Otay River Bridge - International Bridge Technologies, Inc.

Otay River Bridge
San Diego, California
IBT’s Role: Detailed design
and construction engineering.
Technical assistance on site.
Owner: Southbay Expressway
Contractor:
Otay River Constructors
Prime Consultant:
Washington Infrastructure
Services
©2009 Copyright International Bridge Technologies, Inc.
Overall Project
Community Benefits
S
an Diego County’s rapid growth
has outstripped the capacity of the
local freeway network. The region’s
roads face unique challenges, since in
addition to the normal commuter traffic,
there are two busy international border
crossings. One of the two crossings,
at San Ysidro, is connected directly to
Interstate 5. The Otay Mesa crossing to
the east discharges directly into the local
arterial system, creating a congested and
dangerous concentration of traffic.
To address this bottleneck, an 11.2-mile
extension of state highway SR125 was
constructed. This north-south highway
will provide a link to the Otay Mesa
crossing at its southern end and provide
direct access to three major east-west
highways.
The southern portion of this highway
is being built as a Public-Private
Partnership between the State of
California and South Bay Expressway
(SBX), a private entity. Development
was financed by SBX, who will operate
the highway as a toll road for the
duration of their concession period. At
the end of that period, ownership will
revert to Caltrans.
©2009 Copyright International Bridge Technologies, Inc.
Project Description
C
onstruction of the highway
was carried out under a designbuild contract awarded to Otay River
Constructors (ORC), a joint venture
of Washington Group International
and Fluor. It was the responsibility
of ORC to develop the design of the
entire highway, win approval for
the designs, and then construct the
new facilities. As part of that effort,
International Bridge Technologies
(IBT) was hired as a subconsultant by
Washington Infrastructure Services,
part of Washington Group. IBT’s
role was the detailed design and
construction engineering of the Otay
River Bridge.
The Otay River Bridge is a critical
link at the southern end of the
highway. It carries four lanes of traffic
across the wide Otay River Valley, a
seasonal river and environmentally
sensitive area. Spanning a total
of 1012 meters from abutment to
abutment, the bridge is broken into
ten spans of 90.5 meters and two end
spans of 53.5 meters. The bridge
has a twin box girder configuration,
with two trapezoidal box girders
connected by a longitudinal cast-inplace closure pour.
Environmental sensitivity was a
key driver behind the selection
of a segmental bridge at this site.
The bridge crosses one of the few
open spaces remaining in the city
of San Diego. The Otay River
Valley is the home to a number of
protected plant and animal species
and serves as a vital natural oasis
in a rapidly developing area. These
considerations guided the type
selection towards a segmental
bridge, which could be erected with
minimal disruption.
©2009 Copyright International Bridge Technologies, Inc.
Technical Innovation
T
he design of the bridge was
largely influenced by two defining
requirements. The first was the need
to integrate a precast segmental bridge
with the existing design standards of the
State of California. The second was to
accommodate the seismic demands on the
structure in a way that did not impact the
benefits of segmental construction.
The Otay River Bridge is only the
second precast segmental bridge to be
designed and built in California, where the
predominant bridge type is post-tensioned
concrete constructed on falsework.
Caltrans has developed an extensive set
of standards for design detailing and
construction, largely based on this type
of construction. The first step in the
design was to develop Design Criteria that
included these standards but still addressed
the specific needs of precast segmental
construction. The project team worked
closely with Caltrans’ Office of Specially
Funded Projects to ensure that the
segmental requirements were successfully
integrated within the existing framework.
Caltrans has also been at the forefront of
seismic design of bridges. Much of that
work has been distilled into the Caltrans
Seismic Design Criteria (SDC), a set of
standards for seismic analysis, design and
detailing. The SDC was the governing
standard for seismic design of the Otay
River Bridge.
There were two important concepts that
influenced the seismic design. The first
was that the design was ductility based,
rather than force-based. That is, the bridge
substructure was not designed to withstand
a specific set of seismic forces; rather, it
was designed to accommodate a set of
seismic displacements while exhibiting
limited damage. The effects of this
design philosophy were largely limited
to the bridge columns, where seismic
forces are no longer a controlling factor in
determining the vertical steel but influence
the design of the confining steel.
The second important concept was that
of capacity-protected elements. In this
philosophy, bridge elements such as
foundations and pier caps are designed
for the failure load of the column.
Therefore, the more heavily reinforced
the columns are, the more robust the
surrounding elements must be. The
design goal, then, was to limit loads
due to service-level actions such as live
load, wind, temperature and creep. This
minimized the required reinforcement in
the columns and, therefore, the demand on
the foundations and superstructure. This
was the chief reason why the bridge was
designed with three mid-span expansion
joints. While the columns could have
been reinforced to accommodate greater
demand from creep and temperature, the
increase in strength would have rippled
throughout the structure and increased the
overall cost.
©2009 Copyright International Bridge Technologies, Inc.
Design Solutions
E
xpansion joints for balanced
cantilever bridges are a design and
construction challenge. In this case, the
design of the expansion joints had to
accommodate significant longitudinal
movements due to seismic demand
well beyond the typical temperature
movements. For these reasons, a
mid-span joint was chosen. In this
configuration, the expansion joint is
placed at the middle of a span, where the
two cantilever tips meet. The cantilevers
are joined by steel beams placed on the
interior of the box girder. The beams are
housed in concrete diaphragms, which
are configured to transfer shear and
moment across the joint, while allowing
longitudinal movement.
This type of joint has several advantages
for this bridge. From a construction
perspective, it is compatible with
balanced cantilever construction, as it
introduces relatively few changes into
the construction sequence. In addition,
it can easily accommodate significant
longitudinal movement simply by
making the beam as long as necessary on
the sliding end. The remaining design
challenge was to design a beam that would
remain elastic in a seismic event due to
the logistical difficulties in replacing the
beam.
One common concern with precast
segmental bridges located in a seismic
environment is the absence of steel
across the top or bottom flange of some
joints. Many alternatives for addressing
this issue have been proposed, including
secondary concrete pours across the joint
and supplemental post-tensioning. On
the Otay River Bridge, it was decided to
add post-tensioning steel across otherwise
un-reinforced joints. The main effect of
this change was the addition of cantilever
tendons in the bottom flange. Typically,
the bottom flange of a balanced cantilever
bridge is highly compressed near the piers
by the dead load moments, resulting in
little or no steel in that area. In this case,
pairs of four-strand tendons were placed
symmetrically about the pier. These
tendons were installed and stressed to a
nominal seating force at the same time
as the main cantilever tendons. The
supplemental tendons were arranged to
ensure that seismic requirements were met
at every joint. This addition was easily
integrated into the standard balanced
cantilever tendon configuration and
segment production.
©2009 Copyright International Bridge Technologies, Inc.
Construction Innovation
S
uperstructure erection was
achieved by the balanced cantilever method. A self-launching
overhead gantry was used to erect the
segments, which were delivered from
the north abutment over the completed
portion of the viaduct. By delivering
the segments over the completed deck,
disruption to the site was minimized.
Segments were generally stored behind
the north abutment and rarely touched
the floor of the valley.
via temporary post-tensioning
bars. The truss would then slide
transversely to the other alignment. Then, while the truss was
occupied erecting a second pair
of segments, the post-tensioning
crew would install and stress tendons on the first alignment. This
proved to be an efficient method
for maintaining a smooth work
flow and allowed erection rates of
up to six segments a day.
The significant height of the viaduct
also made an overhead gantry a logical
choice. While the construction rightof-way did allow access along the entire alignment, hoisting segments from
below would have been unnecessarily
time-consuming. The truss was also
configured so that it could erect both
box girder alignments in a single pass.
The two main supports for the truss
consisted of steel beams that spanned
transversely across both box girders.
The truss could then slide back and
forth to erect on either alignment.
A typical cycle consisted of erecting
a pair of segments on one alignment
©2009 Copyright International Bridge Technologies, Inc.