Benicia-Martinez Bridge, Northern California
Owner: State of California, Caltrans
General and Concrete Contractor: Kiewit Pacific, Concord, Calif.
Engineer: T.Y. Lin, San Francisco; CH2M Hill, Sacramento, Calif.
California bridges are designed and built with one overriding concern: the ability to withstand strong earthquakes. The Benicia-Martinez Bridge, located on the Carquinez Strait about 50 miles east of San Francisco, is no exception. This new bridge was designed to withstand an earthquake of magnitude 8 on the San Andreas fault, 7.25 on the Hayward fault, and 6.75 on the Green Valley fault, based on the distance from the fault to the bridge. It is considered a Lifeline Structure—meaning that after an earthquake, the bridge will be able open quickly to emergency vehicles.
Construction of this $1.3 billion bridge started in July 1999 and it is scheduled to open by December 2007. It will carry five lanes of traffic and include space for a future light-rail transit system. The bridge is a cast-in-place balanced cantilever segmental bridge with 660-foot spans that are 160 feet above the water. Each cast segment is 82 feet wide by 15.75 feet long and between 16 and 39 feet deep, according to Richard Foley, senior bridge engineer, California Department of Transportation (Caltrans).
In a state where environmental concerns can sometimes control a project's fate, Foley says they had only one significant issue that shut down construction. When workers started driving piles for the pier foundations, the impact vibration traveled through the water killing fish. The contractor installed a “bubble curtain” to isolate the vibrations and that solved the problem.
One unusual feature of this project is that the segments are being cast with 10,000 psi sand lightweight concrete, which means the course aggregate is lightweight but the sand is not. The goal is to reduce the weight of the deck for seismic reasons—less mass equals less force on the structure in the event of an earthquake.
The concrete mix has a unit weight of 125 pounds per cubic foot with 160 to 220 cubic yards of concrete required for each segment, depending on location. Starting from each pier, forms are held in position on both sides by a “traveler.” Kiewit moved fresh concrete by barge to the location then placed it into the forms by either crane bucket or pump. When the concrete reaches the proper strength, post-tensioning tendons are stressed to secure the segment to the nearest pier. Foley says that unsupported segments extended as much as 330 feet from piers before they were joined to the segments coming the other direction to complete a span—a sight that appears to defy gravity.
The project requires Kiewit to cast 344 segments, with only 17 left to cast as of August 27, 2006.
Owner: Colorado Department of Transportation (CDOT); Regional Transportation District (RTD)
Design-build contractor: Southeast Corridor Constructors (a Kiewit/Parsons joint venture)
Sound wall design: Surface Strategy, Denver
Planning: Carter & Burgess, Denver
Expanding and rebuilding 17 miles of a city's most heavily traveled interstate is a big job. Add 19 miles of double-track light rail with 13 stations, seven pedestrian overpasses, three underpasses, and three parking structures; reconstruction of eight major highway intersections; reconstruction and widening of more than 20 bridges; and miles of decorative sound walls, and you have a real concrete mega project. T-REX (for Transportation Expansion Project) has transformed Interstate 25 from down town Denver to the southern edge of the metro area from an outdated congested high-way to an ultra-efficient 21st century transportation corridor. And concrete—a total of more than 630,000 cubic yards—is the most prevalent material on the project. T-REX stands as a testament to concrete's strength and versatility.
With the highway's official opening in September 2006 and the light rail system scheduled to start up in November, T-REX has completed its work ahead of schedule and under its $1.67 billion budget—remarkable for a project of this size and complexity. One reason is the design-build and partnering philosophy used throughout the job. The Colorado Department of Transportation (CDOT), the Regional Transportation District (RTD)—which owns and will operate the light rail— Southeast Corridor Constructors (SECC), and the many other participants, combined to form the T-REX organization that established and maintained goals for safety, quality, budget, and schedule.
All reconstructed pavement on T-REX was concrete. The concrete used for paving was supplied by Aggregate Industries from their Morrison, Colo. quarry, under a CDOT specification, which uses a statistical approach to quality control. With very little admixture used, the specification calls for a minimum cement content and a maximum water-cement ratio. More than 99.9% of the concrete fell within the limits for strength and air content. Pavements are generally 12 inches thick with little subbase, since the underlying layer is alluvial gravel.
“The pavements are designed for a 30-year life, which should be easy to achieve,” said Al Eastwood, T-REX segment manager and the top concrete expert on the project. That implies maintenance work after 20 years, he explains, with sealing and some diamond grinding and replacement of no more than 5% of the panels. In his experience, though, on similar pavement, little replacement was needed at the 20-year mark.
The majority of sound walls were precast and set in place—some as façade over drilled caissons that were then excavated on one side, some as freestanding cavity walls, others as mechanically stabilized earth. Much of the sound wall incorporates cast-in wall art designed by Surface Strategy and created using form-liners from Scott Systems (see “Concrete at its Best 2005,” January 2006, p. 44).
The numerous transit stations and interchanges have led to a development boom along the corridor—transit-oriented development—that resulted in the addition of several pedestrian overpasses and parking structures to accommodate the anticipated business. The overpasses are steel structures with walkways that are 5 inches of concrete on metal deck. Bridges are also concrete, coated with a methyl methacrylate sealer. One section of off-ramp that had been asphalt was whitetopped with a 5-inch overlay—it reopened to traffic within 48 hours and has a 20-year life expectancy.
Renee and Henry Segerstrom Concert Hall, Costa Mesa, Calif.
Owner: Orange County Performing Arts Center, Calif.
Architect: Cesar Pelli, New Haven, Conn. General Contractor: Fluor Enterprises Inc., Irving, Texas
Concrete Contractor: Largo Concrete, Tustin, Calif.
Reinforcing Steel Placement: Franklin Reinforcing Steel Co., Santa Fe Springs, Calif.
Once in a lifetime for a contractor, a project comes along that seems to add one complexity atop another in an increasingly intricate iteration. Start with construction in a high seismic zone, which requires extreme amounts of reinforcing steel. Onto that add impossibly tight tolerances—±1/8 inch for most wall and column locations. Add in walls, decks, and beams that are curving in an undulating serpentine pattern, that the entire structure is designed to provide a virtually soundproof space for concerts, and that there is an absolute deadline for completing the job since 2000 people are coming to see a concert in the completed hall on September 15, 2006.
These are some of the challenges that Largo Concrete faced in construction of the Segerstrom Concert Hall, part of the Orange County Performing Arts Center. “There were 18 chapters of drawings, which led Largo to issue 1400 RFIs and bulletins during construction,” said Largo's onsite project manager Garrett Greer. “Locating vital information such as dimensions often meant reviewing up to six chapters of drawings, wall schedules, shop drawings, and requests for information (RFIs). Decks were sloped and had three-dimensional bowl shapes, grid lines were skewed, and curves required multiple radiuses and work points that often were not tangent to one another due to imprecise dimensions and angles that were rounded off (8.5 degrees instead of 8.4865 degrees, which is the kind of precision that was needed).” Although background CAD files were provided to Largo, they had to overlay and compare a number of different sheets. Largo then generated 3-D drawings where needed to illustrate design problems. This pointed out numerous contradictions between sets of plans and pinpointed where changes had to be made. Having the drawings in this format also allowed Largo to respond quickly to the many changes that came up, mostly due to the architect's aesthetic concerns and to soundproofing concerns. The architect/engineering firm had a full-time onsite architect and engineer. The general contractor also employed two full-time architects and a structural engineer. Largo had two detailers onsite along with two project engineers.
Construction started with a heavily reinforced 3-foot deep mat slab (consuming roughly 7000 cubic yards of concrete) atop 1500 precast concrete piles with pile caps as large as 21x12x5 feet deep. There were nine different mat foundation elevations in the basement. All electrical and plumbing was placed atop the mat and then covered with a 13-inch base course of bird's eye fill. A 5-inch slab on grade was then placed atop the base course.
Beyond the slab is where the complexity really took over. “There was more than 26,700 lineal feet of cast-in-place (CIP) beams,” said Greer, ‘that's over 5 miles of beams. And the 34-inch diameter columns at the north end of the building were freestanding to 60 feet tall.” With few square corners, nearly all form-work was custom built and could seldom be reused. For vertical form-work, Largo used Peri forms. Positioning reinforcing steel was a constant challenge and often required welding and mechanical couplers, since there wasn't room for lap splices. Lenton terminators and formsavers were also used in many locations. Rebar was not only congested, but also had to curve into radius walls and serpentine beams. In some cases, concrete was used to encase structural steel and the reinforcement was welded to the steel.
Sound reduction and control required that the HVAC system be oversized to reduce static air pressure and that every seat had its own individual air vent which required a 5.5-inch hole through the concrete. In most cases, those holes were cored out of the concrete rather than being formed with block-outs. Even the coring required extra steps: since the core could not be allowed to drop to the seating level below, Largo workers installed a plank below to catch the cores. In many cases to increase sound attenuation, elevated slabs were supported on neoprene isolators.
The concrete was typically a 5000 psi mix with 3/8-inch aggregate and superplasticizer to get it through the tight rebar spacing and to eliminate rock pockets. The seating areas were all placed with a placing boom on a 50-to 60-foot-high pedestal.
One nail-biting phase was placement of the 12-foot-deep, 127-foot-span steel trusses that supported the roof. Each truss was supported on its ends by a tall freestanding concrete column with embedded anchor bolts. The tolerance for the bolts and trusses to fit was ±1/8 inch. Thanks to superb detailing and layout work, the trusses fit with little rework and were positioned at a rate as high as two per day. Largo had as many as six layout workers on-site, sometimes shooting locations from nearby rooftops.
On September 15, the hall was ready, despite two of the wettest years in Southern California history. The Pacific Symphony Orchestra and Placido Domingo took the stage to perform the world premier of a new work by Wiliam Bolcom; on the 16th it was the Pacific Symphony Orchestra with Midori performing the world premier of a Phillip Glass work. The concrete in most cases was nowhere to be seen and mostly unappreciated by the audience, but without it, nothing else would have been possible.
Hoover Dam Bypass, Colorado River Bridge
Owner: U.S. Department of Transportation, Federal Highway Administration, Central Federal Lands Highway Division, Washington D.C.
Engineer: HDR, Las Vegas and TY Lin, Denver
General contractor: Obayashi Corp./PS Mitsubishi-USA Joint Venture, South San Francisco
Erection engineers: OPAC, San Francisco and McNary, Bergeron & Associates, Westminster, Colo.
The location is stunning and the construction challenging. Located 1500 feet downstream of Hoover Dam and 350 feet higher, connecting Nevada and Arizona, the new bridge will divert traffic from crossing on top of the dam. The deck of the bridge will be 900 feet above the Colorado River with construction starting 600 feet above the water. The $114 million, 1960-foot-long bridge is scheduled to open for traffic in mid-2008.
But this bridge is unique in more ways than just the challenging location. The bridge will be supported by twin 1060-foot concrete arches, according to Jim Stevens with general contractor Obayashi Corp./PS Mitsubishi-USA. “They will be the longest concrete arch spans in the United States and the fourth longest in the world,” he says. “The cast-in-place arches are supported by temporary stay cables and are connected together by cross bracing, so every time an adjustment is required to one arch, the other is influenced.” He believes this is the first construction involving two arches linked together.
Obayashi erected two 50 ton cableway cranes to move forms, concrete, and people for the construction. The cables cross the gorge from 350-foot-tall steel lattice towers on either side that are 2500 feet apart. Dave Zanetell, project director, Federal Highway Administration, says that using arches as the main support for the bridge represents a classic solution for tight canyon crossings because an arch easily fits within the canyon's walls, with no structure above the rim. By contrast, a suspension bridge requires space for back spans and stays and extends well above the rim of the canyon. “And the arches' shape and form match more closely with the dam,” says Zanetell.
The foundations for the arches are substantial. Referred to as skew back foundations, each is constructed of approximately 2000 cubic yards of 4500 psi concrete. Workers placed approximately 5000 lineal feet of cooling tubes in each foundation to control concrete temperatures in order to avoid thermal cracking. The contractor has recently started forming the arches.
The concrete is specified to be 10,000 psi compressive strength and placement will be completed over the next year. Construction has also begun on the precast concrete columns that will extend from the arches to support the deck. The tallest columns will be 300 feet tall. The columns will support steel tub girders, which will in turn support the concrete deck. The precast column segments are being cast by the joint venture companies at a site 10 miles from the bridge.
The project also includes construction of eight bridges and 4.5 miles of roadway to route traffic to the new bridge. Given the terrain, this is another significant challenge.
Met 3, Miami
Group USA, Miami
Foundation Contractor: HJ Foundation, Miami, Fla.
The MDM Development Group's success is soaring to new depths with the groundbreaking record of the largest auger cast pile by volume in the United States at its Met 3 building in Miami. The auger cast pile—a shaft drilled into the ground while simultaneously pumping concrete into the bottom of the hole through the large crane-sized drill as it pushes the soil up and out—is 3 feet in diameter and 120 feet deep. Before the Met 3 building, a 30-inch pile at a depth of 150 feet was considered large. With the Met 3 set to be the tallest residential building south of New York, the massive piles are critical to stability.
Each 1000-ton capacity pile takes about two hours to drill and five ready-mix trucks to fill. The colossal 308-pile project will use 1540 truck loads—380,000 tons of concrete—to complete. “Improvements in equipment design and leading edge installation techniques have made it possible for us to install larger and deeper piles that enable us to provide foundations for large, tall buildings in South Florida,” said Edwin Hickey, president of foundation contractor HJ Foundation.
In addition to the size of the auger piles, another milestone was reached with introduction of the two-level O-Cell test. Up until now, only one O-Cell test had been used in an auger pile. Because of the size of the Met 3 piles, though, two O-Cells tests were used in each pile. The test is conducted with two hydraulic jacks, one to push up from the bottom and the second to push down on the top. The test is conducted to ensure that the piles have enough capacity to support the building. This type of testing is relatively new to auger cast piles—before 2004 it was only used on drilled shafts.
Metropolitan Miami, the largest mixed-use development in downtown Miami, features three high-rise towers; MET 1, MET 2, and MET 3. There will also be Met Square, a retail, lifestyle and entertainment venue.