To ABC or Not?
A new decision tool for Accelerated Bridge Construction is helping project planners assess the applicability and effectiveness of this technique at specific locations.
According to the Texas Transportation Institute, congestion in U.S. urban areas causes motorists to waste an estimated 4.8 billion hours in traffic and 3.9 billion gallons of gas annually. The economic impact was $115 billion in 2009 and is increasing every year. Although bridge and highway construction and maintenance projects are needed to increase capacity and maintain performance, these projects can add to the congestion problem. According to the Federal Highway Administration's (FHWA), 2010 National Bridge Inventory, 156,289 publicly owned bridges are structurally deficient or functionally obsolete—25.9 percent of all U.S. bridges. Whether those bridges are rehabilitated or new ones are built, the construction itself can exacerbate congestion and result in additional costs to users, sometimes can be associated with safety issues, and often limits road access.
Responding to the public's demand for a "get in, get out, and stay out" approach to bridge construction, the American Association of State Highway and Transportation Officials (AASHTO) and FHWA encourage the use of various Accelerated Bridge Construction (ABC) strategies, including prefabricated bridge elements and systems, state-of-the-art equipment, new material technologies, and innovative contracting methods. Where applicable, these strategies can help reduce onsite construction time, minimize traffic and environmental impacts, improve work zone safety, and deliver longer lasting and more durable bridges.
Bridge owners and engineers need to determine early in the planning phases of a construction project whether elements of ABC are appropriate and effective for a specific bridge replacement or rehabilitation project. New technologies and accelerated construction techniques, however, can introduce risk and uncertainties into a project. Existing cost analysis tools often require significant training and typically address only limited aspects of a project. In addition, cost data often are not readily available in the early phases of a project, making traditional engineering economic analyses difficult to apply reliably.
In December 2009, the Oregon Department of Transportation (ODOT) initiated a pooled fund study, TPF 5(221), with the charge of developing a tool that could assist decisionmakers in identifying whether ABC should be applied to a specific project. To define the study's objectives, ODOT convened a 15-member technical advisory committee. The committee members included a principal investigator from Oregon State University and participants from FHWA and eight States with substantial experience in bridge replacement and rehabilitation. Many had direct experience in the use of ABC in their own States. To support the project, the researchers conducted a comprehensive review of ABC techniques and decisionmaking approaches. Ultimately, the researchers created an easy-to-use analysis tool that bridge owners can turn to in the early planning stages to determine whether to deploy ABC on individual projects.
ABC Approaches And Benefits
ABC approaches include the application of technical innovations and management techniques. Technical innovations include rapid embankment construction, specialized structural placement methods, and prefabricated bridge elements and systems, such as superstructure systems (composite units, truss spans), substructure systems (abutments, caps/columns, piers), and totally prefabricated bridges. Examples of management practices used as part of ABC include staged construction, A+B contracting (otherwise known as cost-plus-time), incentive/disincentive contracting, and lane rentals in which contractors must include the cost to the public as well as construction costs.
"To reduce congestion and improve safety, many communities, industry, and Federal organizations would like to see ABC tools and techniques become standard practice," says FHWA Associate Administrator for Infrastructure King W. Gee. "September 11 and subsequent threats to the U.S. transportation system also have highlighted the need to develop emergency response plans to react quickly to consequences of extreme events. This need also has been cited as an important reason for bringing ABC tools and techniques to more replacement and rehabilitation projects."
Federal organizations have supported several initiatives to foster the development, implementation, and promotion of ABC by departments of transportation (DOTs). Because of the success of ABC projects to date, FHWA has provided resources to further advance ABC's deployment nationwide.
Addressing the challenge of reducing congestion through accelerated reconstruction of obsolete and deficient bridges, States and localities have undertaken successful ABC projects. These projects provide valuable insight for decisionmakers who are considering ABC for the first time or who have had limited exposure to the variety of ABC tools and techniques. A summary of 54 completed ABC projects using various types of prefabricated bridge elements and systems is available on FHWA's "Accelerated Bridge Construction" Web site at www.fhwa.dot.gov/bridge/prefab/all.cfm.
The Need for ABC Decisionmaking Tools
The U.S. Department of Transportation's (USDOT) strategic plan for 2006–2011 identified use of decisionmaking tools as a key strategy to reduce congestion and deliver longer lasting, high-performance infrastructure. Use of decisionmaking tools in the early stages of planning provides a way to help decisionmakers assess alternatives with greater confidence and prevent investment in more costly alternatives. In addition, data-driven decisionmaking tools are consistent with cost-saving recommendations from the U.S. Government Accountability Office.
The decision to use ABC must be made on a project-by-project basis as each bridge location is unique, and factors constraining each project are different. Toward this end, FHWA developed a qualitative decisionmaking framework to determine whether prefabricated bridge elements should be considered for a given project (see www.fhwa.dot.gov/bridge/prefab/framework.cfm). This framework includes three formats: a flowchart, a matrix, and a list of categorical considerations. The formats may be used independently or in combination. For example, to use the framework, decisionmakers respond to questions about a project's specific characteristics. The questions relate to the appropriateness of rapid, onsite construction; safety and environmental issues; standardization and construction site issues; the costs of traffic management and contracting; owner costs; and service life. The framework provides a starting point for evaluating prefabricated bridge elements and systems for a given project but does not help decisionmakers evaluate the broader scope of ABC strategies nor estimate the economic impact of selecting various elements of ABC over traditional methods.
A Decisionmaking Tool for ABC
Users of the FHWA decision framework recognized that decisionmaking for bridge construction involves both quantitative and qualitative criteria. Pooled fund study 5(221) was established to develop a complete set of evaluation criteria. Many of the relevant criteria are difficult to measure, estimate, or evaluate. The pooled fund's technical advisory committee determined that a multi-criteria decisionmaking approach was necessary to guide the development of a decisionmaking tool. That is, multiple criteria would have to be considered simultaneously.
Key assumptions to guide the tool's development were identified. First, the tool had to be usable by transportation specialists and decisionmakers who have a thorough understanding of the technical and nontechnical issues associated with a particular project, but who may not be familiar with multicriteria decisionmaking methods. Second, the tool had to be user-friendly and accommodate a range of construction situations. Third, to ensure that the decision rationale could be readily communicated to a broad base of stakeholders, the tool's methods of calculation and decision criteria had to be transparent.
After a comprehensive review of decisionmaking tools, the technical advisory committee identified the Analytic Hierarchy Process (AHP) as the best technique for this project. This process prioritizes multiple criteria, integrates both quantitative and qualitative criteria, and provides a summary ranking of alternatives, based on the multiple criteria. Despite the long-time use of this process in other domains, particularly manufacturing, AHP has not been widely used in civil and structural engineering applications. As a result, many transportation personnel may be unfamiliar with AHP.
The method uses a multilevel hierarchical structure of criteria, subcriteria, and alternatives. The pertinent data used for an AHP analysis are generated by performing pairwise comparisons between criteria, subcriteria, and alternatives. These pairwise comparisons are made using both a numerical and qualitative scale. The results are used to obtain importance weights for decision criteria and to identify the extent to which various alternatives meet these decision criteria. The committee found AHP attractive due to the rigor of its method, which is mathematically based, and the fact that the required comparisons can be made based either on available data or the user's knowledge, familiarity, and experience with the alternatives being evaluated.
The simplest AHP form consists of three levels: the overall goal of the decision, the criteria by which alternatives will be evaluated, and the available alternatives. A hierarchical structure of decision criteria helps the user decompose the decision into comparisons between each set of criteria. The criteria and, if applicable, subcriteria, are organized in gradual stages from more general criteria, in the upper levels of the hierarchy, to more specific criteria, in the lower levels. A decisionmaker can insert or eliminate levels and elements as necessary. Sometimes, the user can drop a less important criterion from further consideration, if the prioritization shows a relatively small impact on the overall objective.
The first step in using AHP is to develop a hierarchy of criteria. In the second step, the user carries out pairwise comparisons. These comparisons are used to create a set of matrices, which then are evaluated mathematically to produce a recommendation. All pairwise comparisons are performed qualitatively using a predefined set of values or quantitatively using estimated values for the criteria being considered. The predefined set of values, called the Fundamental Scale, represent the importance or weight of one criterion over another or the extent to which a particular alternative meets a particular criterion.
|Examples of Benefits and Barriers to the Application of ABC Tools and Techniques|
|Benefits of ABC||Barriers to the Use of ABC|
Source: TPF 5(221) pooled fund study.
Developing AHP for ABC Decisionmaking
For this project, the technical advisory committee developed a hierarchy of criteria relevant to decisions about determining the best construction methods to apply to bridge replacement and rehabilitation projects. The committee refined the criteria over several months and tested them using previously completed and in-process bridge projects. The final hierarchy consists of two levels. The highest level consists of five criteria, each of which is further specified by two to nine subcriteria.
Definitions for all criteria and subcriteria were based on the members' experiences and expertise. A definition list helps users understand the decision hierarchy and to provide consistency between users who are completing the pairwise comparisons.
To check the completeness and robustness of the criteria, committee members conducted validations of the process using actual project cases. Considering the nature of the decision problems in a typical ABC project, these cases were used to confirm the suitability of the AHP approach in making decisions about whether various elements of ABC should be applied to a particular project. In addition to comparing each criterion and subcriterion in terms of its importance to a particular project, the technical experts also identified at least two different construction alternatives being considered for a particular project. They evaluated each alternative relative to all criteria and subcriteria. After that, they collected data for the cases by conducting interviews with DOT experts. Applications of AHP to two of these projects—Copano Bay Bridge in Texas and Keg Creek Bridge in Iowa—validated existing decisions about the best construction alternatives to apply to a specific project.
|The Fundamental Scale Used For Pairwise Comparisons in AHP|
|Equal importance||Two criteria contribute equally to the objective.|
|Moderate importance||Experience and judgment slightly favor one criterion over another.|
|Strong importance||Experience and judgment strongly favor one criterion over another.|
|Very strong importance||One criterion is favored very strongly over another; its dominance is demonstrated in practice.|
|Extreme importance||The evidence favoring one criterion over another is of the highest possible order of affirmation.|
|Scale values of 2, 4, 6, and 8 can be used to express intermediate values. Scale values such as 1.1, 1.2, 1.3, etc. also can be used for criteria that are very close in importance.|
Source: Adapted from Triantaphyllou, E. and Mann, S.H. (1995). Using the Analytic Hierarchy Process for Decision Making in Engineering.
|Definitions for All Criteria Included in the ABC Decisionmaking Hierarchy|
|Construction||This factor captures the estimated costs associated with the construction of the permanent structure(s) and roadway. Included are the premiums associated with new technologies or innovative construction methods. Premiums might result from factors such as contractor availability, materials availability, and contractor risk. They might include incentive/bonus payments for early completion and other innovative contracting methods.|
|Maintenance of Traffic||This factor captures the maintenance of traffic costs at the project site. Maintenance of traffic costs might affect preference due to its impact on total costs. This factor includes all costs associated with the maintenance of detours before, during, and after construction. Examples include installation of traffic control devices; maintenance of detours during construction, including flagging; shifting of traffic control devices during staged construction; and restoration associated with the temporary detours upon completion of construction.|
|Design and Construct Detours||This factor captures the costs to design and construct temporary structures and roadways to accommodate traffic through the project site.|
|Right-of-Way||This factor captures the cost to procure rights-of-way and includes either permanent or temporary procurements/easements.|
|Project Design and Development||This factor captures the costs associated with the design of permanent bridge(s) and costs related to project development based on the construction method.|
|Maintenance of Essential Services||This factor captures the costs associated with the need to provide essential services that may be affected by the construction selected. Examples of this factor include alternate routes or modes of transportation to provide defense, evacuation, and emergency access to hospitals, schools, fire stations, and law enforcement. This criterion is for situations where measures need to be implemented beyond those already considered in the “Maintenance of Traffic” and “Design and Construct Detours” criteria.|
|Construction Engineering||This factor captures the costs associated with the owner’s contract administration of the project.|
|Inspection, Maintenance, and Preservation||This factor captures the life-cycle costs associated with the inspection, maintenance, and preservation of individual bridge elements.|
|Toll Revenue||This factor captures the loss of revenue due to the closure of a toll facility.|
|User Delay||This factor captures costs of user delay at a project site due to reduced speeds and/or offsite detour routes.|
|Freight Mobility||This factor captures costs of freight delay at a project site due to reduced speeds and/or offsite detour routes.|
|Revenue Loss||This factor captures lost revenues due to limited access to local businesses resulting from limited or more difficult access stemming from the construction activity.|
|Livability During Construction||This factor captures the impact on the communities resulting from construction activities. Examples include noise, air quality, and limited access.|
|Road Users Exposure||This factor captures the safety risks associated with user exposure to the construction zone.|
|Construction Personnel Exposure||This factor captures the safety risks associated with worker exposure at the construction zone.|
|Calendar, Utility, Railroad, or Navigational||This factor captures the constraints placed on the project that might affect the timing of construction as a result of weather windows, significant or special events, railroad tracks, or navigational channels.|
|Marine and Wildlife||This factor captures the constraints placed on the project by resource agencies to comply with marine or wildlife regulations. Examples include in-water work windows, migratory windows, and nesting requirements.|
|Resource Availability||This factor captures resource constraints associated with the availability of staff to design and oversee construction. For example, a State may be required to outsource a project, which may result in additional time requirements.|
|Bridge Span Configurations||This factor captures constraints related to bridge span configurations. This element may affect owner preference regarding bridge layout, structure type, or aesthetics.|
|Horizontal/Vertical Obstructions||This factor captures physical constraints that may affect construction alternatives. Examples include bridges next to fixed objects such as tunnels, right-of-way limitations, sharp curves or steep grades, or urban area structures that constrain methods and/or bridge locations.|
|Environmental||This factor captures the constraints placed on the project by resource agencies to minimize construction impacts on natural resources, including marine, wildlife, and flora.|
|Historical||This factor captures historical constraints existing on a project site.|
|Archaeological Constraints||This factor captures archaeological constraints existing on a project site.|
|Public Perception||This factor captures both the public’s opinion regarding the construction progress and its overall level of satisfaction.|
|Public Relations||This factor captures the costs associated with the communication and management of public relations before and during construction.|
Using ABC Software for Analysis of a Texas Bridge
The Copano Bay Bridge will replace an existing causeway on State Highway 35 at the mouth of Copano Bay. The bridge connects the cities of Rockport/Fulton and Lamar on the Gulf Intracoastal Waterway. Copano Bay is home to oyster colonies and migratory birds, attracting birdwatchers year-round. Two peninsulas frame the bay opening, limiting right-of-way and dictating phased construction.
The bridge is 11,010 feet (3,356 meters) long, with a 129-foot (39-meter)-wide and 75-foot (23-meter)-tall navigation channel. The existing structure suffered severe corrosion damage from marine exposure requiring extensive repairs to many substructure elements. Providing corrosion protection, in the form of high-performance concrete, stainless reinforcing steel, and cylinder pile foundations, was of high importance. The superstructure consists of 100-foot (31-meter), 120-foot (37-meter), and 150-foot (46-meter)-long prestressed concrete girders. A majority of the piers consist of cast-in-place caps on trestle piles, with the tallest piers around the navigation channel being cast-in-place bent caps on cast-in-place columns and waterline pile caps. The bridge owner, the Texas Depart--ment of Transportation (TxDOT), allowed the contractors to propose precast bent caps as an alternate construction method to reduce the duration of construction activities over open water.
Experts from TxDOT provided the data for the AHP analysis. The analysis compared two construction alternatives: precast caps and cast-in-place. The utility value—degree of benefit or preference—for precast caps was 0.72, whereas the degree of benefit or preference for the cast-in-place alternative was 0.28. The results for the criteria weights indicate that site constraints and schedule constraints had the greatest impact on the selection of precast caps as the best construction alternative. Based on the results of the AHP analysis and the importance weightings for each criterion in the ABC model, the analysis validated TxDOT's decision that precast caps are the alternative that best satisfies the project's objectives.
Using ABC Software for Analysis of an Iowa Bridge
The Keg Creek Bridge carries U.S. 6 over Keg Creek in Pottawattamie County, Iowa. The existing 180-foot (55-meter) by 28-foot (8.5-meter) continuous concrete girder bridge was constructed in 1953 and currently is classified as structurally deficient with a sufficiency rating of 33. The Iowa Department of Transportation's proposed bridge replacement aims to increase the structural capacity of the bridge, improve roadway conditions, and enhance safety by providing a wider roadway. The replacement structure will be a three-span steel/precast modular bridge with precast bridge approaches.
Iowa DOT provided the data for the AHP analysis. The analysis compared two construction alternatives: ABC modular bridge (all components prefabricated offsite) and a traditional bridge (conventional). Both alternatives are on the same alignment and use the same offsite detour. The utility values for the modular and conventional alternatives were 0.68 and 0.32, respectively. The results for the criteria weights indicate that direct costs and customer service had the greatest impact on the selection of the modular bridge as the best construction alternative. Based on the results of the AHP analysis, the ABC modular bridge alternative best satisfied the project's objectives.
ABC Decisionmaking Software
In parallel with testing and validating the approach, the AHP software was developed using Microsoft Visual Studio®.NET as a stand-alone application. The software can run on a variety of Windows operating systems, including Microsoft® Windows XP, Vista, and Windows® 7.0. Two alternatives may be compared in an analysis.
The initial interface manages the hierarchy of criteria. When first opening the software, the committee's default hierarchy loads. The user can add or eliminate criteria and subcriteria using the functionality provided in the criteria tab. In this tab, the user also can load, save, and/or modify previously developed hierarchies.
The second tab provides the interface needed for the user to complete the pairwise comparisons of criteria, subcriteria, and alternatives. The user can save an analysis at any time and later return to that specific position, without losing data.
After completing all pairwise comparisons, the user goes to the third tab to review the results. For each criterion in the hierarchy, the software generates a set of two plots: a bar chart indicating the overall utility level for each alternative and a pie chart showing the importance weights for each criterion. The software also provides the option of completing an additional cost-weighted analysis in which decisionmakers can evaluate the impact of the direct cost criterion apart from all other criteria. The cost-weighted analysis may be used only after the user has eliminated all cost criteria from the decision hierarchy.
In the early stages of a construction project, decisionmakers have the difficult task of assessing whether elements of ABC are achievable and effective for a specific bridge location. These decisions are even more difficult as multiple criteria and diverse (sometimes opposing) perspectives need to be considered. The use of appropriate decisionmaking tools in these early stages can help promote dialog and ultimately foster effective solutions.
Applications of the AHP software tool have provided evidence that it can help decisionmakers identify and communicate the rationale behind a decision to select a particular construction method. The tool has been tested on projects from seven States (California, Iowa, Montana, Oregon, Texas, Utah, and Washington). A final version of the software will be available for download at FHWA's Web site at the completion of the project in January 2012. Case study results for all the projects that have been analyzed to date are posted as well.
First Tab in ABC Software
Interface for Completing All Pairwise Comparisons
Interface for Reviewing Results of AHP Analysis
Toni Doolen, Ph.D., is a professor and associate dean at Oregon State University. Her research uses both quantitative and qualitative methodologies to study organizational processes. Doolen received her B.S. in electrical engineering and materials science and engineering at Cornell University, M.S. in manufacturing systems engineering from Stanford University, and Ph.D. in industrial engineering at Oregon State University.
Benjamin Tang, P.E., is an ODOT bridge preservation manager in Salem, OR. He manages the statewide preservation program for all coastal, historical, and movable bridges. Tang retired in 2008 from FHWA after 33 years of Federal service but continues to champion and contribute to the advancement of ABC technology. He holds a master's degree in structural engineering from the University of Illinois, Urbana-Champaign.
Amirali Saeedi is a Ph.D. student in the industrial engineering program at Oregon State University. He holds a B.S. from Sharif University of Technology and an M.S. from Oregon State University.
Samin Emami has a B.S. in industrial engineering from Sharif University of Technology. Currently, he is working on a master's in manufacturing systems engineering at Oregon State University.
For more information, see www.pooledfund.org/projectdetails.asp?id=449&status=6 and www.fhwa.dot.gov/bridge/abc or contact Toni Doolen at 541–737–5641 or email@example.com or Benjamin Tang at 503–986–3324 or firstname.lastname@example.org.