Every bridge construction method has its own advantages and weak points. In the absence of requirements that make one solution immediately preferable to the others, the evaluation of the possible alternatives is always a difficult task.
Comparisons based on the quantities of structural materials are poor indicators of the efficiency of design and may even mislead. The technological costs of processing raw materials (labor, investments for special equipment, shipping and site assembly of equipment, energy) and the indirect costs related to project duration (staff, depreciation of investments, financial exposure) often govern in industrialized countries. In other words, higher quantities of raw materials due to an efficient and rapid construction method rarely make a solution anti-economical.
Steel trusses are light and offer excellent flexural efficiency, as the chord area is far from the cross-sectional neutral axis. Built-up I-girders are heavier and structurally less efficient, but they allow robotized welding of web to flanges with profile-tracking equipment. In industrialized countries, the cost savings in raw materials of a truss are soon offset by the higher labor costs of hand welding, and the built-up I-girders have long replaced the trusses in medium-span bridges despite their higher weight and lower structural efficiency.
Two decades ago, the built-up I-girders were designed with thin web plates, and buckling of web panels was controlled with hand-welded longitudinal and vertical stiffeners. Modern built-up I-girders use thicker web plates to avoid welded stiffeners. Also in this case an unstiffened I-girder is heavier than a stiffened girder but less expensive, because of the savings in hand welding.
In prestressed-concrete bridges, tendon splicing by crossing and overlapping is often less expensive than the use of tendon couplers, even if the tendons are longer and the quantity of strand (which typically measures the cost of post-tensioning) is higher. Rebar with multiple bents are structurally more efficient than straight bars, but the latter are preferred because of lower labor costs. Bridge decks with varying cross-section may diminish the quantity of concrete, but they may also end up with prohibitive forming costs.
Similar considerations apply for bridge construction equipment. Ground falsework may lead to optimum quantities of structural materials due to the absence of design constraints from the construction process; these savings, however, are soon offset by the high labor costs of poorly industrialized construction. High crane demand and long construction duration add indirect and financial costs, and despite cost savings in materials, ground falsework is used only for small bridges in industrialized countries, or for specific tasks within more industrialized construction processes.
Prestressed-concrete bridges built by incremental launching are good indicators of the complexity of the problem. The quantities of concrete and prestressing of a launched bridge are higher than those achievable with ground falsework due to the presence of temporary launch stresses. Casting long deck segments on the ground, however, is simple and repetitive and requires small crews. The learning curve is short, the segments do not require handling, the risks of construction are minimized, the prestressing tendons are long are require less anchorages and stressing operations, and special construction equipment is inexpensive and easily reusable in other projects. Although the quantities of raw materials are higher, incremental launching is a first-choice solution for medium-span bridges of medium length all over the world.
Several parameters influence the cost-effectiveness of bridge design and project organization, the break-even point between different solutions is different in different countries, and often in different regions of the same country, and the choice is hardly standardize-able:
- Length, uniformity and sequence of the spans have a major impact on bridge construction. The span length influences the construction method and the cost of the erection equipment, but the span sequence may have an even greater impact. A longer span between two sequences of regular spans interrupts span continuity and may even lead to different types of bridges for the two approaches.
- For a given span length, the weight of the deck is directly proportional to its width and governs the cost of special construction equipment. The use of twin box girders, for example, may end up less expensive than the use of a single, wide box girder because of the savings in the erection equipment.
- The radius of plan curvature has a major impact on the operations of all types of self-launching erection equipment.
- Bridge context, accessibility of the area beneath the bridge, and height of the piers influence the erection method as well. When the deck can be reached from the ground throughout the bridge, ground crane erection often leads to the most cost-effective erection procedures.
- Cost and availability of skilled labor, project time-schedule, bridge length and logistics influence the optimal level of industrialization of the construction process.
- The number of bridges and their geometric features influence the choice of special erection equipment.
The investment for special equipment generates direct costs and financial exposure. The service life of these machines, however, is much longer than the duration of a typical bridge project, and different strategies are available to diminish the impact of the investment on the project. Leasing the machine for the project duration, selling the machine when no longer necessary, or modifying the machine and reusing it in new projects are typical strategies. Local fiscal rules also play an important role by defining the number of years for full fiscal depreciation of the investment.
The bridge contractors estimate these costs during the bidding process, keep the actual costs monitored during construction, and use the data thus acquired to refine future bids. This information is not of public domain - actually it is kept tightly confidential as it directly affects the contractor's capability of making business. However, comparisons of alternatives based on raw material quantities, labor, investments, energy, indirect costs and risks of construction are often a good starting point in defining the cost-effectiveness of a construction method.
Few has been written on mechanized bridge construction despite its fundamental role in the modern bridge engineering. These are the rationale, mission and values of the bridge engineering eManuals offered by BridgeTech. The eManuals convey trans-disciplinary information and guidance on modern bridge design and construction technology to bridge owners, designers and construction professionals interested in mechanized bridge construction.
- Bridge owners and designers will find comprehensive information on how the bridge will be built and will interact with special equipment during construction.
- Educators will find a solid technology basis for theoretical courses of bridge engineering.
- Contractors will find information on procurement, operations, performance and productivity of special equipment and a guide to value-engineering, time-scheduling, risk analysis, bidding, safety planning and the formation of management and site personnel for the risks and the opportunities of mechanized bridge construction.
- Designers and manufacturers of special equipment will find comprehensive information on design loads and load combinations, calibration of load and resistance factors, design for robustness and redundancy, numerical modelling and analysis, out-of-plane buckling and prevention of progressive collapse, human error, failure of materials and systems, repair, reconditioning and industry trends.
- Construction engineers, resident engineers, inspectors and safety planners will find information on operations, casting cycles, cycle times, loading and structure-equipment interaction.
- Forensic engineers will find numerous case studies on failed equipment.
The construction cost of a modern bridge is a mix of direct costs of labor, structural materials, expendable materials, amortization of the investment for special equipment, energy, and indirect costs related to project duration. The courses of modern bridge design and construction technology that BridgeTech teaches for the Continuing Education Program of the American Society of Civil Engineers and face-to-face in the offices of bridge owners, designers and constructors explore new and emerging bridge technology and modern construction methods. Five courses are available to explore the available construction methods in great detail.
- Mechanized Bridge Construction (2 days) explores configurations, operations, kinematics, loads, performance, productivity, structure-equipment interactions and industry trends for every family of bridge construction machines. The course examines beam launchers and shifters, self-launching gantries and lifting frames for precast segmental bridges, movable scaffolding systems (MSS), form travelers for balanced cantilever decks and arches, forming carriages, span launchers and portal carriers with underbridge for full-span precasting, and the special equipment used for bridge launching. The course also explores the design of bridge piers, abutments and superstructures for safe and efficient use of special equipment.
- Launched Bridges (2 days) provides exhaustive coverage of the design, construction, technology and industry trends of incrementally launched bridges. Richly illustrated with almost 300 photographs, the course discusses the circumstances that make incremental launching a competitive solution and explores and compares the available alternatives. It explains how to control self-weight bending and shear with launch noses, temporary piers and front cable-stayed systems. It explores geometric launchability criteria, the RTM method for parametric launch stress analysis with a spreadsheet, the stability of steel girders, launch post-tensioning and casting yard organization for prestressed-concrete bridges, launch bearings and guides, and thrust and control systems for uphill and downhill launching.
- Precast Segmental Bridges (1 day) explores the geometric design of precast segmental bridges, the casting curve analysis, the fabrication of standardized atypical segments combined with geometry correction in the short-line molds, and the geometry control of short- and long-line casting. For each family of self-launching gantries and lifting frames, the course explores configurations, operations, kinematics, loads, performance, productivity, stiffness interactions to consider for bridge design, the stability of tall bridge piers, and staged application of post-tensioning to avoid joint decompression and the risk of brittle span failure. The course also discusses new solutions for simultaneous erection of adjacent bridges, and the full potential of macro-segmental construction.
- Movable Scaffolding Systems (1 day) explores the use of MSS for span-by-span and balanced cantilever casting. You will learn under which circumstances is span-by-span casting a competitive alternative to incremental launching and precast segmental construction, will compare the use of telescopic MSS for macro-segmental balanced cantilever bridges with in-place casting with form travelers, and will explore bridge design and detailing for effective use of MSS. The course also explains configurations, operations, loading, kinematics, performance, productivity, structure-equipment interactions and industry trends of overhead, OPS and underslung MSS for span-by-span casting, telescopic MSS and form travelers for balanced cantilever casting, and forming carriages for segmental slab casting on steel girders.
- Launched Bridges (1 day) explores the design, construction, technology and industry trends of steel and prestressed-concrete bridges built by incremental launching. It explains the use of launch noses, temporary piers and front cable-stayed systems for control of self-weight bending and shear, the web stability and lateral torsion-flexure buckling of steel girders, and launch post-tensioning, deck segmentation and casting yard organization of prestressed-concrete bridges. For both types of bridges, the course examines launch bearings and guides, thrust systems and how to control the movements of the deck during uphill and downhill launching.
The eManuals further expand and integrate the courses for an exhaustive coverage of the topic.
Photo: courtesy Povoas.