Cost Analysis of Precast Segmental Bridges

construction cost of precast segmental bridges

Precast segmental bridges compete with other types of prestressed-concrete bridges that typically require smaller investment for specialized equipment. On medium spans, span-by-span construction of precast segmental bridges competes with balanced cantilever erection of constant-depth segments, incremental launching and in-place span-by-span casting with Movable Scaffolding Systems (MSS). On longer spans, balanced cantilever construction of precast segmental bridges competes with incremental launching over temporary piers and in-place balanced cantilever casting with form travelers or telescopic MSS. All of these solutions also compete with steel-composite decks over the entire range of spans.

The construction cost of a bridge deck can be schematically divided into three major components: construction materials, labor and technology. Different structural solutions and construction methods may generate savings or extra costs of structural materials, but this rarely is a major differentiator. What makes a solution more cost-effective than others, especially within design-build procurement, is minimizing the combined cost of labor and technology.

The rationale of precast segmental technology is investing capitals to set up a precasting facility and to procure special transportation and erection means to save labor costs and accelerate project construction. Acceleration of the project time-schedule offers savings in indirect costs but often requires multiple production lines for foundations and piers to keep the rapid erection rate of the deck, while alternate construction methods (incremental launching in particular) offer low labor demand with smaller investments. The break-even point between investing in technology and spending in labor depends on the dimensions of the project and on the cost and availability of the skilled labor needed to implement sophisticated construction technologies. Choosing the most appropriate deck erection method for a project, in other words, requires thorough analysis of the construction cost of the different alternatives and of the related risks and opportunities.

Analyzing the construction cost of a precast segmental bridge is a complex task that includes hundreds of cost items and requires accurate planning not to become overwhelming. The most efficient approach is working with a spreadsheet where the cost items are recorded row by row. Four columns are used for each item: the first column identifies the construction cost, the second column identifies the opportunities (potential of cost savings), the third column identifies the risks (potential of extra costs), and the fourth column is the total (construction cost minus opportunities plus risks). Row by row, the sum of the second and third column identifies the net combined risk profile for that cost item.

The cost items are grouped into three major blocks: segment fabrication, segment delivery and on-site deck assembly. Each block includes a repetitive sequence of items:

  • Setup of the production line (precasting yard, staging areas for segment delivery, and erection lines). These items are kept separated (instead of being accounted through their components, such as direct labor, materials, etc.) for two reasons. Firstly, because these activities are performed at the beginning of the project when a stable project organization is not yet available, and they are therefore mostly subcontracted. Secondly, because these activities generate financial exposure and their total cost may therefore be accrued initially and depreciated during the development of the project.
  • Direct labor. This is the work force directly employed by the contractor and does not include labor employed by subcontractors, manufacturers and suppliers of semi-worked materials, components and equipment.
  • Permanent materials and subcontractors. The permanent materials are materials incorporated into the bridge such as concrete, reinforcement, post-tensioning, etc. The subcontractors may be kept separated or may be combined with the permanent materials for simplicity. For example, supply of ready-mix concrete is a mix of permanent materials (fresh concrete delivered in mixer) and subcontracts (pumping), and combining the two items simplifies analysis of the construction cost and cost recording during production.
  • Amortization of equipment. For equipment owned by the contractor, the amortization cost for every item is the difference between the value of the equipment item at the moment of its entry in the project and the value at the end of its use in the project. Entry and exit values are defined by the contractor and may differ from the correspondent fiscal values in relation to the equipment cost charging strategy implemented by the contractor. For rental equipment, the cost is the monthly rent multiplied by the number of months on the project.
  • The expendable materials are custom components of construction equipment that because of their specific nature cannot be reused in future projects. Whatever their use duration on the project may be, the cost of these components is accounted as the difference between the initial cost and the final salvage value.
  • The cost of energy includes electric energy purchased from suppliers; fuel and lubricants for machines, generators, air compressors, trucks and cars; and hydraulic fluid for hydraulic systems.
  • The indirect costs include staff, engineering, surveying, quality control, dust control, safety, security, maintenance of equipment and other such costs not directly related to production.
  • Other costs include weather downtime, redesign of bridge components and contingencies.

    In 134 pages in full A4/letter format, Construction Cost of Precast Segmental Bridges explains how to perform such a challenging task. The fabrication cost of the segments includes the setup cost of precasting facilities and the production costs of the short- and long-line method. Segment transportation includes trucking, trains and barges, with or without intermediate staging areas. Segment erection includes span-by-span and balanced cantilever construction and the setup and production costs of the different types of self-launching gantries and lifting frames.

    The estimation spreadsheet that accompanies the eManual includes 1004 cost items (yes, you have read well: one-thousand and four) and four columns for each cost item. The first column identifies the construction cost, the second column identifies the opportunities (potential of cost savings), the third column identifies the risk (potential of extra costs), and the fourth column is the total (construction cost minus opportunities plus risks). Row by row, the sum of the second and third column identifies the net combined risk profile for that cost item.

    For the three production lines, costs, opportunities and risks are grouped into multiple cost components:

    • Setup of production lines
    • Direct labor
    • Permanent materials and subcontractors
    • Amortization of equipment
    • Expendable materials
    • Energy
    • Indirect costs
    • Other costs

    Manual and estimation spreadsheet are indispensable working tools for bridge owners, designers and constructors interested in the design, bidding and construction of precast segmental bridges. For professionals with consolidated experience of precast segmental bridges, the publication will help to rationalize cost and quantified risk analysis during the bidding process and cost recording during production. For professionals with less experience, eManual and estimation spreadsheet are also a precious guide not to forget cost, opportunity and risk items during the bidding process.

    Combined with Span-by-Span Construction of Precast Segmental Bridges (134 pages), the monographs provide 268 pages of exhaustive coverage of span fabrication and the different types of self-launching gantries. They cover the segment fabrication process, explain how segment erection influences bridge design and segment fabrication, and explore bridge design for modularity and the factors that drive the choice between precast segmental technology and in-place casting.

    The eManuals explain the short-line method, the operations of casting cells for constant- and varying-depth segments, and the geometric design of the deck for standardized production of atypical segments and geometry correction with the typical segments. They also explain how to generate the casting curve in relation to precasting sequence, erection sequence and time-dependent effects, the geometry control of short-line casting (inclusive of commercial software programs and how they work) and the progressive correction of geometry errors.

    The eManuals also explore the long-line method, post-casting operations, different organizations of the stockyard, and segment delivery and epoxy gluing at the erection site. Last but not least, the monographs explain loading, kinematics, performance, productivity and structure-equipment interactions of all types of self-launching gantries (twin-truss overhead, single-girder overhead, telescopic and underslung) and provide exhaustive guidance on the analysis of construction costs, risks and opportunities of precast segmental bridges.

    If then one adds Balanced Cantilever Construction of Precast Segmental Bridges (81 pages), the eManuals Project provides 349 pages of coverage of all the construction methods and all the types of erection equipment for precast segmental bridges. If you are interested in the design, bidding, construction and inspection of precast segmental bridges, the eManuals Project will provide you with a unique wealth of knowledge, learning, insights, and unique working tools as well.

    If you thought that ASBI Construction Practices Handbook for Concrete Segmental and Cable-Supported Bridges was the international reference for the design and construction of precast segmental bridges, you will be greatly surprised.

    The eManuals of BridgeTech complement Precast Segmental Bridges, the 1-day course that Dr. Rosignoli teaches on-demand in the offices of bridge owners, designers and constructors. The bridge courses of Dr. Rosignoli originated within the ASCE Continuing Education Program. For more than 40 years, the American Society of Civil Engineers has ensured high-quality professional development and the latest innovations for bridge engineers. The ASCE Continuing Education Program is accredited by IACET to meet the premier benchmark for adult learning and undergoes review by a committee of professionals to select instructors that are authorities in their field, have decades of practical experience, and provide an outstanding level of expertise, in-depth training, and dedication to engineering.

    The courses that Dr. Rosignoli teaches for the ASCE Continuing Education Program and on-demand in the offices of bridge owners, designers and constructors are true learning experiences to train bridge teams in modern bridge design and construction technology while meeting continuing education objectives. The courses foster personal research, innovation and professional development; promoting technical culture is indeed an excellent way to motivate, train and retain staff.

    Learning is very effective with small groups of bridge professionals; 10-30 attendees is often the best compromise between interaction and the time constraints of hundreds of slides, although a 2010 seminar for the IABSE gathered 162 attendees in Singapore with excellent results. Richly illustrated with hundreds of photographs, the courses are constantly top-rated for material and presentation.