This summer, a mere 14-day road closure will be required to replace the 1953-constructed U.S. 6 Bridge over Keg Creek in Council Bluffs, Iowa, with a bridge that is new from its foundations to a modular concrete-steel composite superstructure.
The original design by the Iowa Department of Transportation would have required an estimated six-month road closure to construct. Instead, HNTB Corp., in association with Structural Engineering Associates Inc. and Iowa State University, created an accelerated bridge construction (ABC) project that may provide a rapid renewal tool for highway departments across the country.
“What HNTB has done is pretty remarkable,” said Monica Starnes, Ph.D., senior program officer, the National Academies, Transportation Research Board, Strategic Highway Research Program (SHRP 2). “HNTB has created a new design for complete bridges, so we have an innovative solution that bridge owners can use.”
The U.S. 6 Bridge design is an ABC project in the renewal research focus area of SHRP 2, a Congressionally funded program that is tasked with finding innovative ways to save lives, reduce congestion and improve quality of life on America’s roads and highways.
SHRP 2 includes 92 projects overall and four areas of research: reliability, renewal, capacity and safety. The program is sponsored by the Federal Highway Administration in cooperation with the American Association of State Highway & Transportation Officials.
Most main arteries can trace their origins back to the Interstate Highway System work championed by the Eisenhower administration in the mid-1950s. Back then, engineers estimated a useful road life of 30-40 years, based on approximated traffic volumes that did not even begin to anticipate the scope of today’s multicar families, lengthy daily commutes and consumer demand for truck-delivered goods, according to Neil Hawks, director of SHRP 2.
“Today’s work needs to be done within a far more complex culture, balancing not only the perpetual financial and political challenges, but also environmental, legal, safety, neighborhood, historical, archaeological and other issues that were all but unheard of 60 years ago,” Hawks said.
SHRP 2 represents a key philosophical shift in transportation development from the historical emphasis on lowest initial cost to meet design criteria to a broader estimation of cost that encompasses construction impact on traffic congestion, safety to the contractors and the traveling public, the environment and quality of life.
Stream of reasons
The overwhelming, widespread deterioration of our nation’s transportation infrastructure is forcing highway departments, designers and contractors to develop innovative strategies for rapid renewal. Gone are the days of building one bridge at a time using conventional methods. These days every department of transportation in the nation has hundreds if not thousands of bridges that need rebuilding.
Business as usual will not work. The old approaches will not support the technology and delivery innovations required to fix what is now broken. The industry must find smarter and faster ways of rebuilding the nation’s transportation system using standardized approaches that allow rapid renewal, economies of scale in manufacturing and construction, reduced traffic disruption and increased safety. ABC has the potential to become a very powerful tool in the transportation industry.
Ahmad Abu-Hawash, P.E., chief structural engineer, Iowa Department of Transportation Office of Bridges and Structures, reports that the U.S. 6 Bridge was chosen as the SHRP 2 ABC test project for several reasons. The SHRP 2 team was looking for a project that met several criteria. It needed to be a project that was ready for construction in 2011, met a certain size and span configuration that was representative of typical bridge replacements needed across the state and was a project that needed to be built quickly, with as little disruption to traffic as possible.
“In Iowa, we have a lot of average-size streams that require an average of three spans,” Hawash said.
The existing bridge comprises three spans and is 28 ft wide and 180 ft long. Located about 20 miles outside of Council Bluffs, the bridge is part of a main artery with about 9% truck traffic.
Approaches to ABC typically focus on the use of bridge-movement technology to move completed bridges into place or use of prefabricated elements to construct in place. In addition to focusing on design and construction technology, HNTB has defined three tiers of ABC projects based on the duration of traffic closure. Total project completion time frames, which do not affect traffic, may be longer. A Tier 1 project is one that can be completed over a weekend closure, usually requiring some sort of bridge-movement technology (launching, sliding, etc.). A Tier 2 project is one that can be delivered within one month. Finally, a Tier 3 project is defined as a large reconstruction contract in which the application of ABC design and construction technology will reduce the overall project schedule and user impacts by months or even years.
The 14-day, SHRP 2 R04 demonstration project in Iowa makes it a Tier 2 project. The design team made all the elements as standardized as possible so they could be prefabricated off-site, easily transported and quickly assembled.
“In order to find the right solution, HNTB worked very collaboratively with the owners and contractors,” Starnes said. Contractors indicated they did not want to bid on ABC jobs that they would have to sub out to specialty contractors or buy special equipment or technology to perform. Special emphasis was placed on achieving constructability that could be mastered by local contractors and keeping prefabricated elements within a 50-ton range so they could be lifted using conventional equipment.
“We don’t want to put plans out for bridges that nobody can construct,” Hawash said. “We want to make sure someone can build it and at a reasonable cost.” In fact, the February letting for SHRP 2 R04 attracted seven local bidders—an above-average number of bidders for a typical DOT bridge project. The ABC test project design calls for the prefabrication of almost 100% of the project. Precast elements include: modular superstructure units with precast barriers and backwalls; precast suspended backwall to create a semi-integral abutment; precast concrete pier caps and columns; precast abutments and wingwalls; and precast concrete approach slabs.
The only components that will be cast in place will be the drilled-shaft foundations.
Iowa State University has developed an ultra-high-performance concrete (UHPC) with compressive strengths ranging from 24,000 to 30,000 psi and a short bond development time to complete the real innovation of the project—the connections and deck joints.
Although UHPC joints have been used elsewhere between decks or modular systems, this is one of the first applications in the country for longitudinal UHPC joints and the first for a full-depth transverse joint over the pier carrying the live-load negative moment between adjacent spans. The project also features UHPC joints to connect precast approach slabs. Many owners have shied away from ABC because of concerns about joints not being as durable as cast-in-place construction. HNTB’s design takes jointless technology to another level, and every joint performs like a cast-in-place, full-moment connection. As a result, the bridge is essentially jointless, providing good strength and seismic performance, redundancy and long-term durability.
Indeed, no individual element of the project is an innovation in itself, but the design team put all the elements together in a package that creates a template for DOTs wanting to deliver ABC projects in the future. According to Starnes, that alone makes this project a success.
Cost of doing different
SHRP 2 R04 came in about 25% higher than the Iowa DOT’s original estimate for construction. The winning bidder was Godberson-Smith Construction of Ida Grove, Iowa.
Because it supports the innovation requirement for ABC and because it was to be included in SHRP 2, the Iowa DOT did not balk at the additional cost. “We knew this was a demonstration project, so it was picked to meet all the goals of the project, not simply the criteria for cost,” Hawash said. As an academic exercise, Hawash did use an ABC decision tool that he has been developing with other DOTs to determine if ABC is appropriate for a project. “If you have a site where there is no traffic and a bridge can be done for half price, you don’t need ABC,” Hawash said. “However, when I tested this project using the tool we are developing, the analysis showed ABC was preferred for this site.”
The original cost estimate for the Iowa DOT design was made using the old formula, which only looks at construction cost. If ABC is to be widely embraced, there must be a way to quantify the savings that ABC offers to users in time savings, quality of life, construction safety and environmental benefits. The tide is turning, and owners are beginning to consider the added value of safety and quality of life and the other less tangible costs, such as the political impacts of long-term road closures for construction, in determining the total cost of a project. But, there still is no generally applicable and uniformly accepted algorithm for quantifying those “costs.”
ABC will gain significant savings in economies of scale, particularly in the area of standardization of prefabricated elements. Significant cost and time savings can be realized by clustering similar bridges into a single ABC contract. Consider the significant cost savings of programs to rebuild 500 or 1,500 bridges using ABC rather than stick-built or cast-in-place. HNTB has been working on standardized design for bridges grouped in programs like Missouri’s Safe & Sound. Similar programs are being discussed in Pennsylvania, Massachusetts and New York.
The SHRP 2 R04 test project is progressing in parallel with other transportation innovation efforts like the FHWA’s Every Day Counts initiative. Before ABC is widely accepted, however, some institutional inertia must be overcome on the part of owners, designers and constructors. The engineering and design communities must shift their focus to the desired outcomes, i.e., the metrics of reduced construction in-situ, which in turn saves time, money and enhances work-zone safety. This paradigm shift will allow contractors to embrace different approaches to both design and construction, including prefabricated components and off-site assembly. One of the more important aspects of this paradigm shift is to promote collaboration between all three entities during design, i.e., designers need to have a better understanding of construction means and methods and the ways in which contractors assign risk and associated cost. Owners need to clearly define the allocation of risk so there is no ambiguity during bidding. If ambiguity exists, it will represent a risk in itself and can result in unnecessary cost increases.
Engineers can no longer deliver designs and simply expect the contracting community to assume all of the construction risk. Historically, owners and designers have attempted to divorce themselves from risk on the construction side, but public safety, for example, is very much a risk that belongs to everyone: the owner, the designer and the contractor. Owners are frequently subject to lawsuits. Construction means and methods, interruption to traffic, associated user impacts and duration of construction are very much a part of ABC design and construction.
Another significant issue is self performance. In the future, owners, designers and contractors will ideally focus on solutions that can be manufactured and delivered by local and regional contractors. ABC is not intended to create an exclusive club. If we are to rebuild the nation’s infrastructure, we will have to work as an inclusive team, not an elite few, focusing not only on how to design bridges, but how to build them faster and safer.
