By: Arthur Schurr
Philosopher Lao Tzu said, “In the world there is nothing more submissive and weak than water. Yet for attacking that which is hard and strong nothing can surpass it.”
For the past 75 years, the elements—and water in particular—have been attacking structures spanning the Indian River Inlet. The Delaware Department of Transportation (DelDOT) is responding to that challenge with an innovative concrete bridge solution.
“This will be the fifth bridge built in the last 75 years over this natural breach within the peninsula,” explained DelDot Indian River Inlet Bridge (IRIB) Project Manager Douglass Robb. “Like every bridge before it, the current S.R. 1 bridge over the inlet faces an extreme scour condition.”
Built in the 1960s and widened in the 1970s, the existing steel-girder bridge encounters constant, acute scouring forces. The inlet channel depth around the piers was 28 ft at the time of construction; with two supporting piers positioned in the turbulent inlet channel bed, scour holes adjacent to the piers have now surpassed 100 ft.
“It’s one of the worst scour situations in the country,” added Robb. “And that’s the primary purpose for this project. We need a bridge with piers outside of the water, because the violent flows through the channel are too destructive. There are several boater safety issues, and periodic drownings in the inlet. The environment beneath that bridge is plain nasty.
“Another key requirement was for the bridge to be predominantly a concrete structure, which largely precluded the use of structural steel. Because of the environment—saltwater exposure, high winds, a very corrosive area—a lot of our project criteria have revolved around the durability and longevity of the structure.”
DelDOT had good reason for adopting those stringent criteria. This bridge is a critical transportation element in the region.
“S.R. 1 is a lifeline route, a critical link in our roadway network. A durable, reliable structure is very important. If this bridge is out, the detour around the bay takes at least 45 minutes. This bridge has considerable life-safety and economic impact on our schools, hospitals and communities.”
Slim and fast
Scheduled to open to traffic in April 2011, the new IRIB will boast four traffic lanes with shoulders, sidewalks and a sand bypass system. The 2,600-ft-long, $150 million cable-stayed bridge will have one 950-ft main span and two 400-ft back spans. Featuring both cast-in-place and precast concrete elements, the IRIB is expected to have a 100-year design life. To meet DelDOT’s needs, the bridge work was awarded as a design-build project to a Skanska/AECOM team.
“Design-build is a great delivery method for owners that need to augment their resources. And it served this project perfectly,” explained Skanska IRIB Project Manager Jay Erwin Jr. “DelDOT was concerned about speed of design and construction because of the existing scour holes; they wanted an effective replacement structure in place as quickly as possible. They opted for design-build to compress the schedule.
“The project was awarded in June 2008. We were mobilized at the site in September 2008. And the bridge will open to traffic in April 2011. In effect, the bridge will be procured, designed and built in a period of 40 months. That efficiency is no accident. For example, we’re using a temporary shoring system to support all of the back span of the cable-stay structure and a portion of the main span. Through methods like that, we can concurrently construct large structural components while the tower construction is going on. Because of our team’s extensive design-build experience and the close partnership with DelDOT, design-build effectively reduced the schedule by an entire year.”
Ken Butler agreed. And as IRIB design manager and AECOM’s national director of bridge services, he believed the use of design-build—and the tough requirements for the bridge—fostered innovation in design and construction.
“The stipulations on this bridge actually spurred creativity. For example, no piers could be located within the inlet because of scour. The inlet itself is currently 500 ft wide at the crossing, and the U.S. Army Corps of Engineers may eventually widen it to 800 ft. Therefore, DelDOT put a horizontal clearance envelope restriction of 900 ft around the inlet. That virtually dictated what the bridge would look like. A cable-stayed bridge was the most economical solution; our span became 950 ft from center of pylon to center of pylon.
“Another critical specification is the use of high-performance and low-permeability concrete. Use of structural steel is not permitted except for specific structural components like expansion joints, bearing plates and the cable-stay anchorage boxes inside of the pylons. Any exposed and nonreplaceable structural steel components, like bearing plates, have to be stainless steel. The cable-stay anchorage boxes are encapsulated in the pylons and therefore protected from the elements. These components are further protected with a three-part zinc paint system. But critical specifications were not the only driving force behind the design of the new bridge.
“The community wanted an unobtrusive bridge, one that didn’t overpower the pristine landscape or block the spectacular views of the Atlantic Ocean. The area surrounding the bridge is beautiful, and the community wanted the vista undisturbed. So we focused on slender pylons that were not too high, and we avoided heavy cross struts between the pylons. We wanted the bridge to be very open and opted for vertical planes of cable stays in a semi-harped pattern to replicate sails. Also, we went with a thin, graceful superstructure that wouldn’t overpower the view. We wanted to complement the surroundings, not intrude upon them.”
To achieve this unique blend of form and function, the project team optimized every possible element of design and construction. For the bridge’s most copious element, the team chose low-permeability concrete from Thorogoods Concrete Inc., a local concrete supplier. According to DelDOT’s Robb, the project will ultimately use more than 35,000 cu yd of that critical element.
“We have a minimum of a 100-year design life. We have low-permeability requirements and very stringent concrete cover requirements. We’ve gone through extensive testing to make sure we don’t develop a concrete mix that will have an alkali-silica reaction problem, which is unfortunately prevalent in Delaware. We put a lot of attention and focus on the materials to make sure we’ll get longevity and durability in this new structure.”
Chief construction
Using predominantly concrete down to the foundations, the foundations themselves are 3 ft square, prestressed, precast concrete piles. Voided to reduce weight, they measure approximately 100 ft long and provide 1,800 tons of ultimate load-carrying capacity per pile. The piles have an 8,500-psi concrete strength. Forty-two piles support each of the four pylons. All precast components for the project were cast at Bayshore Concrete Products (a Skanska company) and trucked to the project site. The footings are 4,000-psi concrete and the abutments are 4,500-psi concrete. So overall, the substructure uses approximately 4,500-psi concrete, as do the approach columns and caps.
Supporting 19 stays each, the 240-ft-tall concrete pylons are voided sections that taper from 16 ft at the base to 12 ft at the first stay. From there on, they are a consistent 12-ft width to the top. Transversely, they are only 11 ft wide. So the nominal cross section of the pylon itself is about 12 ft by 11 ft. Conventionally reinforced and not post-tensioned, the pylons are 6,500-psi concrete—with structural steel anchor boxes encased within them to anchor each stay. The pylons are cast in place in 18-ft lifts.
The cable-stayed bridge is 2,600 ft long, with two 400-ft side spans and one 950-ft main span. Flanking the cable-stayed spans are four 106-ft-long prestressed concrete edge girder spans on either side of the bridge. The cable-stayed superstructure is composed of cast-in-place concrete edge girders that are conventionally reinforced (with some post-tensioning). These girders are 6 ft deep by 5 ft wide. Transverse floor beams connect the edge girders and rest on 12-ft centers. The transverse floor beams are a combination of precast and cast-in-place concrete. All floor beams over land are precast; over the water, floor beams will be cast in place. The edge girders, floor beams and deck are all 6,500-psi concrete. The central 200 ft of the 950-ft cable-stayed main span is composed of 7,000-psi concrete. The precast concrete approach spans are 70-in.-deep, prestressed concrete bulb-tee girders. Also of low-permeability concrete, the bulb-tee girders boast 8,500-psi strength. The bulb-tee girders are spaced at 10-ft 10-in. centers.
Construction of the superstructure over land is being done on falsework. The portion over the water will be executed with cast-in-place form travelers. Two Strukturas Design form travelers—weighing 260 tons apiece—will start on each shore and work toward the center, with final closure at the midpoint of the bridge scheduled for January 2011.
Providing two travel lanes southbound and two travel lanes northbound, the low-permeability concrete deck also will offer interior and exterior shoulders. There is a sidewalk on the northbound or ocean side. The deck will be 8.5 in. thick (reduced because of the 12-ft center-to-center floor-beam spacing). The closer floor-beam spacing allows for the thinner deck, reducing the dead load in the structure by about 20%. That resulted in significant savings in the pylons and the pylon foundations. The deck will be topped with a 15?8-in. latex-modified concrete (LMC) overlay. The LMC is a wearing course to protect the deck from the elements.
Along with comprehensive concrete specifications and testing, great consideration was given to the aboveground effects of constant wind on the structure. The structure had to be designed to withstand a “normal” 100-year wind event. In addition, the project team had to test for a 2,000-year return period “extreme event.” And even that wasn’t the ultimate test.
“We had to prove aerodynamic stability for a 10,000-year return period,” added AECOM’s Butler. “The wind-tunnel testing and coastal engineering/scour analysis were key aspects controlling the design of the bridge. We examined 20 hurricanes that have struck the coast, as well as 15 nor’easters. In fact, we had two heavy nor’easters this year; one of them caused a breach between the ocean and the bay. From the wind perspective—in addition to the normal design wind speeds of 91 mph—the bridge had to be able to handle 140-mph winds for the 10,000-year event. That’s a Category 5 hurricane.”
Third of the way
The IRIB project also highlights another invaluable component in the design and construction of such structures: partnering. “We developed a really close partnership with the design-build team and the owner,” added Skanska’s Erwin. “That’s a critical part of making design-build and this project successful. We have a formal partnering process and it’s working very well.”
“We’re pleased with the progress we’re making,” said Robb. “This was our third attempt at getting this project under way. We’re happy to see the construction proceeding. The design phase is virtually complete, and we’re almost 50% through construction. All indications are that we’re going to meet our schedule and our budget. From our standpoint, we’re pretty happy right now.”
Tzu said, “In the world there is nothing more submissive and weak than water. Yet for attacking that which is hard and strong nothing can surpass it.”
The Indian River Inlet Bridge is proving that philosophy to be true, albeit incomplete. While respecting the power of water and the elements, DelDOT and the IRIB team are demonstrating that exemplary design, construction and partnering can create something that will confront those elements successfully—for the next 100 years.
About The Author: Schurr is a New York-based freelance writer who covers transportation infrastructure.
