Building Bridges: How One Modern Drawbridge Project Is Addressing Urban Challenges
Although people most often think of insulation in terms of its applications in buildings, it plays a role in preserving the integrity and efficient function of other types of structures as well. This article focuses on one such structure—the new Woodrow Wilson Bridge in the Washington, D.C., area—and how insulation plays a role in its future success. It also serves as a case study, because as the nation’s infrastructure ages and traffic volumes increase, many of the concerns here are applicable to structures throughout the United States. This article provides background on the original bridge and the recent project to replace it.
The Need for a Bridge
Road transportation in the metropolitan Washington, D.C., area is not merely an “inside-the-beltway” concern. While it is true that commuters from Virginia, Maryland, and the District of Columbia move back and forth between jurisdictions during their work days, as the nation’s capital and a key point on Interstate I-95, the area also supports personal and business travel in the mid-Atlantic region. This travel is complicated by the existence of the Potomac River, which separates Virginia from Maryland and the District.
In the mid-1950s, Congress approved construction of what was originally known as the Jones Point Bridge to connect Alexandria, Virginia, and Oxon Hill, Maryland. When the bridge opened in December 1961, it consisted of six lanes—three in each direction—to serve an anticipated daily traffic volume of 75,000 vehicles. To accommodate river traffic, the 6,000-foot-long, multispan, girder design included a bascule drawbridge located approximately 500 feet east of the Virginia shoreline. It is one of just nine movable-span bridges in the U.S. interstate system.
The bridge provided the first direct connection across the Potomac River for Washington, D.C., suburbs in Virginia and Maryland. At the time, it was neither anticipated nor designed to be the busy commuter route and East Coast interstate travelway into which it eventually evolved.
Outgrown Nearly Immediately
Unfortunately, the bridge reached its design volume in just 8 years. Today, traffic volume is three times the original estimate, resulting in about 7 hours’ worth of backed-up traffic that frequently runs several miles in either direction. It is not just local-area commuters who suffer from this congestion. Consider that in 1993 $58 billion in trucked gross national product (GNP) —at least 1.3 percent of total—passed over the bridge. Today, some 11 percent of the daily traffic is large trucks.
To make matters worse, the narrow shoulders provided in the original design, coupled with the increased traffic, led to a disproportionately high accident rate. Although the bridge was widened to accommodate 7-foot-wide emergency shoulders on each side when it was re-decked in 1983, its accident rate is still nearly twice that of similar stretches of highway in Virginia and Maryland, and of the Washington, D.C., beltway’s other main bridge, the American Legion Bridge. As the highway leading to the bridge was widened to eight lanes, the decrease to six lanes on the bridge created a bottleneck and more congestion.
Excessive Volume Stresses the Structure
All of that excess traffic (and weight) took its toll. In 1994, Hardesty and Hanover, a New York engineering firm, issued a report summarizing the results of a publicly funded study conducted by 12 bridge engineers, including representatives from the Federal Highway Administration, the District of Columbia, Maryland, and Virginia. The report warned that the higher-than-expected volume of traffic (including large numbers of trucks with maximum legal loads higher than those permitted on highways back when the original bridge was designed) was causing excessive vibrations that were damaging the bridge supports.
The bottom line: Without major upgrades to the structure, the bridge would need to be closed to truck traffic by 2004. Given the cost to upgrade, the traffic disruption it would cause, and the fact that the rehab would not result in more lanes of traffic, the bridge suffered from such extreme wear and tear that it needed to be replaced just 40 years after it was unveiled.
The Search for a Solution
In 1988, the federal government, along with the District of Columbia, Maryland, and Virginia, initiated a study aimed at replacement solutions for the decaying and outgrown bridge. With so many jurisdictions—and local public opinion—involved, the project planning process grew, with panel studies conducted into the mid-1990s. The options under consideration included a tunnel, a new drawbridge, and a fixed, high-level bridge. The tunnel represented the costliest of the alternatives considered. And residents of nearby Old Town Alexandria raised concerns about the imposing appearance of a new, high-span bridge, whose steeper grades might have posed a challenge for trucks to run at normal speeds (possibly slowing bridge traffic more).
In the end, the solution chosen was a $2.43-billion project encompassing a 7.5-mile stretch including roadways and interchanges in both Maryland and Virginia, as well as a new bridge. The new bridge, which represents $826.1 million of the total project cost, is a twin-span drawbridge that is 20 feet higher than the old. The 70-foot vertical navigation clearance over the river will decrease the number of bridge openings (and resulting roadway traffic interruptions) from 260 down to 65 per year (about 1 per week), a 75-percent reduction.
In addition, the new bridge will accommodate 12 lanes of traffic, which breaks down as follows:
- Eight general-purpose driving lanes
- Two lanes for merging and diverging traffic
- One express and local lane
- One lane set aside for future rail and transit lines (which will open once the connecting systems are established on both sides of the river)
Roles for Insulation
Insulation plays an important role in optimizing performance and longevity of the new bridge. During the construction phase, insulation was critical in ensuring the structural integrity of the new bridge by controlling the temperature of the concrete as it set. In some areas of the bridge (the deck surface, for example), insulation was required to keep the concrete warm as it set, protecting it from the cold of the outside environment. In other areas—surrounding the forms for the massive concrete shapes of the foundation for the draw span—insulation was used to prevent the formation of cracks caused by thermal shrinkage. In the latter case, insulation essentially protected the concrete from itself.
In a recent interview discussing the role of insulation in the Wilson Bridge Project, Construction Manager Jim Ruddell, P.E., vice president at Parson Brinckerhoff Construction Services, began by focusing on the basic ingredients (sand, cement, water, and rock), likening the process to baking a cake. Different types of applications, much like different types of cake (think chocolate torte versus cheesecake, for example) require different ingredients. Professionals in the construction industry are familiar with how the chemical reaction from mixing the ingredients of concrete generates heat. For the new bridge, consideration was given to designing a mixture that would meet requirements for strength and chloride permeability. With co-author Paul Gudelski, Rudell described the results of heat-flow analysis of the mass concrete mix in “A Capital Bridge” (Concrete Engineering International, Autumn 2004) as follows:
The mix replaced 75 percent of the Portland cement with ground granulated blast furnace slag (ggbs), and included air entraining and high-range water reducing agents to enhance freeze-thaw resistance, workability, and provide minimum 56-day strength of 27.6 megapascals (MPa).
In addition to the need to design the best possible concrete mix to begin with, a project such as this presents issues and considerations related to the structure’s unique shape and size. Ruddell described the challenges of working with the large, geometric shapes used in different sections of the bridge. (To give an idea of the extremities of scale, the four blocks representing the foundation for the drawbridge used 6,400 yards of concrete and measured 89 x 119 x 16 feet.) In the Wilson Bridge Project, custom-fabricated forms for the large concrete structures were treated with spray-on isocyanate foam insulation in an effort to keep the temperature toward the exterior of the structure as consistent as possible with that at the core. According to Ruddell, the goal was to maintain a differential no greater than 35 degrees between the core and the outside. Along with the insulation, cooling pipes were used to circulate water within and cool the core as needed. Thermocouples embedded in the concrete regulated temperature.
For large concrete structures, the main environmental factor was ambient temperature. By preventing the concrete faces from cooling too quickly, the foam insulation, in part, allowed work to take place regardless of season. For the smaller structures (those smaller than 6 x 6 x 6 feet) and for the bridge deck, the primary environmental challenge came from the wind. For these sections, insulation was used to protect against freezing, as the concrete was kept covered.
For the elevated slabs of the bridge deck, an insulating blanket system was employed to keep the concrete from freezing, as shown in Photos 1 and 2. The bottom of the blanket is reflective metal, designed to reflect and conduct heat down and out throughout the covered structure (in this case, the deck). In addition to the blankets themselves, the system included heating tubes that circulated hot fluid under the blankets. Although the blankets are full of air bubbles, they are durable enough to allow workmen to walk on them.
Interestingly, the Potomac River was used to insulate one area of the bridge as it was constructed. One of the first steps in the bridge-construction process was construction of temporary watertight structures called cofferdams, where the bridge pilings were built. Once the cofferdams were ready, water was pumped out to create a dry environment. The river water just outside the cofferdams actually created a stable environment, from a temperature standpoint.
Ruddell says the most difficult of the structures were the so-called knuckle pieces: the blocks of concrete at the top of the V-piers. The sections are full of reinforcing steel, post-tensioning bars, “a whole bunch of stuff … that we need structurally,” says Ruddell. The concrete job was already compounded by the existence of so many critical elements in one place, but then there was the shape, which extends out from the foundations to form an upside-down triangle. The end pieces at the top were especially vulnerable to temperature differential. These factors represented something of a “perfect storm” of challenges. Because the knuckles carry so much load, quality and structural integrity of these sections was critical. According to Ruddell, the knuckles took about 1 month to construct, were cast in 1 day, and then took another month for thermal monitoring.
Concrete—the attention to developing the perfect blend and using insulation to protect it when setting—is just one area where designers and engineers looked for innovative ways to extend the new bridge’s life span. Gudelski and Ruddell describe how more-durable materials will extend the life of the new draw span decks here:
… the new draw span decks are designed with stainless steel (Unified Numbering System designation S31803 or S31653) reinforcing and lightweight (1,890 kilograms per cubed meter), and chloride-resistant (less than 2,000 coulombs) concrete with design strength of 31.0 MPa.
In key areas such as the V-piers’ post-tensioning ducts, high-performance thixotropic grout is used instead of conventional grout. Thixotropic grout pulls water, substantially reduces segregation, and serves as a barrier to protect against corrosion.
The Wilson Bridge Project can serve as both a “lesson-learned” warning and a case study for improving the nation’s infrastructure. The federal government and local jurisdictions will not be able to afford replacing substantial interstate infrastructure elements every 40 years. Along with studying the demographics that will drive use and demand, planners need to work with engineers and designers to use the best technology from all systems—not just insulation—to optimize performance and longevity in the finished product.
- Woodrow Wilson Bridge Project, www.wilsonbridge.com