and have a depth varying from 4.5 to 14.5 m (15 to 48 ft). both approach structures have typical spans of 93 m (305 ft) and a depth varying from 3.0 to 5.1 m (10 to 17 ft). Water depths in the Strait vary along the alignment, with a maximum depth of about 35 m (115 ft). 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 11.0 m (36 ft) barrier-to-barrier; total bridge width is 12 m (40 ft). The profile grade of the main bridge is 40.80 m (134 ft) above the project datum, increasing to 59 m (194 ft) 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 250 m-long (820 ft). 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 172 m (564 ft) wide navigation channel with 49 m (160 ft) vertical clearance and a minimum 13 m (43 ft) 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.
 Concrete Specifications
With a design life of 100 years, the use of high performance concrete and careful attention to production and construction practices were imperative. Over 400,000 cubic meters (520,000 cu yd) 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:
http://www.confederationbridge.com/
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