Unique Construction Aspects of the Maumee River Bridge

Transcription

Unique Construction Aspects of the Maumee River Bridge: Foundations and Substructure
By Wade S. Bonzon, P.E.
Wade S. Bonzon, P.E.
Figg Bridge Inspection, Inc.
424 N. Calhoun St.
Tallahassee, Florida 32301
Telephone: (850) 224-7400
Facsimile: (850) 224-8745
E-mail: [email protected]
Word Count = 7413 (3913 Words and 14 Figures)
TRB 2003 Annual Meeting CD-ROM
Paper revised from original submittal.
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Unique Construction Aspects of the Maumee River Bridge: Foundations and Substructure
The new I-280 Maumee River Bridge, currently under construction near downtown Toledo, Ohio, is the centerpiece
of the largest single project ever undertaken by the Ohio Department of Transportation (ODOT). This cable-stayed
river crossing and its extensive approach and ramp structures incorporate unique engineering solutions to a variety
of challenges, including an ODOT goal to maintain four lanes of interstate traffic during peak use times throughout
the narrow I-280 corridor.
To meet these challenges Figg Engineering Group utilized precast, segmental, concrete box girders built with the
span-by-span method. To minimize disruption to traffic, the design accounted for “top-down” erection techniques
with delivery of segments across previously erected spans.
The foundation and substructure design minimize construction impacts on traffic and the local community. Single
drilled shaft foundations reduce disruptions to interstate traffic as well as alleviate construction noise for the
surrounding neighborhoods. Drilled shaft operations are fully inspected before and after concrete placement using
down-hole cameras and cross-hole sonic logging. The Contractor’s construction methods eliminate the need for
crane-mounted drilling equipment.
In some areas of the project, the narrow right-of-way and existing structures necessitated unique design solutions for
the approach piers. Large, aesthetically obtrusive permanent straddle bents were eliminated by the use of T-shaped
piers made integral with the superstructure and through the design of innovative temporary straddle bents. The
temporary straddle bents incorporate portions of the permanent piers eventually to be constructed at those locations.
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INTRODUCTION
The new $220 million I-280 Maumee River Bridge, currently under construction near downtown Toledo, Ohio, is
the centerpiece of the largest single project ever undertaken by the Ohio Department of Transportation (ODOT).
This cable-stayed river crossing incorporates unique engineering solutions to provide the city of Toledo and the
State of Ohio with a durable and aesthetically striking signature bridge (See Figure 1).
In 1988 the Toledo Metropolitan Area Council of Governments (TMACOG) identified the replacement of
the existing I-280 bridge over the Maumee River as its highest priority transportation project. This 1950’s era
bascule lift span, known as the Craig Memorial Bridge, is one of only a few movable bridges still remaining on the
nation’s interstate highway system. Interstate 280 runs north and south through eastern Toledo in a narrow corridor
surrounded by residential areas of ethnic and historical importance. Because of this, right-of-way acquisitions were
held to a minimum. The City of Toledo sits at the crossroads of both land- and water-based commerce in the Great
Lakes area, and Interstate 280 provides a major link between northwest Ohio and southern Michigan. Therefore,
ODOT set a goal to maintain four lanes of interstate traffic during peak use times. During non-peak hours, a
minimum of one lane was to be kept open in each direction. This requirement added to the challenge of erecting
elevated structures within the existing I-280 corridor.
To meet these challenges, and to provide an aesthetically pleasing urban viaduct, Figg Engineering Group
utilized precast, segmental, concrete box girders built using the span-by-span method. To minimize disruption to
traffic, the design accounted for “top-down” erection techniques with delivery of segments across previously erected
spans (See Figure 2).
The foundation and substructure design also accommodated the strict maintenance of traffic requirements
while providing the aesthetic features of a world-class signature bridge. Drilled shaft foundations were selected to
minimize construction impacts on interstate traffic as well as the surrounding neighborhoods. In some areas of the
project, the narrow right-of-way and preexisting structures necessitated unique design solutions for the approach
piers. The use of large, aesthetically obtrusive permanent straddle bents was eliminated using T-shaped piers made
integral with the superstructure and through the design of innovative temporary straddle bents. These temporary
structures incorporate portions of the permanent piers eventually to be constructed at those locations.
PROJECT OVERVIEW
Main Span Structure
The cable-stayed main-span crosses the Maumee River with two 612.5-foot (187m) spans that flank a single pylon
that rises 404 feet (123m) above the water. Twenty stay cables in a combined fan/harp arrangement support the
precast concrete segments of the superstructure with deck-level anchorages located at 28’-0” (8.53m) intervals. The
stays are aligned in a single plane along the axis of the pylon. The pylon is founded on 17 drilled shafts each with a
diameter of 8’-0” (2.44m). The 104’-0” (31.7m) diameter circular pylon footing is located approximately 10 feet
(3.05m) below the Maumee River mud line.
Approach and Ramp Structures
The Maumee River Bridge has a total deck area of approximately 1.2 million square feet (111,500m2). The 14,550
linear feet (4430m) of approach structures and 9350 linear feet (2850m) of on- and off-ramps account for 85% of
this total.
Superstructure
The northbound and southbound approach structures each comprise 25 spans on the south side of the river and 29
spans to the north. Spans vary in length from 106 feet to 150 feet (32m to 46m) and typically consist of 13 to 16
precast concrete box girder segments. These segments are 58 feet (17.7m) wide, 9’-2” (2.79m) deep, and vary from
6’-0” to 10’-0” (1.83m to 3.05m) in length.
Four ramps provide access to and from city streets on the north and south sides of the Maumee River.
Ramps A and D, located north of the river crossing, consist of 18 and 12 spans respectively. The 20-span Ramp Y
and 24-span Ramp Z provide access for local traffic on the south side of the river. The 9’-2” (2.79m) deep ramp
segments are 29’-0” (8.84m) wide and vary in length from 5’-0” to 9’-6” (1.52m to 2.90m).
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Altogether, 3015 precast segments comprise the main span, approaches, and ramp superstructures.
Substructure
Precast segmental and cast-in-place alternate designs for the typical approach and ramp piers were presented in the
bid documents. Between the two options cross-sectional shapes, heights, and exterior dimensions remain consistent.
The Contractor’s winning bid included the cast-in-place option for all piers on the project.
Typical approach piers have an octagonal cross-sectional shape with maximum exterior dimensions of 8’0” and 12”-0” (2.44m and 3.66m). These hollow box cross-sections have a wall thickness of 1’-0” (310mm). The
ramp piers have similar architectural details with an octagonal cross-section and nominal exterior dimensions of 8’0” and 10’-0” (2.44m and 3.05m). (See Figures 3a and 3b.)
FOUNDATION DESIGN AND CONSTRUCTION
Except in a few special cases, each approach and ramp pier is founded on a single, large-diameter drilled shaft
(monoshaft). Approach piers feature 8’-0” (2.44m) diameter shafts with design depths ranging from 70 to 120 feet
(21m to 37m) to competent rock. All ramp piers were designed with 7’-0” (2.13m) diameter monoshafts up to 100 ft
(30m) deep. Monoshafts for the typical ramp and approach piers were designed to resist vertical loads through tip
bearing capacity only.
Foundation Type Selection
The selection of large diameter monoshaft foundations for the Maumee River Crossing project was driven by several
issues. Many pier locations for the new bridge structure were located very close to existing I-280 and ramp
alignments. The use of large footings needed for multiple drilled shaft or driven pile foundations would have
resulted in a greater number of lane closures on these existing roadways. For the monoshaft-to-pier connections,
only small circular footings with radii up to 14’-0” (4.27m) were needed.
The large number of pier locations indicated a need to simplify foundation construction operations to keep
the project on schedule. The use of multiple drilled shaft groups or large groups of driven piles would have added a
significant number of construction operations to each pier location.
Concerns regarding construction noise and vibration were also accommodated in the foundation type
selection process. Given the close proximity of residential neighborhoods to the project corridor on both sides of the
river, extensive pile driving operations could have been highly disruptive to the surrounding community.
Innovative Drilled Shaft Installation Techniques
Shaft Reinforcement
The original design called for standard mild deformed reinforcement sizes to maximize the number of available
suppliers and garner the lowest possible price through increased competition. The Contractor elected to replace the
Grade 60 longitudinal shaft reinforcement with proprietary threaded bars with a yield strength of 80 ksi (551 MPa).
This provides several advantages during fabrication and installation of the rebar cage. First, the use of #20
threadbars with a nominal diameter of 2.48 inches (63mm) alleviates the congestion associated with the coupling of
bundled reinforcement of smaller, standard, diameters. In addition, the threaded deformations allow for relatively
simple coupling procedures during reinforcing cage fabrication off-site and just prior to cage installation at the site.
In addition, the use of threaded nuts and anchor plates eliminates the need for standard bar hooks for embedment of
the drilled shaft rebar in the pier footings. Finally, installation of the cages is quicker and simpler because threaded
nuts and plates can be installed to quickly anchor a picking frame capable of securely holding the entire length of the
rebar cage vertically for insertion in the shaft as shown in Figure 4.
Excavation Methods
Drilled shafts for the Maumee River Bridge are being constructed using a fully cased method with excavation
performed by a spherical grab clamshell. Two different types of drilling machines are in use that allow the
advancement of the casing while excavation is underway. A hydraulic “rotator drilling machine” (RDM) is being
used for the approach pier monoshafts. The RDM unit uses hydraulic motors to spin the casing through full 360º
turns with up to 5460 k-ft (7400 kN-m) of torque. With the bottom of the first section of casing equipped with
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carbide drilling bits, and a maximum downward retaining force of 337 kips (1500 kN), the RDM is capable of
inserting casing sections with a rotational speed of 1 rpm (See Figure 5).
The Contractor is also using a hydraulic “oscillator machine” (VRM) for the ramp pier monoshafts. This
equipment is slightly smaller than the RDM unit but works in a similar manner. Instead of turning a full 360º, the
VRM oscillates the casing sections approximately 25º in each direction to drive them into the ground. Two hydraulic
arms attached at opposite points to the machine’s frame alternate the torque force and direction applied to the casing.
Neither machine requires crane-mounted drilling equipment to assist their operations. Instead, the crane is
free to operate the spherical grab at the same time the casing is being advanced. Also, the crane can be used to
supply the RDM or VRM with additional sections of casing as needed throughout the drilling operations. After the
casing and excavation has advanced to bedrock, a steel chisel is used to flatten the bottom of the shaft in preparation
for final cleaning. Designed and built in Germany, the use of these hydraulic drilling machines is rare in the United
States. However, they are well suited to operating in confined urban environments (1).
Comprehensive Inspection Requirements
ODOT specified a complete inspection program to ensure the construction of high quality drilled shafts. This
combination of pre- and post-pour measures is designed to verify shaft cleanliness and verticality as well as the
proper placement of concrete.
Pre-Pour Shaft Inspection
Due to the tip-bearing nature of the shafts on this project, ODOT mandated the use of a down-hole inspection
camera known as a miniature Shaft Inspection Device (Mini-SID). This system uses a stainless steel inspection
“bell” equipped with a high resolution digital video camera and a bank of LED lights to visually inspect the bottom
of the drilled shafts (See Figure 6). The 12 inch (305mm) diameter camera and LED housing acts like a diving bell,
trapping a pocket of air through which the camera can view the bottom regardless of whether the shaft is filled with
water or slurry (See Figures 7a and 7b). The system enables the Contractor and the Owner to work as a team during
the inspection process because both can watch the process in real-time using the video monitor housed in an
attached trailer.
The sensitivity of the mini-SID system allows the Engineer the ability to evaluate the strict tip cleanliness
requirements for this project. No more than 50% of the shaft bottom can be covered with a sediment layer 0.5 inches
(13mm) thick. In addition, no debris more than 1 inch (25mm) deep are acceptable. Although these requirements are
strict, the Contractor has had relatively little problem meeting them using the fully cased method described above.
Cross-Hole Sonic Logging
The project specifications also require cross-hole sonic logging (CSL) tests to be performed after the shaft concrete
has been placed and cured for a minimum of 24 hours. The rebar cage for each approach and ramp monoshaft
includes eight 2 inch (50mm) diameter Schedule 40 steel tubes that run the entire length of the shaft. These tubes are
spaced at even increments around the inside perimeter of the rebar cage, capped at the bottom, and filled with water
prior to concrete placement. The water-filled steel pipes are less susceptible to debonding from the shaft concrete
because the water acts to minimize the temperature gradient across the wall of the pipe induced by the heat of
hydration during concrete curing.
CSL testing uses ultrasound emitter and receiver units to measure the density of the material between them
by measuring the time required for the signal to travel across the intervening medium. The presence of a void in the
concrete between the emitter and receiver alters the propagation speed. By placing the units in various combinations
of tubes, a complete “picture” of the shaft concrete can be obtained and interpreted.
The ultrasonic pulses typically travel through sound concrete at speeds of at least 12,000 feet per second
(3650m/sec). Transmission speeds less than 10,000 fps (3050m/sec) may indicate the presence of an anomaly (2).
As an example, one type of output from the CSL test from another project is shown in Figure 8.
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DESIGN AND CONSTRUCTION OF SPECIALTY PIERS
The physical limitations of the narrow I-280 corridor, required minimum number of traffic lanes, and locations of
surrounding existing structures all contributed to the need for special pier designs for the approach structures on both
sides of the Maumee River. In addition, the community’s aesthetic preferences heavily influenced the final
appearance of these unique substructures. These solutions eliminated the need for large, unsightly straddle bents as a
part of the permanent structural configuration.
Integral T-Piers
Design Considerations
The primary factor in the development of the pier layout near the south abutment was the position of the Norfolk
Southern Railroad Bridge across the existing I-280 alignment. At that point Interstate 280 narrows to a cross-section
approximately 90 feet (27m) wide. The alignment of the new approach structures was centered directly over the
existing I-280 roadways through this bottleneck. Consequently, no room existed for the construction of typical piers
for the separate northbound and southbound approach bridges without prolonged closures to I-280 traffic lanes.
Several concepts to mitigate this issue were discussed during conceptual design. First, the design team
considered placing the approach structure abutment far enough to the north to avoid having to rebuild the railroad
bridge with longer spans. This could also significantly shorten the overall length of the approach bridges. However,
this option required unacceptably steep grades on the approach structures to provide the minimum required vertical
clearance above the new Ramp Y structure.
The second option involved positioning the approach bridge abutment close to the existing railroad bridge.
This would require reconstruction of the railroad bridge with longer spans to accommodate the geometry of the
Interstate 280 ramp alignments necessary to skirt the abutment area during its construction. This effectively added
the railroad bridge reconstruction work to the project’s overall critical path. The coordination process with the
railroad was expected to be complicated and time-consuming, potentially delaying the completion of the project’s
design phase.
The design team elected to place the southern abutment of the new approach structures approximately 675
feet (206m) south of the existing Norfolk Southern Railroad Bridge. This option eliminated the need to reconstruct
the railroad bridge, and allowed adequate space for the temporary I-280 alignments needed to construct the new
approach bridge abutment. Permanent, two-span straddle bents were initially considered for this confined area but
were ultimately rejected based on aesthetic concerns regarding their potentially large size and obtrusive appearance.
Structural Configuration
The resulting structural solution is a single pier with a large T-shaped cap (See Figure 9). Both the north- and
southbound approach structures are integrally connected to the cap, allowing a significant reduction in depth in
comparison to a non-integral superstructure/cap connection. The pier shaft’s hollow box section varies in height
from 29 feet (8.8m) at Pier 4 to 42 feet (12.8m) at Pier 8. The pier cap’s exterior shape is similar to the typical piers
used for the remainder of the project.
A group of six 4~0.6” (15.24mm) and fourteen 27~0.6” (15.24mm) transverse tendons support the cap
cantilevers. These tendons are designed to be stressed in two stages: a portion of the tendons prior to release of the
cap forms and the remainder before erection of the integral superstructure spans begins.
Construction Phasing
During Phase 2 of the Maintenance of Traffic plan all four lanes of I-280 will be carried by the southbound approach
superstructure. The integral T-piers were designed to account for the transverse moments resulting from this live
load condition, but only if both the north and southbound superstructures have been erected for the entire length of
that six-span continuous unit. Due to the large unbalanced transverse moments during superstructure erection, the Tpiers were designed to carry no more than one span out-of-balance transversely during the erection process.
A schematic of the erection sequence assumed during design is shown in Figure 10. Based on this erection
plan, the construction loads represent the critical loading condition for the T-piers. However, the scheme still allows
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for delivery of precast segments over the previously completed portions of the superstructure to minimize impacts to
traffic flow.
Temporary Straddle Bents
Design Considerations
The required maintenance of traffic and aesthetic issues were the primary factors behind the development of the four
temporary straddle bents that support the southbound approach structure north of the Maumee River. Approximately
650 feet (198m) north of the river the existing I-280 alignment drops into a narrow “trench” and passes under a
number of neighborhood streets. While the new approach structures will be elevated above these crossing streets,
they still share the same corridor with I-280 through this area, creating conflicts with the existing southbound lanes
of I-280 traffic.
Due to aesthetic concerns, the large permanent straddle bents that would have been required at several pier
locations were not preferred as a part of the final structural configuration. Integral T-piers like those designed for the
area near the southern abutment would have been much larger here because the northbound and southbound
approach bridges have a greater separation between them.
Structural Configuration
An innovative plan was developed to construct temporary straddle bents in four locations to support the southbound
approach structure during the second phase of the Maintenance of Traffic plan. At this stage of construction, all four
lanes of I-280 will be carried by the southbound approach bridge. These straddle bents incorporate the upper portion
of the permanent pier and are designed to be removed following construction of the permanent foundation and
remainder of the permanent column.
The straddle bent beams consist of precast concrete sections with steel bracing for weight reduction. The
sections of the beam are match-cast against one another and post-tensioned with 19~0.6” (15.24mm) and 27~0.6”
(15.24mm) tendons. One of the beam sections is also match-cast around a central column segment that is designed
to remain as a part of the permanent pier. To achieve a friction connection with adequate shear capacity, the central
column segment and beam section are temporarily post-tensioned together with 44 1-3/8” (35mm) diameter P.T.
bars.
Construction Methods and Traffic Phasing
The following is the process envisioned during design for construction and eventual removal of the temporary
straddle bents.
Phase I – Construct precast straddle beam elements:
•
•
•
Cast the central column segment and match-cast the central beam section around the central column segment.
Match-cast the beam end sections with the central beam section.
Cast the temporary piers.
Phase II – Post-tension and erect the straddle bent beam (See Figure 11):
•
•
•
Post-tension the central column segment to the central beam section using P.T. bars.
Partially stress the beam tendons to post-tension the beam sections together.
Switch I-280 traffic to one lane each direction and erect the beam on to the temporary piers.
Phase III – Prepare the straddle bent to receive the superstructure spans:
•
•
•
Restore I-280 traffic to two lanes in each direction.
Cast the permanent pier cap.
Stress additional beam tendons prior to erection of superstructure onto straddle bent.
Phase IV – Erect the superstructure spans (See Figure 12):
•
•
Erect the first span supported by the straddle bent using the span-by-span method.
Stress additional straddle beam tendons.
•
•
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Erect the second span using the span-by-span method.
Completely stress the remainder of the straddle beam tendons.
Phase V – Construct the permanent foundation and pier:
•
•
Move all four lanes of I-280 traffic to the southbound approach structure.
Cast the lower portion of the permanent pier, leaving an 8’-0” (2.44m) gap under the straddle bent beam.
Phase VI – Connect lower and upper portions of the permanent pier (See Figure 13):
•
•
Place concrete for the cast-in-place closure segment between the permanent pier cast in Phase V and the central
column segment attached to the straddle bent beam.
Stress 16 vertical P.T. bars to achieve structural continuity throughout the permanent pier.
Phase VII – Remove the temporary straddle bent (See Figure 14):
•
•
•
•
•
•
Attach cranes to the straddle bent beam end sections.
Saw-cut the joints between the beam’s end and central sections.
Saw-cut joints in the central beam section and de-tension the friction connection P.T. bars.
Remove the central beam section.
Demolish the temporary straddle bent piers.
Remove the shear keys on the outside faces of the central precast column segment and finish the surface to the
neat lines of the permanent pier.
Using this procedure, the removal of the temporary straddle bents will leave behind permanent piers that
are aesthetically consistent with the remainder of the bridge substructure. The elimination of permanent straddle
bents will serve to enhance the aesthetic value of the new structure.
CONCLUSIONS
The drilled monoshaft foundations selected for this project help to shorten and eliminate interstate lane closures, and
serve to reduce construction noise in surrounding neighborhoods. The need for large, aesthetically obtrusive
permanent straddle bents was eliminated by the design of T-shaped piers made integral with the superstructure and
innovative temporary straddle bents. Portions of these temporary straddle bents are designed to remain as part of the
permanent structure.
The selected substructure designs and construction techniques successfully accommodate the maintenance
of traffic through the busy I-280 corridor, minimize construction impacts on the surrounding neighborhoods, and
provide ODOT and the City of Toledo with the aesthetic features of a world-class signature bridge.
ACKNOWLEDGEMENTS
The author wishes to thank the Ohio Department of Transportation and the Federal Highway Administration for the
opportunity to take part in the creation of a new landmark for the city of Toledo, Ohio.
REFERENCES
1.
Angelo, William J., Rosenbaum, David B. Americans Learn to Do the Twist With European Drilling
Technology. Engineering News-Record. August 27, 2001, pp. 38-40.
2.
Felice, Conrad W. Cross-Hole Sonic Logging (CSL). Lecture Presented at the Maumee River Crossing Field
Site. August 21, 2002.
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LIST OF FIGURES
FIGURE 1 Rendering of Maumee River Bridge cable-stayed main span.
FIGURE 2 “Top-down” span erection technique.
FIGURE 3a) Typical approach bridge pier and b) typical ramp pier.
FIGURE 4 Picking frame for drilled shaft rebar cages secured with anchor nuts to threadbars.
FIGURE 5 Hydraulic “rotator drilling machine” (RDM) used to install drilled shaft temporary casings.
FIGURE 6 Mini-SID stainless steel inspection bell.
FIGURE 7 Mini-SID camera views: a) clean shaft bottom b) shaft bottom with rock fragments and debris.
FIGURE 8 Sample cross-hole sonic logging output.
FIGURE 9 Integral T-Pier.
FIGURE 10 Unit 2 erection sequence for integral piers 4 through 8.
FIGURE 11 Temporary straddle bent construction phase II.
FIGURE 12 Temporary straddle bent construction phase IV.
FIGURE 13 Temporary straddle bent construction phase VI.
FIGURE 14 Temporary straddle bent construction phase VII.
10
FIGURE 1 Rendering of Maumee River Bridge cable-stayed main span.
11
FIGURE 2 “Top-down” span erection technique.
12
FIGURE 3a) Typical approach bridge pier and b) typical ramp pier.
13
FIGURE 4 Picking frame for drilled shaft rebar cages secured with anchor nuts to threadbars.
14
FIGURE 5 Hydraulic “rotator drilling machine” (RDM) used to install drilled shaft temporary casings.
15
FIGURE 6 Mini-SID stainless steel inspection bell.
16
(a)
(b)
FIGURE 7 Mini-SID camera views: a) clean shaft bottom b) shaft bottom with rock fragments and debris.
17
FIGURE 8 Sample cross-hole sonic logging output (2).
18
FIGURE 9 Integral T-Pier.
19
FIGURE 10 Unit 2 erection sequence for integral piers 4 through 8.
20
FIGURE 11 Temporary straddle bent construction phase II.
21
FIGURE 12 Temporary straddle bent construction phase IV.
22
FIGURE 13 Temporary straddle bent construction phase VI.
23
FIGURE 14 Temporary straddle bent construction phase VII.

Unique Construction Aspects of the Maumee River Bridge

Transcription

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