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BART-SFO Extension Aerial Wye Structure
San Francisco, California
Transit
Home > Bart-SFO Extension
 The
Aerial Way Structure is an integral component of the $1.48 billion
BART San Francisco Airport Extension Project, one of four Federal
Transit Administration (FTA) sponsored design-build turnkey demonstration
transit projects. Designed using a state-of-the-art analysis to meet
the most stringent seismic standards, and built using innovative techniques
that enabled construction in a wetlands area, the aerial wye is a
sleek, new, cast-in-place elevated track structure that connects with
the Airport constructed Highway 101 overpass trackway to bring Bay
Area Rapid Transit District (BART) service directly to the San Francisco
International Airport.
The structure, called the aerial wye, is composed of two legs that carry BART
trains both north and south, to and from the airport trackway. The
two legs have a total of 46 spans. The spans are 80 feet long and
the columns are up to 60 feet high. The north split aerial leg is
2,240 feet long with a minimum radius of 1,150 feet and the south
let is 1,440 feet long with a minimum radius of 600 feet. The complexity
of the structure as well as its location, two and one quarter miles
from an active seismic area, the San Andreas Fault, presented significant
design challenges.
The designers of the structure used a “non-linear 3D time
history analysis”—the most current seismic analysis
technique available—to achieve the District’s objective
of minimal structural distress resulting from a “maximum credible”
seismic event. Although BART seismic requirements have always been
stringent, the “maximum credible” seismic standard was
adopted after the Loma Prieta Earthquake in 1989.
Creativity
To meet the challenges of construction in the environmentally sensitive
“West of Bayshore” area where the project is located,
the project team developed special construction techniques that
prevented any negative impact to the habitat of one endangered and
one threatened species, the San Francisco Garter Snake and the California
Red-Legged from respectively. The erection of exclusion fencing
kept the endangered and threatened species out of the work areas
and the construction of a temporary steel and timber trestle to
support heavy equipment during construction was key to minimizing
disturbance to the ground. The trestle provided all access for construction.
After the pile caps were cast, the support for the girders was located
entirely on top of the pile caps. After the girder concrete had
cured, the form supports were lowered back onto the trestle and
removed. Then the trestle was removed.
 Environmental
monitoring programs also safeguarded the endangered species. Additionally,
the contractor was not permitted to use any mechanized equipment
to clear the area, and had to hand clear vegetation using non-mechanized
tools.
Transferability
In the aftermath of Loma Prieta, BART functioned as a lifeline for
the Bay Area when the Bay Bridge linking the East Bay with San Francisco
and the West Bay was significantly damaged. The importance of keeping
BART operational after any potential future earthquake became apparent.
Since earthquakes larger than the Loma Prieta are anticipated in
the Bay Area, BART decided to adopt a standard that would withstand,
with minimal damage, even the maximum credible earthquake that could
occur.
The techniques used by the project team to design for the most
stringent active seismic zone requirements as well as to develop
a construction method for building in wetlands can be easily applied
to other structures facing similar challenges.
Aesthetics
Since the BART aerial wye is highly visible, its appearance is very
important. Travelers on Highway 101 and airport- and BART-users
all pass the new structure that brings service to and from the airport
from the BART main line. Designed as a multi-cell cast-in-place
concrete box girder section, the structure is a graceful combination
of horizontal and vertical curves supported on variable height columns
with piles anchored below the Bay mud.
Economics
With its long life, and low maintenance compared with steel, concrete
was the natural choice for this affordable and beautiful aerial
structure.
Functionality
The structure connects BART to the airport, with split tracks that
transport passengers out of the airport, north to San Francisco,
and south to the Millbrae Intermodal Station. Using the innovative
trestle technique demonstrates that important concrete structures
can be built with minimum disturbance to the environment. Total
cost of the Aerial Wye Structure portion of the BART project is
approximately $14 million.
Project Credits
Owner: S.F. Bay Area Rapid Transit District (BART)
Engineer: BATC JV (Bechtel, PBQ&D, John Warren Assoc. &
Don Todd Assoc. for PE, and HNTB for Final Design
Contractor: Tutor-Saliba/Slattery JV
Concrete Supplier: A&A Ready Mix
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